bgp load sharing with ebgp and ibgp

[abarakat77]

Hi,

I see the topic was discussed many times but I can't find a solution to my problem

so I have two routers, RT1 and RT2, the routers can reach each by two ways:

* a ISP L3 service, the two routers made a ebgp session with the ISP's CE router

* a ISP L2 service, the two routers made a ibgp session between them directly

I can't figure out to made load sharing on the two links, I see ebgp is preferred, I'm looking for the tricks

to tell bgp to use the two links

a example of prefix selection on RT1:

RT1#sh ip bgp 10.132.128.100

BGP routing table entry for 10.132.128.0/22, version 24

Paths: (2 available, best #2, table default)

Multipath: eBGP iBGP

  Not advertised to any peer

  Refresh Epoch 1

  Local, (received & used)

    10.132.192.1 from 10.132.192.1 (10.132.97.1)

      Origin incomplete, metric 0, localpref 100, valid, internal

  Refresh Epoch 1

  65000 65500, (received & used)

    172.22.190.1 from 172.22.190.1 (172.22.190.1)

      Origin incomplete, localpref 100, valid, external, best

on RT2, the network 10.132.128.0/22 is coming from OSPF redistribution

I already tried to add "bgp bestpath as-path ignore", enable multipath on ebgp/ibgp

Thanks for any tips

[Joseph W. Doherty]

Each of routers, RT1 and RT2, each have a single eBGP link?  They share their BGP via iBGP?

eBGP, from RT1 and RT2 are to same or different AS?

How does your outbound traffic get (route) to RT1 and/or RT2?

What platforms (and IOS version) are RT1 and RT2?

[abarakat77]

each router have only one ebgp link

they don't share, at least it's not the goal, the two routers are in distinct locations.

ebgp from the two routers made with  the same ISP's AS

RT1 is a cisco router 2901

RT2 is a nexus 5500 with L3 daughter card

thanks

[Joseph W. Doherty]

they don't share, at least it's not the goal, the two routers are in distinct locations.

Was this intended as an answer to my question "How does your outbound traffic get (route) to RT1 and/or RT2?"

If the two routers are at distinct locations, and the goal is not to share, what kind of load sharing are you trying to accomplish?

[abarakat77]

a diagram would be better

[Joseph W. Doherty]

Yes, the diagram helps.

Unclear what/why you're trying to load balance from RT1 AND RT2?

If you do it statically, like you're trying to solve, consider traffic from Site A hits RT1 and it routes half the outbound to the ISP and half to RT2.  Now if RT2 tries the same, it might take half the traffic received from RT1 and send it to your ISP and routes the other half back to RT1!  Normally when you load balance , you don't want to go "sideway", to avoid just this issue.

This is a principle reason why, when AS hops are equal, eBGP is preferred over iBGP.

[abarakat77]

Thanks  for the explanation, not sure to understand because AS hops are not equal (ebgp has one more),

so basically I want to load share (50% on each,  like round robin) on the two links on each side, any traffic

[milan.kulik]

Hi,

AFAIK, there is one possibility to load share between eBGP and iBGP, but it's avaliable only under the IPv4 VRF address family configuration mode.

See http://www.cisco.com/en/US/docs/ios/12_2sx/feature/guide/fsxeibmp.html for details.

Here is another nice page discussing different BGP load sharing possibilities:

http://stack.nil.com/ipcorner/LoadBalancingBGP/

Best regards,

Milan

[abarakat77]

Thanks milan,

I have already enabled multipath eiBGP on RT2 (Cisco N5K) inside a vrf but ibgp is prefered :

RT2%MPLS# sh ip bgp 172.22.190.130
BGP routing table information for VRF MPLS, address family IPv4 Unicast
BGP routing table entry for 172.22.190.128/25, version 161
Paths: (4 available, best #1)
Flags: (0x804001a) on xmit-list, is in urib, is best urib route, exported
Multipath: eiBGP

  Path type: internal, path is valid, is best path
  AS-Path: NONE, path sourced internal to AS
    10.132.192.3 (metric 0) from 10.132.192.3 (10.132.97.2)
      Origin incomplete, MED 1, localpref 100, weight 0
      Extcommunity:
          RT:3:3

  Path type: internal, path is valid, received only
  AS-Path: NONE, path sourced internal to AS
    10.132.192.3 (metric 0) from 10.132.192.3 (10.132.97.2)
      Origin incomplete, MED 1, localpref 100, weight 0

  Path type: external, path is valid, not best reason: AS Path
  AS-Path: 65000 65500 , path sourced external to AS
    10.132.96.1 (metric 0) from 10.132.96.1 (151.164.121.30)
      Origin incomplete, MED not set, localpref 100, weight 0
      Extcommunity:
          RT:3:3

  Path type: external, path is valid, received only
  AS-Path: 65000 65500 , path sourced external to AS
    10.132.96.1 (metric 0) from 10.132.96.1 (151.164.121.30)
      Origin incomplete, MED not set, localpref 100, weight 0

  Advertised to peers:
    10.132.96.1        10.132.192.10 

Your link is nice but no setup with mixed eBGP/iBGP

Thanks

[milan.kulik]

Hi,

I'm afraid you need to get the iBGP and eBGP prefix with the same attributes (AS_PATH length, weight, MED, local preference) to get the load sharing active for the particular prefix?

You could try some route-map applied on the iBGP session I guess?

And/or "bgp bestpath as-path ignore"?

Best regards,

Milan

[Rizwan Khan]

Hi barakat can you send the bgp configurations so that I can help you out. I configured bgp load sharing between 2 ISPs terminated on 2 different routers successfully. Besides what networks you want to advertise to the outside world ?


Sent from Cisco Technical Support Android App

[abarakat77]

Hi rizwan,

You use only ebgp I think with your 2 ISP  right ?

the problem is my case is the mix eBGP/iBGP

[neil grant]

Simply answer is you cant do this simply relying on BGP, you cannot load balance across a Ibgp and Ebgp path (at least without a lot of route maps (per prefix etc) and administritive over head.

You will need to configure / implement one of the following:

1. Prf (performance routing)  previously (OER) optimized edge routing.(preferred)
2. Policy based routing
3. Implement a L2 connection between the routers and run GLBP / HSRP (groups)

Regards Neil

[neil grant]

References:

http://docwiki.cisco.com/wiki/Performance_Routing_FAQs

http://www.cisco.com/en/US/prod/collateral/vpndevc/ps6525/ps9370/ps7180/white_paper_c11-643822_ps6599_Products_White_Paper.html

[abarakat77]

thanks for your reply neil

I forgot the idea for the moment unitl someone can give me a "simple" solution with bgp