BGP maximum paths

[Jonathan0410]

Hello,

I am hoping one of you can help me with a problem I am having relating to BGP and load balancing.  I have a network configured as

per the diagram with maximum-paths ibgp 2 configured on each of my core switches all routes are learned via R3 and R4 and have

no manipulation from our routers.  All routers and switches are connected and established via IBGP.

The problem is that I should have two routes to each destination in the core switches yet I only have a single route, and from

what I can see this is because our telco is injecting a metric into our AS, however, I only see this metric on the core switches

not on the routers and I would have thought I would see it on the router?

Show IP BGP and show IP routes

R3>sh ip bgp 29.12.0.0
BGP routing table entry for 29.12.0.0/21, version 859691
Paths: (4 available, best #3, table Default-IP-Routing-Table)
  Advertised to peer-groups:
     peer1
  2856 2856 2856 34140, (aggregated by 34140 29.12.7.68)
    10.9.35.29 from 10.9.35.29 (62.6.192.137)
      Origin IGP, localpref 100, valid, external, atomic-aggregate
      Community: 34140:222
  2856 34140, (aggregated by 34140 29.12.7.68), (received-only)
    10.9.35.29 from 10.9.35.29 (62.6.192.137)
      Origin IGP, localpref 100, valid, external, atomic-aggregate
      Community: 34140:222
  2856 34140, (aggregated by 34140 29.12.7.67), (received & used)
    10.9.35.17 from 10.9.35.17 (62.6.192.137)
      Origin IGP, localpref 100, valid, external, atomic-aggregate, best
      Community: 34140:111
  2856 34140, (aggregated by 34140 29.12.7.67), (received & used)
    161.163.164.249 (metric 156416) from 161.163.164.249 (161.163.164.249)
      Origin IGP, metric 0, localpref 100, valid, internal, atomic-aggregate
      Community: 34140:111

Core-1>sh ip bgp 29.12.0.0
BGP routing table entry for 29.12.0.0/21, version 396742
Paths: (2 available, best #1, table Default-IP-Routing-Table)
Multipath: iBGP
  Advertised to update-groups:
     1
  2856 34140, (aggregated by 34140 29.12.7.67), (received & used)
    161.163.164.247 (metric 153856) from 161.163.164.247 (161.163.164.247)
      Origin IGP, metric 0, localpref 100, valid, internal, atomic-aggregate, best
      Community: 2237399151
  2856 34140, (aggregated by 34140 29.12.7.67), (received & used)
    161.163.164.249 (metric 154112) from 161.163.164.249 (161.163.164.249)
      Origin IGP, metric 0, localpref 100, valid, internal, atomic-aggregate
      Community: 2237399151

Core-1>sh ip ro 29.12.0.0
Routing entry for 29.12.0.0/21
  Known via "bgp 65356", distance 200, metric 0
  Tag 2856, type internal
  Last update from 161.163.164.247 7w0d ago
  Routing Descriptor Blocks:
  * 161.163.164.247, from 161.163.164.247, 7w0d ago
      Route metric is 0, traffic share count is 1
      AS Hops 2
      Route tag 2856

======================
Core 2

R4#sh ip bgp 29.12.0.0
BGP routing table entry for 29.12.0.0/21, version 422102
Paths: (4 available, best #4, table Default-IP-Routing-Table)
  Advertised to peer-groups:
     peer1
  2856 2856 2856 34140, (aggregated by 34140 29.12.7.68)
    10.9.35.25 from 10.9.35.25 (62.6.192.138)
      Origin IGP, localpref 100, valid, external, atomic-aggregate
      Community: 34140:222
  2856 34140, (aggregated by 34140 29.12.7.68), (received-only)
    10.9.35.25 from 10.9.35.25 (62.6.192.138)
      Origin IGP, localpref 100, valid, external, atomic-aggregate
      Community: 34140:222
  2856 34140, (aggregated by 34140 29.12.7.67), (received & used)
    161.163.164.247 (metric 156416) from 161.163.164.247 (161.163.164.247)
      Origin IGP, metric 0, localpref 100, valid, internal, atomic-aggregate
      Community: 34140:111
  2856 34140, (aggregated by 34140 29.12.7.67), (received & used)
    10.9.35.21 from 10.9.35.21 (62.6.192.138)
      Origin IGP, localpref 100, valid, external, atomic-aggregate, best
      Community: 34140:111

Core-2>sh ip bgp 29.12.0.0
BGP routing table entry for 29.12.0.0/21, version 180821
Paths: (2 available, best #2, table Default-IP-Routing-Table)
Multipath: iBGP
  Advertised to update-groups:
     2
  2856 34140, (aggregated by 34140 29.12.7.67), (received & used)
    161.163.164.247 (metric 154112) from 161.163.164.247 (161.163.164.247)
      Origin IGP, metric 0, localpref 100, valid, internal, atomic-aggregate
      Community: 2237399151
  2856 34140, (aggregated by 34140 29.12.7.67), (received & used)
    161.163.164.249 (metric 153856) from 161.163.164.249 (161.163.164.249)
      Origin IGP, metric 0, localpref 100, valid, internal, atomic-aggregate, best
      Community: 2237399151

Core-2>sh ip ro 29.12.0.0
Routing entry for 29.12.0.0/21
  Known via "bgp 65356", distance 200, metric 0
  Tag 2856, type internal
  Last update from 161.163.164.249 7w0d ago
  Routing Descriptor Blocks:
  * 161.163.164.249, from 161.163.164.249, 7w0d ago
      Route metric is 0, traffic share count is 1
      AS Hops 2
      Route tag 2856

Many thanks in advance,

Jonathan

[rais]

Per this doc:

http://www.cisco.com/en/US/docs/ios/12_2t/12_2t2/feature/guide/ftbgpls.html

All attributes must be the same. The attributes include weight, local preference, autonomous system path (entire attribute and not just length), origin code, Multi Exit Discriminator (MED), and Interior Gateway Protocol (IGP) distance.

In case of your Core1, the IGP metrics are not shown above.

HTH.

[Jonathan0410]

Hi Rais,

To the best of my knowledge everything is the same apart from the MED, would I not see the MED on R3 & R4 though?

Thanks,

Jonathan

Sent from Cisco Technical Support iPhone App

[milan.kulik · Accepted solution]

Hi,

2856 34140, (aggregated by 34140 29.12.7.67), (received & used)

161.163.164.249 (metric 153856) from 161.163.164.249 (161.163.164.249)

Origin IGP, metric 0, localpref 100, valid, internal, atomic-aggregate, best

I believe MED=0 here, metric 153856 is the IGP metric to the BGP next-hop router?

BR,

Milan

[Jonathan0410]

Thanks Milan, yes that was the issue. Question I have is I thought that field was reserved for the MED? I take it that it is for MED and IGP metric?

Thanks,

Jonathan

Sent from Cisco Technical Support iPhone App

[milan.kulik]

Hi,

IMHO, Cisco is creating a lot of confusion by using "metric" in this kind of outputs where you have to understand from the context what it does mean exactly - MED usually, but IGP metric sometimes.

It would be much easier to understand if they would use "MED" instead of "metric" in the "show ip bgp ..." outputs.

BR,

Milan