extra ip address from isp

[suthomas1]

Good day everyone,

we have two different links to different isp's on the same device. Due to some requirement of hosting services at one of our data center's, ISP-B has given an extra small subnet for use (192.168.1.0 /28). This subnet will be advertised in our configuration so that it can be used & we want it divided into two further /29 subnets.

we were caught in the discussion, when doubts came in which is outlined as follows.

1) Does this advertisement from our side has to be only done towards ISP-B as they own this subnet given to us & not towards ISP-A?

2) What happens if we also advertise this towards ISP-A?

3) If this is divided into two /29 subnets, should it be advertised as two different /29's or as one whole /28?

Appreciate all inputs to this please.

[Giuseppe Larosa]

Hello @suthomas1 ,

1) if the new subnet belongs to ISP-B and has a prefix-length /28 or even /29 you shouldn't advertise it to ISP-A unless you make agreeements with both providers about this. There are two reasons : a /28 is generally too speciifc too much long to be accepted by an ISP and the second reason is that this subnet is actually owned by ISP-B.

2) Without prior agreements ISP-A is likely not going to accept this advertisement first for being too much specific, second because it does not belong to you but to ISP-B

3) If you have made an agreement with ISP-B for a /28 they expect a /28, so again you should advertise the /28 prefix even if you have subnetted it in two /29 subnets.

You can use the aggregate command in BGP  to generate the right prefix with option summary-only.

you will need also a network command for each of the two /29 subnets in order to effectively have the prefix /28 generated.

Hope to help

Giuseppe

[suthomas1]

Thank you Giuseppe.

should this /28 subnet be configured to be advertised as equal to /28 or ge /28.

Does it make any difference with they subnetted as two /29? 

[Giuseppe Larosa · Accepted solution]

Hello @suthomas1 ,

thanks for your kind remarks

what I was suggesting is the following:

router bgp 65000

network x.y.z.0 255.255.255.248

network x.y.z.8 255.255.255.248

aggregate-address x.y.z.0 255.255.255.240 summary-only

in this way you will advertise exactly the prefix x.y.z.0/28 that is what ISP-B would expect to receive from your side.

Hope to help

Giuseppe