CCIE Study : IGP Summary Default Metric

A lot of confusion arises from the default metric in OSPF when ABR summarizes the routes. Consolidating some documents I found on the internet in a single blogpost.






Accd to the above post, everything (by default) uses lowest IGP metric (of all the component/subordinate routes) except OSPFv3. This is what is the observed behavior on Cisco Routers by default. This is where the confusion arises. RFC 2328 says that OSPF should use the highest metric of all the component routes for the summary route. I found an explanation for this behavior (copied directly from Cisco's website):

 RFC 1583  called for calculating the metric for summary routes based on the minimum       metric of the component paths available.  RFC       2178  (now obsoleted by       RFC 2328 )       changed the specified method for calculating metrics for summary routes so the       component of the summary with the maximum (or largest) cost would determine the       cost of the summary.

Prior to IOS 12.0, Cisco was compliant with the then-current       RFC 1583 . As       of IOS 12.0, Cisco changed the behavior of OSPF to be compliant with the new       standard, RFC       2328 . This situation created the possibility of sub-optimal routing if       all of the ABRs in an area were not upgraded to the new code at the same time.       In order to address this potential problem, a command has been added to the       OSPF configuration of Cisco IOS that allows you to selectively disable       compatibility with RFC 2328 . The new configuration command is under       router ospf, and has the following syntax:

[no] compatible rfc1583 

So, in case of OSPFv2, Cisco follows the older RFC 1583 and not the latest RFC 2328.