BGP Network Advertisement - Break up /22 to 4 /24's

[Carl Duvall]

I'm going to try to keep this simple, but if need be I can map out a topology or upload some configs to look at.  Thanks in advance for the help.

2 sites - one for production, one for DR

Both sites use same AS, both sites have same ISP.

Production and DR site are interconnected via MPLS (layer 2).

At production site, currently advertising a /22 public IP subnet that is owned by the company.  Want to break it up into 4 /24 networks:  Advertise 3 of the 4 at the production site, 1 at the DR site.

ISP is accepting prefix of /22 le 24, so that part is already done.

There is an interface on the Production router and DR router in the /22 network space, so there is a connected route in the routing table for /22.  The network command in BGP config is set for the /22 network.

I have tried in a lab to test this to see if I can advertise the individual /24's at each site, but I never see them on the ISP router in the BGP table.  I have tried using the individual /24's in the network command in BGP, as well as advertising (route-map/prefix-list) the individual /24's....it doesn't work.  The only way I can make it work is if I manually add static routes to each individual /24 network through the connected /22 interface.  I don't think this is the best way and am wondering why I have to do this.

I realize this may not be explained well enough, so if you need a diagram/config, let me know and I will draft one and attach it.

Thanks,

Carl

[Richard Burts · Accepted solution]

Carl

One important thing to remember about BGP is that when you put a network statement into BGP that includes a mask (which you would do to break your /22 into /24s) then BGP must find in the local routing table an exact match - same network/subnet and same mask. So if you put in network statements specifying /24 then BGP must find those networks with /24 into the routing table. It sounds from your description that the best way to do this is to put in the static routes.

I would suggest a different approach. If I understand correctly what you are trying to accomplish it is that one of the /24s be routed from the ISP to your DR site. I suggest that you leave the advertisement from production site as /22 and that from the DR site that you have a network statement for the /22 (which allows you to back up the production site if there are problems) and put a network statement for the single /24 that you want to go directly to DR. This way the single /24 prefers DR all other networks prefer production and each site is able to back up the other in case of problems.

HTH

Rick

[Carl Duvall]

Thanks, Rick for the quick response.

So it would be acceptable to have the network statement for the /22 as well as a /24 that is inclusive in the /22?  Wouldn't it also be required to have the static route statement on the DR router for the individual /24?

Carl

[Richard Burts]

Carl

Yes it would be acceptable to have both the /22 and a /24 in BGP at the DR. And yes it would require a single static route for the /24 at the DR.

HTH

Rick

[Carl Duvall]

Thanks again, Rick.

Last question:  Should I make it a "floating static" with a high AD so that it wouldn't impact any other routing, but still show up in the table?

Regards,

Carl

[Richard Burts]

Carl

Floating static with higher AD is a nice thought but probably not effective. We do not know much about your environment but I have the impression that there is likely not an existing route for the /24 network (and if there was an existing route for the /24 in the routing table then you do not need the static). And if there is not an exact match for the floating static in the routing table then the floating static will be inserted into the routing table.

HTH

Rick

[Richard Burts]

Carl

I am glad that my suggestions have been helpful. Thank you for using the rating system to mark this question as answered. This will help identify for readers in the forum that there is helpful information here.

HTH

Rick