BGP Thought Check

[visitor68]

Conventional wisdom tells us that if we have 1 edge router that is multihomed to 2 different ISPs, the edge router should have its own AS and use eBGP to peer to both ISP routers.

Why cant we just configure 2 default routes on the edge router, each one pointing to a different ISP?

Thanks

[Atif Awan · Accepted solution]

ex-engineer wrote:
Conventional wisdom tells us that if we have 1 edge router that is multihomed to 2 different ISPs, the edge router should have its own AS and use eBGP to peer to both ISP routers.
Why cant we just configure 2 default routes on the edge router, each one pointing to a different ISP?
Thanks

In the case you mention, if you are using provider assigned address space and are able to configure your edge router properly so your traffic going out each interface is NAT'd to the respective provider's address space then you do not need BGP. If, however, you are using provider independent address space (you acquired your own IP Block) and you want to have control over how traffic flows into your network then BGP is the protocol of choice. While more complex Multi-homed topologies (with provider independent address space) will most likely mandate the use of BGP, if we are to only consider the scenario you mention then you can get away without using BGP but you need to understand the following:

1. You will need to work with both your upstream providers to have them advertise your address space and route it properly to your network.
2. Traffic flow can be asymmetric. While you can control how traffic flows out of your network, return traffic flow will not be in your control.
3. Failover will depend on your upstream providers' configuration. If an upstream provider somehow continues to advertise your address space (configuration error) while your link with them is down then you will have a traffic blackhole situation.

Atif

[Mahesh Gohil]

Hello,

It depends upon the customer requirement.

The proposal will be useful to...

> Customer who do not require primary/backup or load-share type of scenario:

    ISP will provide their own IP address(Like ISP1 will provide  x.x.x.x/x and ISP2 will

   provide y.y.y.y/y. And ISP1 will not permit y.y.y.y/y which is assigned by other provider and vice versa. In short you can not create load balance or

   primary/backup kind of thing)

> If you think you will have your own public IP and you will ask your provider to announce over the internet. well here also if you are thinking of

   default route then most of the provider will deny such request because if you are not running bgp then provider will have to advt. the prefix with their  

   own  AS which might lead to blacklisting of provider AS

I hope above explanation is meaningful to you.

Regards

Mahesh

[visitor68]

Atif and Mahesh:

Sorry for taking so long to get back to you....

Everything you are saying makes sense....I appreciate your feedback. Good stuff. Rated.

I may get back at you wiht another thought or 2....

Thanks again.