Content Overview Dynamic routing
protocols can help simplify the life of a network
administrator. Dynamic routing makes it possible to avoid the
time-consuming and exacting process of configuring static
routes. Dynamic routing also makes it possible for routers to
react to changes in the network and to adjust their routing
tables accordingly, without the intervention of the network
administrator. However, dynamic routing can cause problems.
Some of the problems associated with dynamic distance vector
routing protocols are discussed in this module, along with some
of the steps that designers of the protocols have taken to
solve the problems. Routing Information Protocol (RIP) is a
distance vector routing protocol that is used in thousands of
networks throughout the world. The fact that RIP is based on
open standards and is very simple to implement makes it
attractive to some network administrators, although RIP lacks
the power and features of more advanced routing protocols.
Because of its simplicity, RIP is a good beginning protocol for
the networking student. This module also introduce RIP
configuration and troubleshooting. Like RIP, Interior Gateway
Routing Protocol (IGRP) is a distance vector routing protocol.
Unlike RIP, IGRP is a Cisco-proprietary protocol rather than a
standards-based protocol. While remaining very simple to
implement, IGRP is a more complex routing protocol than RIP and
it is able to use a number of factors to determine the best
route to a destination network. This module will introduce IGRP
configuration and troubleshooting. Students completing this
module should be able to: - Describe how routing loops
can occur in distance vector routing
- Describe several
methods used by distance vector routing protocols to ensure
that routing information is accurate
- Configure
RIP
- Use the ip classless command
- Troubleshoot RIP
- Configure RIP for load
balancing
- Configure static routes for RIP
- Verify RIP
- Configure IGRP
- Verify IGRP
operation
- Troubleshoot IGRP
Content
7.1 Distance Vector Routing 7.1.1
Distance vector routing updates Routing table updates
occur periodically or when the topology in a distance vector
protocol network changes. It is important that a routing
protocol be efficient in updating the routing tables. As with
the network discovery process, topology change updates proceed
systematically from router to router. Distance vector
algorithms call for each router to send its entire routing
table to each of its adjacent neighbors. The routing tables
include information about the total path cost as defined by the
metrics and the logical address of the first router on the path
to each network contained in the table. Web Links
Dynamic Routing http://cisn.metu.edu.tr/2001-3/ dynamic.php
Content 7.1 Distance Vector Routing
7.1.2 Distance vector routing loop issues Routing
loops can occur when inconsistent routing tables are not
updated due to slow convergence in a changing network.
- Just before the failure of Network 1, all routers have
consistent knowledge and correct routing tables. The network is
said to have converged. Assume for the remainder of this
example that Router C's preferred path to Network 1 is by way
of Router B, and the distance from Router C to Network 1 is
3.
- When 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 of the failure. When Router A sends out its
update, Routers B and D stop routing to Network 1. However,
Router C has not received an update. To Router C, Network 1 is
still reachable via Router B.
- Now Router C sends a
periodic update to Router D, indicating a path to Network 1 by
way of Router B. Router D changes its routing table to reflect
this good, but incorrect, information, and propagates the
information to Router A. Router A propagates the information to
Routers B and E, and so on. Any packet destined for Network 1
will now loop from Router C to B to A to D and back to again to
C.
Web Links Distance Vector Protocols
http://www.mcpprep.com/ WebHelp/ccna/ ccna_obj_42.htm
Content 7.1 Distance Vector Routing
7.1.3 Defining a maximum count The invalid updates
of Network 1 will continue to loop until some other process
stops the looping. 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 count to infinity 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. Distance vector routing algorithms are self-correcting,
but a routing loop problem can require a count to infinity. To
avoid this prolonged problem, distance vector protocols define
infinity as a specific maximum number. This number refers to a
routing metric which may simply be the hop count. With this
approach, the routing protocol permits the routing loop to
continue until the metric exceeds its maximum allowed value.
The graphic shows the metric value as 16 hops. This exceeds the
distance vector default maximum of 15 hops so the packet is
discarded by the router. In any case, when the metric value
exceeds the maximum value, Network 1 is considered
unreachable. Web Links Avoiding Counting To Infinity In
Distance Vector Routing http://www.uni-koblenz.de/~steigner/
labor/papers/ ripmti.pdf
Content 7.1 Distance
Vector Routing 7.1.4 Elimination routing loops
through split horizon Another possible source for a routing
loop occurs when incorrect information that has been sent back
to a router contradicts the correct information that the router
originally distributed. Here is how this problem occurs:
- Router A passes an update to Router B and Router D,
indicating that Network 1 is down. Router C, however, transmits
an update to Router B, indicating that Network 1 is available
at a distance of 4, by way of Router D. This does not violate
split-horizon rules.
- Router B concludes, incorrectly,
that Router C still has a valid path to Network 1, although at
a much less favorable metric. Router B sends an update to
Router A advising Router A of the new route to Network 1.
- Router A now determines that it can send to Network 1
by way of Router B; Router B determines that it can send to
Network 1 by way of Router C; and Router C determines that it
can send to Network 1 by way of Router D. Any packet introduced
into this environment will loop between routers.
- Split-horizon attempts to avoid this situation. If a
routing update about Network 1 arrives from Router A, Router B
or Router D cannot send information about Network 1 back to
Router A. Split-horizon thus reduces incorrect routing
information and reduces routing overhead.
Web
Links Configuring RIP http://www.cisco.com/en/US/products/sw/
iosswrel/ps1831/products_configuration_guide_
chapter09186a00800d97f7.html#xtocid20
Content 7.1
Distance Vector Routing 7.1.5 Route
poisoning Route poisoning is used by various distance