Multiple ospf abr routers

[carl_townshend]

Hi all

if we have 2 abr routers between areas with redistribution, what needs to be done to stop suboptimal routing or loops?

do we need to do filtering etc or would it generally work without any further config?

for example if we have area 0 with 2 routers connecting to both area 0 and area 1, would we need to do anything ?

[balaji.bandi]

yes you need to have route-map to filter those routes not to announce back.

you can have simple example s.

https://blog.ine.com/2009/08/17/ospf-route-filtering-demystified

[carl_townshend]

Hi 

what if we were not doing redistribution? And just one area into an other? Would we not need to do any filtering then ?

[balaji.bandi]

If there is no Loop then you do not need  redistribution  - until you have any other reason to filter ?

[carl_townshend]

In what scenario would we need to filter then ?

[Joseph W. Doherty]

ABRs and redistribution?  Did you mean ASBRs?

[carl_townshend]

Hi Joseph

I have heard in the past that you have to be careful when doing mutual redistribution with multiple asbrs as it can cause loops or routing issues, when would this case occur ? And how would you fix it? I hear you would have to tag routes and filter etc? 

[Joseph W. Doherty]

Correct, you need to be careful when doing mutual redistribution (between routing protocols), as it can cause issues.

An example given two ASBRs, both running OSPF and EIGRP.  If ASBR X redistributes its OSPF routes into EIGRP and if ASBR Y redistributes its EIGRP routes into OSPF, you've setup a "circular" redistribution.  I.e. each routing protocol's routes will come back into that routing protocol via the other routing protocol.

The way to avoid this is to filter out routes that originated in a routing protocol.  So, for example, if ASBR Y somehow "knows" that some of its EIGRP were sourced from its OSPF (i.e. those redistributed by ASBR X), it doesn't redistribute those routes back into OSPF.  Likewise ASBR X somehow "knows" that some of its OSPF routes were sourced from its EIGRP (i.e. those redistributed by ASBR Y), it doesn't redistribute those routes back into EIGRP.

What attributes are used for this filtering depends on the routing protocol.  For example, in OSPF you can "tag" routes as you redistribute them into OSPF.  These tags can then be used to filter them out when doing redistrubtion.  (BTW, often when I redistribute into OSPF, even when not doing mutual redistribution, I tag the OSPF routes with a unique tag for each router.  Then if I need to "know" which router sourced a particular external route, it's easy to identify it.)