Routing - seed metrics

The seed metric is an initial value that a routing protocol uses when learning about routes from another routing protocol. Each routing protocol has a particular seed metric that is used by default when being redistributed into another routing domain. The reason for different seed metrics in different routing protocols is due to the differing methods they use to calculate the best path.

RIP and EIGRP use a seed metric of infinity because they are distance-vector protocols. In these protocols, a route with a metric of infinity is considered unreachable. By initially setting the seed metric to infinity, the protocol essentially considers the new route as unreachable. This prevents the route from being used until the protocol can assign a more accurate metric to it, based on subsequent updates or configuration changes. In practice, a network administrator will configure a more useful seed metric based on design and network requirements. More info about seed metrics in EIGRP can be found in the note about EIGRP seed metrics when redistributing.

In OSPF, the maximum cost is 65535, and anything beyond that is considered infinite. However, OSPF is a link-state protocol whose metric is based on link bandwidth. It uses a seed metric of 20 when redistributing routes from other protocols. This seemingly arbitrary number is likely a design decision made to provide a “middle of the road” value, as the OSPF cost value is typically much higher than that of RIP or EIGRP.

Now when redistributing BGP into OSPF we have an exception where the default seed metric used is 1. The likely reason for this is that BGP uses a very complex path selection process and doesn’t really have a comparable metric to OSPF’s cost. Therefore, setting the seed metric to 1 ensures that the routes redistributed from BGP are not immediately considered less favorable than OSPF’s internal routes simply because of a higher cost.

Ultimately, these values have been chosen by design to ensure the most appropriate behavior of redistributed routes. Keep in mind however that these default seed metrics should ultimately be manually configured by an administrator to more appropriate values depending on the specific requirements of the particular network.