How to aggregate a pair of WAN links provided by ISP?

Unanswered Question

So, the ISP has agreed to replace our current 2MB x 2 Serial WAN links of xDSL technology to 4MB x2 links, probably MPLS based Ethernet offering from their side.

The links are basically used for Internet based http/SMTP traffic.

In the old scenario, we utilised our 2MB x 2 links by bringing them both into serial interfaces of our 1800 router while the iSP did simple packet switching from their side while we ran IP CEF on our router. In case one line failed, we had a fallback of 2 MB. The FE interface was connected to PIX side.

Now, in the new scenario, the ISP techies are saying that the two new 4MB x 2 links will terminate on a new 2811 router having 4 Ethernet interfaces. They will run SVI on two of the FE interfaces as our 2811 HWIC doesn't support 4 individual real WAN IPs.

Also, in order to give us a combined bandwidth of 4MB x 2 links, they will run BGP on their side of the routers and also on our 2811.

My concerns:

1. Is this the correct way to deploy 4Mb x 2 WAN links? Do I really get a combined bandwidth of 8MB, both inwards and outwards.

2. What would be the effect of running BGP on our side of the router? With 64 MB of onbaord RAM, is this doable?

3. Is it better to keep 4MB live and keep the other 4 MB as backup and run some sort of OSPF for failover?

Any other ideas?

PS: For some reason, the ISP is unwilling to give an 8MB, single line to make things easier for us.

I have this problem too.
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Paolo Bevilacqua Sat, 04/19/2008 - 07:11

A single circuit would make things easier, but as it seems the isp is willing configure you router, let they do and check it does all what is supposed to do. now for the questions:

1 if you want a certain redundancy, yes.

2 you would be benefit of 256MB, you will no take full tables however

3 not really

cisconoobie Sun, 04/20/2008 - 19:07

Well what you can do is setup 2 default routes on the 2811 that point to each SVI with a equal cost. So the 2811 will load-balance your outgoing traffic over both links.

Then talk to the ISP about load-balancing the incomming traffic back to you over both links.

This way you can utilize both of the links and not kill your router by running BGP. The 2811 is a bit weak.

Thanks CN, your alternative makes much sense. I was wondering, just by employing equal cost to two interfaces, would I achieve adequate load balancing without involving any other routing protocol (HSRP, GLBP etc)?

Also, does the only option for ISP to load balance incoming traffic from their side, rests at running BGP or is there anything better we can ask them to do?



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