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Given the following network, with all sites' network routing tables built via OSPF:

  |           \
  |      Intermediate_site_C_router -- Other_sites
  |                       |
  |      Intermediate_site_D_router
  |                       |
Site_B_router_1     Site_B_router_2
             \       /

Currently, there is no link between Site A and Site C. I'm looking at adding that in place and seting up unequal cost load balancing between Site A and Site B to provide some redundancy as well as get more effective bandwidth between Site A and hosts on the Internet and at Site B. From a significant amount of Googling, it appears I want to set up MPLS TE tunnels between the Site A router and one of the Site B routers to direct all traffic between the two sites over one of the two routes.

I'm stuck at trying to wrap my head around how the MPLS tunnels will fit into the scheme of the routed network. From my reading, MPLS TE layers on top of the OSPF derived routes and simply defines paths to take between two routers.

Do I need to advertise their existence at all via OSPF? I wouldn't think so, as I suspect that'd just end up routing all traffic from anywhere on the network destined to Site A through Site B then back across one of the two paths to Site A.

Alternately, would I just set the tunnels up on a Site B router but not advertise their existence and just allow them to sort any trafifc that arrives into one of the two tunnels paths? If this works in this manner, I'd expect hosts in site D, site C, and the other sites connected via site C to just route the traffic over the shortest OSPF route, ignoring the tunnels.

Complicating this is that many of the hosts in Site B build their routing tables via OSPF and thus there is no single gateway router that the hosts use to determine the next hop. Which of the three routers should I be setting the Site B -> Site A direction of the MPLS TE tunnel up on in order to get the appropriate load balancing?

If I set it up on router 3, I suspect only Internet-sourced traffic would be load balanced, as hosts in site B are going to be sending site A-destined packets through to either router 1 or 2 directly. Alternately, I could set it up on router 1, as that's almost certain to be the OSPF shortest route. However, then traffic is going to be routed from router 1 through to router 2 somewhat needlessly as traffic takes the alternate tunnel. I suspect this is overthinking things somewhat and is propably premature optimization.

Given this situation, what's the best practice way for doing what I'm trying to do?

share|improve this question
You have a link on this map between A and C, but then you say there is not one there - is that planned? Also, what's the meaning of 'intermediate' on this map - it doesn't seem to add anything to the question. Which links are you actually trying to load-share between? Are you trying to do both ways or one way (MPLS-TE tunnels only carry traffic 1 way) – Aaron Dec 14 '11 at 18:36
Yeah, it's on the diagram to show a planned link. The main purpose of this is to route types of traffic between A and site B and A and the internet over one of the two available connections. Yes, it is intended to be bidirectional and I do realize I need to set up each half on the appropriate locations. – Oesor Dec 15 '11 at 18:09
up vote 1 down vote accepted

Yes, you can create MPLS tunnels between the routers to make OSPF load balance between them (the tunnels) as OSPF does not support UCLB itself.

You do need to add a few mpls commands to your ospf config. See here

You ought to be able to create two tunnels between the site A router and the "main" router at site B (probably router 1) One tunnel would be between A-1 and B-1 directly and the other - through router B-2. Routers A-1 and B-1 will perform the load balancing of traffic

In this scenario, the hosts would need to be configured to use router B-1 as their default gateway in site B.

*Edited for correct router names.

share|improve this answer
In fact, that article is what I read prior to doing this. There is no real 'main' router at site B and if at all possible I'd like to have redundancy as well so that in case the router that is load balancing the traffic goes down that Site A is still routable from the network. Would this be best served by just announcing the tunnels with a small cost, letting it serve as the shortest path from site B, then if the router goes down letting the other OSPF routes take over? i.e., should I be viewing the tunnels as a shortcut of sorts advertising least cost between sites? – Oesor Dec 15 '11 at 18:16
Also, if I simply advertise Site A from Site B in the three locations, the two routers directly connected to Site B and the router in Site A that handles the two Site A -> Site B tunnels, I should be able to get around configuring a default gateway in site A, if I'm visualizing this correctly? – Oesor Dec 15 '11 at 18:18
The router with the MPLS tunnels (A-1) will perform the UCLB, therefore, hosts need to be configured to send traffic to that router. If you want redundancy as well in case that router dies, you can use GLBP to have the routers share an IP with router A-1 being the main holder of that IP. – Paul Ackerman Dec 15 '11 at 20:12
Not sure about your second comment. If Internet is being fed from Site B, you must configure a default gateway in site A but you can create a second set of MPLS tunnels to send traffic back to site B using both it's routers. – Paul Ackerman Dec 15 '11 at 20:13
So basically, to get the UCLB, all routes need to be announced to flow through a single device, and thus needs to handle all external announcements into the OSPF LSA fabric. These tunnels are simply one-way interfaces from a routing standpoint, and without explicit routing thable entries the routers aren't going to do anything 'magical'. Think I get it, thanks. – Oesor Dec 19 '11 at 15:47

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