When & Why do we need BGP

[amit-singh]

Hi Experts,

I am pretty confused about when and why do we need BGP. I have been thinking a lot about it, but my doubts couldnt get clear.

1. I have a single connection to my ISP : I dont need BGP in this case as a signle static or defualt route can help me.

2. Two redundent links to my ISP: I can configure multilink or static route redundency for this, so there is no need for BGP here.

3. Multihoming to 2 ISP's: In this case, I can use say GLBP on my LAN and have static or default route do the load-balancing to my ISP, so I dont need BGP here.

So when, under which condition I need BGP. Is it only when I use multihoming with 2IPS's and use one link for outgoing traffic and another for incoming traffic. Please put some light on it with an example.

regards,

-amit singh

[tcordier]

Hi Amit,

in my experience you need BGP in at least two situations:

1) You do want to exercise control about the prefixes which you advertize to the Internet (assuming you have public IP ranges). BGP allows you to control which prefixes you advertize without the need to request your provider to add or remove routing statements.

2) Your situation 3). It will work as you describe it, but you need BGP if you want to make routing decisions in your router, or to implement other policies based in BGP parameters.

For instance, you can use BGP to make sure that you always use the ISP with the shortest AS-path to the destination, and in this way provide possibly better response times to your users. Another example would be the possibility to implement filters for unwanted destinations etc.

I am sure there are other situations where BGP is useful, but these two are examples I often see.

HTH, Thomas