BGP subneting and advertising

[igor.hamzic]

Hi all,

I'm in a need for some help regarding BGP subneting and advertising question. The situation is as follows.

Currently we have an x.x.x.x /23 public IP address range, 2 ISP internet links(connected to two routers) and we are advertising the x.x.x.x /23 range through BGP to the ISPs without any problem. The need has arisen in our company that a remote location should have internet access and should be in our public address range and AS number and use existing internet links to reach the internet.

What I was thinking was of doing was to subnet the larger /23 range into two /24 ranges and then subneting one /24 range into smaller /28 subnets for current and further uses. I will also be adding an additional network module in one of the routers on which one of the new /28 subnets will be connected through a L2 link to the remote location. The current router network interfaces and all other network equipment with /23 subnet masks will get the /24 subnet masks from the range not being split further into /28 subnets

What I need help with is how do I configure the BGP on the two exsiting routers. Do I leave the current network statements ie. network x.x.x.x mask 255.255.254.0 or should I do something else as I will be subneting the current range? Any help is greatly welcomed.

[Haris P]

can you post a network diagram including the ip address assigned for the router interface.?

If i understand clearly you are saying that yor current network interfaces are with /23 subnet masks ? If so how you can route some IP's from this range to remote location ?

You can route a /24 in your Head office (2 ISP links internet location ) and route next /24 to remote branch .and then establish a ibgp relationship between your head office and remote branch .

[igor.hamzic]

I was thinking of something like this in the attached pictures.

[Kishore Chennupati · Accepted solution]

Hi Igor,

You can defintly do this that way you have suggested. You can run iBGP as Harish suggested on the new router at your branch and the existing Router at HO where this will connect to and it should work.

Now, you got 2 X /24's. Would you like to load share these between the 2 ISP's?. Your network is a perfect candidate.

What you can do is this

1. The /24 which you will use in your HO network, you can advertise it with AS_PATH prepend via ISPA so that upstream will always choose ISP B for the return traffic and in the case that the link to ISP B fails it will be routed via ISP A and also increase the local-pref on the link to ISP B to say 200 so that the outgoing traffic will be via ISP B as well

2. The other /24 which you will be using for the HO-Branch office connecitivy, that you can advertise via ISP A.

In this way both your links will be utlised.

HTH

Regards,

Kishore

Please rate if helpful

[igor.hamzic]

Just to clarify something. Both of the routers on the pictures are in the home branch and already up and running. The remote location I am talking about is the one with the firewall.

[Haris P · Accepted solution]

The remote branch is the one with firewall but the ouside IP of the firewall and router is in /24 network and this /24 you are trying to advertise to outside .

The below confg. should work fine . R1 is connected to ISP1 and R2 is connected to ISP2 . Now also as Kishore suggested you can add AS-Path prepending for load balancing and failover

R1

router bgp 45455
network 192.168.1.0 mask 255.255.255.0
neighbor 2.2.2.2 remote-as 45455
neighbor 2.2.2.2 soft-reconfiguration inbound
neighbor 2.2.2.2 next-hop-self

neighbor e-bgp-peer remote-as 8855
neighbor e-bgp-peer soft-reconfiguration inbound

R2

router bgp 45455
network 192.168.2.0 mask 255.255.255.0
neighbor 1.1.1.1 remote-as 45455
neighbor 1.1.1.1 soft-reconfiguration inbound

neighbor 1.1.1.1 next-hop-self

neighbor  e-bgp-peer remote-as 8855
neighbor  e-bgp-peer soft-reconfiguration inbound

[igor.hamzic]

Thanks for the replies guys. I think that will do the trick nicely.