Source/Destination routing

[ahoangphan]

Hi all,

My company is gettting a bundle of T1s from the same ISP to create a big pipe of 6mb since we can't get a T3 here. The thing is this IPS doesn't support Multilink PPP and CEF... I was told that I need to use Source/Destination routing to create equal cost default routes and span the data between 4 T1s. I've never set a router up for this before and would love to get some help from you all...

Thank you in advance,

Anthony.

[Richard Burts]

I am not familiar with Source/Destination routing but I think I know what you can do that will work.

Normal routing decisions in IP are destination based. The router looks in the packet and finds the destination address and looks in the routing table to find the best path to the destination. That is normal routing.

There is an option in IOS to influence the routing decision based on the source address. This is called policy routing and can over ride the normal forwarding decision based on the destination address and the routing table.

I do not see anything in your description of your environment that indicates that you need policy routing. My suggestion to you is to set up four default routes with each one pointing to one of the T1s. That will create four equal cost routes and your routing logic will use all four paths to get to the ISP. I do not see any need for something more complicated.

[ahoangphan]

Thank you for replying... I was just wondering if you could provide an example of the configuration. So on one of the serial ports I have to specify the ip address and everything that my isp provides? then on the rest of the serial interface I dont have to specify the address but I have to bring them up and set up the default routes with each pointing to the serial interface that has an ip address?

[Richard Burts]

Each of the serial interfaces will need an IP address. For an example config assume that your serial interfaces were serial 1/0, 1/1, 1/2, and 1/3. You would need to configure each of the serial interfaces with IP addresses and any other parameters that your ISP provides. Then you would configure:

ip route 0.0.0.0 0.0.0.0 serial 1/0

ip route 0.0.0.0 0.0.0.0 serial 1/1

ip route 0.0.0.0 0.0.0.0 serial 1/2

ip route 0.0.0.0 0.0.0.0 serial 1/3

Your router would then have 4 entries in its routing table for the default route.

This will set it up so that your router sends traffic to the ISP using all 4 of the serial links.

This will take care of how you send traffic outbound. The ISP will need to have something similar to manage how they send traffic to you.

One other aspect of how to make this work is to make sure that all the routers in your network send their Internet bound traffic to this router for forwarding to the ISP. So the other routers in your network will need a default route pointing to this router. How you accomplish this will depend on what routing protocol you run within your network.