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Dynamic Routing Protocols
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Static Routing
Easy to configure
 Easy to troubleshoot
 Doesn’t scale well
 Limited redundancy options
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Routing Protocols
A routing protocol is the language a router
speaks with other routers in order to share
information about the reachability and
status of networks.
 Routing protocols also determine the next
best path if the current best path to a
destination becomes unusable.
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Routing Algorithm specifies:
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A procedure for passing reachability information
about networks to other routers
A procedure for receiving reachability information
from other routers
A procedure for determining optimal routes based
on the reachability information it has and for
recording this information in a routing table
A procedure for reacting to, compensation for, and
advertising topology changes in an internetwork
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Example: 3-router internetwork
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Metrics
When there are multiple routes to the same
destination, a router must have a mechanism
for calculating the best path
 A metric is a variable assigned to routes to
rank them from best to worst
 Different protocols use different, and
sometimes multiple, metrics
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Hop Count
A hop count metrics simply counts router
hops.
 Is the A-B link really the best path?
What if the A-B link is a DS-0 and the A-C
and C-B links are T-1s?
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Bandwidth
A bandwidth metric would choose a higherbandwidth path over a lower-bandwidth
link.
 What if the higher-bandwidth link has a
higher delay and we transfer voice traffic?
 What if the T1 links are highly loaded and a
56Kb link is lightly loaded?
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Load
This metric reflects the amount of traffic
utilizing the links along the path. The best
path is the one with the lowest load.
 Unlike hop count and bandwidth, the load
on a router changes, and therefore the
metric will change. If the metric changes
too frequently, route flapping - the frequent
change of preferred routes - may occur.
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Delay
Delay is a measure of the time it takes a
packet to travel a route.
 A routing protocol using delay as a metric
would choose the path with the least delay
as the best path.
 Delay changes with traffic load.
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Reliability
Reliability measures the likelihood that the
link will fail in some way and can be either
variable or fixed.
 Examples: the number of times a link has
failed or the number of errors it has
received within a certain period of time.
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Cost
Cost is configured by the network
administrator to reflect more- or lesspreferred routes.
 The term cost is often used as a generic
term when speaking of route choices.
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Convergence
The process of bringing all route tables to
the state of consistency is called
convergence.
 The time it takes to share information across
an internetwork and for all routers to
calculate best paths is called the
convergence time.
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Distance Vector Routing Protocols
Most routing protocols fall into one of two
classes: distance vector or link state.
 Distance vector algorithms are based on the
work done of Bellman, Ford, Fulkerson and
occasionally referred to as Bellamn-Ford or
Ford-Fulkerson.
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Distance Vector Routing Protocols
(cont’d)
The name distance vector is delivered from
the fact that routes are advertised as vectors
of distance, where distance is defined in
terms of metric, and direction is defined in
terms of the next-hop router.
 Example: Destination A is at a distance of 5
hops away, in the direction of next-hop
router X.
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Distance vector routing protocols:
Routing Information Protocol (RIP) for IP
 Cisco’s Internet Gateway Routing Protocol
(IGRP)
 Novell’s IPX RIP
 AppleTalk’s Routing Table Maintenance
Protocol (RTMP)
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Common Characteristics of DVPs
Periodic updates
 Neighbors
 Broadcast Updates
 Full Routing Table Updates
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Periodic Updates
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Periodic Updates mean that at the end of a
certain time period, updates will be
retransmitted.
These periods typically range from 10 to 90
seconds.
If updates are sent too infrequently,
convergence time may be unacceptably high; if
updates are sent too frequently, network traffic
may be unacceptably high.
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Neighbors
In the context of routers, neighbors always
mean routers sharing a common data link.
 A distance vector routing protocol sends its
updates to neighboring routers and depends
on them to pass the update information
along to their neighbors.
 For this reason, distance vector routing is
said to use hop-by-hop updates.
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Broadcast Updates
When a router first becomes active on a
network, how does it find other routers and
how does it announce its own presence?
 The simplest method - to send the updates
to the broadcast address 255.255.255.255.
 Hosts and other devices uninterested in the
routing updates will simply drop the
packets.
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Full Routing Table Updates
Most distance vector protocols take the very
simple approach of telling their neighbors
everything they know by broadcasting the
entire routing table
 Neighbors receiving these updates glean the
information they need and discard
everything else.
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Routing by Rumor
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Route Invalidation Timers
What if a router goes down? Information
about a path is no longer valid, but there is
no router to inform about this fact.
 The problem is handled by setting a route
invalidation timer for each entry in the
routing table.
 Typical periods for route timeouts range
from 3 to 6 update periods.
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Split Horizon
Each router broadcasts its entire route table
to every neighbor every update period.
 Is this really necessary?
 A router pointing back to the router from
which packets were received is called a
reverse route.
 Split horizon is a technique for preventing
reverse routes between two routers.
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Split Horizon (cont’d)
Split Horizon prevents routing loops
problems.
 2 categories of split horizon: simple split
horizon and split horizon with poisoned
reverse.
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Poisoned Reverse
When sending updates out of a particular
interface, designate any network that was
learned from updates received on that
interface as unreachable.
 Split horizon with poisoned reverse is
considered safer and stronger than simple
split horizon - it helps to remove corrupted
routing information.
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Counting to Infinity
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Sometimes split horizon doesn’t resolve the loop problems.
The way to alleviate the effect of counting to infinity is to
define infinity.
For example, RIP defines infinity as 16 hops. As soon as
hop count increases to 16, the network is considered
unreachable.
Setting a maximum hop count to 15 helps to solve the
counting-to-infinity problem, but convergence will be very
slow. Given an update period of 30 seconds, a network
could take up to 7.5 minutes to reconverge.
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Triggered updates
Triggered updates, also known as flash
updates, are very simple:
 If a metric changes for better or for worse, a
router will immediately send out an update
without waiting for its update timer to
expire.
 Reconvergence will occur quicker.
 Regular updates may still occur along with
triggered updates.
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Holddown Timers
Holddown timers introduce a certain
amount of skepticism to reduce the
acceptance of bad routing information.
 If the distance to a destination increases (for
example, the hop count increases from 2 to
4), the router sets a holddown timer for that
route.
 Until the timer expires, the router will not
accept any new updates for the route.
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Asynchronous Updates
As a few routers become synchronized,
collisions will begin to occur, further
contributing to system delays, and
eventually all routers sharing the broadcast
network may become synchronized.
 Asynchronous updates may be maintained
by a small random time, or timing jitter,
which is added to each update period as an
offset.
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Link State Routing Protocols
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Unlike the routing-by-rumor approach of
distance-vector, link state routers have
firsthand information from all their peer
routers.
Each router originates information about itself,
its directly connected links, and the state of
those links.
This information is passed around from router
to router, each router makes a copy of it, but
never changes it.
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Link State Routing Protocols
Every router has identical information about
the internetwork, and each router will
independently calculate its own best path.
 Link state protocols, sometimes called
shortest path first or distributed database
protocols, are built around a well-known
algorithm from graph theory, E.W.
Dijkstra’s shortest path algorithm.
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Link-state routing protocols:
Open Shortest Path First (OSPF) for IP
 The ISO’s Intermediate System to
Intermediate System (IS-IS) for IP
 Novell’s NetWare Link Service Protocol
(NLSP)
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Basic functionality of LSRP
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Each router establishes a relationship - an adjacency - with
each of its neighbors.
Each router sends link state advertisements (LSAs),
sometimes called link state packets (LSPs), to each
neighbor.
Each router stores a copy of all the LSAs it has seen in a
database.
The completed topology database, also called link state
database, describes a graph of the internetwork.
Each router calculates the shortest path to each network
and enters this information into the route table.
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Autonomous System (AS)
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On the Internet, an autonomous system
(AS) is the unit of router policy, either a
single network or a group of networks that
is controlled by a common network
administrator.
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An autonomous system is assigned a
globally unique number, called an
Autonomous System Number (ASN).
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Interior and Exterior Gateway Protocol
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Networks within an autonomous system
communicate routing information to each
other using an Interior Gateway Protocol.
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An autonomous system shares routing
information with other autonomous systems
using an Exterior Gateway Protocol.
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Want to know more?
Jeff Doyle ‘Routing TCP/IP, volume 1’
 Perlman, R. ‘Interconnections: Bridges and
Routers.’
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