Thursday, September 25, 2008

OSPF routing part II - discard routes

The default behaviour for OSPF (in releases 12.1(6) and higher) when creating a summary route with the 'area x range' command is to auto-generate the summary pointing to the null interface. Cisco calls these discard routes as they prevent routing loops when summarization occurs i.e. if a packet is received for the summary route without a more granular present it will be discarded.

This default behavior can be turned off with the use of the 'no discard-route' at the ospf config router prompt.

config-router#no discard-route internal
config-router#no discard-route external

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