Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
New Member

Peering BGP Routers over OSPF Network?

Would there be any issues peering 2 BGP routers over an OSPF Network or would this cause any major issues or even work? 

We will have 2 separate ISP's we are connected to with routers that are about 5 hops apart (Internet Routing via OSPF).  For redundancy I need to peer these routers via iBGP so they can exchange BGP routes with each other.  Can I just setup bgp neighbor's between the two routers that are not on the same subnets or will need to run BGP on every router in between them? 

What do you think?

Jim

11 REPLIES
Hall of Fame Super Blue

Peering BGP Routers over OSPF Network?

Jim

IBGP does not really care about the number of hops between peers as long as each peer is reachable that is all that matters.

Obviously though depending on the redundancy of the network in between you may have a few single points of failure in between.

Jon

Hall of Fame Super Blue

Peering BGP Routers over OSPF Network?

Jim

Apologies but my first reply was a bit general.

If the network was MPLS for example then the routers in between would not need to know about the external routes being advertised between IBGP peers only the routers to get from one IBGP peer to another.

But if this is not MPLS then the intermediate routers would need to know about the external networks because if one IBGP peer sent traffic to the other for a remote network then the intermediate routers need to know about these networks to be able to route the traffic.

Jon

Hall of Fame Super Blue

Peering BGP Routers over OSPF Network?

Jim

Not one of my better posts

Following on from my last post you have two options for network reachability -

1) redistribute routes into OSPF. If this is full internet routes not a good idea

2) have each router running IBGP. However you will then hit the rule that states if an IBGP router receives a route from an IBGP peer it cannot then advertise this route to another IBGP peer.

The way round this is to use route reflectors where that rule is relaxed ie. route reflector clients can advertise the routes to the router reflector and it in turn can advertise them to other router reflector clients.

Jon

New Member

Peering BGP Routers over OSPF Network?

This is non MPLS on the internal network and full internet routes.  What I'm thinking is option 3 (not listed in your last email) that I am going to use one of my BGP Internet connections as a primary route for most traffic and inject 2 default routes into OSPFat the BGP routers that will also be running OSPF , one pointing to my primary connection bgp router, and one to the secondary.  I definately would not want to infject full internet routes into OSPF and also route reflectors are not something I really want to do.

What do you think of this idea?

Jim

Hall of Fame Super Blue

Peering BGP Routers over OSPF Network?

Jim

Not sure exactly how this would work.

Do you mean two default routes but prefer the one going to the primary router ? If so how would that direct most traffic ie. it would be an all or nothing approach.

In addition if you are going to use default routes it could be argued you then wouldn't need full routing internet routing tables as they give you nothing.

Jon

New Member

Peering BGP Routers over OSPF Network?

Do you mean two default routes but prefer the one going to the primary router?

That is correct

In addition if you are going to use default routes it could be argued you then wouldn't need full routing internet routing tables as they give you nothing.

You are technically correct here, I could probably have our providers just send a default route, however what this does give us if we keep full internet routes is the capability if one of our providers loses connectivity to a major (or minor) network our BGP router would know about it and route the request to the other router that does have the route available. 

Your Thoughts?  How would you do this.  I really don't want to enable BGP with route reflectors across the entire network between these 2 routers.  These 2 connections are for geographically redundancy more than load sharing

Jim

Hall of Fame Super Blue

Peering BGP Routers over OSPF Network?

Jim

however what this does give us if we keep full internet routes is the capability if one of our providers loses connectivity to a major (or minor) network our BGP router would know about it and route the request to the other router that does have the route available.

If i understand correctly it wouldn't though because you are not running IBGP between those routers so each BGP router is not aware of the other routes.

If they really are for redundancy rather than load sharing then i am struggling to see the advantage of receiving full routes. That said you make a valid point about your pimary provider losing connectivity to a portion of the internet but then as already said without being aware of the alternate path via the other router it would still simply use it's default route to the primary provider because the provider is still up.

It is a choice basically between simplicity and potentially not being able to route to certain networks in some failure scenarios and more complexity with more redudancy.

A lot does depend on how reliable and well connected your ISPs are.

Perhaps others can add their thoughts to this.

Jon

New Member

Peering BGP Routers over OSPF Network?

I actually am talking about peering the 2 BGP routers (with iBGP between them) so they will know if the other router has or loses a network.  The OSPF network won't be aware of the failure and thus a few extra hops to certain networks would be expected as they would go to the primary router first and then turn around and go back to the other secondary router.

Does that make sense?  What would you (or anyone else) do if this was your network? 

Jim

Hall of Fame Super Blue

Peering BGP Routers over OSPF Network?

Jim

You can peer the routers together but we are back to the initial issue ie.

1) the primary router loses a route to a specific network via it's provider

2) it sees it has the same route via IBGP to the other router

3) if traffic then arrives at the primary router for this network it sends it to the other IBGP router via the intermediate OPSF routers.

4) the first hop router from the primary router (OSPF only) looks at the destination address and drops the traffic because it has no route for that network.

The reason it has no route is because you haven't redistributed BGP into OSPF (which you shouldn't do anyway) and the router that drops the traffic is not running IBGP so it is completely unaware of the remote network.

In terms of what to do it is a diffcult question to answer because it depends entirely on the company's requirements etc.  That's why i said it is a choice between simplicity and complexity and the tradeoffs you have to make.

If as you say you only want one link as a backup and not load sharing then it becomes more of a simple choice ie -

1) receive a default from both providers and influence the metric so one is preferred. You gain simplicity but you lose granularity ie. if your primary provider has limited connectivity your default route would still send traffic that way. Only if the provider failed completely would it then failover to the backup link.

In addition you may not always be taking the optimal path to a destination network by going via only one provider all the time.

2) receive full routes from both providers and setup IBGP because without it you are not really achieving anything as each router is unaware of the alternate paths via the other router. You get far more granularity but with quite a bit more complexity.

If you wanted load sharing there are other options as well eg. partial and a default from one provider and only a default from another.

Like i say some of it comes down to how reliable your providers are as well but you may find that you can accept the loss of precise routing information. And it doesn't sound as though you want to do any specific outbound traffic manipulation ie. send some one way and some the other so again having full routing tables is not as important.

If it helps generally speaking we see more posts on here with people who are receiving full routing tables and don't really need them than we do the reverse.

By all means feel free to ask further.

Jon

Peering BGP Routers over OSPF Network?

Hi Jon,

what about iBGP peering via a GRE tunnel between the primary and secondary BGP routers?

Then it might be enough to advertise a default route with a better mertic to OSPF from the primary router and with  a worse metric from the secondary.

And as Jim  said "..a few extra hops to certain networks would be expected", wouldn't that work as requested?

There might be a question of MTU decrease by the tunnel headers, of course. But with some tunning it should not be a major problem.

And this scenario does not fix possible asymmetric routing for the returning traffic, too. But this was not requested in the original question, was it?

Best regards,

Milan

Hall of Fame Super Blue

Peering BGP Routers over OSPF Network?

Hi Milan

I did actually think of this but i just wasn't sure it would work and have never come across it before so i didn't feel comfortable suggesting it as a possible solution.

You are right though, there is no reason, as far as i can see, as to why it wouldn't work and it would provide the failover needed for specific networks.

For some reason i always think of GRE tunnels as a sort of last resort but i think that's often a mistake because in some scenarios they are just what is needed.

Jon

873
Views
5
Helpful
11
Replies
CreatePlease to create content