I'm hoping someone can clarify the selection criteria for which ASBR to use for E2 routes in OSPF.
I'm injecting BGP routes in to a network at two points and redistributing them in to OSPF as E2s. I've decided to let OSPF determine which route to install, i.e. devices should just route to their nearest exit point.
As the routes have an identical metric of 1, then the route installed should be determined by the cost to the ASBR. This appears to be happening, but I'd appreciate clarification that the ASBR selection is based purely on the cost to the ASBR and not on whether the ASBR route is intra over inter area, i.e. an inter area route to an ASBR with a lower cost will be preferred over an intra area route with a higher cost.
By default when redistributing into OSPF , the default type is 2 and metric 20 ( except when redistributing from bgp , when the metric is 1 ).
When we have type 2 external , the best path selection is made from the cost set on the redistribution only and also the cost to the ASBR (LSA4) because this type of external route does not add to its metric, the cost along the path. In this case the forwarding address is set to 0.0.0.0 - the next-hop is the ASBR. This can be check from the show ip route output and foward metric
Cost = LSA5 + LSA4 ( ASBR cost )
There is one more case when the forwarding address is not null. This means the the next hop used is the one specified in the Fowrding address.
Thats interesting, so from that if I had a router with interfaces in Area 0 and Area 1 and one of my ASBR was also in Area 1 then you would expect the route from the ASBR in Area 1 to be used, even if it had a higher cost?
This is actually a pretty cool feature, i didn't even know it existed until I was looking for a solution to advertise a subnet (prefix in BGP talk), only if a certain condition existed. This is exactly what conditional advertisements does
j ai une question j ai achete un routeur cisco 887VA-k9 , je le configuré avec la configuration ci- dessous
si je le lier avec mon pc portable sur l un de ses ports directement ça marche toute est bien ( la connexion internet + m...
Attached policy provides CLI access to the Cisco 4G router over text messaging. Two files are in the attached .tar file:
2. PDF with instructions on how to load and use the .tcl file.