[c-nsp] eliminating asymmetric routes

[David Barak]

--- 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 :)

what routing protocol are you using?  

Are all of the below routers under your administrative
control?  Are the metrics symmetric in all cases?  

If the answers to the above two questions are "yes,"
then where is the asymmetry coming from?  

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?  

Also, where are the NAT and FW boundaries which are
causing the constraint?



> 
> 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: 
http://www.listentothefranchise.com

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