

Two types
Distance Vector
◦ Examples:
 RIP v1 and RIPv2 (Routing Information Protocol)
 IGRP (Interior Gateway Routing Protocol)

Link State
◦ Examples
 OSPF (Open Shortest Path First)
 IS-IS (Intermediate System - Intermediate System)
 NLSP (Netware Link Services Protocol)

Path Vector
◦ Example
 BGP (Border Gateway Protocol)
Link
State (LS) advantages:
More stable (aka fewer routing
loops)
Faster convergence than
distance vector
 Easier to discover network
topology, troubleshoot
network.

Routing table shows the
◦ route source
Directly connected networks
Static routes
Dynamic routing protocols
Parent and Child Routes
◦ A Level 1Parent route does not
contain any next-hop IP address or
exit interface information
 Level 2 child routes contain route
source & the network address of the route

Diagram illustrates 2 child networks
belonging to the parent route 172.16.0.0 / 24
level 1 route
level 2 route

Longest Match: Level 1 Network Routes
–Best match is also known as the longest match
–The best match is the one that has the most number of
left most bits matching between the destination IP address
and the route in the routing table.



Classful routing protocols do not send subnet mask information
with their routing updates.
A router running a classful routing protocol will react in one of two
ways when receiving a route:
◦ If the router has a directly connected interface belonging to the
same major network, it will apply the same subnet mask as that
interface.
◦ If the router does not have any interfaces belonging to the same
major network, it will apply the classful subnet mask to the route.
◦ Example: 10.3.1.0 and 10.5.5.0 belong to the same major
network (10.0.0.0)
What happens when Router B sends routing update to Router A?
◦ What subnet mask will be used by Router A?
When using classful routing
protocols, the subnet mask
must remain consistent
throughout your entire
network
Scenario: Routing Behaviour- Classful/Classless Routing
What happens to a packet with destination 172.16.4.0/24 for
• Classful routing and
• Classless routing?

If no match is found in child routes of previous slide
then router continues to search the routing table for a
match that may have fewer bits in the match

why the router drops the Packet destined to
172.16.4.0/24
None
of the child routes left most bits
match the first 24 bits.




The use of CIDR and VLSM not
only reduces address waste, but
it also promotes route
aggregation, or route
summarization.
route summarization reduces
the burden on upstream
routers.
Example: Figure 1
variable-sized networks and
subnetworks is summarized at
various points using a prefix
address until the entire network
is advertised as a single
aggregate route of
192.168.48.0/20
Figure 1





Route flapping occurs when a
router interface alternates
rapidly between the up and
down states. This can be caused
by a number of factors,
including a faulty interface or
poorly terminated media.
Summarization can effectively
insulate upstream routers from
route-flapping problems.
Example: Figure 1
If the RTC interface connected to
the 200.199.56.0 network goes
down, RTC removes that route
from its table
What if routers were not
configured to summarise?
Figure 1




Steps to calculate a
route summary
List networks in binary
format
Count number of left
most matching bits to
determine summary
route’s mask
Copy the matching
bits and add zero bits
to determine the
summarized network
address

Default routes
◦ Packets that are not defined specifically in a
routing table will go to the specified interface
for the default route
◦ Example: Customer routers use default
routes to connect to an ISP router.
◦ Command used to configure a default route
is
◦
#ip route 0.0.0.0 0.0.0.0 s0/0/1

When network topology changes, network traffic
must reroute quickly. The phrase "convergence
time" describes the time it takes a router to start
using a new route after a topology changes.

Routers must do three things after a topology
changes:

Detect the change

Select a new route

Propagate the changed route information

EASE OF IMPLEMENTATION

SPEED OF IMPLEMENTATION
Three key issues determine the amount of
bandwidth a routing protocol consumes:
1. When routing information is sent--1. Periodic updates are sent at regular intervals.
Flash updates are sent only when a change
occurs.
2. Complete updates contain all routing
information. Partial updates contain only
changed information.
3. Flooded updates are sent to all routers.
Bounded updates are sent only to routers that
are affected by a change.
Note: These three issues also affect CPU sage.





CPU usage is protocol dependent.
Some protocols use CPU cycles to compare new
routes to existing routes.
Other protocols use CPU cycles to regenerate
routing tables after a topology change.
In most cases, the latter technique will use more
CPU cycles than the former.
◦ Example: For link-state protocols, keeping areas small
and using summarization reduces CPU requirements by
reducing the effect of a topology change and by
decreasing the number of routes that must be
recomputed after a topology change.



Routing protocols use memory to store
routing tables and topology information.
Route summarization cuts memory
consumption for all routing protocols.
Keeping areas small reduces the memory
consumption for hierarchical routing
protocols


The ability to extend your internetwork is
determined, in part, by the scaling
characteristics of the routing protocols used
and the quality of the network design.
Network scalability is limited by two factors:
◦ operational issues and
 Operational scaling concerns encourage the use of
large areas or protocols that do not require
hierarchical structures.
◦ technical issues
 When hierarchical protocols are required, technical
scaling concerns promote the use of small areas




Some routing protocols provide techniques that
can be used as part of a security strategy.
Some routing protocols allow filter on the routes
being advertised so that certain routes are not
advertised in some parts of the network.
Some routing protocols can authenticate routers
that run the same protocol. Authentication
mechanisms are protocol specific
Authentication can increase network stability by
preventing unauthorized routers or hosts from
participating in the routing protocol, whether
those devices are attempting to participate
accidentally or deliberately.
Uses hop count as metric (max: 16 is infinity)
 Tables (vectors) “advertised” to neighbors every 30 s.
Each advertisement: upto 25 entries
 No advertisement for 180 sec: neighbor/link
declared dead
routes via neighbor invalidated
new advertisements sent to neighbors (Triggered

updates)
neighbors in turn send out new advertisements (if
tables changed)
link failure info quickly propagates to entire net
poison reverse used to prevent ping-pong loops
(infinite distance = 16 hops)

Split horizon/poison reverse does not
guarantee to solve count-to-infinity problem
◦ 16 = infinity => RIP for small networks only!
◦ Slow convergence


Broadcasts consume non-router resources
RIPv1 does not support subnet masks (VLSMs)
◦ No authentication


Why ? Installed base of RIP routers
Provides:
◦
◦
◦
◦
◦


VLSM support
Authentication
Multicasting
“Wire-sharing” by multiple routing domains,
Tags to support EGP/BGP routes.
Uses reserved fields in RIPv1 header.
First route entry replaced by authentication
info.
CISCO proprietary; successor of RIP (late 80s)
 Several metrics (delay, bandwidth, reliability, load
etc)
 Uses TCP to exchange routing updates
 Loop-free routing via Distributed Updating Alg.
(DUAL) based on diffused computation
 Freeze entry to particular destination
 Diffuse a request for updates
 Other nodes may freeze/propagate the diffusing
computation (tree formation)
Unfreeze when updates received.
Tradeoff: temporary un-reachability for some
destinations







Key: Create a network “map” at each node.
1. Node collects the state of its connected links and
forms a “Link State Packet” (LSP)
2. Flood LSP => reaches every other node in the
network and everyone now has a network map.
3. Given map, run Dijkstra’s shortest path
algorithm (SPF) => get paths to all destinations
4. Routing table = next-hops of these paths.
5. Hierarchical routing: organization of areas, and
filtered control plane information flooded.


Reliable Flooding: sequence #s, age
LSA types, Neighbor discovery and
maintainence (hello)
◦ Efficiency in Broadcast LANs, NBMA, Pt-Mpt
subnets: designated router (DR) concept

Areas and Hierarchy
◦ Area types: Normal, Stub, NSSA: filtering
◦ External Routes (from other ASs), interaction with
inter-domain routing.

Advanced topics: incremental SPF algorithms
OSPF

OSPF Network Topology
OSPF Addressing and Route Summarization

OSPF Route Selection

OSPF Convergence

OSPF Network Scalability

OSPF Security
