BGP preferred route

Unanswered Question
Nov 20th, 2007

I have two connections coming in, one is a 45M link and the other is a 10M link, bgp for the most part is running fine, but the 10M link is getting maxxed out and am staring to experience problems. I would prefer traffic would "default" to the 45 meg connection on the first hssi.

Any ideas on how to accomplish this, I just cant quite get over this little hump.

I have this problem too.
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Kevin Dorrell Tue, 11/20/2007 - 07:37

To give a proper answer, I need to ask one or two things about your topology.

Do these connections terminate on the same router? That would determine whether you do it with weight or with local preference.

Do these come from the same AS, or from different AS?

Does the BGP terminate at the border router(s), or does it penetrate further into your network?

Do they give the same set of prefixes? Or do they each give you just a default summary?

There are lots of things you can do to control the outbound traffic. The inbound traffic is a bit more tricky. Which one is getting maxed out?

For inbound traffic, the same sort of questions apply. However, there might be rather less you can do about it.

Could we have some more details please?

Kevin Dorrell


sburckhard Tue, 11/20/2007 - 08:15

one router 7206VXR

Hssi1 is AS 209 (Qwest 45M)

Hssi2 is AS 1239 (Sprint 10M)

FastEthernet0/0 is AS 21531 (connection to my core switch)

BGP does go further into the network, as some of the neighbors are announced on the core.

The outbound is my main priority right now, that is getting maxed more often.

Richard Burts Tue, 11/20/2007 - 08:22


This information is helpful. One question that Kevin asked becomes critical now: do the EBGP peers advertise more than just a default route. If each EBGP peer advertises some prefixes in addition to any default that they may advertise then you can set weight or local-perference for some of those prefixes to prefer the higher capacity link. This would address outbound traffic but not inbound traffic.

You would almost certainly want to configure a route map and apply it inbound to the peer so that you can prefer SOME (not all) routes from that peer.



sburckhard Tue, 11/20/2007 - 08:28

Ok I will admit, BGP is new to me, so here it goes. You ask "do the EBGP peers advertise more than just a default route"

How do I verify that?

Richard Burts Tue, 11/20/2007 - 08:34


The easy way to do that is to do show ip bgp summary and post the output. This will display each peer and how many prefixes they have advertised. For more detail use show ip bgp. This command will show the BGP working table and show what prefixes have been advertised, and by looking at the AS path and the next hop you can figure which peer has advertised which prefix.



sburckhard Tue, 11/20/2007 - 09:47

show ip bgp summary

BGP router identifier, local AS number 21531

BGP table version is 362359, main routing table version 362359

233787 network entries and 465696 paths using 39442395 bytes of memory

82752 BGP path attribute entries using 4967100 bytes of memory

73572 BGP AS-PATH entries using 1926998 bytes of memory

582 BGP community entries using 38124 bytes of memory

15 BGP extended community entries using 2122 bytes of memory

0 BGP route-map cache entries using 0 bytes of memory

0 BGP filter-list cache entries using 0 bytes of memory

Dampening enabled. 244 history paths, 445 dampened paths

9 received paths for inbound soft reconfiguration

BGP activity 484015/309727 prefixes, 966782/501086 paths, scan interval 60 secs

Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd 4 209 1198028 21076 362345 0 0 19:17:19 232129 4 21531 10446 10566 362358 0 0 19:17:26 0 4 21531 10410 10587 362358 0 0 19:17:25 1 4 26671 10444 10564 362346 0 0 19:17:09 1 4 1239 426976 14068 362354 0 0 19:17:15 233302

first neighbor is Qwest DS3

second neighbor is Customer Aggregation Router (another 7206VXR that feeds T1's and DSL subscribers)

third neighbor is layer3 routing on the Core Switch

fourth neighbor is BGP we are announcing for one of our customers

fifth neighbor is Sprint 10MB connection

Thanks again


Richard Burts Tue, 11/20/2007 - 10:23


This shows that Qwest is advertising 232129 prefixes to you and that Spring is advertising 233302 prefixes to you. This certainly gives you something to work with if you want to move some of the traffic to the higher capacity outbound link.




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