[c-nsp] eliminating asymmetric routes

[Joe Maimon]

David Barak wrote:
> 
> --- Joe Maimon <jmaimon at ttec.com> wrote:
> 
> 
>>A not so hypothetical question.
>>
>>How to go about eliminating asymmetric routes from
>>all routers/sites 
>>mentioned below while using a routing protocol.
>>
> 
> 
> Okay, I'll bite :)
Thanks
> 
> what routing protocol are you using?  
Whichever will do the job most naturally/simply on a cisco platform.
> 
> Are all of the below routers under your administrative
> control?  Are the metrics symmetric in all cases?  
> 
They will be. Metrics are symmetric and equal cost.

> If the answers to the above two questions are "yes,"
> then where is the asymmetry coming from?  
If you count the hops, as a distance vector, A<->H have two equal cost 
paths. A->C->E->F->H and H->F->D->C->A would be the asymmetricity.
> 
> You mentioned that "warm standby routing" is not
> preferred - do you mean that you want packets going
> from C -> F to traverse both E and D, or would you be
> willing to accept determinism in that case?  
I want the shortest path between any two points, but the return path to 
be guaranteed identical in reverse.
> 
> Also, where are the NAT and FW boundaries which are
> causing the constraint?
> 
> 
C<->E
C<->F
D<->F
D<->E
> 
> 
>>A   B
>>|   |
>>C---D
>>|   |
>>E---F
>>| | |
>>G | H
>>   |
>>   I
>>
>>Or
>>
>>
>>A   B
>>|   |
>>C---D
>>|\ /|
>>| \ |
>>|/ \|
>>E---F
>>| | |
>>G | H
>>   |
>>   I
>>
>>E,F,I are all on a common subnet/vlan.
>>
>>Asymmetricity is a problem because of FW with
>>natting OR without natting.
>>
>>This would occur on links
>>
>>C<->E
>>C<->F
>>D<->F
>>D<->E
>>
>>The second design, while harder to scale eliminates
>>inefficiencies.
>>
>>Warm standby routing (where only one link was used
>>for ALL sites e.g. 
>>C<->E is very much not preferred.
>>
>>So far on the table is-
>>
>>A)
>>Extending connections (ethernet vlan/subnets) to
>>ensure that all sites 
>>have a one hop to all other sites across the nat/fw
>>boundaries. 
>>Difficult to scale.
>>
>>B)
>>
>>Tagging routes that cross
>>
>>
>>C<->E
>>C<->F
>>D<->F
>>D<->E
>>
>>And announcing them with higher cost on links
>>
>>C<->D
>>E<->F(,I)
>>
>>Any ideas welcome.
>>
>>Joe
>>
> 
> 
> -David
> 
> David Barak
> Need Geek Rock?  Try The Franchise: 
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