Overlapping Addresses in ONE VRF

[kfarrington]

Hello, I am an MPLS newbie ad I have a design

question maybe someone could help with.

If a client outsources its WAN to an MPLS provider and

all of the clients global routing goes into one VRF.

The client has an external connections to a third

party in two locations, say Sydney and Pakistan and

on this third party network, there is a host that the

client needs to access ie, 211.194.21.2.

The MPLS network is a BGP core network.

Now, a third/forth site, say India, and NZ are

connected to the BGP core, needs to access this host

on the third party network via India, or Sydney, the

BGP core will have only ONE BGP active route that will

point to either India or Sydney.

Ideally, we want the users in NZ to access this host

via Sydney and users in India to use the link via

Pakistan.

Question is, how can BGP decide this as only one

route in this single VRF will be the active route?

Many thx for your time and I hope this is not to much

of a dumb question.

Kindest regards,

Ken

[romccallum]

You should try using maximum-paths 2 or 3 or whatever under the main config and maximum-paths import under the ipv4 config.

[vdadlaney]

Hi Robert,

Would this solve the problem of having overlapping addresses in the same VRF. Using maximum paths you can achive load balancing but can u actually advertise the same subnet from 2 different CE routers to 2 totally different PE Routers.

I didn't think this was possible? If I understood the question correctly than it would seem that Ken is referring to using the most preferred route to get to the destination and not load balance. In this case assuming that the RD's are different that would make the route unique in MBGP. But again assuming that the Service Provider is using RR would it be possible to have the Provider do some filtering to advertise only for that one subnet the most preferred route based on the customer requirements towards the site that needs to use it and deny the other. Please advise if I am totally off base over here. I was just under the impression that having the same address space in the same VRF is not possible. Thx

[kfarrington]

YOU ARE SPOT ON :))))))))

This is the problem I have, one network subnet coming from two locations becuae we have two paths to this customer at different locations. And I want to tell other sites in the vpn, to use their local regional path. ie pakistan users use the network advertised from india to the customer, and NZ user the route advertised from sydney to the customer.

Just need to get my head round what I ask the carrier to do for me?

Many thx,

Ken

[romccallum]

Folks, Although this is a duplicate network there is no real problem with advertising both. I have a customer who uses the same RP address and advertise it into the mpls network via 4 different locations. This RP address is exactly the same address and all that happens is the closest address (cost in this case) is the one chosen for multicast joins. This should be no difference just because you are using BGP. The customer I have is using OSPF. In BGPs case you could use local pref.

[kfarrington]

Hi There :)

So if i have 4 networks being advertised into the MPLS BGP cloud, I could ask the provider to either do one of the following :-

User different RDs so in MiBGP there would be 4 routes (ip prefix:RD)

Ask the provider to increase the max-paths under BGP. (This would then use the metrically closest)

Is this correct, and is there a preferred method?

Many thx all to the valued input :)

Ken

[vdadlaney]

Hi There,

I still haven't quite understood Rob's solution since i don't see how if you have the same prefix it can be guaranteed that one site would use the best path without affecting the others.

I would think that you would need a combination of both the RD and the maximum path if that solution does actually work.

Example:

Site A advertises to its PE A:ipprefix

Site B Advertises to its PE B:ipprefix

Site C needs to prefer the route from Site A

The PE routers are peering with the Route-Reflector. The Route-reflector gets both the routes from Site A and Site B and advertises it to Site C. I would think that depending on the provider they could do some filtering using communities to tag that specific route so that the RR advertises to Site C only the route from Site A and not the route from Site B. This is just a logical guess? I am not sure if this is possible. Rob can you please advise if this is possible? Thx.

I am still not sure how using local preference in your AS will it influence the Service Provider AS. When you mentioned that your customer is advertising addresses from different locations using ospf isn't it true that ospf metric is preserved in the BGP med but how does that affect the other sites. The Metric doesn't change in the Service Provider Backbone (I think??). Won't all the other sites also prefer that same route even when you don't want one site to prefer that route. I guess I am missing something extremely simple over here. Thx for your help.