BGP Advertisement Help

[brentgunn]

I am attempting to advertise a subnet to my ISP and have hit a little snag and would appreciate any assistance.

I am attempting to advertise a /24 subnet to my ISP but i want to break the subnet into two /25s (So i can have 2 DMZs -one public IP/routed and one private IPs / NAT'd).

Therefore my router (ISR4431) has a /25 subnet mask on the backside interface and if i redistribute connected it will attempt to advertise the /25 (they will only accept /24 BGP advertisements.

I have tried using access lists, redistribute static and having a static route for the /24 to null0, but i cannot get a successful advertisement to my ISP.

Any help is appreciated. Thanks,

So my setup
router bgp 12345

 bgp log-neighbor-changes

network 250.250.250.0

int gig0/1

ip address 250.250.250.1 255.255.255.128

[Jon Marshall]

Using the static route to null0 should have done the trick. ie.

ip route 250.250.250.0 255.255.255.0 null0

router bgp 12345

network 250.250.250.0 mask 255.255.255.0

when you added the static route did it show up in the IP routing table ?

Jon

[brentgunn]

Thanks for the quick reply-

I backed everything else out and got it down to what you said and i do see in the routing table 

S 250.250.250.0/24 is directly connected, Null0

C 250.250.250.0/25 is directly connected, Gi0/1

L 250.250.250.1/32 is directly connected, Gi0/1

It is worth mentioning that it looks like it is being advertised, but the ISP isnt seeing it, when i do a

sh ip bgp neighbors X.X.X.X advertised-routes

   Network               Next Hop  Metric Weight Path

*> 250.250.250.0    0.0.0.0   0 32768  i

[Jon Marshall]

I was just about to add to this to say you could also use the aggregate-address command to do this as well but it looks like it is being advertised from your above output.

Not sure why the ISP is not seeing it.

Is that the only route being advertised or did you just cut out the rest ?

Jon

[brentgunn]

* It was the only route advertised

[brentgunn]

*Actually - i just simplified this.

I changed the interface to /24 and did redistribute connected.

That really should be working and its not - maybe it is actually on my ISP. I am awaiting a callback from the ISP.

[Jon Marshall · Accepted solution]

I think it may well be your ISP end because as far as I can see from your output your router is advertising that route.

Jon

[brentgunn]

Jon,

Thanks again.

I spoke to the ISP (for the 5th time) and they were going to look at a few things and call me back. Miraculously while they were 'looking' my traffic began passing. 

They owned up to it that they had a reject where they should have had an accept.

Frustrating that they didn't find it before. I simply lacked the confidence to push them until i had your reassurance.

Thanks again,

**Once i got it working with the /24 it was easy to change it - add the static route for null0 and 'redistribute static'.

[Jon Marshall]

I spoke to the ISP (for the 5th time) and they were going to look at a few things and call me back. Miraculously while they were 'looking' my traffic began passing. 

Amazing how that happens isn't it :-)

Glad to hear you got it working.

Jon