[nsp] BGP Best Path Selection Algorithm
Bob Tinkelman
bob at tink.com
Fri May 7 09:52:25 EDT 2004
Hi Scott,
A more specific (e.g., a /17 inside a /14) *always* wins.
The rules (regarding the order in which the various bgp metrics
are considered) apply only in comparing routes for exactly the
same prefix.
--
Bob Tinkelman <bob at tink.com>
ISPnet, Inc. 718-464-4747
> Hi,
> According to :
> http://www.cisco.com/warp/public/459/25.shtml
> The weight is first, local pref is second. But I'm not sure
> how it handles :
> TTSG-6509b>sho ip bgp 62.253.162.218
> BGP routing table entry for 62.253.128.0/17, version 1699823546
> Paths: (3 available, best #3, table Default-IP-Routing-Table)
> Advertised to non peer-group peers:
> 199.224.14.29
> 4436 3549 1239 5089
> 69.31.95.53 from 69.31.95.53 (69.31.95.240)
> Origin IGP, metric 200, localpref 100, valid, external
> Community: 3549:2082 3549:30840 4474:3111 13945:1000 13954:1006
> 4436 3549 1239 5089, (received-only)
> 69.31.95.53 from 69.31.95.53 (69.31.95.240)
> Origin IGP, metric 0, localpref 100, valid, external
> Community: 3549:2082 3549:30840 4474:3111
> 4436 3549 1239 5089, (received & used)
> 216.231.107.225 (metric 2) from 216.231.107.225 (216.231.107.225)
> Origin IGP, metric 100, localpref 110, valid, internal, best
> Community: 3549:2082 3549:30840 4474:3111 13945:1000 13954:1006
> and then
> BGP routing table entry for 62.252.0.0/14, version 1677738739
> Paths: (7 available, best #7, table Default-IP-Routing-Table)
> Advertised to non peer-group peers:
> 199.224.14.29
> 6461 5089
> 198.32.160.22 from 198.32.160.22 (209.249.254.55)
> Origin IGP, metric 200, localpref 170, valid, external
> Community: 6461:1001 6461:2004 6461:2107 6461:2215 6461:2434 13945:2000 13
> 945:2001
> 6461 5089, (received-only)
> 198.32.160.22 from 198.32.160.22 (209.249.254.55)
> Origin IGP, metric 4241, localpref 100, valid, external
> Community: 6461:1001 6461:2004 6461:2107 6461:2215 6461:2434
> 1273 5089
> 198.32.160.112 from 198.32.160.112 (195.2.1.4)
> Origin IGP, metric 200, localpref 170, valid, external
> Community: 1273:12826 13945:2000 13945:2001
> 1273 5089, (received-only)
> 198.32.160.112 from 198.32.160.112 (195.2.1.4)
> Origin IGP, metric 0, localpref 100, valid, external
> Community: 1273:12826
> 4436 1273 5089
> 69.31.95.53 from 69.31.95.53 (69.31.95.240)
> Origin IGP, metric 200, localpref 100, valid, external
> Community: 1273:12826 4474:1111 13945:1000 13954:1006
> 4436 1273 5089, (received-only)
> 69.31.95.53 from 69.31.95.53 (69.31.95.240)
> Origin IGP, metric 0, localpref 100, valid, external
> Community: 1273:12826 4474:1111
> 1273 5089, (received & used)
> 216.231.107.225 (metric 2) from 216.231.107.225 (216.231.107.225)
> Origin IGP, metric 100, localpref 180, valid, internal, best
> Community: 1273:12826 13945:2000 13945:2001
> Is it using the one with the /17 because its more specific,
> correct? But where does it say it takes that into account? It seems like
> it talks about it per entry, but not between entries.
> Thanks, Tuc/TTSG Internet Services, Inc.
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