Redistribution and Summarization

What it is

Redistribution copies routes from one routing source into another, and summarization replaces many specific routes with one shorter prefix.

A routing protocol only knows its own routes. When a network runs two protocols, such as after a merger, a router in the middle must redistribute so each side learns the other's prefixes.

Summarization keeps routing tables small and hides instability. A flapping subnet inside a summary does not trigger updates outside it.

How it works

Redistribution needs a command, a seed metric the target protocol understands, and often a filter.

Redistribution commands

Each protocol has its own metric type, so a seed metric is needed.

R1(config)# router ospf 1
R1(config-router)# redistribute eigrp 100 subnets
R1(config-router)# router eigrp 100
R1(config-router)# redistribute ospf 1 metric 10000 100 255 1 1500
R1(config-router)# router rip
R1(config-router)# redistribute ospf 1 metric 3

The EIGRP metric values are bandwidth in Kbps, delay in tens of microseconds, reliability, load and MTU. In named mode, the same redistribute command sits under topology base inside the address family.

Connected and static routes redistributed into EIGRP get a metric derived from the interface or next hop, so those usually work without a metric.

Routes redistributed into OSPF appear as E2 by default. An E2 route keeps the same cost across the domain. An E1 route adds the internal cost to reach the ASBR.

R1(config-router)# redistribute eigrp 100 subnets metric-type 1 metric 50

Administrative distance and loops

Redistributed routes get the AD of their new protocol. External EIGRP routes use 170 and OSPF routes use 110.

If you redistribute in two directions at two routers, a route can return and win because of its AD. This is suboptimal routing or a loop. Use route tags, as the previous page describes, and filter with route maps.

Troubleshooting redistribution

Work through these checks in order.

• Is the route in the source table?
• Is the redistribute command present, and for OSPF is subnets present?
• Does the target protocol have a seed metric?
• Does a route map or distribute list drop the route?
• Is a lower AD route from another source winning?

R1# show ip route eigrp
R1# show ip protocols
R1# show ip ospf database external
R1# show ip eigrp topology

show ip protocols lists which sources are redistributed into each process.

Manual and automatic summarization

Manual summaries are configured per protocol, and each creates a discard route to Null0 on the router that originates it.

The EIGRP summary route toward Null0 has an AD of 5. This stops traffic for missing subnets from looping.

Auto-summarization summarizes at classful network boundaries. It was on by default in older EIGRP releases, and the default differs by protocol and software version, so check show ip protocols. It breaks discontiguous networks, where the same classful network exists on two sides of another network.

R1(config)# router eigrp 100
R1(config-router)# no auto-summary

The BGP summary-only keyword suppresses the more specific routes. Without it, both the aggregate and the specifics are advertised.