Routing/Routed Protocols
Remember:
• A Routed Protocol – defines logical
addressing. Most notable example on the
test – IP
• A Routing Protocol – fills the routing table
with routing information. Examples on the
test – RIP, IGRP, EIGRP, OSPF, IS-IS
Main Goals of Routing Protocols
• To fill the routing table with current best,
loop-free routes
• To notice when routes in the table are no
longer valid and remove them from the
routing table
• To add new routes or replace lost routes
– The time for finding a working route is called
convergence.
Two Categories of Routing
Protocols
• Exterior Routing Protocols – used for use
between different organizations such as
ISPs or ISPs and their customers.
– Ex: Border Gateway Protocol (BGP)
• Interior Routing Protocols – used to
distribute routing information inside a
single organization.
– Ex: RIP, IGRP, EIGRP, OSPF, IS-IS
Key Points of Interior Routing
Protocols:
• Each interior routing protocol can be
characterized based on the underlying
logic used by the routing protocol.
• The underlying logic is referred to as the
TYPE of routing protocol.
• The three types are:
1)Distance vector
2)Link-state
3)Hybrid
Support for VLSM
• Variable-length subnet masking (VLSM)
means that, in a single Class A, B, or C
network, multiple subnet masks can be
used.
• The advantage of VLSM is that it enables
you to vary the size of each subnet, based
on the needs of that subnet.
• Some routing protocols support VLSM,
and some do not
Classless or Classful
• Classless routing protocols transmit the
subnet mask along with each route in the
routing updates sent by that protocol.
• Classful routing protocols do not transmit
mask information.
• Only classless routing protocols support
VLSM. To say that a routing protocol is
classless is to say that it supports
No VLSM with RIPv1 or IGRP
• Neither RIPv1 nor IGRP routing protocols
has a field for subnet information.
• If a router running RIPv1 has a subnet
mask of a certain value, it assumes that all
interfaces interfaces within the classful
address space have the same have the
same subnet mask.
• This is called classful routing, and RIP and
IGRP are both considered classful routing
protocols.
VLSM with RIPv2, EIGRP, & OSPF
• Classless routing protocols do support the
advertisement of subnet information.
• Use VLSM with routing protocols such as
RIPv1, EIGRP, or OSPF.
• The advantage with this type of network is
that bunches of IP addresses are saved.
More Routing Protocol
Basics
Static vs. Dynamic Routing
• Static routing occurs when you manually
add routes in each router’s routing table. It
will be covered in subsequent routing
command procedures since it is mainly
demonstrated through routing commands.
• Dynamic routing is when protocols are
used to find networks and update routing
tables on routers.
Interior vs. Exterior Gateway
Protocols
• IGPs are used to exchange routing
information with routers in the same
autonomous system (AS).
• An AS is a collection of networks under a
common administrative domain, which
basically means that all routers sharing the
same routing table information are in the
same AS.
• EGPs are used to communicate between
Ass such as in WAN links.
Administrative Distances
• The administrative distance (AD) is used
to rate the trustworthiness of routing
information received on a router from a
neighbor router.
• An administrative distance is an integer
from 0 to 255, where 0 is the most trusted
and 255 means NO traffic will be passed
via this route.
Default Administrative Distances
Route Source
Connected Interface
Static Route
EIGRP
IGRP
OSPF
Default AD
0
1
90
100
110
RIP
External EIGRP
Unknown
120
170
255 (no traffic)
The Three Classes of Routing
Protocols
• Distance Vector – finds the best path to a
remote network using hop count. (RIP, IGRP)
• Link State – (also called shortest-path-first
protocols) – the routers each create three
separate tables. 1) keeps track of directly
attached neighbors, 2) topology of network, 3)
the routing table. (OSPF, IS-IS)
• Hybrid – uses aspects of both distance vector
and link state. (EIGRP)
Distance Vector Terms
• See board for routing loop example.
• Maximum Hop Count – prevents counting
to infinity.
– RIP maximum hop count = 15 (16 = infinity)
– IGRP maximum hop count default = 100
• Split Horizon – reduces incorrect routing
information and routing overhead by
enforcing the rule that routing information
cannot be sent back in the direction from
which it was received.
Distance Vector Terms
• Route Poisoning – occurs when an
“unreachable,” or “infinite,” message is
distributed when a link is down.
• Holddown Timer – A holddown prevents
regular update messages from reinstating
a route that is going up and down (called
“flapping”). Typically, this happens on a
serial link that is losing connectivity and
then coming back up. Network might never
converge otherwise.
Holddown Timers
• There are 3 instances when triggered
updates will reset the holddown timer:
1) The holddown timer expires
2) Another update is received with a better
metric
3) A flush time, which is the time a route would
be held before being removed, removes the
route from the routing table when the timer
expires.
Routing Information Protocol (RIP)
• A true distance-vector routing protocol.
• Sends the complete routing table to all
active interfaces every 30 seconds.
• RIP only uses hop count to determine the
best way to a remote network – max hop
count = 15, with 16 being “infinite,” or
unreachable.
• Works well in small networks.
• RIPv1 uses only classful routing.
Interior Gateway Routing Protocol
(IGRP)
• Cisco-proprietary distance-vector routing
protocol (must use only Cisco routers).
• Classful
• Default max hop count = 100.
• Can be used in large networks.
• Uses a different metric than RIP – IGRP
uses bandwidth and delay of line by
default. This is called a “composite metric.”
– Reliability, load, and MTU can also be used,
although they are not by default.
Open Shortest Path First (OSPF)
Basics
• An open standards routing protocol (nonCisco proprietary
• Good for large networks (over 15 routers)
• Uses the Dijkstra algorithm
• Only supports IP routing
• Classless
OSPF Logic
1) Each router discovers its neighbors on
each interface. The list of neighbors is
kept in a neighbor table.
2) Each router uses a reliable protocol to
exchange topology information with its
neighbors.
3) Each router places the learned topology
information in its topology database.
OSPF Logic (cont.)
4) Each router runs the SPF algorithm
against its own topology database to
calculate the best routes to each subnet in
the database.
5) Each router places the best route to each
subnet in the IP routing table.
OSPF
• A link-state protocol uses a neighbor table
and a topology database in addition to
adding routes to the routing table.
• OSPF converges more quickly than do
distance-vector protocols.
• Uses “Hello” packets to keep in touch with
neighbors.
• Defined in RFC 2328
Enhanced Interior Gateway
Routing Protocol (EIGRP)
• Cisco proprietary
• Classless
• Uses autonomous system numbers
– A number assigned to a group of routers
under mutual administration.
• Referred to as a “hybrid routing protocol”
• Provides support for IP, IPX, and Appletalk
• Best path selection using the Diffusing
Update Algorithm (DUAL)
EIGRP
•
•
Before EIGRP routers exchange routes
with each other, they must become
neighbors.
There are three conditions that must be
met for neighborship establishment:
1) Hello or ACK received
2) AS numbers match
3) Identical metrics
EIGRP Feature Comparison
Link-state Features
• Converges quickly
• Discovers neighbors
via “Hello” packets
• Builds topology table
• After learning its
neighbor’s routes,
only changes to the
routing table are
propagated.
Distance-vector Features
• Uses autonomous
system number (like
IGRP)
• Uses metric based on
bandwidth & delay
• Advertises entire
routing table to new
neighbors.