From: Carlos G Mendioroz (tron@xxxxxxxxxxx)
Date: Tue Jun 04 2002 - 18:33:47 GMT-3
Jonathan,
Q1: yes...
BGP only informs (advertises) the best path to each CIDR, so when
the best path to 1.0.0.0 is via RouterB, it does not tell RouterB
that (kind of split horizon).
This is explicit by the show ip bgp 1.0.0.0:
BGP routing table entry for 1.0.0.0/8, version 2
Paths: (2 available, best #1, table Default-IP-Routing-Table)
Advertised to non peer-group peers:
192.1.1.2
100
193.1.1.2 (metric 12476) from 195.1.1.2 (195.1.1.1)
Origin IGP, metric 50, localpref 100, valid, internal, best
100
192.1.1.2 from 192.1.1.2 (2.0.0.1)
Origin IGP, metric 100, localpref 100, valid, external
Q2: don't know but counters say "Policy routing matches: 0 packets, 0
bytes"
so I guess counters only count packets being switched, not routing info.
Jonathan Hays wrote:
>
> Hi Group,
>
> I have a question about MED operation. [No, I have not read all the RFCs
> from start to finish. Sorry. ;-) ]
>
> I am studying Lab #50 in Hutnik and Satterlee (2nd Edition),
> specifically the MED configuration. In brief, for those without the
> book, RouterA is in AS100 advertising networks 1.0.0.0 and 2.0.0.0 to
> RouterB (AS200) and RouterC (AS200). Router A has direct serial links to
> RouterB (192.1.1.1) and RouterC (193.1.1.1). There is also a RouterD
> (AS200) with direct links to RouterB and RouterC (the topology is
> diamond-shaped).
>
> RouterA is configured with route maps so that updates for 1.0.0.0 sent
> to RouterB will set MED=100 and updates for 1.0.0.0 sent to RouterC will
> set MED=50 (which is preferred since it is lower).
>
> RouterB's BGP table:
> Network Next Hop Metric LocPrf Weight Path
> *>i1.0.0.0 193.1.1.1 (RouterC) 50 100 0 100 i
> * 192.1.1.1 (RouterB) 100 0 100 i
> * i2.0.0.0 193.1.1.1 0 100 0 100 i
> *> 192.1.1.1 0 0 100 i
>
> RouterC's BGP table:
> Network Next Hop Metric LocPrf Weight Path
> *> 1.0.0.0 193.1.1.1 50 0 100 i
> *> 2.0.0.0 193.1.1.1 0 0 100 i
> * i 192.1.1.1 0 100 0 100 i
>
> Note the absence of the route to 1.0.0.0 via 192.1.1.1 in RouterC's
> table. I started 'debug ip bgp updates" and then a "clear ip bgp *".
> After some updates the following debug message popped up, which is
> consistent with the BGP table:
>
> 02:59:20: BGP(0): 194.1.1.2 rcv UPDATE about 1.0.0.0/8 -- withdrawn
>
> Q1. Would I be correct in assuming that this is normal behavior, that
> after RouterC informs RouterB that it has a path to 1.0.0.0 with a
> metric of 50, RouterB then withdraws its own advertisement to 1.0.0.0?
>
> Q2. Although RouterB and RouterC are obviously receiving updates with
> the MED set, I note that the output of "show route-map" on the
> originating router (RouterA) does not indicate any matches. Why?
>
> Thanks in advance for the advice,
>
> Jonathan
This archive was generated by hypermail 2.1.4 : Tue Jul 02 2002 - 08:12:24 GMT-3