[c-nsp] BGP best path

[Oliver Boehmer (oboehmer)]

Mikisa Richard <mailto:rmikisa at bushnet.net> wrote on Tuesday, May 02,
2006 4:26 PM:

> Oliver Boehmer (oboehmer) wrote:
> 
>> Could this be a routing recursion? BGP next-hop 64.152.195.33 learnt
>> via BGP? What is the 64.152.195.33 address? Can you use
>> next-hop-self on the neighbor peering with AS23549?
>> 
>
telia(AS1299)----------BUSH-UPLINK(AS29032)-------------UPLINK(29032)---
---------Customer(AS23549)
> 213.248.97.89                        217.212.242.1
>                         217.212.242.2                       
> 64.152.195.33 
> 
> That's the lay out. The session between 217.212.242.2 and the customer
> (64.152.195.33) seems ok ..
> 
> UPLINK#show ip bgp nei 64.152.195.33 ro
> BGP table version is 309, local router ID is 217.212.243.253
> Status codes: s suppressed, d damped, h history, * valid, > best, i -
> internal,
>               r RIB-failure, S Stale
> Origin codes: i - IGP, e - EGP, ? - incomplete
> 
>    Network          Next Hop            Metric LocPrf Weight Path
> *> 63.103.129.0/24  64.152.195.33            0             0 23549 i
> *> 64.152.195.0/24  64.152.195.33            0             0 23549 i
> 
> Total number of prefixes 2
> 
> Then the UPLINK ( 217.212.242.2) does iBGP with 217.212.242.1...
> 
> UPLINK#show ip bgp neighbors 217.212.243.254 ad
> BGP table version is 316, local router ID is 217.212.243.253
>    Network          Next Hop            Metric LocPrf Weight Path
> *> 41.220.0.0/20    0.0.0.0                  0         32768 i
> *> 41.220.1.0/26    217.212.242.23           0         32768 ?
> *> 41.220.13.0/24   217.212.242.14           0         32768 ?
> *> 41.220.14.0/23   217.212.242.13           0         32768 ?
> *> 63.103.129.0/24  64.152.195.33            0             0 23549 i
> *> 64.152.195.0/24  64.152.195.33            0             0 23549 i
> *> 64.152.195.33/32 0.0.0.0                  0         32768 ?
> 
> and ....
> 
> BUSH-UPLINK-01#show ip bgp ne 217.212.242.2 ro
> BGP table version is 558467, local router ID is 217.212.243.254
> Status codes: s suppressed, d damped, h history, * valid, > best, i -
> internal
> Origin codes: i - IGP, e - EGP, ? - incomplete
> 
>    Network          Next Hop            Metric LocPrf Weight Path
> * i41.220.0.0/20    217.212.242.2            0    100      0 i
> *>i41.220.13.0/24   217.212.242.14           0    100      0 ?
> *>i41.220.14.0/23   217.212.242.13           0    100      0 ?
> * i63.103.129.0/24  64.152.195.33            0    100      0 23549 i
> * i64.152.195.0/24  64.152.195.33            0    100      0 23549 i

Here's your problem: The "UPLINK" originates and advertises
64.152.195.33/32 to "BUSH-UPLINK-01". But it is not received there. Does
BUSH-UPLINK knows this address via IGP?
You see that BUSH-UPLINK knows 64.152.195.0 through 64.152.195.33/32,
and unless the /32 is learned as a more specific network (preferably via
IGP), BUSH-UPLINK can't resove this next-hop.

If you add "neighbor 217.212.243.254 next-hop-self" on "UPLINK", this
router will advertise the networks using its own address
(217.212.243.2), and the recursion stops. Or you advertise the /32 (or
/30) via IGP, but next-hops-self is preferred..

	oli