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Community Member

Configuring BGP


I'm in the process of setting up BGP between 2 ISPs at 2 data centers. From ISP A to data center A, I have 4 T1 lines configured with "ip load-sharing per-packet", from ISP B to data center B I have 1 T1, and I have a /24 assigned to me from ISP B to advertise. Between the 2 data centers I have a point to point T1 line going. My question is: Is it possible to use IP addresses from private class C to assigned to the interfaces that carry the point to point connection between the 2 date centers, then create loopback interfaces on the routers, assign these interfaces with ip addresses from the IP block that I'm advertising, then use "update-source loopback 0" to send routing updates between the two routers at the 2 data center (iBGP). Is it possible also to use an ip address from this IP block to assign to the ethernet interface that's facing with my internal firewall while this ip block is being advertised?

Since I have 4 T1 lines coming from ISP A, they asked to have a loopback interface set up which will be used to connect to their loopback interface on their router, is it still neccessary to set up "ebgp multihop".

We're mostly setting this up as failover, with as-prepending applied on the route out from IPS B, so that data center A will be the primary data center (traffic to this /24 IP space will go to data center A) until it goes offline, then data center B will take over all traffic to this /24.



Community Member

Re: Configuring BGP


You can use private IP address for connection between DC-A and DC-B without BGP "update-source lookback" - you do not actually use BGP, if you run your own IGP internally.

It is not a problem to assign the IP block to your Ethernet facing external IF of Firewall.

For 4xT1 load balanceing, you do need "ebgp multihop" command and also "update-source loopback". Your ISP will do the similar configuration as well. static routes(4) with Serial IFs as next hop to your ISP ebgp neighbor will be required too.

For incoming traffic, AS-Prepend should be applied at router facing ISP-B. However, for outgoing traffic, you need to play with IGP metric with higher metric toward router connecting to ISP-B. This can be accomplished by redistribution from BGP to IGP.


Re: Configuring BGP

You can sure assign private class C addresses to your intranet T1.

You have to use ebgp-multihop.

You can assign your public IPs to your loopbacks and peer using ieBGP with no problems.

Just keep in mind that when traffic leaves the network to ISP, it should be properly NATed to that ISP's space.

Hope this helps.

Community Member

Re: Configuring BGP


Yes, you can use private address between your 2 data-centers and use loopback address too. But if your intention is to exchange only internal traffic between your 2 datacenters, use OSPF (IGP). Use iBGP if you want to provide some external redundancy to both the data-centers.

The use of loopback interface is for security. If you set up a loopback address in your eBGP session, you always have to use the "ebgp multihop". For iBGP session with loopback address, you don't have to use the "ebgp multihop" command. The "ebgp multihop" command is only for eBGP sessions.

The use of "as-prepend" is only for incoming traffic. For outgoing traffic you have to use another attributes from BGP, like local-preference.

I hope this can help you..



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