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OSPF default route question

chrisbell1
Beginner
Beginner

Hello,

In a practice lab I have two routers configured with OSPF directly connected.

I have established an OSPF adjacency and advertised a default route from each router with the "default-information originate always" command.

Each router will not inject the default route pointing to the other router into the routing table.

I have checked the OSPF database and can see two default routes listed.

Can anyone explain why each router will not inject a default route to point to the other?

Cheers,

3 Replies 3

Seb Rupik
VIP Advisor VIP Advisor
VIP Advisor

Hi there,

What are the sources for the static routes? If both routers have static default routes configured then it will have a much lower AD (1) than an OSPF route (110) and will not be installed in the routing table.

http://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/15986-admin-distance.html

cheers,

Seb.

Rolf Fischer
Engager
Engager

Hi,

this seems to be an additional loop prevention check in IOS. When the 'always'-keyword is used on the local router, the default routes originated from other routers are removed from the local OSPF RIB. This has, of course, no effect on the LSDB.

When we use dynamic routing protocols we don't want them to create the "mother of all routing loops" - two routers with default-routes pointing to each other, right? So I think this is a very reasonalbe check but I couldn't find any documentation about it so far.

HTH
Rolf

chrisbell1
Beginner
Beginner

Gidday,

 

Yes I understand why this check is good, I was just looking for documentation/reference to the protection. 

 

AD does not come into play with my test as each OSPF router did not actually have a static default. 

 

In summary, if OSPF router advertises a default route with the "always" keyword, it will not inject a default route learnt via OSPF into its routing table.

 

Cheers,

 

Cheers,

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