ROUTE REDISTRIBUTION SEED METRICS

What is a Seed metric?

Seed metric has an important role to play during the route redistribution. When a router does the redistribution, it must assign a metric to the redistributed routes. Redistributed routes are learned from routing protocols and not physically connected. If a boundary router want to redistribute few prefixes between routing protocols, it must be capable of translating the metric of the received route from the source routing protocol into the other routing protocol. For example, if a boundary router receives a RIP route, the route has hop count as a metric. Similarly, if a route has to be redistributed into OSPF, the boundary router must translate the hop count into a cost metric that the other OSPF routers will understand.This metric, referred to as the Seed or default metric, is defined during redistribution configuration. After the seed metric for a redistributed route is established, the metric increments normally within the autonomous system, however there is an exception to this rule with regards to OSPF. OSPF E2 routes, hold their initial metric regardless of how far they are propagated across an autonomous system.

Below is  the route redistribution Seed metric chart:


Few Important Points to notice:
  • A route/prefix must be present in the routing table for it to be redistributed in a routing protocol.
  • When a router advertises a directly connected link( using 'redistribute connected'), the initial, or seed, metric used is derived from the characteristics of that interface, and the metric increments as the routing information is passed to other routers.For EIGRP, the default seed metric is based on the interface bandwidth and delay. For RIP, the seed metric starts with a hop count of 0 and increases in increments as the prefixed is advertised from router to router. For OSPF, the seed metric is based on the interface’s bandwidth. For IS-IS, each interface has a default IS-IS metric of 10. 
  • The 'default-metric' router configuration command establishes the seed metric for all redistributed routes. Cisco routers also allow the seed metric to be specified as part of the 'redistribute' command, either with the metric option or by using a route map.
  • When redistributing routing information, set the seed metric to a value larger than the largest metric within the receiving autonomous system, to help prevent sub-optimal routing and routing loops.
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