Routing Redistribution on Cisco Routers (Cheat Sheet)

In order for two Layer 3 devices (routers or layer 3 switches) to be able to exchange routing information, it is necessary to use the same routing protocol, such as RIP, EIGRP, OSPF, BGP, etc.

Different routing protocols, or protocols configured differently (eg different EIGRP autonomous system) do not exchange information.

However, when a device learns routing information from different sources (eg static routes or using different protocols) Cisco IOS allows the information learned from a specific source to be published to other devices using a different protocol.

For example, a route learned through RIP can be advertised to other devices using OSPF. This is what is called “redistribution” of routes: Using a routing protocol to advertise routes that are learned through other means (other protocol, static routes or directly connected).

To configure route redistribution some rules must be in place:

• The redistributed route must be present in the routing table.
• The redistributed route will be received by the neighbouring device with a new metric as configured by the redistributing router.

What is it used for?

In principle it is desirable that a network should use a single routing protocol. However, in some cases we may require the use of redistribution: two companies merged, different departments of a company managed by different teams, multi-vendor environments, migration, etc.

When addressing a redistribution of routes scenario we should take into account particular aspects of routing: different metrics, administrative distance of each protocol, the capabilities of classful and classless routing, and network topology.

Metrics

Each routing protocol uses a different metric. This causes the routes redistributed to lose the original metric of the protocol and the metric is redefined in terms of the new protocol.

For example, if an OSPF route is redistributed with a metric of 1642 in RIP, RIP metric uses number of hops (between 1 and 15). So you must change the metric before redistributing to RIP.

The metric with which a protocol receives the routes learned by another metric is called seed metric.
Each protocol uses a default seed metric:

RIP – default seed metric: infinity.
EIGRP – default seed metric: infinity.
OSPF – default seed metric: 20.
The default seed metric can also be modified using the “default metric” command.

The basic commands

When you configure redistribution of protocols, you should indicate how to redistribute routing information, and how we want to measure these routes (metric) when they are redistributed. If we do not indicate anything, the routes are redistributed with the default metric.

Router (config) # router rip
Router (config-router) # network 129.100.0.0
Router (config-router) # redistribute ospf 1 metric 2

In this example we tell the router to redistribute routing information into RIP when learned through the OSPF process 1 which is in the routing table, with a metric of 2 hops.

Redistribution in EIGRP

To redistribute routing information into EIGRP, it should be noted that the default metric is infinite. Therefore, if you do not specify metric for redistributed routes, they will not appear in the routing table of the neighbouring device.

Furthermore, by defining the metric it should be noted: bandwidth, delay, reliability, load and MTU.

An example:

Router (config) # router eigrp 100
Router (config-router) # redistribute static
Router (config-router) # redistribute rip
Router (config-router) # default-metric 10000 100 255 1 1500

Redistribution in OSPF

The default metric used by OSPF is 20, so it does not require us to specify a metric for the route learned by the adjacent devices.

However, when there are multiple subnets on the same network and you want to publish routes for each subnet, you must configure a metric otherwise OSPF will summarize all subnets in the class boundary and publish a single route.

An example:

Router (config) # router ospf 1
Router (config-router) # redistribute static metric 200 subnets
Router (config-router) # redistribute eigrp 100 metric 500 subnets

RIP redistribution

Like EIGRP, RIP redistributes the protocols using a default metric of infinity, so it is necessary to specify a different metric in order for the neighbour router to incorporate the routing information in its table.

An example:

Router (config) # router rip
Router (config-router) # redistribute static metric 1
Router (config-router) # redistribute ospf 1 metric 2