7.5 Summarization on ABRs

CCNP RS 300-101, Optimizing OSPF Behavior, Uncategorized

Summarization on ABRs affects Type 3 LSAs. Here are some characteristics of summarization on an ABR:

  1. The summary will be entered into the ABRs routing table with a route to null if there is at least one subnet that falls within the specified range; if all subnets that are a part of the range are removed, the summary route is removed from the routing table.
  2. The null interface ensures that traffic is not forwarded to any subnets that are not defined within the summary range.
  3. Only the summary range will be advertised to OSPF neighbors as a Type 3 LSA.
  4. The lowest cost of any of the subnets within the specified range is automatically assigned as the metric to the advertised summary route, though the cost can be changed in the summary command.
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