Migration from OSPF to BGP

Unanswered Question
Jan 9th, 2009
User Badges:

Hi all.

We have a headquarter, a disaster recovery site and 21 branches. We are planning on migrating our FR WAN to MPLS. Some branches have DMVPN for link backup. We run OSPF in both LAN and WAN, including the DMVPN. There are about 35 routers.

However, MPLS service providers prefer BGP as the routing protocol for the WAN and the one with best prices even doesn't support OSPF.

So I want to hear from you what you think about migrating from OSPF to iBGP as our WAN routing protocol.


I forgot to mention ... the HQ and DR will have different service providers. Some branches will have two service providers.


Paulo Roque


  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 0 (0 ratings)
Loading.
Giuseppe Larosa Fri, 01/09/2009 - 11:52
User Badges:
  • Super Silver, 17500 points or more
  • Hall of Fame,

    Founding Member

Hello Paulo,

the scenario is rather complex for the presence of the backup DMVPN.


If that wasn't present the approach would be:


CE routers facing MPLS provider PE routers will be running both OSPF and BGP.

You run eBGP sessions with PE routers.

From the eBGP session routes about all other sites are learned and can be redistributed into OSPF.

Inside the BGP process network commands representing the local site and lea via OSPF can be used to advertise in BGP local networks.


Doing so all other sites ip subnets will be some form of O E1 or O E2.

So on the central site if the DMVPN headends are different from the MPLS CE devices you need to use a trick:

you need to use two different OSPF processes on the DMVPN headends to make backup routes of the same type O E1 or O E2 with greater metric otherwise DMVPN OSPF routes if not external would be preferred


On remote sites if multiple routers are present you need to use the same trick if the CE router and the DMVPN are on the same router the less AD of EBGP in comparison with OSPF should work.


Edit:

on remote sites connected to both providers to reach HQ and DR you need to provide preference to HQ primary link

you can use


neigh SP1_to_HQ.ipaddr weight 500

to prefer to route to HQ


I'm not sure if something has to be done for the return path, being two different service providers should be enough to prefer SP1 everywhere including HQ and DR sites


Hope to help

Giuseppe

pauloroque Sat, 01/17/2009 - 14:06
User Badges:

Thank you Giuseppe.

I took some days off just after posting this question.


You've got my idea: "the scenario is rather complex". And I am just wondering if I can make it simple by using only BGP instead of OSPF+BGP.


Paulo Roque

Giuseppe Larosa Sat, 01/17/2009 - 23:48
User Badges:
  • Super Silver, 17500 points or more
  • Hall of Fame,

    Founding Member

Hello Paulo,

moving to an iBGP only implementation can provide advantages in your scenario in terms of complexity.


Comparing BGP routes you can use all the tools that the protocol provide local preference, MED and so on to implement the routing policy.


the aspects to be investigated are related to routing convergence:

depending on application and services (voip for example) you may need to tune BGP convergence changing BGP session timers.


Big sites like HQ can still implement an IGP in the internals but with no redistribution of BGP into the IGP


Hope to help

Giuseppe


Actions

This Discussion