eBGP advertisement question (null0)

[4everlearning]

Hi Guys,

I have a CE (one global routing table) connected to different networks (Network A and B) and I am advertising a big subnet /20 to network A using Static route Null0 . Now i have requirements to advertise to reach some specific IPs /29 out of that range through network B. 

So I will make specific static routes to reach the /29 through network B. In this case how would that affect the /20 advertisement to network A? would the specific static routes to network B affect the Null0 advertisement of /20 that is going to Network A?

Thanks,

[Jon Marshall]

On rereading your post I'm not sure I follow.

From your CE you are advertising a summary address to network A.

But then you say you want to reach some of the subnets within that summary range  via network B.

Do you mean that or do you mean you want to advertise some of the subnets to network B ?

It's not clear whether these subnets and summary range are part of your network or are reachable via the other networks ?

Jon

[4everlearning]

Yes I am advertising null0 /20 to Network A.... However, some specific IPs from that range /29 are configured on devices that connect to Network B. so I need to reach them through Network B hence configuring a specific static route to reach them through network B. 

[Jon Marshall · Accepted solution]

Right.

So if understand your CE advertises the summary to network A. But within that summary are some devices that are on network B. And you simply want to add statics to the CE for those subnets pointing to network B.

And you use BGP to peer with network A but not network B.

If so then no, the addition of the statics will not affect the summary range being advertised to network A.

Depending on how you advertise that summary range will determine whether the more specific subnets are advertised to network A ie.

if you use a network statement for the summary address in your BGP configuration they won't be

if you redistribute static into BGP and don't filter which statics are redistributed they will be.

Jon

[4everlearning]

Thanks, that's exactly the situation. 

I advertise using network statement with a null0 static route configured since the /20 is not in the routing table. 

so in this case, the whole /20 will still be advertised to network A but the /29 subnet will be excluded from it ? or the /29 within the /20 range will still be advertised to ebgp (i'm just curios to know even though it would not matter in my case since defining a more specific static route to network B will allow me to reach the server in network B even if the subnet is still being advertised to ebgp as part of the summary). 

[Jon Marshall]

so in this case, the whole /20 will still be advertised to network A but the /29 subnet will be excluded from it ? or the /29 within the /20 range will still be advertised to ebgp

If you are using network statements under BGP and not redistributing statics then only the summary range should be advertised.

This is because when you use the network statement under BGP there must be an exact match in the IP routing table for it to be advertised. An exact match means both the network and the subnet mask.

So your network statement will match the summary route but it won't match the more specific route because, even if the network part were the same ie. the smaller subnet was at the start of the summary range, the subnet mask would be different.

Jon

[4everlearning]

I agree with the exact match statement, and this is why i believe everything would be advertised to ebgp. correct me if i'm wrong:

If I use the network statement to route 10.220.0.0/20, and have a null0 statement in the routing table for 10.220.0.0/20 (this includes 10.220.0.0 to 10.220.15.255).

so I was thinking even if i make a specific static route to reach devices configured within that range (10.220.10.0/29) through network B, this will still be sent to the ebgp because it is part of the summary range (10.220.0.0 to 10.220.15.255). isn't that the case here?

[Jon Marshall]

No it isn't because it is not an exact match.

For an exact match the network prefix and the subnet mask must match and the static routes will have different subnet masks.

Jon

[Jon Marshall]

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[4everlearning]

Thanks for the reply Jon,

Yes my CE is peering with different routers associated with different networks. I am connected to network A with eBGP and network B with VRRP where i can control exactly what to advertise to network B. 

I don't care if the specific subnets from the big /20 range is still advertised through eBGP to network A. I just need to have them preferred to go through Network B which should be the case since static (more specific) is preferred over eBGP (null0 static). 

However, in this case would Network A still receives the whole /20 range? or would making more specific static routes from that range to network B affect the null0 advertisement?

and would the more specific static routes to network B be excluded from the /20 null0 advertisement to network A? or they will still be sent without any effect on the null0 statement or the /20 eBGP advertisement?

Thanks,

[Jon Marshall]

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[4everlearning]

Thanks Jon sorry for the confusion it's kind of complicating to explain those things without drawing :) 

well, the issue is that there are internal devices and servers connected to the CE and have a big dedicated range reserved for them (10.220.0.0/20) which is not fully utilized yet. Now those devices usually communicate with network A therefore the whole /20 range was advertised with null0 so that no further advertisement needed from CE side to network A when assigning new IPs from that range to the internal servers. Now some of those servers need to communicate with a server in network B for reporting where a small range (10.220.10.0/29) will be dedicated for this connectivity.

Now the CE does not run any routing protocol with network B and only has a default route where anything not specified to be sent to network A will go to network B. but since the /29 subnet that will be used for the new server in network B connectivity is part of the summary advertised to network A it will not go through the default route. therefore, I was thinking to create a specific static route /29 to reach the server in network B through network B. 

But i need to make sure that by doing this i will not affect the /20 advertised to network A since nothing else from that range will be used for any connectivity with network B.

I hope that makes sense though it's just hard to put it in words :)

[Jon Marshall]

No need to apologise, I think it really was just me after a long day :-)

Not sure you've seen it but the last post I made was assuming exactly the situation you have just described.

In short, no it should not have any affect on the existing advertisement.

For the longer answer see the last post I made which is further up in the thread.

Jon