Route Redistribution

Terms

Administrative Distance

How trusted a route is, if two routes have the same subnet and mask.

One-way Redistribution

Useful for WAN edges, to learn a few subnets and add them into the core. The WAN network or branch site gets a default route.

Two-way Redistribution

Used nearer the core, the router redistributes both ways. This can cause feedback loops without techniques like route filters.

Seed Metric

The values given to the receiving routing protocol.

Route Loop

Traffic bounces back-and-forth between two routers until the TTL reaches 0.

Route Feedback

A redistributed route in a protocol with a better AD, is fed back into a protocol with a lower AD.

Route feedback can be fixed by modifying Administrative Distance with distance commands.

Rules

Non-Transitive

If a router is running three routing protocols, for full-reachability, each routing protocol needs the other two protocols redistributed into each other.

Routes Must Be In the RIB

The router doing the redistribution needs to have the route installed.

Seed Metrics Matter

Fast links should get better seed metrics that slower links.

Always provide a meaningful seed metric.

Troubleshooting

debug ip routing

Show changes to the routing table.

Seed Metric Preference

If multiple seed metrics are applied, this is the order of preference.

1. Route Map applied redistribute
2. redistribute
3. default-metric

References

B. Edgeworth and R. Lacoste, CCNP Enterprise Advanced Routing ENARSI 300-410 Official Cert Guide, 2nd ed. Indianapolis, IN: Cisco Press, 2023.