Configuring BGP for Backup Route

[johnbin]

I have 2 routers, A and B, connected via T1 PTP. They both have T1's out to the internet also. If my internet connection goes down on router A I want to reroute traffic through router B and out to the internet. Router B is running EBGP and peering with my internet provider. I already have the traffic rerouting on router A to B, but need help configuring BGP on router B to allow this to happen. So traffic coming back in from the internet on router B destined for network on router A knows how to get back to router A from router B.

Please Help....

 

[baddos]

What is the ISP for router A? Is it the same ISP or a different one?

Also what is the size of the networks you are announcing (I.E. a /24, /20, etc)?

Are you running any routing protocols already or are you using static routes?

 

[johnbin]

Router A ISP is Sprint, Router B ISP is AT&T. I'm running EBGP on router B with AT&T

Router A has static routes, B is running BGP. The size of the networks are /24.

 

[baddos]

Ok... What you need is your own AS number if you don't have one. You can't use on of AT&T's AS numbers, you'll need to get one from ARIN

http://www.arin.net

if you don't have one.

Then you'll need to contact sprint and tell them that you want to connect to them with BGP and announce your /24 from them and AT&T's /24. You'll also need to notify AT&T to accept the /24 from sprint.

One you do that, then you can start configuring router A with eBGP for the sprint connection, and iBGP with router B. On both routers A&B you'll annouce both subnets (/24's). You'll make a route-map to ensure that sprint's /24 primarily goes down the sprint pipe and vs.

Example route-map:

ip prefix-list preferred description My preferred routes
ip prefix-list preferred seq 5 permit 1.xxx.xxx.0/24
!
ip prefix-list notpreferred description My least preferred routes
ip prefix-list notpreferred seq 5 permit 2.xxx.xxx.0/24
!
route-map myroutes permit 10
match ip address prefix-list preferred
set metric 20
!
route-map myroutes permit 20
match ip address prefix-list notpreferred
set metric 10
set as-path prepend 27468 27468 27468 27468

This tells your router to tell your eBGP peer that I have subnets 1.xxx.xxx.0/24 and 2.xxx.xxx.0/24 but the 1.xxx.xxx.0/24 is a really good route. When you setup your other router, you would reverse these prefix-lists so that your telling the other ISP that your other subnet is really good.

It's pretty advanced subject, but this is the basics of what you want.

 

[baddos]

One thing... under the "set as-path prepend" command, the number repeated many times will be your AS number (the one you got from ARIN). You repeat it many times and it basically tells the neighbor router that in order to get to this router you have to get to me 4 times (which makes the route long and not preferred).

 

[NetEng631]

You also need portable address or the service providers will not advertise the Network. Just becuase it's a /whatever doesn't mean it will be advertised to the rest of the internet.

 

[baddos]

You don't need a direct allocation for them to announce it. As long as the ISP swipes it to you, you can announce. To verify if it's done, just put your network address into Arin's Whois lookup. It should match your company's information.