IPv4 Routing Protocol Selection

woonsocketpoliticalNetworking and Communications

Oct 28, 2013 (3 years and 11 months ago)

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IPv4 Routing Protocol Selection
This section describes the process of choosing routing protocols for your network and discusses the
concepts of redistribution, filtering, and administrative distance.
Choosing Your Routing Protocol
To decide which routing protocol is best for your network, you need to first look at your
requirements. You can then compare your requirements to the specifications for the available
routing protocols, as detailed in the previous sections and summarized earlier in Table 3
-
2
, and
choose the routing protocol that best meets your needs.
Recall that Chapter 1
, "Network Design," described the hierarchical model in which a network is
divided into three layers: core, distribution, and access. Because each layer provides different
services, they typically have different routing requirements and therefore use different routing
protocols. The specific network function performed at each of these layers is as follows:

Access layer Provides end-user and workgroup access to the resources of the network.

Distribution layer Implements the organization's policies (including filtering of traffic) and
provides connections between workgroups and between the workgroups and the core.

Core layer Provides high-speed links between distribution-layer devices and to core
resources. The core layer typically uses redundancy to ensure high network availability.
Thus, the different routing protocols suitable at each layer are as follows:

In the core layer, a fast-converging routing protocol is required: EIGRP, OSPF, and IS-IS are
the possible choices. OSPF and IS-IS require a hierarchical topology with areas defined;
EIGRP supports a hierarchical topology but doesn't require it. EIGRP is Cisco-proprietary, so it

can only be supported if all routers are Cisco routers. IS-IS requires OSI addresses to be
configured, which is not a common skill.

In the distribution layer, any of the interior routing protocols are suitable, depending on the
specific network requirements. For example, if it is an all-Cisco network and has a mixture of
link types so that VLSMs would be appropriate, EIGRP would be the logical choice. Because
the distribution layer routes between the core and access layers, it might also have to
redistribute (share with) and/or filter between the routing protocols running in those layers,
as described in the next section.

The access layer typically includes RIPv2, IGRP, EIGRP, OSPF, or static routes. The devices in

this layer are typically less powerful (in terms of processing and memory capabilities) and
therefore support smaller routing tablesthus, the distribution layer should filter routes sent to

this layer. Remember that EIGRP is not suitable for use in a dial-up network and that
distance vector routing protocols have issues in NBMA networks.
Redistribution, Filtering, and Administrative Distance
Key Point
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If two (or more) routing protocols are run in the same network, information from one
routing protocol can be redistributed with, or shared with, another routing protocol.
Routers that participate in more than one routing protocol perform the redistribution.

Redistribution can be bidirectionalthe information from each routing protocol is shared with the
other. It can also be performed in only one direction, with default routes used in the other
direction. You must be careful not to introduce routing loops when you use redistribution.
Key Point
Routes can be filtered to prevent specific routes from being advertised. In other words,
the router can exclude specific routes from the routing updates it sends to other specific
routers.

Route filtering is useful when redistribution is being used, to help prevent routing loops.
For example, consider the network in Figure 3
-
17
, with IGRP running in the upper part and RIPv2
running in the lower part. Both Routers A and B are configured to pass IGRP information into the
RIPv2 network, and RIPv2 into the IGRP network, with the intention that all devices can reach all
networks.
Figure 3-17. Routers A and B Are Redistributing Between IGRP and RIPv2

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A problem can occur if both Routers A and B redistribute the full content of their routing tables,
because more than one path exists between the IGRP and RIPv2 networks. For example, Router B
can pass information about network 10.0.0.0 to Router E, which can pass it to Router D, which can
pass it to Router C, which can pass it to Router A. Router A is connected to network 10.0.0.0, but
depending on how the redistribution is configured, Router A might think that the path to some of
the subnets of network 10.0.0.0 through Router C is betterthrough the IGRP network. If Router A
passed this information to Router F, and so on, traffic from the RIPv2 part of the network might
loop around the entire network before ending up where it startedin other words, the potential
exists for a routing loop. Specific route filtering can be configured to avoid thisyou must know your
network and ensure that you are not introducing problems.
Because each routing protocol uses different metrics, you can't compare one metric with anotherfor

example, how do you compare whether 3 RIP hops are better than an OSPF cost of 10? Thus, when

multiple routing protocols are run on Cisco routers, another parameter, called the administrative
distance, compares the routing protocols.
Key Point
Cisco routers use the administrative distance in the path selection process when they
learn two or more routes to the same destination network or subnet from different
sources of routing information, such as different routing protocols. The administrative
distance rates the believability of the sources of routing information.
The administrative distance is a value between 0 and 255; the lower the value, the higher
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the believability of the source of the routing information.

Table 3-4 lists the default administrative distance of the sources of routing information, including
routing protocols, supported by Cisco routers.

For example, consider a router that receives a route to network 10.0.0.0 from RIPv2 (with an
administrative distance of 120) and also receives a route to the same network from IGRP (with an
administrative distance of 100). The router uses the administrative distance to determine that IGRP

is more believable; the router therefore puts the IGRP route into its routing table.
The administrative distance can be changed from its default value, either for all routes from a
routing protocol or for specific routes. This can help to eliminate routing loops.
Note
Many details about routing protocol operation and configuration, including redistribution,
filtering, and administrative distances, are provided in the book CCNP Self-Study: Building
Scalable Cisco Internetworks (BSCI), Second Edition, by Paquet & Teare, Cisco Press,
2003.

Table 3-3. Administrative Distance of Routing Protocols
Route Source
Default Distance
Connected interface, static route out
an interface
0
Static route to an address
1
EIGRP summary route
5
EBGP
20
Internal EIGRP
90
IGRP
100
OSPF
110
IS-IS
115
RIPv1, RIPv2
120
External EIGRP
170
Internal BGP
200
Unknown
255

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