[c-nsp] eliminating asymmetric routes

[Joe Maimon]

A not so hypothetical question.

How to go about eliminating asymmetric routes from all routers/sites 
mentioned below while using a routing protocol.


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