[Xorp-users] How to AS prepend ?

[Sébastien Namèche]

Bruce,

Le 26 juin 09 à 15:10, Bruce Simpson a écrit :
> What is your desired as-path on the redistributed route(s), in the
> output you'd expect to see in the BGP routing table ?
.../...
> So your intention is in fact to announce the same prefix X.Y.W.0/23,
> from the XORP BGP router, but with your AS path prepended to the
> existing AS path advertised by another BGP router which already
> announces prefix X.Y.W.0/23?
>
> This isn't going to work if you're directly applying the policy to the
> route redistributed from OSPF, as described earlier; it sounds as  
> though
> you need to inject this prefix announcement manually, and/or specify  
> the
> AS of your peer with the shortest path explicitly.



Following is an exemple of what we would want to do :


                                           Big, wild, Internet
                                           /                 \
                                          /                   \
                                       |-----|             |-----|
                        (AS 123) peer2 |     |             |     |  
peer1 (AS 456)
                                       |-----|             |-----|
                                          |                   |
                prefered link to 1.2.3/23 |                   |  
prefered link to 4.5.6/23
                                          |                   |
                                       |-----|             |-----|
            (ASBR, OSPF area 0) st2rt1 |     |   AS 7890   |     |  
st1rt1 (ASBR, OSPF area 0)
1.2.3/23 gw st1cr1 learned from OSPF  |-----|             |-----|  
4.5.6/23 gw st1cr1 learned from OSPF
   => redistribute to BGP                 |                   |       
=> redistribute to BGP
4.5.6/23 gw st1cr1 learned from OSPF     |                   |     
1.2.3/23 gw st1cr1 learned from OSPF
   => redistribute to BGP with AS prepend |                   |       
=> redistribute to BGP with AS prepend
                                          |                   |
                                       |-----|   ~400km    |-----|
         (ABR, OSPF area 0 & 2) st2cr1 |     |-------------|     |  
st1cr1 (ABR, OSPF area 0 & 1)
                                       |-----|    link0    |-----|
                                         /|\                 /|\
                                        / | \               / | \
                       (OSPF area 2) to 1.2.3/23         to 4.5.6/23  
(OSPF area 1)



st1rt1 and st2rt1 are our Xorp routers.


The AS-path seen on BGP routing table should be :

In normal state:
   1.2.3/23 ... 123 7890
   1.2.3/23 ... 456 7890 7890 7890
   4.5.6/23 ... 456 7890
   4.5.6/23 ... 123 7890 7890 7890

If link0 breaks:
   1.2.3/23 ... 123 7890
   4.5.6/23 ... 456 7890

If link to peer1 breaks:
   1.2.3/23 ... 123 7890
   4.5.6/23 ... 123 7890 7890 7890

If link to peer2 breaks:
   1.2.3/23 ... 456 7890 7890 7890
   4.5.6/23 ... 456 7890


Thanks,

-- 
Seb