BGP Question

[Krelian]

Hi guys,

Hope some experts here can shine some light on simple question on BGP protocol.

We recently got a single router with (2) serial T1 connections to the Internet. As a fault tolerance measure, the (2) T1 are connected to different network segments located at (2) different ISP POPs. Our consultant told us in order to achieve this fault tolerance scenario and so that if one T1 fail, it will automatically failover to the second link, the router has to be configured to run BGP4 I understand that BGP is a routing protocol used for ISPs to advertise their routes to the entire Internet. But I don't understand why do we have to run BGP4 to have the router failover to the second link. What is so magical about BGP?

A

 

[hexn]

I guess the first question is, do you own your own address space?

If yes, then you should run BGP anyway to advertise it out both ISPs. If each ISP just advertises it for you, this would also work, and failover shouldnt be a problem. As for outbound traffic, 2 static routes will do the job just fine.

If no, then BGP wont help very much. If one of your providers goes down, then whatever ips you get from them will also go down. You will need to get permission from them if you want to advertise their IP space with BGP to your other provider.

If you dont have any IP space at all, and are just doing NAT, then BGP wont help you at all. Just 2 static routes will be a perfect failover/load balancer.

 

[majestik]

Krelian,

Ask your ISP's if they can support a shared private Autonomous System (AS) number (unlikely). If not, then you will need to own at least one Class-C of public address space, and you will also need to apply for a public AS Number.

BGP should be configured to prepend one ISP link with your new ASN several times, therby advertising it on the Internet as a longer AS path and making it the less preferred route. If the primary ISP link fails, the primary AS path will no longer be advertised and incoming traffic will be forced to use the backup route.

Without BGP, your primary ISP would have to advertise static routes on your behalf and traffic would never enter your site through the backup link.

Cheers,
-maj

 

[edil]

Hi!

Krelian, Where you able to setup BGP as Majestik suggest? I'm using a 6509 with a router module with two DS-3 links to 2 different providers. I have my own ASN and IP address range, receiving full BGP routing table. I want to have full failover, plus load sharing and balance.

Majestik, I don't know if I'm asking too much but can you please give me an example of how to setup the BGP to "prepend one ISP link with your new ASN several times"?

I will appretiate your help.