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

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. And see here for current known issues.

New Member

BGP not Working Correctly

I have two routers 1 with an OC-12 link and the other with a DS3 link. I want the OC-12 to be primary and the DS3 as backup. I set the preferred link on the OC-12 and in my route map statement I set as-path prepend AS# AS# AS# AS# on the DS3 link. All of my outgoing traffic goes up the OC-12 link but the incoming traffic is split between both links 60% OC-12 and 40% DS3. What other statements can I use to make all traffic go over the OC-12 link? Or is that controlled by my ISP’s?


Re: BGP not Working Correctly

You are prepending the AS Path on your DS3 link to make it less desirable for incoming traffic to choose the DS3 path. However, your DS3 ISP mightn't advertising your NET to the INTERNET core servers with the same attribute. Telnet to a route server on the Internet and check how your NET shows up from the core server perspective.

To resolve the situation you would have to contact your ISP and work with them.

Hope that helps.



Re: BGP not Working Correctly


One other thing you might want to try is te split up the address range that you are advertising so that you are advertising more specific prefixes over the OC12 and the aggregate over the DS3. Since routers route based on the longest-prefix match, all your traffic should come in via the OC12.

For example, say you had the following block: You could split it up into two smaller blocks: and Then, advertise over the DS3 and advertise the two /21s over the OC12.

Pls do remember to rate posts.


Re: BGP not Working Correctly

If the ISP lets you advertise smaller blocks then, YES. The way the internet routing table is growing the ISP mightn't be inclined to accept smaller blocks as it takes up more router resources.

Every ISP would have their own set of policies/rules when it comes to accepting routes from customers. They normally wouldn't deviate from those rules for one customer. ISP would tell the customer what they can/can't do. Personally, AS Path Prepend is a good choice to influence incoming traffic.



New Member

Re: BGP not Working Correctly

For the outgoing traffic you can assign a higher weight for the default route that received from ISP that connected on OC-12, then all the outgoing traffic will be going through this interface

CreatePlease to create content