OSPF Series 10: Everything on Redistribution: Part 1- Basics, RIP, IGRP, EIGRP, Static and Connected

Everything on Redistribution- Basics, RIP, IGRP, EIGRP, Static and Connected

Route Redistribution Basics

It is preferable to employ a single routing protocol in an internetwork environment, for simplicity and ease of management. Unfortunately, this is not always possible, making multi-protocol environments common.

Route Redistribution allows routes from one routing protocol to be advertised into another routing protocol. The routing protocol receiving these redistributed routes usually marks the routes as external. External routes are usually less preferred than locally-originated routes.

At least one redistribution point needs to exist between the two routing domains. This device will actually run both routing protocols. Thus, to perform redistribution in the following example, RouterB would require at least one interface in both the EIGRP and the OSPF routing domains:

It is possible to redistribute from one routing protocol to the same routing protocol, such as between two separate OSPF domains (distinguished by unique process ID’s). Static routes and connected interfaces can be redistributed into a routing protocol as well.

Routes will only be redistributed if they exist in the routing table. Routes that are simply in a topology database (for example, an EIGRP Feasible Successor), will never be redistributed.

Routing metrics are a key consideration when performing route redistribution. With the exception of IGRP and EIGRP, each routing protocol utilizes a unique (and thus incompatible) metric. Routes redistributed from the injecting protocol must be manually (or globally) stamped with a metric that is understood by the receiving protocol.

(Reference: http://www.cisco.com/warp/public/105/redist.html)

Redistributing into RIP

RIP is a standardized Distance-Vector routing protocol that uses hop-count as its distance metric. Consider the following example:

RouterB is our redistribution point between IGRP and RIP. To redistribute all IGRP routes into RIP:

RouterB(config)# router rip

RouterB(config-router)# network 172.16.0.0

RouterB(config-router)# redistribute igrp 10 metric 2

First, the router rip process was enabled. Next, RIP was configured to advertise the network of 172.16.0.0/16. Finally, RIP was configured to redistribute all igrp routes from Autonomous System 10, and apply a hop- count metric of 2 to the redistributed routes. If a metric is not specified, RIP will assume a metric of 0, and will not advertise the redistributed routes.

Redistributing into IGRP

IGRP is a Cisco-proprietary Distance-Vector routing protocol that, by default, uses a composite of bandwidth and delay as its distance metric. IGRP can additionally consider Reliability, Load, and MTU for its metric.

Still using the above example, to redistribute all RIP routes into IGRP:

RouterB(config)# router igrp 10

RouterB(config-router)# network 10.0.0.0

RouterB(config-router)# redistribute rip metric 10000 1000 255 1 1500

First, the router igrp process was enabled for Autonomous System 10. Next, IGRP was configured to advertise the network of 10.0.0.0/8. Finally, IGRP was configured to redistribute all rip routes, and apply a metric of 10000 (bandwidth), 1000 (delay), 255 (reliability), 1 (load), and 1500 (MTU) to the redistributed routes.

 

Redistributing into EIGRP

EIGRP is a Cisco-proprietary hybrid routing protocol that, by default, uses a composite of bandwidth and delay as its distance metric. EIGRP can additionally consider Reliability, Load, and MTU for its metric.

To redistribute all OSPF routes into EIGRP:

RouterB(config)# router eigrp 15

RouterB(config-router)# network 10.1.2.0 0.0.0.255

RouterB(config-router)# redistribute ospf 20 metric 10000 1000 255 1 1500

First, the router eigrp process was enabled for Autonomous System 15. Next, EIGRP was configured to advertise the network of 10.1.2.0/24. Finally, EIGRP was configured to redistribute all ospf routes from process- ID 20, and apply a metric of 10000 (bandwidth), 1000 (delay), 255 (reliability), 1 (load), and 1500 (MTU) to the redistributed routes.

It is possible to specify a default-metric for all redistributed routes:

RouterB(config)# router eigrp 15

RouterB(config-router)# redistribute ospf 20

RouterB(config-router)# default-metric 10000 1000 255 1 1500

RIP and IGRP also support the default-metric command. Though IGRP/EIGRP use only bandwidth and delay by default to compute the metric, it is still necessary to specify all five metrics when redistributing. If the default-metric or a manual metric is not specified, IGRP/EIGRP will assume a metric of 0, and will not advertise the redistributed routes.

Redistribution will occur automatically between IGRP and EIGRP on a router, if both processes are using the same Autonomous System number.

EIGRP, by default, will auto-summarize internal routes unless the no auto- summary command is used. However, EIGRP will not auto-summarize external routes unless a connected or internal EIGRP route exists in the routing table from the same major network of the external routes.

Redistributing Static and Connected Routes

Redistributing static routes into a routing protocol is straightforward:

RouterB(config)# router eigrp 15

RouterB(config-router)# redistribute static

Redistributing networks on connected interfaces into a routing protocol is equally straightforward:

RouterB(config)# router eigrp 15

RouterB(config-router)# redistribute connected

The above commands redistribute all connected networks into EIGRP. Route-maps can be used to provide more granular control:

RouterB(config)# route-map CONNECTED permit 10

RouterB(config-route-map)# match interface fa0/0, fa0/1, s0/0, s0/1

RouterB(config)# router eigrp 15

RouterB(config-router)# redistribute connected route-map CONNECTED

Connected networks can be indirectly redistributed into a routing protocol. Recall that routes will only be redistributed if they exist in the routing table, and consider again the following example:

 

If RouterB is configured as follows:

RouterB(config)# router eigrp 15

RouterB(config-router)# network 10.1.2.0 0.0.0.255

RouterB will advertise the 10.1.2.0/24 network to RouterA, but it will not have an EIGRP route in its routing table for that network, as the network is directly connected.

Despite this, when redistributing EIGRP into OSPF, the 10.1.2.0/24 is still injected into OSPF. The network 10.1.2.0 0.0.0.255 command under the EIGRP process will indirectly redistribute this network into OSPF.