Please forgive the simplistic nature of the query..
Basically my company is multi-homed with 2 different providers in the UK, and advertising a /18. Now some colleaguges in another part of the world want to break that /18 into two /19's and advertise one /19 and we advertise the other. This is fine, however we are NOT running IBGP in the core, therefore the UK customers in the /19 will not be able to reach the other /19 as there would be a loop detected through EBGP.
Now someone mentioned that we could use AS-LOOP-IN feature which will overcome this problem and allow us to route to each other via EBGP. I really think this is a bad idea but until we get an internal link - I dont see a way forward. So... anyone doing this currently in their network or have any "best practices" way round this. I want our company to be good Netizens but still be able to pass traffic between the 2 /19's.
Please forgive the simplistic nature of the query..
Basically my company is multi-homed with 2 different providers in the UK, and advertising a /18. Now some colleaguges in another part of the world want to break that /18 into two /19's and advertise one /19 and we advertise the other. This is fine, however we are NOT running IBGP in the core, therefore the UK customers in the /19 will not be able to reach the other /19 as there would be a loop detected through EBGP.
Pardon my simplistic solution, try dropping the /18, and -only-
advertise the corresponding /19 from each region.
Now someone mentioned that we could use AS-LOOP-IN feature which will overcome this problem and allow us to route to each other via EBGP. I really think this is a bad idea but until we get an internal link - I dont see a way forward. So... anyone doing this currently in their network or have any "best practices" way round this. I want our company to be good Netizens but still be able to pass traffic between the 2 /19's.
Please forgive the simplistic nature of the query..
Actually, it is refreshing to see _operational_ questions on the list.
Basically my company is multi-homed with 2 different providers in the UK, and advertising a /18. Now some colleaguges in another part of the world want to break that /18 into two /19's and advertise one /19 and we advertise the other. This is fine, however we are NOT running IBGP in the core, therefore the UK customers in the /19 will not be able to reach the other /19 as there would be a loop detected through EBGP.
Pardon my simplistic solution, try dropping the /18, and -only-
advertise the corresponding /19 from each region.
This will only work if you have separate ASNs, which would be my suggested solution. In fact, even if you announce the /18 + both /19s, as long as each site as a separate ASN, it will work.
If they must have the same ASN for some reason, have your upstreams send you default route as well as a full table. You will not see the "other" /19, but you will send traffic to the upstream because of the default and they will route it properly.
Now someone mentioned that we could use AS-LOOP-IN feature which will overcome this problem and allow us to route to each other via EBGP. I really think this is a bad idea but until we get an internal link - I dont see a way forward. So... anyone doing this currently in their network or have any "best practices" way round this. I want our company to be good Netizens but still be able to pass traffic between the 2 /19's.
I've never used AS-LOOP-IN. Sorry.
But I have used the above solution (and static defaults), and it works fine.
Hi there,
If I understand your predicament correctly, our company has a similar situation. We have two locations from which we need to advertise routes from our AS, but our internal link between these two locations is a very high cost satellite link. This means we can not afford to advertise our whole IP allocation equally from both locations.
We have a /19 allocated, and we advertise both the /19 from each location, and the more specific /20 particular to each location. To circumvent the loop detection, we use the hidden Cisco command, neighbour x.x.x.x allow-as in. This allows each location to accept the remote's advertised /20 to be inserted into the routing table. Should connectivity ever be lost across the public networks in between, there is a higher cost static route over the satellite link.
Perhaps in a more complex and more meshed AS, this loop dodging would be a bad thing(tm). In our simple two location, semi-discontiguous network layout, it has been a problem-free solution. Hope this helps in some way.
Regards,
Graham Blake
SSI Micro Network Services
1. Acquire a second ASN, and announce each site's /19 from
a different asn.
2. Announce each locations /19 from it's respective
location, using the same asn.
Use the cisco BGP command Allow-as-in to permit each AS to
hear the remote site's network advertisement.
3. If the remote site will not be multihomed, ask their ISP
to announce the /19 for you.
My gut says that if you are advertising a block in the
territory of another RIR, your irr entries will need to be
correct to save filtering issues.
Well another way for solve this problem is that both parts advertise
their /19. From the transit provider take your normal routing table and
a default route. The default route allows the two /19's to reach each
other over the transit provider(s).
Some gear doesn't send updates with AS X in the path to AS X. So depending on the type of routers your upstreams have, you may not see the routing information from the other instance of your AS.
