BGP Conditional Advertisement

[richardmcmahon]

Hi,

We are constructing a new network for our datacentres. We plan to have two peering points with the facilites being connected to each. Each peering router will run BGP and EIGRP. The datacentre routers will only run EIGRP. I plan to inject a default route from each peering router into eigrp. The datacentre routers will be configured to load balance accross the two received defaults from the peering routers. To enable me to disable bgp anouncements of unreachable networks from the peering routers, ie if the worst happens and the router looses all connections except its ebgp links I want it to stop advertising the unreachable network(s) to the world at large. To do this it looks like I need to use conditional advertisement. The documentation seems to suggest that conditional advertisement is based on what networks are in the bgp routing table. Thus to get networks in the data centre that would only be in the eigrp table would I need to redistribute eigrp into bgp? I have attached a quick pdf sketch to help get the idea of the layout.

[vladrac-ccna]

I believe the conditional advertisement is based on the contents of your BGP table.

You need to match it inside a route-map and it must exist (or not-exist) in your BGP table.

vlad

[network.king]

Hi Richard,

I want to clarify two things from you

1.Is that you are going to do static advertisments of subnets ( Network + /24 route, so that advertisments happens from the peering routers eventhough the subnet at datacenter router may be down .

or

2.Advertisment is done to internet with only Network and route would be based on the routing table entry in the Eigrp table (/24 and higher )

There would not be any need for redistribution of Eigrp prefix if option 2 is worked out , but care must be taken as flapping of Eigrp route can lead to route dampening in the internet .

Pls let me know if you had a different design in consideration

regards

vanesh k

[mheusinger]

Hi,

in your case it would be advisable to use the network statement in BGP. This could look like:

router bgp 65000

network 10.1.0.0 mask 255.255.0.0

network 10.2.0.0 mask 255.255.0.0

neighbor ...

You need to adjust AS number and network(s).

The networks would only be announced IF there is a matching entry in the IP routing table. Subnets from 10.1/16 would not lead to BGP announcement of 10.1/16 in my example.

So to get the required routing table entry you can

1) announce this through EIGRP

2) use a static route "ip route 10.1.0.0 255.255.0.0 Null0"

The second approach would not meet your requirements. So I suggest to creat an EIGRP summary for your whole official IP address block and announce it to the BGP routers, which will in turn announce that to the internet. In case connectivity to your EIGRP summary is lost, BGP would withdraw the network from the internet.

Hope this helps! Please rate all posts.

Regards, Martin

[richardmcmahon]

Hi Martin,

I have tried this in the lab and by exporting only specific routes and not everything (ie the noc lan segment in each datacentre) from eigrp in bgp and creating the conditional line it drops the netblock in and out of service depending on whether it is reachable through eigrp on each core node. Another question that you may be able to help me with is that I am currently advertsing a high metric default route through eigrp to the datacentre routers. The only thing is that it is not installing both into the routing table, only 1. I have dual paths for all other routes on the peering router to other facilities but the default only seems to have one. Any ideas, if you remeber you answered the q previously on wether or not I should export /8's from bgp.

Thanks,

Richard

[richardmcmahon]

Hi Vanesh,

I am advertising aggregates /18, /20 and /19 to the internet. These are assigned to each dc, ie not overlapping, I want to drop announcments of each aggregate depending on network visibility.

Martin has pretty much answered the question below.

Cheers,

Richard

[pushkar1782]

If your networks go down it wont be advertised , they will be adverstied only if they are being installed in the table .. if n/w goes down and you have higher metric router to NULL0 , then it will be advertised..otherwise it wont .. so i think you dont have to worry at-all .. but i say this is not at all perferable ? is there any specific reason ??

I hope this helps . Kindly rate

[richardmcmahon]

The specific reason is that in order to broadcast the network out on bgp there will be a network statement. Should the fibre carrier links connecting the locations go down but no the upstream carriers I do not want to advertise routes for the entire netblock as the router will blackhole all traffic it gets. I want it withdraw the announcement for the unreachable block. The same is also true if one of the datacentres becomes unreachable but not the other from one of the core routers. I would still want traffic for the reachable centre to get there but not for the unreachable one. Hopefully that makes sense.

Cheers,

Richard

[network.king]

Hi Richard,

With the network statement under the BGP and a matching route for the network is present in your EIGRP table , announcement of route would automatically happen and In case of a problem , route would get vanished and bgp advertisement would stop .

Also , pls check whether you have /24 or greater networks in each location , so that there is no problem with advertisement to Internet

regards

vanesh k

[pushkar1782]

Dear richard ,

Thats what we all say ,

If you loose your routes in routing table BGP wont advertise them so u need not worry . nothing special has to be configured.

rate if helps .

TC

prs