>Hi all,
>
>Need to find out if there is a method for avoiding asymmetric routing
>between 2 areas both with 2 ABRs and equal capacity links (E1 - 2.048Mbps).
First, what is the problem you are trying to solve? Asymmetric
routing is a reality in most hierarchical networks that use dynamic
routing. In most cases, it's best to work within its limits.
When there are strict latency or other requirements that really
demand symmetrical routing, you probably need explicit static routes
or MPLS paths in both directions.
>
>I'm looking to influence flow of traffic to certain destinations to remain
>on a single link, and not have traffic to lets say 10.32.0.0/12 go out on
>link A (ABR A) and return on link B (ABR B).
In the OSPF world, a starting point would be to avoid all stubby
areas, so the ingress routers can consider the end-to-end path to the
destination.
>If I have a range of addresses 10.0.0.0/12, I know I can split it between
>ABR A (10.0.0.0/13) and ABR B (10.8.0.0/13) - using area summary statements.
>But the difficulty is actually configuring the other ABR as a backup for the
>same routes. I.e. ABR A backs up ABR B's range 10.8.0.0/13 and vice versa.
Cisco and Bay/Nortel took different approaches to ABR summarization.
Both approaches have good and bad features, and I really wish each
vendor would offer both methods.
Cisco's assumption is that stability is most important. Once an ABR
is programmed with less-specific summaries, it will always announce
those to area 0.0.0.0 even if some or all of the intra-area
more-specifics are down. It will announce them even if that means
some traffic is blackholed.
Bay/Nortel's approach is that precision is most important. If some
of the constituent more-specifics of an aggregate go down, the ABR
will stop advertising the summary and only advertise the
more-specifics.
>
>Is the above possible?
>
>regards,
>Ming.
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