Ch. 6– Routing Theory – Part 2
CCNA Semester 2
Originally by Rick Graziani, Instructor
Modified by Prof. Yousif
Ch. 6 Routing - Part II
Routing Theory and
The success of dynamic routing depends on two basic router
functions:
1. maintenance of a routing table
2. timely distribution of knowledge, in the form of routing updates,
to other routers
Dynamic routing relies on a routing protocol to share knowledge among
routers.
A routing protocol defines the set of rules used by a router when it
communicates with neighboring routers. For example, a routing
protocol describes:
 how to send updates
 what knowledge is contained in these updates
 when to send this knowledge
 how to locate recipients of the updates



When a routing algorithm updates a routing table, its primary
objective is to determine the best information to include in the
table.
Each routing algorithm interprets what is best in its own way.
The algorithm generates a number, called the metric value, for
each path through the network.
Typically, the smaller the metric number, the better the path.
These metrics will be discussed later or in CCNP Advanced
Routing, with their appropriate Routing Protocols:
 RIP – hop count
 IGRP – bandwidth, delay, reliability, load
 EIGRP – bandwidth, delay, reliability, load
 OSPF – bandwidth
 BGP – attribute values and shortest path
Most routing algorithms can be classified as one of two basic
algorithms:
 distance vector
 link state.
The distance-vector routing approach determines the direction
(vector) and the cost or metric (distance) to any link in the
internetwork.
 RIP, IPX RIP and IGRP (CCNA)
 AppleTalk, RTMP and others (non-CCNA)
The link-state (also called shortest path first) approach re-creates
the exact topology of the entire internetwork (or at least the
portion in which the router is situated).
 OSPF
 IS-IS
The balanced hybrid approach combines aspects of
the link-state and distance-vector algorithms.
These are really distance-vector routing protocols which
apply some of the advantages of a link-state routing
protocols, and also known as advanced-distancevector routing protocols.
 EIGRP


When all routers in an internetwork are operating with the same
knowledge, the internetwork is said to have converged.
Fast convergence is a desirable network feature because it
reduces the period of time in which routers would continue to
make incorrect/wasteful routing decisions.
Topics – (Continued)
Part II. Routing Theory and Dynamic Routing Operations
 Dynamic Routing Operations
– Routing Metrics
– Classes of Routing Protocols
– Convergence
 Distance Vector Routing Protocols
– Distance Vector Concepts
– Distance Vector Network Discovery
– Simple Split Horizon (Introduction)
– Distance Vector Network Discovery with Split Horizon
– Network Discovery FAQs
– Triggered Updates
– Routing Loops
– Count to Infinity
– Defining a Maximum
– Split Horizon
– Split Horizon with Poison Reverse
– Holddown Timers
– TTL – IP’s Time-To-Live Field
Distance Vector Concepts
 The mathematical basis of the distance-vector routing protocols
is the Bellman-Ford algorithm.
 Pure distance-vector routing protocols suffer from long
convergence times and possible temporary routing loops
(more in a few moments).
 There are remedies to some situations that may cause these
problems which we will examine in a moment.
172.16.0.0/16
Network Discovery and Routing Table Maintenance





Distance-vector-based routing algorithms pass periodic copies of a
routing table between adjacent routers, from router to router. (RIP
every 30 seconds, IPX RIP every 60 seconds, IGRP every 90
seconds).
These regular updates between routers help routers discover each
other’s networks and communicate topology changes.
Routers only learn about other networks from adjacent routers, their
directly connected neighbors.
Router D learned about Router A’s network 172.16.0.0/16 from Router
C, who learned it from Router B, who learned it from Router A.
This is why distance-vector routing protocols are also known as
routing by rumor.

Distance-vector routing protocols do not allow routers
to know the topology of the network, as they only
know how far a network is (distance: hops) and which
way to forward the packet (vector: exit interface).
(Link-state routing protocols allow routers to see the
exact network topology – later.)
– Distances (hops) are cumulative from one router
to the next
How do routers learn about other networks and
determine the best routes to these networks?


We will now look at the concepts of how this happens.
In Chapter 7 Routing Protocols, we will look at how RIP does
this and view it happening!
Distance-Vector Network Discovery
RTA
Network W
Network X
Routing Table
(Distance) (Vector)
Net. Hops
Exit-int.
W
0
<-X
0
-->
RTB
Network Y
Routing Table
(Distance) (Vector)
Net. Hops
Exit-int.
X
0
<-Y
0
-->
RTC
Network Z
Routing Table
(Distance) (Vector)
Net. Hops
Exit-int.
Y
0
<-Z
0
-->
Initial Routing Tables

The first routes entered in the routing table are directly connected
networks, assigning a metric or cost of “0” (hop count for RIP).
– See previous slides on IP Routing Table and Directly Connected
Networks.
–
–
00:28:56: RT: add 192.168.2.0/24 via 0.0.0.0, connected metric [0/0]
00:28:56: RT: interface Ethernet0 added to routing table
Remember, these interfaces must be “up” and “up”
 The next step is for routers to share their complete routing tables with
any and all directly connected neighboring routers.
 Distance-vector routing protocols do not maintain formal relationships
with neighboring routers, I.e. they do not know who their neighboring
routers are.
 So how do they know who to send their routing tables to? Distancevector routing protocols use a broadcast or multicast address to send
out routing updates, although you can specify a unicast address. (later)

Distance-Vector Network Discovery
RTA
Network W
Routing Update
Net. Hops Next-hop-add
W
1
RTA
X
1
RTA
Network X
RTB
Network Y
Routing Update
Net. Hops Next-hop-add
X
1
RTB
Y
1
RTB
RTC
Network Z
Routing Update
Net. Hops Next-hop-add
Y
1
RTC
Z
1
RTC
Sent via broadcast or multicast (later)
Sending Routing Tables to Neighbors

(Later, we will see another example with real IP addresses.)
 In their routing updates to the neighboring router(s), distance-vector
routing protocols include the following information for each network in
their routing table:
– Network address – This would normally be an ip network address.
– The metric or cost (with RIP this is the number of hops, but can be
other metrics for other distance-vector routing protocols).
• Since we are using hops in this example, RIP increments the
hop count by one in its routing table before sending out the
routing update.
– Next-hop address – This would normally be the ip address of the
interface from which the routing update was sent.
Distance-Vector Network Discovery
RTA
Network W
Routing Update
Net. Hops Next-hop-add
W
1
RTA
X
1
RTA
Routing Table
(Distance) (Vector)
Net. Hops
Exit-int.
W
0
<-X
0
-->
Y
1
RTB
Network X
RTB
Network Y
Routing Update
Net. Hops Next-hop-add
X
1
RTB
Y
1
RTB
Routing Table
(Distance) (Vector)
Net. Hops
Exit-int.
X
0
<-Y
0
-->
W
1
RTA
Z
1
RTC
RTC
Network Z
Routing Update
Net. Hops Next-hop-add
Y
1
RTC
Z
1
RTC
Routing Table
(Distance) (Vector)
Net. Hops
Exit-int.
Y
0
<-Z
0
-->
X
1
RTB
Routers Update their Routing Tables


A router will enter this update into its routing table if:
– It is a new route, a network which is not currently in its routing table.
– It is an existing route, a network which is currently in its routing
table, but this update has a better (smaller) metric (fewer hops).
– Note: If the update contains a route to an existing route, with the
same metric (hops), but via a different interface, the router may or
may not add it to its routing table depending upon whether or not
the routing protocol is providing load balancing (later). RIP does
provide this.
A router will not enter this update into its routing table if:
– It is an existing route with a worse metric.
Distance-Vector Network Discovery
RTA
Network W
Routing Update
Net. Hops Next-hop-add
W
1
RTA
X
1
RTA
New
Routing
Tables
Routing Table
(Distance) (Vector)
Net. Hops
Exit-int.
W
0
<-X
0
-->
Y
1
RTB
Network X
RTB
Network Y
Routing Update
Net. Hops Next-hop-add
X
1
RTB
Y
1
RTB
Routing Table
(Distance) (Vector)
Net. Hops
Exit-int.
X
0
<-Y
0
-->
W
1
RTA
Z
1
RTC
RTC
Network Z
Routing Update
Net. Hops Next-hop-add
Y
1
RTC
Z
1
RTC
Routing Table
(Distance) (Vector)
Net. Hops
Exit-int.
Y
0
<-Z
0
-->
X
1
RTB
Routers Update their Routing Tables (continued)
RTA:
 Ignores the route to X because it has an existing, better route with a cost=0.
 Accepts the route to Y because it didn’t exist in the routing table.
RTB (from RTA):
 Accepts the route to W because it didn’t exist in the routing table.
 Ignores the route to X because it has an existing, better route with a cost=0.
RTB (from RTC):
 Ignores the route to Y because it has an existing, better route with a cost=0.
 Accepts the route to Z because it didn’t exist in the routing table.
RTC:
 Accepts the route to X because it didn’t exist in the routing table.
 Ignores the route to Y because it has an existing, better route with a cost=0.
Distance-Vector Network Discovery
RTA
Network W
Routing Update
Net. Hops Next-hop-add
W
1
RTA
X
1
RTA
Y
2
RTA
Network X
RTB
Network Y
Routing Update
Net. Hops Next-hop-add
X
1
RTB
Y
1
RTB
W
2
RTB
Z
2
RTB
RTC
Network Z
Routing Update
Net. Hops Next-hop-add
Y
1
RTC
Z
1
RTC
X
2
RTC
Next Round of Routing Updates

We have still not reached convergence, because all of the routers do not
have complete and accurate network information.
 Which router does have complete information? Which ones do not?
 In the next round, routers must forward their new routing tables in the form
of routing updates, to their directly connected neighbors.
 Remember, with the distance-vector routing protocol RIP, the router
increments the number of hops in its own routing table by one, before
sending out the routing update. - “If it is one hop for me to get there, and
you are getting there via me, then it is two hops for you.”
Distance-Vector
Network
Discovery
RTA
RTB
RTC
Network W
Existing
Routing
Tables
Routing Table
(Distance) (Vector)
Net. Hops
Exit-int.
W
0
<-X
0
-->
Y
1
RTB
Routing Update
Net. Hops Next-hop-add
W
1
RTA
X
1
RTA
Y
2
RTA
New
Routing
Tables
Routing Table
(Distance) (Vector)
Net. Hops
Exit-int.
W
0
<-X
0
-->
Y
1
RTB
Z
2
RTB
Network X
Network Y
Routing Table
(Distance) (Vector)
Net. Hops
Exit-int.
X
0
<-Y
0
-->
W
1
RTA
Z
1
RTC
Routing Update
Net. Hops Next-hop-add
X
1
RTB
Y
1
RTB
W
2
RTB
Z
2
RTB
Routing Table
(Distance) (Vector)
Net. Hops
Exit-int.
X
0
<-Y
0
-->
W
1
RTA
Z
1
RTC
Network Z
Routing Table
(Distance) (Vector)
Net. Hops
Exit-int.
Y
0
<-Z
0
-->
X
1
RTB
Routing Update
Net. Hops Next-hop-add
Y
1
RTC
Z
1
RTC
X
2
RTC
Routing Table
(Distance) (Vector)
Net. Hops
Exit-int.
Y
0
<-Z
0
-->
X
1
RTB
W
2
RTB
Routers Update their Routing Tables - again
RTA: (Advertised Cost Compared to current local cost)
 Ignores the route to X because it has an existing, better route with a cost=0.
 Ignores the route to Y because it already has that route, with the same cost.
 Ignores the route to W because it has an existing, better route with a cost=0.
– “I am not going to send you the packet, so you can send it back to me, …”
– Later, we will see that split-horizon prohibits this route from being sent.

Accepts the route to Z because it didn’t exist in the routing table.
Distance-Vector
Network
Discovery
RTA
RTB
RTC
Network W
Existing
Routing
Tables
Routing Table
(Distance) (Vector)
Net. Hops
Exit-int.
W
0
<-X
0
-->
Y
1
RTB
Routing Update
Net. Hops Next-hop-add
W
1
RTA
X
1
RTA
Y
2
RTA
New
Routing
Tables
Routing Table
(Distance) (Vector)
Net. Hops
Exit-int.
W
0
<-X
0
-->
Y
1
RTB
Z
2
RTB
Network X
Network Y
Routing Table
(Distance) (Vector)
Net. Hops
Exit-int.
X
0
<-Y
0
-->
W
1
RTA
Z
1
RTC
Routing Update
Net. Hops Next-hop-add
X
1
RTB
Y
1
RTB
W
2
RTB
Z
2
RTB
Routing Table
(Distance) (Vector)
Net. Hops
Exit-int.
X
0
<-Y
0
-->
W
1
RTA
Z
1
RTC
Network Z
Routing Table
(Distance) (Vector)
Net. Hops
Exit-int.
Y
0
<-Z
0
-->
X
1
RTB
Routing Update
Net. Hops Next-hop-add
Y
1
RTC
Z
1
RTC
X
2
RTC
Routing Table
(Distance) (Vector)
Net. Hops
Exit-int.
Y
0
<-Z
0
-->
X
1
RTB
W
2
RTB
Routers Update their Routing Tables – again (continued)
RTB:
 Ignores all routes from both RTA and RTC, because it already has those
routes, with the same costs.
 No new information.
Distance-Vector
Network
Discovery
RTA
RTB
RTC
Network W
Existing
Routing
Tables
Routing Table
(Distance) (Vector)
Net. Hops
Exit-int.
W
0
<-X
0
-->
Y
1
RTB
Routing Update
Net. Hops Next-hop-add
W
1
RTA
X
1
RTA
Y
2
RTA
New
Routing
Tables
Routing Table
(Distance) (Vector)
Net. Hops
Exit-int.
W
0
<-X
0
-->
Y
1
RTB
Z
2
RTB
Network X
Network Y
Routing Table
(Distance) (Vector)
Net. Hops
Exit-int.
X
0
<-Y
0
-->
W
1
RTA
Z
1
RTC
Routing Update
Net. Hops Next-hop-add
X
1
RTB
Y
1
RTB
W
2
RTB
Z
2
RTB
Routing Table
(Distance) (Vector)
Net. Hops
Exit-int.
X
0
<-Y
0
-->
W
1
RTA
Z
1
RTC
Network Z
Routing Table
(Distance) (Vector)
Net. Hops
Exit-int.
Y
0
<-Z
0
-->
X
1
RTB
Routing Update
Net. Hops Next-hop-add
Y
1
RTC
Z
1
RTC
X
2
RTC
Routing Table
(Distance) (Vector)
Net. Hops
Exit-int.
Y
0
<-Z
0
-->
X
1
RTB
W
2
RTB
Routers Update their Routing Tables - again
RTC:
 Ignores the route to X because it already has that route, with the same cost.
 Ignores the route to Y because it has an existing, better route with a cost=0.
 Accepts the route to W because it didn’t exist in the routing table.
 Ignores the route to Z because it has an existing, better route with a cost=0.
– “I am not going to send you the packet, so you can send it back to me, …”
Distance-Vector Network Discovery
RTA
Network W
Routing
Tables
Routing Table
(Distance) (Vector)
Net. Hops
Exit-int.
W
0
<-X
0
-->
Y
1
RTB
Z
2
RTB
Network X
RTB
Network Y
Routing Table
(Distance) (Vector)
Net. Hops
Exit-int.
X
0
<-Y
0
-->
W
1
RTA
Z
1
RTC
RTC
Network Z
Routing Table
(Distance) (Vector)
Net. Hops
Exit-int.
Y
0
<-Z
0
-->
X
1
RTB
W
2
RTB
Convergence!
 All of the routers now have a consistent and accurate view of the
network.
 Later, we will see how RIP handles this operation.
Note: The on-line curriculum has incorrect
information regarding split horizon.
Split Horizon Rule

Before we continue looking at routing tables and network discovery,
using real ip addresses, let’s take a look at the split horizon rule.
 “The effect of split horizon is that a router will send out different routing
messages on different interfaces. In effect a router never sends out
information on an interface that it learned from that interface.” (Lewis,
Cisco TCP/IP Routing)
 As we will see later in this presentation, split horizon helps prevent
routing loops. (Discussed in much more detail soon.)
 For now, we will see that split horizon means that the router does not
send out all of the information in the routing table to its neighbors.
 Note: Usually, split horizon is enabled and can be disabled.
Network Discovery with Split Horizon
10.1.1.0/24
.1
RTA
10.1.2.0/24
.1
.2
s0
s0
e0
RTB
10.1.3.0/24
.1
s1
RTC
.2
s0
10.1.4.0/24
.1
.2
s1
s0
RTD
.1
10.1.5.0/24
e0
Routing Table
Routing Table
Routing Table
Routing Table
Net.
Hops Ex-Int
10.1.1.0/24 0
e0
10.1.2.0/24 0
s0
Net.
Hops Ex-Int
10.1.2.0/24 0
s0
10.1.3.0/24 0
s1
Net.
Hops Ex-Int
10.1.3.0/24 0
s0
10.1.4.0/24 0
s1
Net.
Hops Ex-Int
10.1.4.0/24 0
s0
10.1.5.0/24 0
e0
Initial routing tables
Network Discovery with Split Horizon





First of all, notice we are using real ip addresses.
In the routing updates, next-hop ip addresses for the networks are sent to
the neighboring router specifying the address it can use to forward
packets to.
The split horizon rule also affects common networks between two
routers.
To the router, a directly connected network is known via its own interface,
so it does not include that network in routing updates sent out that same
interface.
In other words, the router does not send information about a directly
connected network out the interface of that directly connected network.
Network Discovery with Split Horizon
10.1.1.0/24
.1
RTA
10.1.2.0/24
.1
.2
s0
s0
e0
RTB
10.1.3.0/24
.1
s1
.2
s0
RTC
10.1.4.0/24
.1
.2
s1
s0
RTD
.1
10.1.5.0/24
e0
Routing Table
Routing Table
Routing Table
Routing Table
Net.
Hops Ex-Int
10.1.1.0/24 0
e0
10.1.2.0/24 0
s0
Net.
Hops Ex-Int
10.1.2.0/24 0
s0
10.1.3.0/24 0
s1
Net.
Hops Ex-Int
10.1.3.0/24 0
s0
10.1.4.0/24 0
s1
Net.
Hops Ex-Int
10.1.4.0/24 0
s0
10.1.5.0/24 0
e0
Routing Update
Next-hop
Net.
Hops Address
10.1.1.0/24 1 10.1.1.1
-----------------------10.1.2.0/24 1 10.1.1.1
X
Routing Update
Next-hop
Net.
Hops Address
10.1.2.0/24 1 10.1.3.1
-----------------------10.1.3.0/24 1 10.1.2.2
Routing Update
Next-hop
Net.
Hops Address
10.1.3.0/24 1 10.1.4.1
-----------------------10.1.4.0/24 1 10.1.3.2
Routing Update
Next-hop
Net.
Hops Address
10.1.4.0/24 1 10.1.4.2
-----------------------10.1.5.0/24 1 10.1.4.2
X
Network Discovery with Split Horizon - First Round of Updates
“The effect of split horizon is that a router will send out different routing
messages on different interfaces. In effect a router never sends out
information on an interface that it learned from that interface.” (Lewis)
RTA’s routing update sent out serial 0 to RTB
 Includes the network 10.1.1.0/24 which RTB can reach via 10.1.1.1.
 Split horizon: Does not include the 10.1.2.0/24 network because that
network was learned via serial 0 (interface serial 0, ip address 10.1.1.1
…) – a common network between RTA and RTB.

Network Discovery with Split Horizon
10.1.1.0/24
.1
RTA
10.1.2.0/24
.1
.2
s0
s0
e0
RTB
10.1.3.0/24
.1
s1
.2
s0
RTC
10.1.4.0/24
.1
.2
s1
s0
RTD
.1
10.1.5.0/24
e0
Routing Table
Routing Table
Routing Table
Routing Table
Net.
Hops Ex-Int
10.1.1.0/24 0
e0
10.1.2.0/24 0
s0
Net.
Hops Ex-Int
10.1.2.0/24 0
s0
10.1.3.0/24 0
s1
Net.
Hops Ex-Int
10.1.3.0/24 0
s0
10.1.4.0/24 0
s1
Net.
Hops Ex-Int
10.1.4.0/24 0
s0
10.1.5.0/24 0
e0
Routing Update
Next-hop
Net.
Hops Address
10.1.1.0/24 1 10.1.1.1
-----------------------10.1.2.0/24 1 10.1.1.1
X
Routing Update
Next-hop
Net.
Hops Address
10.1.2.0/24 1 10.1.3.1
-----------------------10.1.3.0/24 1 10.1.2.2
Routing Update
Next-hop
Net.
Hops Address
10.1.3.0/24 1 10.1.4.1
-----------------------10.1.4.0/24 1 10.1.3.2
Routing Update
Next-hop
Net.
Hops Address
10.1.4.0/24 1 10.1.4.2
-----------------------10.1.5.0/24 1 10.1.4.2
X
Network Discovery with Split Horizon - First Round of Updates
RTB’s routing update sent out serial 0 to RTA
 Includes the network 10.1.3.0/24 which RTA can reach via 10.1.2.2.
 Split horizon: Does not include the 10.1.2.0/24 network. Split horizon
blocks the 10.1.2.0/24 update from being sent to RTA with a hop count of
“1.” (Note: To keep the diagrams less cluttered, omission of the proper
red/blue arrow means split horizon is in affect, same as the “X.”)
RTB’s routing update sent out serial 1 to RTC
 Includes the network 10.1.2.0/24 which RTC can reach via 10.1.3.1.
 Split horizon: Does not include the 10.1.3.0/24 network.
Network Discovery with Split Horizon
10.1.1.0/24
.1
RTA
10.1.2.0/24
.1
.2
s0
s0
e0
RTB
10.1.3.0/24
.1
s1
.2
RTC
s0
10.1.4.0/24
.1
.2
s1
s0
RTD
.1
10.1.5.0/24
e0
Routing Table
Routing Table
Routing Table
Routing Table
Net.
Hops Ex-Int
10.1.1.0/24 0
e0
10.1.2.0/24 0
s0
Net.
Hops Ex-Int
10.1.2.0/24 0
s0
10.1.3.0/24 0
s1
Net.
Hops Ex-Int
10.1.3.0/24 0
s0
10.1.4.0/24 0
s1
Net.
Hops Ex-Int
10.1.4.0/24 0
s0
10.1.5.0/24 0
e0
Routing Update
Next-hop
Net.
Hops Address
10.1.1.0/24 1 10.1.1.1
-----------------------10.1.2.0/24 1 10.1.1.1
X
Routing Update
Next-hop
Net.
Hops Address
10.1.2.0/24 1 10.1.3.1
-----------------------10.1.3.0/24 1 10.1.2.2
Routing Update
Next-hop
Net.
Hops Address
10.1.3.0/24 1 10.1.4.1
-----------------------10.1.4.0/24 1 10.1.3.2
Routing Update
Next-hop
Net.
Hops Address
10.1.4.0/24 1 10.1.4.2
-----------------------10.1.5.0/24 1 10.1.4.2
X
Network Discovery with Split Horizon - First Round of Updates

Same with updates from RTC and RTD.
Your Turn:
 Write out the new routing tables for each router after this round.
 Also, find any mistakes I might have made 
Network Discovery with Split Horizon
10.1.1.0/24
.1
RTA
10.1.2.0/24
.1
.2
s0
s0
e0
RTB
10.1.3.0/24
.1
s1
.2
s0
RTC
10.1.4.0/24
.1
.2
s1
s0
RTD
.1
10.1.5.0/24
e0
Routing Table
Routing Table
Routing Table
Routing Table
Net.
Hops Ex-Int
10.1.1.0/24 0
e0
10.1.2.0/24 0
s0
Net.
Hops Ex-Int
10.1.2.0/24 0
s0
10.1.3.0/24 0
s1
Net.
Hops Ex-Int
10.1.3.0/24 0
s0
10.1.4.0/24 0
s1
Net.
Hops Ex-Int
10.1.4.0/24 0
s0
10.1.5.0/24 0
e0
Routing Update
Next-hop
Net.
Hops Address
10.1.1.0/24 1 10.1.1.1
-----------------------10.1.2.0/24 1 10.1.1.1
Routing Update
Next-hop
Net.
Hops Address
10.1.2.0/24 1 10.1.3.1
-----------------------10.1.3.0/24 1 10.1.2.2
Routing Update
Next-hop
Net.
Hops Address
10.1.3.0/24 1 10.1.4.1
-----------------------10.1.4.0/24 1 10.1.3.2
Routing Table
Routing Table
Routing Table
Routing Table
Net.
Hops Ex-Int
10.1.1.0/24 0
e0
10.1.2.0/24 0
s0
10.1.3.0/24 1 10.1.2.2
Net.
Hops Ex-Int
10.1.2.0/24 0
s0
10.1.3.0/24 0
s1
10.1.1.0/24 1 10.1.2.1
10.1.4.0/24 1 10.1.3.2
Net.
Hops Ex-Int
10.1.3.0/24 0
s0
10.1.4.0/24 0
s1
10.1.2.0/24 1 10.1.3.1
10.1.5.0/24 1 10.1.4.2
Net.
Hops Ex-Int
10.1.4.0/24 0
s0
10.1.5.0/24 0
e0
10.1.3.0/24 1 10.1.4.1
X
Routing Update
Next-hop
Net.
Hops Address
10.1.4.0/24 1 10.1.4.2
-----------------------10.1.5.0/24 1 10.1.4.2
X
Answer – Do your routing tables look like these?
Now – What do the next round of routing updates look like?
Show the routes which are sent (propagated) and those that
are not sent because of split horizon.
Network Discovery with Split Horizon
10.1.1.0/24
.1
e0
RTA
10.1.2.0/24
.1
.2
s0
s0
RTB
10.1.3.0/24
.1
s1
.2
RTC
s0
10.1.4.0/24
.1
.2
s1
s0
RTD
.1
10.1.5.0/24
e0
Routing Table
Routing Table
Routing Table
Routing Table
Net.
Hops Ex-Int
10.1.1.0/24 0
e0
10.1.2.0/24 0
s0
10.1.3.0/24 1 10.1.2.2
Net.
Hops Ex-Int
10.1.2.0/24 0
s0
10.1.3.0/24 0
s1
10.1.1.0/24 1 10.1.2.1
10.1.4.0/24 1 10.1.3.2
Net.
Hops Ex-Int
10.1.3.0/24 0
s0
10.1.4.0/24 0
s1
10.1.2.0/24 1 10.1.3.1
10.1.5.0/24 1 10.1.4.2
Net.
Hops Ex-Int
10.1.4.0/24 0
s0
10.1.5.0/24 0
e0
10.1.3.0/24 1 10.1.4.1
Routing Update
Next-hop
Net.
Hops Address
10.1.3.0/24 1 10.1.4.1
10.1.2.0/24 2 10.1.4.1
-----------------------10.1.4.0/24 1 10.1.3.2
10.1.5.0/24 2 10.1.3.2
Routing Update
Next-hop
Net.
Hops Address
X10.1.4.0/24 1 10.1.4.2
X10.1.3.0/24 2 10.1.4.2
-----------------------10.1.5.0/24 1 10.1.4.2
Routing Update
Next-hop
Net.
Hops Address
10.1.1.0/24 1 10.1.1.1
-----------------------10.1.2.0/24 1 10.1.1.1 X
10.1.3.0/24 2 10.1.1.1 X
Routing Update
Next-hop
Net.
Hops Address
10.1.2.0/24 1 10.1.3.1
10.1.1.0/24 2 10.1.3.1
-----------------------10.1.3.0/24 1 10.1.2.2
10.1.4.0/24 2 10.1.2.2
Answer – Do your routing updates look like these?
 Again, omission of the red/blue arrow means split horizon is in affect.
 For example, RTB is not sending the route 10.1.1.0/24 to RTA to tell RTA
it can get to 10.1.1./24 in 2 hops via RTB. - This would make sense!
 Split horizon - router never sends out information on an interface that it
learned from that interface
Now – What do the routing tables look like?
Network Discovery with Split Horizon
10.1.1.0/24
.1
RTA
e0
10.1.2.0/24
.1
.2
s0
s0
RTB
10.1.3.0/24
.1
s1
.2
s0
RTC
10.1.4.0/24
.1
.2
s1
s0
RTD
.1
10.1.5.0/24
e0
Routing Table
Routing Table
Routing Table
Routing Table
Net.
Hops Ex-Int
10.1.1.0/24 0
e0
10.1.2.0/24 0
s0
10.1.3.0/24 1 10.1.2.2
Net.
Hops Ex-Int
10.1.2.0/24 0
s0
10.1.3.0/24 0
s1
10.1.1.0/24 1 10.1.2.1
10.1.4.0/24 1 10.1.3.2
Net.
Hops Ex-Int
10.1.3.0/24 0
s0
10.1.4.0/24 0
s1
10.1.2.0/24 1 10.1.3.1
10.1.5.0/24 1 10.1.4.2
Net.
Hops Ex-Int
10.1.4.0/24 0
s0
10.1.5.0/24 0
e0
10.1.3.0/24 1 10.1.4.1
Routing Update
Next-hop
Net.
Hops Address
10.1.3.0/24 1 10.1.4.1
10.1.2.0/24 2 10.1.4.1
-----------------------10.1.4.0/24 1 10.1.3.2
10.1.5.0/24 2 10.1.3.2
Routing Update
Next-hop
Net.
Hops Address
X10.1.4.0/24 1 10.1.4.2
X10.1.3.0/24 2 10.1.4.2
-----------------------10.1.5.0/24 1 10.1.4.2
Routing Update
Next-hop
Net.
Hops Address
10.1.1.0/24 1 10.1.1.1
-----------------------10.1.2.0/24 1 10.1.1.1 X
10.1.3.0/24 2 10.1.1.1 X
Routing Update
Next-hop
Net.
Hops Address
10.1.2.0/24 1 10.1.3.1
10.1.1.0/24 2 10.1.3.1
-----------------------10.1.3.0/24 1 10.1.2.2
10.1.4.0/24 2 10.1.2.2
Routing Table
Routing Table
Routing Table
Routing Table
Net.
Hops Ex-Int
10.1.1.0/24 0
e0
10.1.2.0/24 0
s0
10.1.3.0/24 1 10.1.2.2
10.1.4.0/24 2 10.1.2.2
Net.
Hops Ex-Int
10.1.2.0/24 0
s0
10.1.3.0/24 0
s1
10.1.1.0/24 1 10.1.2.1
10.1.4.0/24 1 10.1.3.2
10.1.5.0/24 2 10.1.3.2
Net.
Hops Ex-Int
10.1.3.0/24 0
s0
10.1.4.0/24 0
s1
10.1.2.0/24 1 10.1.3.1
10.1.5.0/24 1 10.1.4.2
10.1.1.0/24 2 10.1.3.1
Net.
Hops Ex-Int
10.1.4.0/24 0
s0
10.1.5.0/24 0
e0
10.1.3.0/24 1 10.1.4.1
10.1.2.0/24 2 10.1.4.1
Answer – Do your routing tables look like these? Convergence?

Note: Newest routing table entries are at the bottom of the routing tables in these
diagrams.
Now – What do the next round of routing updates look like and the
routing tables? (We’ll finish this up  )
Network Discovery with Split Horizon
10.1.1.0/24
.1
e0
RTA
10.1.2.0/24
RTB
.1
.2
s0
s0
10.1.3.0/24
.1
s1
.2
s0
RTC
10.1.4.0/24
.1
.2
s1
s0
RTD
.1
10.1.5.0/24
e0
Routing Table
Routing Table
Routing Table
Routing Table
Net.
Hops Ex-Int
10.1.1.0/24 0
e0
10.1.2.0/24 0
s0
10.1.3.0/24 1 10.1.2.2
10.1.4.0/24 2 10.1.2.2
Net.
Hops Ex-Int
10.1.2.0/24 0
s0
10.1.3.0/24 0
s1
10.1.1.0/24 1 10.1.2.1
10.1.4.0/24 1 10.1.3.2
10.1.5.0/24 2 10.1.3.2
Net.
Hops Ex-Int
10.1.3.0/24 0
s0
10.1.4.0/24 0
s1
10.1.2.0/24 1 10.1.3.1
10.1.5.0/24 1 10.1.4.2
10.1.1.0/24 2 10.1.3.1
Net.
Hops Ex-Int
10.1.4.0/24 0
s0
10.1.5.0/24 0
e0
10.1.3.0/24 1 10.1.4.1
10.1.2.0/24 2 10.1.4.1
Routing Update
Next-hop
Net.
Hops Address
10.1.2.0/24 1 10.1.3.1
10.1.1.0/24 2 10.1.3.1
-----------------------10.1.3.0/24 1 10.1.2.2
10.1.4.0/24 2 10.1.2.2
10.1.5.0/24 3 10.1.2.2
Routing Update
Next-hop
Net.
Hops Address
10.1.3.0/24 1 10.1.4.1
10.1.2.0/24 2 10.1.4.1
10.1.1.0/24 3 10.1.4.1
-----------------------10.1.4.0/24 1 10.1.3.2
10.1.5.0/24 2 10.1.3.2
Routing Update
Next-hop
Net.
Hops Address
10.1.1.0/24 1 10.1.1.1
-----------------------10.1.2.0/24 1 10.1.1.1 X
10.1.3.0/24 2 10.1.1.1 X
10.1.4.0/24 3 10.1.1.1 X
Routing Update
Next-hop
Net.
Hops Address
X10.1.4.0/24 1 10.1.4.2
X10.1.3.0/24 2 10.1.4.2
X10.1.2.0/24 3 10.1.4.2
-----------------------10.1.5.0/24 1 10.1.4.2
Routing Table
Routing Table
Routing Table
Routing Table
Net.
Hops Ex-Int
10.1.1.0/24 0
e0
10.1.2.0/24 0
s0
10.1.3.0/24 1 10.1.2.2
10.1.4.0/24 2 10.1.2.2
10.1.5.0/24 3 10.1.2.2
Net.
Hops Ex-Int
10.1.2.0/24 0
s0
10.1.3.0/24 0
s1
10.1.1.0/24 1 10.1.2.1
10.1.4.0/24 1 10.1.3.2
10.1.5.0/24 2 10.1.3.2
Net.
Hops Ex-Int
10.1.3.0/24 0
s0
10.1.4.0/24 0
s1
10.1.2.0/24 1 10.1.3.1
10.1.5.0/24 1 10.1.4.2
10.1.1.0/24 2 10.1.3.1
Net.
Hops Ex-Int
10.1.4.0/24 0
s0
10.1.5.0/24 0
e0
10.1.3.0/24 1 10.1.4.1
10.1.2.0/24 2 10.1.4.1
10.1.1.0/24 3 10.1.4.1
Answer – Do your routing tables look like these?
Convergence? – YES!
Good Job!
FAQs – Network Discovery
Q: How often does initial network discovery happen?
A: Only when the network first comes up.
Q: Do routers share routing table information after network
discovery?
A: Yes, distance-vector routing protocols share their entire routing
tables periodically (with or without split horizon enabled).
Distance vector routing protocols on Cisco routers by default
use split horizon with poison reverse (discussed in the next
section). Depending upon the distance-vector routing protocol,
the frequency of the updates will happen for RIP every 30
seconds, IPX RIP every 60 seconds, and IGRP every 90
seconds.
Q: What happens when there is a change in the topology, link goes
down, new network is added, new router, is added, etc.?
A: Let’s take a look.
Triggered Updates
 Routers do not have to wait for the periodic update to hear about
changes in the network topology.
 Improvements to the distance-vector algorithm is typically made
in distance-vector routing protocols, like RIP, to include
triggered updates.
 Even with triggered updates, large distance vector networks can
suffer from long convergence times in some situations.
Triggered updates: (continued)
 Triggered updates are sent whenever a router sees a topology
change or a change in routing information (from another router).
 The router does not have to wait for the period timer, but can
send them immediately.
 Triggered updates do not need to include the entire routing table
but only the modified route(s).
Triggered updates: (continued)
 Triggered updates must still be sent to adjacent routers, from
router to router, like other routing updates.
 Most distance-vector routing protocols limit the frequency of
triggered updates so that a flapping link does not put an
unnecessary load on the network. (RIP: random 1 to 5 seconds)
 Typically, triggered updates can be “triggered” by:
– Interface transition to the up or down state
– A route has entered or exited an unreachable (down) state (later)
– A new route is installed in the routing table
Routing Loops

Distance vector routing protocols are simple in their implementaton and
configuration, but this comes at a price.
 Pure distance vector routing protocols suffer from possible routing
loops.
 Routing loops can cause major network problems, from packets getting
lost (blackholed) in your network, to bringing down your entire network.
 Several remedies to have been added to distance-vector algorithms to
help prevent routing loops including:
– Split horizon
– Hold-down timers
– Defining a maximum metric
Routing Loops (continued)

What can cause routing loops?
 Routing loops can occur when there are:
– Incorrect or inconsistent routing updates due to slow convergence
after a topology change. (Example coming up next.)
– Incorrect or incomplete routing information (see presentation on
Discard Routes)
– Static routes incorrectly configured with an intermediate address
which does not become resolved in the routing table. (see
presentation on Static Routes – Additional Information)
Routing Loop Example
 Assume for the remainder of this example that Router C’s
preferred path to network 1 is by way of Router B.
 Router C’s routing table has a distance of 3 to network 1 via
Router B.
Network 1 Fails
 Router E sends an update to Router A.
 Router A stops routing packets to network 1.
 But Routers B, C, and D continue to do so because they have
not yet been informed about the failure.
 Router A sends out its update.
 Routers B and D stop routing to network1, (via Router A).
 However, Router C is still not updated.
 To router C, network 1 is still reachable via router B.
Router C sends a periodic update to Router D

Router C sends a periodic update to Router D indicating a path to
network 1 (by way) of via Router B. (4 hops).
Router D’s Routing Table information for Network 1





Current path to Network 1 = Unreachable (down)
Information from Router C: Network 1 : 4 hops by way of Router C
Normally, RouterD ignores this routing information because it usually
has a better route, 2 hops, via Router A, but this route is now down.
Router D changes its routing table to reflect this (good) better, but
incorrect information, Network 1 by way of Router C (4 hops)
Router D propagates the information to Router A.
Routers A changes its routing table


Router A adds new route to its routing table, get to Network 1 by way of
Router D (5 hops).
Propagates the information to Routers B and E.
Router B (and Router E) change their routing tables



Router B now believes it can get to Network 1 by way of Router A (6
hops).
Wow! I was about to tell Router C that Network 1 was down via Router
B, but now I have new information!
Propagates the incorrect information to Router C.
Router C changes its routing table

Router C still believes it can get to Network 1 by way of Router B (7
hops).
 Of course now it believes it is 7 hops instead of 3.
 Propagates the newer but still incorrect information to Router D.
Here we go again!


Data packets destined for Network 1 get caught in a routing loop, from
Routers A to D to C to B to A to D etc.
As routing updates continue between the routers, the hop count gets
greater – to infinity? (Not quite – we will see in a moment.)
Counting to Infinity
 The routing loop we just saw creates another problem, known as
“Counting to Infinity.”
 This condition, called count to infinity, loops packets continuously
around the network in spite of the fundamental fact that the destination
network, Network 1, is down.
 While the routers are counting to infinity, the invalid information allows a
routing loop to exist.
 Without countermeasures to stop the process, the distance vector
(metric) of hop count increments each time the packet passes through
another router. - These packets loop through the network because of
wrong information in the routing tables.
Solution to Counting to Infinity: Defining a Maximum metric
 Distance vector routing protocols remedy the problem by limiting the
maximum number of hops for any route in the routing table.
 When the distance vector routing protocol has a route with a metric that
is more than its maximum-value, it is denoted as “infinity” and the
route is considered “unreachable.”
 For RIP the maximum-value is 15 (hops), infinity is 16 (hops).
 For IGRP the maximum-value 100 (hops), infinity is 101 (hops).
– IGRP uses bandwidth, delay, reliability and load for its metric in
determining best path.
– IGRP does not use hop count as this metric. Hop count is only
used by IGRP to stop the counting to infinity behavior. (more later)
Solution to Counting to Infinity: Defining a Maximum metric
 Remember that distance vector routing protocols like RIP, increment
the metric (hop count) before sending the routing update to their
adjacent routers.
– After incrementing the hop count, if the metric (hops) is less than
15, routing updates to other adjacent routes will receive a valid
route for this network from this router.
– After incrementing the hop count, if the metric (hop count) is equal
to 15, this router will be able to route packets to this network, 15
hops away, but routing updates to other adjacent routers will have
the incremented hop count of 16 (infinity). - This means other
routers cannot reach this network via this router.
– After incrementing the hop count, if the metric (hop count) is equal
to 16,“infinity”, this router will not be able to route packets to this
network. Routing updates to other adjacent routers will also have
the hop count of 16 (infinity), which means they cannot reach this
network via this router.

There is another situation where the router itself will modify the hop
count to infinity – split horizon with poison reverse. – Coming up
next!
FAQs – Defining a maximum-value
Q: Why does RIP use a hop count as the route metric, and why is
its maximum value limited to 15?
A: When RIP was designed and implemented, dynamic routing
protocols were not widely used. Instead, networks relied mostly
on static routing. RIP, even with its hop-count-metric – which
seems very poor to us today – was quite a big improvement.
Counting intermediate routes is the simplest method to measure
the quality of routes. Setting the infinity value for the metric is
always a problem of choosing between wider networks and
faster convergence when the protocol starts counting. When RIP
was invented, it seemed unlikely to have a network with the
maximum diameter of more than 15 routers, so 16 was chosen
as the infinity value. (Zinin, Cisco IP Routing)
Split Horizon Rule - Reminder
“The effect of split horizon is that a router will send out different routing
messages on different interfaces. In effect a router never sends out
information on an interface that it learned from that interface.” (Lewis,
Cisco TCP/IP Routing)
 Earlier we saw that split horizon meant that the router does not send
out all of the information in the routing table to its neighbors.
– If a router learned about a network from an adjacent router, it does
not include that network in its routing updates to that neighbor.
(See previous slides, Network Discovery and Split Horizon.)
 As we will now see split horizon also helps prevent routing loops.

Simple Split Horizon
10.1.1.0/24
.1
RTA
10.1.2.0/24
.1
.2
s0
s0
e0
RTB
10.1.3.0/24
.1
e0
Routing Table
Routing Table
Net.
Hops Ex-Int
10.1.1.0/24 0
e0
10.1.2.0/24 0
s0
Net.
Hops Ex-Int
10.1.2.0/24 0
s0
10.1.3.0/24 0
e0
Initial
routing
tables
Split Horizon Rule – Avoiding Routing Loops

Routers RTA and RTB have their initial routing tables and are ready to
exchange routing information via a distance-vector routing protocol like
RIP.
Split Horizon disabled

If split horizon were disabled the routing updates would include all of
the networks in their routing tables including their directly connected
networks and any networks learned from any interface.
10.1.1.0/24
.1
RTA
10.1.2.0/24
.1
.2
s0
s0
e0
RTB
10.1.3.0/24
.1
e0
Routing Table
Routing Table
Net.
Hops Ex-Int
10.1.1.0/24 0
e0
10.1.2.0/24 0
s0
Net.
Hops Ex-Int
10.1.2.0/24 0
s0
10.1.3.0/24 0
e0
Routing Update
Next-hop
Net.
Hops Address
10.1.1.0/24 1 10.1.1.1
10.1.2.0/24 1 10.1.1.1
Routing Table
Net.
Hops Ex-Int
10.1.1.0/24 0
e0
10.1.2.0/24 0
s0
10.1.3.0/24 1
10.1.2.2
Routing Update
Next-hop
Net.
Hops Address
10.1.2.0/24 1 10.1.2.2
10.1.3.0/24 1 10.1.2.2
Routing Table
Net.
Hops Ex-Int
10.1.2.0/24 0
s0
10.1.3.0/24 0
e0
10.1.1.0/24 1
10.1.2.1
Initial
routing
tables
10.1.2.0/24
network is
included because
split horizon has
been disabled
New
routing
tables
Split Horizon Disabled
 After the initial exchange of updates everything in the routing
tables look fine.
 Because split horizon disabled, the 10.1.2.0/24 network is sent
by both routers, but neither router includes the other’s route to
10.1.2.0/24 (1 hop) in the routing table, because it has a current
route with a better metric of 0.
10.1.1.0/24
.1
RTA
e0
10.1.2.0/24
.1
.2
s0
s0
Routing Table
Net.
Hops Ex-Int
10.1.1.0/24 0
e0
10.1.2.0/24 0
s0
10.1.3.0/24 1
10.1.2.2
Routing Update
Next-hop
Net.
Hops Address
10.1.1.0/24 1 10.1.1.1
10.1.2.0/24 1 10.1.1.1
10.1.3.0/24 2 10.1.1.1
Routing Table
Net.
Hops Ex-Int
10.1.1.0/24 0
e0
10.1.2.0/24 0
s0
10.1.3.0/24 1
10.1.2.2
RTB
10.1.3.0/24
.1
e0
Routing Table
Net.
Hops Ex-Int
10.1.2.0/24 0
s0
10.1.3.0/24 0
e0
10.1.1.0/24 1
10.1.2.1
Routing Update
Next-hop
Net.
Hops Address
10.1.2.0/24 1 10.1.2.2
10.1.3.0/24 1 10.1.2.2
10.1.1.0/24 2 10.1.2.2
Routing Table
Net.
Hops Ex-Int
10.1.2.0/24 0
s0
10.1.3.0/24 0
e0
10.1.1.0/24 1
10.1.2.1
Previous
routing
tables
Networks in red
were included
because split
horizon has been
disabled
New
routing
tables
Split Horizon Disabled

After the next exchange of updates everything in the routing tables look fine
and the routing tables are converged.
 Because split horizon disabled, the besides the 10.1.2.0/24 network, the
networks learned from the other router in the previous update is also sent
by both routers.
 However, neither router includes those networks, because it has a current
route with a better metric of 0.
10.1.1.0/24
.1
RTA
e0
10.1.2.0/24
.1
.2
s0
s0
Routing Table
Net.
Hops Ex-Int
10.1.1.0/24 0
e0
10.1.2.0/24 0
s0
10.1.3.0/24 1
10.1.2.2
RTB
10.1.3.0/24
.1
e0
X
Routing Table
Net.
Hops Ex-Int
10.1.2.0/24 0
s0
10.1.3.0/24 0
e0
10.1.1.0/24 1
10.1.2.1
Routing Update
Next-hop
Net.
Hops Address
10.1.1.0/24 1 10.1.1.1
10.1.2.0/24 1 10.1.1.1
10.1.3.0/24 2 10.1.1.1
Routing Table
Net.
Hops Ex-Int
10.1.1.0/24 0
e0
10.1.2.0/24 0
s0
10.1.3.0/24 1
10.1.2.2
Previous
routing
tables
Networks in red
were included
because split
horizon has been
disabled
Routing Table
Net.
Hops Ex-Int
10.1.2.0/24 0
s0
10.1.3.0/24 2
10.1.2.1
10.1.1.0/24 1
10.1.2.1
New
routing
tables
Split Horizon Disabled – 10.1.3.0/24 down


Note: Routing tables are not sent at the exactly same time. We will learn about
this in Ch. 7 Routing Protocols, that this is done on purpose to avoid collisions
on broadcast networks like Ethernet.
Here, the 10.1.3.0/24 network fails, and before RTB sends out its routing
update, RTB receives a routing update from RTA.
10.1.1.0/24
.1
RTA
e0
10.1.2.0/24
.1
.2
s0
s0
Routing Table
Net.
Hops Ex-Int
10.1.1.0/24 0
e0
10.1.2.0/24 0
s0
10.1.3.0/24 1
10.1.2.2
RTB
10.1.3.0/24
.1
e0
X
Routing Table
Net.
Hops Ex-Int
10.1.2.0/24 0
s0
10.1.3.0/24 0
e0
10.1.1.0/24 1
10.1.2.1
Routing Update
Next-hop
Net.
Hops Address
10.1.1.0/24 1 10.1.1.1
10.1.2.0/24 1 10.1.1.1
10.1.3.0/24 2 10.1.1.1
Routing Table
Net.
Hops Ex-Int
10.1.1.0/24 0
e0
10.1.2.0/24 0
s0
10.1.3.0/24 1
10.1.2.2
Previous
routing
tables
Networks in red
were included
because split
horizon has been
disabled
Routing Table
Net.
Hops Ex-Int
10.1.2.0/24 0
s0
10.1.3.0/24 2
10.1.2.1
10.1.1.0/24 1
10.1.2.1
New
routing
tables
Split Horizon Disabled – 10.1.3.0/24 down





RTB notices that it has a route to 10.1.3.0/24 via RTA. Even though it is 2 hops
it is certainly better than its current situation of “unreachable” so it accepts this
better, but incorrect information from RTA.
RTB now forwards all packets destined for 10.1.3.0/24 to RTA at 10.1.2.1.
RTA receives these packets and forwards them to RTB at 10.1.2.2.
RTB forwards them back to RTA at 10.1.2.1.
And so on! The packets get blackholed in this routing loop.
10.1.1.0/24
.1
RTA
e0
10.1.2.0/24
.1
.2
s0
s0
Routing Table
Net.
Hops Ex-Int
10.1.1.0/24 0
e0
10.1.2.0/24 0
s0
10.1.3.0/24 1
10.1.2.2
RTB
10.1.3.0/24
.1
e0
X
Routing Table
Net.
Hops Ex-Int
10.1.2.0/24 0
s0
10.1.3.0/24 2
10.1.2.1
10.1.1.0/24 1
10.1.2.1
Routing Update
Next-hop
Net.
Hops Address
10.1.2.0/24 1 10.1.2.2
10.1.3.0/24 3 10.1.2.2
10.1.1.0/24 2 10.1.2.2
Routing Table
Net.
Hops Ex-Int
10.1.1.0/24 0
e0
10.1.2.0/24 0
s0
10.1.3.0/24 3
10.1.2.2
Routing Table
Net.
Hops Ex-Int
10.1.2.0/24 0
s0
10.1.3.0/24 2
10.1.2.1
10.1.1.0/24 1
10.1.2.1
Previous
routing
tables
Networks in red
were included
because split
horizon has been
disabled
New
routing
tables
Split Horizon Disabled – 10.1.3.0/24 down




Meanwhile, its RTB’s turn to send its routing update.
RTB increments the hop count to 10.1.3.0/24 to 3 hops and sends it to
RTA.
When RTA sends out its next routing table it will increment the hop
count to 10.1.3.0/24 to 4 hops and sends it to RTB.
And on and on, until “infinity” which in RIP is 16 hops.
10.1.1.0/24
.1
RTA
e0
10.1.2.0/24
.1
.2
s0
s0
Routing Table
Net.
Hops Ex-Int
10.1.1.0/24 0
e0
10.1.2.0/24 0
s0
10.1.3.0/24 16 10.1.2.2
RTB
10.1.3.0/24
.1
e0
X
Routing Table
Net.
Hops Ex-Int
10.1.2.0/24 0
s0
10.1.3.0/24 16 10.1.2.1
10.1.1.0/24 1
10.1.2.1
Split Horizon Disabled

Once both routers have 16 hops for 10.1.3.0/24, they will both mark this
network as unreachable and discontinue forwarding, drop, packets to
this network.

This temporary routing loop can be easily avoided by enabling
split horizon on the serial 0 interfaces.
Split horizon rule states that router never sends out information
on an interface that it learned from that interface
Let’s see!


10.1.1.0/24
.1
RTA
10.1.2.0/24
.1
.2
s0
s0
e0
RTB
10.1.3.0/24
.1
e0
Routing Table
Routing Table
Net.
Hops Ex-Int
10.1.1.0/24 0
e0
10.1.2.0/24 0
s0
Net.
Hops Ex-Int
10.1.2.0/24 0
s0
10.1.3.0/24 0
e0
Routing Update
Next-hop
Net.
Hops Address
10.1.1.0/24 1 10.1.1.1
Routing Table
Net.
Hops Ex-Int
10.1.1.0/24 0
e0
10.1.2.0/24 0
s0
10.1.3.0/24 1
10.1.2.2
Routing Update
Next-hop
Net.
Hops Address
10.1.1.0/24 1 10.1.1.1
Split Horizon Enabled
Previous
routing
tables
Routing Update
Next-hop
Net.
Hops Address
10.1.3.0/24 1 10.1.2.2
Routing Table
Net.
Hops Ex-Int
10.1.2.0/24 0
s0
10.1.3.0/24 0
e0
10.1.1.0/24 1
10.1.2.1
Routing Update
Next-hop
Net.
Hops Address
10.1.3.0/24 1 10.1.2.2
New
routing
tables
10.1.1.0/24
.1
RTA
10.1.2.0/24
.1
.2
s0
s0
e0
RTB
10.1.3.0/24
.1
e0
Routing Table
Routing Table
Net.
Hops Ex-Int
10.1.1.0/24 0
e0
10.1.2.0/24 0
s0
Net.
Hops Ex-Int
10.1.2.0/24 0
s0
10.1.3.0/24 0
e0
Routing Update
Next-hop
Net.
Hops Address
10.1.1.0/24 1 10.1.1.1
Routing Table
Net.
Hops Ex-Int
10.1.1.0/24 0
e0
10.1.2.0/24 0
s0
10.1.3.0/24 1
10.1.2.2
Routing Update
Next-hop
Net.
Hops Address
10.1.1.0/24 1 10.1.1.1
Previous
routing
tables
Routing Update
Next-hop
Net.
Hops Address
10.1.3.0/24 1 10.1.2.2
Routing Table
Net.
Hops Ex-Int
10.1.2.0/24 0
s0
10.1.3.0/24 0
e0
10.1.1.0/24 1
10.1.2.1
New
routing
tables
Routing Update
Next-hop
Net.
Hops Address
10.1.3.0/24 1 10.1.2.2
Split Horizon Enabled
 As you can see, with split horizon enabled, RTA does not send RTB (out
s0) information about 10.1.3.0/24 because it learned it from RTB (same
s0), and RTB does not send RTA (out s0) information about 10.1.1.0/24
to RTA because it learned it from RTA (same s0). (This also includes the
common network between them.
10.1.1.0/24
.1
RTA
e0
10.1.2.0/24
.1
.2
s0
s0
Routing Table
Net.
Hops Ex-Int
10.1.1.0/24 0
e0
10.1.2.0/24 0
s0
10.1.3.0/24 1
10.1.2.2
RTB
10.1.3.0/24
.1
e0
X
Routing Table
Net.
Hops
Ex-Int
10.1.2.0/24 0
s0
10.1.3.0/24 (down) e0
10.1.1.0/24 1
10.1.2.1
Previous
routing
tables
Routing Update
Next-hop
Net.
Hops Address
10.1.3.0/24 16 10.1.2.2
Routing Table
Net.
Hops
Ex-Int
10.1.1.0/24 0
e0
10.1.2.0/24 0
s0
10.1.3.0/24 (down) 10.1.2.2
Routing Table
Net.
Hops
Ex-Int
10.1.2.0/24 0
s0
10.1.3.0/24 (down) e0
10.1.1.0/24 1
10.1.2.1
New
routing
tables
Split Horizon Enabled – 10.1.3.0/24 down



RTB notices 10.1.3.0/24 is down and puts this route into hold-down
state in its routing table. (hold-down coming next)
RTB immediately sends out a triggered update for only this route (if there
were others in the routing table) with a metric of infinity, 16.
RTA receives the triggered update and puts the route for 10.1.3.0/24 into
hold-down state.
10.1.1.0/24
.1
RTA
e0
10.1.2.0/24
.1
.2
s0
s0
Routing Table
Net.
Hops Ex-Int
10.1.1.0/24 0
e0
10.1.2.0/24 0
s0
10.1.3.0/24 1
10.1.2.2
RTB
10.1.3.0/24
.1
e0
X
Routing Table
Net.
Hops
Ex-Int
10.1.2.0/24 0
s0
10.1.3.0/24 (down) e0
10.1.1.0/24 1
10.1.2.1
Previous
routing
tables
Routing Update
Next-hop
Net.
Hops Address
10.1.3.0/24 16 10.1.2.2
Routing Table
Net.
Hops
Ex-Int
10.1.1.0/24 0
e0
10.1.2.0/24 0
s0
10.1.3.0/24 (down) 10.1.2.2
Routing Table
Net.
Hops
Ex-Int
10.1.2.0/24 0
s0
10.1.3.0/24 (down) e0
10.1.1.0/24 1
10.1.2.1
New
routing
tables
Split Horizon Enabled – 10.1.3.0/24 down

Notice that RTA never sends RTB a routing update for 10.1.3.0/24,
because split horizon is enabled on these interfaces.
Split Horizon with Poison Reverse
10.1.1.0/24
.1
RTA
e0
10.1.2.0/24
.1
.2
s0
s0
Routing Table
Net.
Hops Ex-Int
10.1.1.0/24 0
e0
10.1.2.0/24 0
s0
10.1.3.0/24 1
10.1.2.2
Routing Update
Next-hop
Net.
Hops Address
10.1.1.0/24 1
10.1.1.1
10.1.2.0/24 16 10.1.2.1
10.1.3.0/24 16 10.1.2.1
10.1.3.0/24
.1
e0
Routing Table
Net.
Hops Ex-Int
10.1.2.0/24 0
s0
10.1.3.0/24 0
e0
10.1.1.0/24 1
10.1.2.1
Routing Update
Next-hop
Net.
Hops Address
10.1.3.0/24 1
10.1.2.2
10.1.2.0/24 16 10.1.2.2
10.1.1.0/24 16 10.1.2.2
Split Horizon with Poison Reverse

RTB
“Poisoned”
routes in red.
Routing tables
remain the
same.
Many vendor implementations of distance vector routing protocols like
Cisco’s RIP and IGRP apply a special kind of split horizon, called split
horizon with poison reverse.
 “Split horizon with poison reverse means that, instead of not advertising
routes to the source, routes are advertised back to the source with a
metric of 16, which will make the source router ignore the route. It is
perceived that explicitly telling a router to ignore a route is better than not
telling it about the route in the first place.” (Lewis, Cisco TCP/IP Routing)
 One drawback is that routing update packet sizes will be increased when
using Poison Reverse, since they now include these routes.
Split Horizon Enabled
10.1.1.0/24
.1
RTA
e0
10.1.2.0/24
.1
.2
s0
s0
Routing Table
Net.
Hops Ex-Int
10.1.1.0/24 0
e0
10.1.2.0/24 0
s0
10.1.3.0/24 1
10.1.2.2
Routing Update
Next-hop
Net.
Hops Address
10.1.1.0/24 1
10.1.1.1
10.1.2.0/24 16 10.1.2.1
10.1.3.0/24 16 10.1.2.1
RTB
10.1.3.0/24
.1
e0
Routing Table
Net.
Hops Ex-Int
10.1.2.0/24 0
s0
10.1.3.0/24 0
e0
10.1.1.0/24 1
10.1.2.1
Routing Update
Next-hop
Net.
Hops Address
10.1.3.0/24 1
10.1.2.2
10.1.2.0/24 16 10.1.2.2
10.1.1.0/24 16 10.1.2.2
“Poisoned”
routes in red.
Split Horizon Enabled by Default
Split horizon with poison reverse is enabled by default for all interfaces
except:
 Physical interfaces or multipoint sub-interfaces using Frame Relay or
SMDS encapsulation (CCNA Semester 4 and CCNP Remote Access)
To disable split horizon on an interface:
Router(config-if)# no ip split-horizon
To enable split horizon on an interface:
Router(config-if)# ip split-horizon
Holddown timers
 The main function of holddown timers is to prevent the
distance vector routing protocol from establishing routing loops
during periods of network transition (topology changes).
 “The rule: Once a route is marked unreachable, it must stay in
this state for a period of time assumed sufficient for all routers to
receive new information about the unreachable network. In
essence, we instruct the routers to let the rumors calm down
and then to pick up the truth.” (Zinin, Cisco IP Routing)
 The amount of time a router remains in “this state” is determined
by the holddown timer.
Cisco on-line curriculum information
 You can avoid the count-to-infinity problem by using hold-down
timers.
 When a router receives an update from a neighbor indicating
that a previously accessible network is now inaccessible, the
router marks the route as inaccessible and starts a hold-down
timer.
Cisco on-line curriculum information (continued)
Same Route from same neighbor: Network is back up (Correct News)
 If at any time before the hold-down timer expires an update is
received from the same neighbor indicating that the network is
again accessible, the router marks the network as accessible and
removes the hold-down timer.
Cisco on-line curriculum information (continued)
Better Route from different neighbor (Correct News)
 If at any time before the hold-down timer expires an update
arrives from a different neighboring router with a better metric
than originally recorded for the network, the router marks the
network as accessible and removes the hold-down timer.
Cisco on-line curriculum information (continued)
Poorer Route from a different neighbor. (Incorrect News)
 If at any time before the hold-down timer expires an update
arrives from a different neighboring router with a poorer metric
than originally recorded for the network the update is ignored
and the hold-down timer continues.
 Ignoring an update with a poorer metric when a hold-down is in
effect allows more time for the knowledge of a disruptive change
to propagate through the entire network.
Additional Information on Holddown Timers
Flapping routes
 Holddown timers not only help prevent routing loops during transient
periods but also help network stability by dampening unstable, flapping
routes (routes which continuously go up and down).
Holddown Time
 As we will see with both RIP and IGRP, the amount of time the router
remains in the holddown state can be modified (with caution!), even set
to 0.
 We will look at this later in the presentations on RIP and IGRP.
Additional Information on Holddown Timers
Packet forwarding
 Even though routing tables remain constant and routers do not accept
potentially bad updates, an interesting question is whether or not
routers should continue use the existing routes that are in holddown
state for forwarding packets?
 “In practice, routes in the holddown state are used for packet
forwarding.
 In the worst case, packets are forwarded toward the router that was
previously connected to the destination network, which drops them.
 In the best case, they are forwarded along a potentially suboptimal but
valid path.” (Zinin, Cisco IP Routing)
IP’s TTL – Time To Live field
IP Header
0
4-bit
Version
15 16
4-bit
Header
Length
8-bit Type Of
Service
(TOS)
16-bit Total Length (in bytes)
3-bit
Flags
16-bit Identification
8 bit Time To Live
TTL
31
8-bit Protocol
13-bit Fragment Offset
16-bit Header Checksum
32-bit Source IP Address
32-bit Destination IP Address
Options (if any)
Data
Let’s look at a related item in IP, the TTL field.
IP’s TTL – Time To Live field
IP Header
0
4-bit
Version
15 16
4-bit
Header
Length
8-bit Type Of
Service
(TOS)
16-bit Total Length (in bytes)
3-bit
Flags
16-bit Identification
8 bit Time To Live
TTL
31
8-bit Protocol
13-bit Fragment Offset
16-bit Header Checksum
32-bit Source IP Address
32-bit Destination IP Address
Options (if any)
Data



When a packet is first generated a value is entered into the TTL
field.
Originally, the TTL field was the number of seconds, but this was
difficult to implement and rarely supported.
Now, the TTL is now set to a specific value which is then
decremented by each router.
IP’s TTL – Time To Live field
IP Header
0
4-bit
Version
15 16
4-bit
Header
Length
8-bit Type Of
Service
(TOS)
16-bit Total Length (in bytes)
3-bit
Flags
16-bit Identification
8 bit Time To Live
TTL
31
8-bit Protocol
13-bit Fragment Offset
16-bit Header Checksum
32-bit Source IP Address
Decrement by 1, if 0 drop the
packet.
32-bit Destination IP Address
Options (if any)
Data

If the router decrements the TTL field to 0, it will then drop the packet
(unless the packet is destined specifically for the router, I.e. ping, telnet,
etc.).
 Common operating system TTL values are:
–
–
–
–
UNIX: 255
Linux: 64 or 255 depending upon vendor and version
Microsoft Windows 95: 32
Other Microsoft Windows operating systems: 128
http://www.switch.ch/docs/ttl_default.html
TTL Overview - Disclaimer:
The following list is a best effort overview of some widely used TCP/IP stacks. The
information was provided by vendors and many helpful system administrators. We would
like to thank all these contributors for their precious help ! SWITCH cannot, however,
take any responsibility that the provided information is correct. Furthermore, SWITCH
cannot be made liable for any damage that may arise by the use of this information.
+--------------------+-------+---------+---------+
| OS Version
|"safe" | tcp_ttl | udp_ttl |
+--------------------+-------+---------+---------+
AIX
n
60
30
DEC Pathworks V5
n
30
30
FreeBSD 2.1R
y
64
64
HP/UX
9.0x
n
30
30
HP/UX
10.01
y
64
64
Irix 5.3
y
60
60
Irix 6.x
y
60
60
Linux
y
64
64
MacOS/MacTCP 2.0.x
y
60
60
OS/2 TCP/IP 3.0
y
64
64
OSF/1 V3.2A
n
60
30
Solaris 2.x
y
255
255
SunOS 4.1.3/4.1.4
y
60
60
Ultrix V4.1/V4.2A
n
60
30
VMS/Multinet
y
64
64
VMS/TCPware
y
60
64
VMS/Wollongong 1.1.1.1
n
128
30
VMS/UCX (latest rel.)
y
128
128
MS WfW
n
32
32
MS Windows 95
n
32
32
MS Windows NT 3.51
n
32
32
MS Windows NT 4.0
y
128
128
Assigned Numbers (RFC
1700, J. Reynolds, J.
Postel, October 1994):
IP TIME TO LIVE
PARAMETER
The current
recommended default
time to live (TTL)
for the Internet
Protocol (IP) is 64.
Safe: TCP and UDP
initial TTL values
should be set to a
"safe" value of at
least 60 today.
IP’s TTL – Time To Live field
IP Header
0
4-bit
Version
15 16
4-bit
Header
Length
8-bit Type Of
Service
(TOS)
16-bit Total Length (in bytes)
3-bit
Flags
16-bit Identification
8 bit Time To Live
TTL
31
8-bit Protocol
13-bit Fragment Offset
16-bit Header Checksum
32-bit Source IP Address
Decrement by 1, if 0 drop the
packet.
32-bit Destination IP Address
Options (if any)
Data


The idea behind the TTL field is that IP packets can not travel
around the Internet forever, from router to router.
Eventually, the packet’s TTL which reach 0 and be dropped by the
router, even if there is a routing loop somewhere in the network.