Route-Map with IP prefix list that does not exist?

I was experimenting with a lab I created to test BGP to OSPF redistribution, and just want to clarify the behaviour I have seen or find some documentation

Basic setup R1 redistributing BGP routes into R2's OSPF. I wanted to set the metric of one subnet and set the type for another 2 x subnets.


!
route-map BGP_to_OSPF permit 30
 match ip address prefix-list 1_routes
 set metric 200
!
route-map BGP_to_OSPF permit 40
 match ip address prefix-list 2_routes
 set metric-type type-1
!
route-map BGP_to_OSPF permit 50

During my config the prefix list 1_routes did not exist.

The behaviour I saw was that the 2_routes networks were leaving R1 with a metric of 200?

Once I added the prefix-list 1_routes in this went away.

I have proved the behaviour but need to clarify the logic?

If you reference a prefix list that does not exist is that the same as having an empty line so it will set metric of 200 on everything?

 

Thanks

Roger

http://www.rogerperkin.co.uk/ccie

Comments

  • it is not just the case with prefix-list it is the case with route-map, access-list, prefix-list. If there is a statement there will be a implicit deny at the end but if the list is empty you can say it has the opposite implicit permit. That is a good behavior as you can put a access-list or route-map and acciently refer a list that is not there and block everything.

     

     

    HTH 

  • Thank you for clarifying, I have not done much with route-maps to date so it is all good learning!

     

    Roger

  • IF you match in a route-map on a prefix-list or ACL which does not exists, it ignores the command. What happens ina  route-map entry when you have no match commands? It has an  implicit/invisible match any. If you apply an ACL to an interface, direction is irelevant, and the ACL has no ACE entries, there is an implicit/invisible permit any. The implicit deny any gets activated in an ACL as soon as you add at least one ACE to the ACL.

    Good luck with your studies!

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