CCNP Routing and Switching ROUTE 300-101 Official Cert Guide

LSAs and Metrics for External Type 2 Routes

To appreciate how OSPF calculates the possible routes for each E2 route, you need to take a moment to think about the Type 5 LSA in more detail. First, by definition, the router that performs the redistribution into OSPF becomes an autonomous system border router (ASBR), because it injects external routes into OSPF. For each such route, that ASBR creates a Type 5 LSA for that subnet. The Type 5 LSA includes the following fields:

LSID (Link-state ID): The subnet number

Mask: The subnet mask

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