Which multihoming should be used in BGP ?

[eigrpy]

Hi As we know, there are three kinds of Multihoming in BGP: 1 default routes form each provider, 2 default route plus some more specific routes, 3 all routes from all providers. I think only ISP use third one, and general company use first one where BGP is configured only at edge routers. Am I right ? Can you give some comments ? Thank you

[Masoud Pourshabanian]

Hello,

Depending on your service provider and your company devices and configuration, all three are possible.

If you get service from a service provider, your service provider can advertise a default route or some routes or BGP full routes.

Does the company devices support a full BGP routes? Do you have any special need to receive those many internet routes If you ISP gives you full BGP routes.

Nation and internation and regional ISPs usually receive full BGP routes from their peering and  upstream ISP.  A local ISP might receive just a default route or even just have a static default route to upstream ISP depending on the size of that ISP.

Local ISPs might have only a router and several servers, so ISP does not always have a large network.

You can not say which is more common. It is different case by case.

Masoud

[eigrpy]

Thank you for your reply. 

"Does the company devices support a full BGP routes? Do you have any special need to receive those many internet routes If you ISP gives you full BGP routes. "

Yes The router support full BGP. The company can reach internet by using default route. Why do they use full BGP routes ? In what situation the company need to receive default route plus some special routes ? 

[Masoud Pourshabanian · Accepted solution]

Suppose you have two links to two different service providers. If you receive only two default routes, you are not able to give priority to different networks. if you receive full BGP routes, you are able to manipulate BGP attribute and utilizes your links better. Sometimes, you have downstream neighbour, so you need to advertise some routes to your neighbour with different attributes.

As for default route plus some special routes, suppose you have two links connected ISP1 and ISP2.

You have two options. Load balancing across those two links or failover. Some networks are more important for you and you just want to go to those networks through ISP2. You ask ISP2 to send you a default route and some specific routes. In routing, specific roures(smaller subnets) are prefered, so you always go to those network through ISP2. If ISP2 getsdisconnected, you go through ISP1 by default route.

It was just one case. As I said before, It is different case by case.

Masoud

[eigrpy]

Exllent explanation! Thank you

[Masoud Pourshabanian]

Thanks,

Happy it helped.

[eigrpy]

You are welcome to the post:

https://supportforums.cisco.com/discussion/12693111/why-do-we-use-bgp-non-isp-company

[Venkatesh Perumal · Accepted solution]

This is somthing where you have to go through the documentation to get the clear idea of multihoming types since this cannot be explained here.

would suggest to go through the Cisco docs for the same and have a look on the below doc:

https://www.nanog.org/meetings/nanog41/presentations/BGPMultihoming.pdf

Do rate helpful post.

[eigrpy]

Very good link! Thank you