Wednesday, June 19, 2013

Cisco Routing Protocols Redistribution Initial Metrics

One Interesting topic is redistribution between various routing protocols. Every routing protocol has a way to calculate the best path to the end destination, RIP for example uses number of hops whether the links between the routers is slow or fast. EIGRP uses it's complicated metric utilizing reliability, MTU, delay etc.. to calculate the best path to the destination. to put that into prospective, if we have the same topology and we decided to run OSPF instead of RIP, most certainly the traffic path will be changed due to the way the metric sees the best path.

Our topic here is what metric will a route have if it's redistributed from one protocol to another? 

The table below shows the default initial metrics that are assigned by protocols incase of redistribution between different protocols.

There are some facts when it comes to redistribution, like:
  1. Redistribution between similar protocols will not do any change to the metric.
  2. EIGRP and RIP will not redistribute other protocols unless the initial metric is set manually.
  3. Connected and Static Routes will always have initial metrics to any protocols since their distribution is only one way redistribution and the fact that they're always local to the router.
  4. BGP Multi-Exit Discriminator ( MED ) will be used as metric in case of redistribution.