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ISP Expansion

Frequent Contributor
Frequent Contributor

Hi All,

I'm after some advice on a network design for a small ISP please. Currently this ISP has one POP in location1 and they are looking to expand into location2.

Transit bandwidth is cheaper in location2 so it would be advantageous to connect both POPs together and advertise all prefixes via location2 however have location1 as a backup link incase of any outages in location2. Both locations will participate in private peerings with various providers via Internet Exchange Points and these prefixes will be distrubuted to both locations.

The ISP has two 7204 VXR routers currently in location1, one with the primary feed and also a connection into an IXP, the second router is connected to a seconday feed and also a different IXP.

The plan is to connect both locations together using a 10 Mbps link and move the secondary 7204 VXR router from location1 to location2. The 7204 VXR at Location2 will then connect into the new primary provider and the location1 will then become the seconday provider. A second smaller router (3600) will be installed at each location which will be connected to the 10 Mbps link for redundancy. The 7204 VXR's will peer with each other using IBG via the 3600's and the 10 Mbps link. (Please see attached diagram). The ISPs prefixes will be advertised from location 2 to the primary provider and also from location1 to the seconday provider with as-path prepend set.

What i don't full understand is the best way to provide a default route to the distribution layer at each location, whether it's best to inject the default route into EIGRP (or OSPF) or configure the distribution layer switches as IBGP peers and advertise the default route to them from the 7204 VXR using "neighbor {ip-addres x.x.x.x default-originate [route-map map-name]" and perhaps a route-map with some conditional statements.

My other question is what kind of bandwidth utilisation would be required for the routing updates between the two sites as the first stage of the plan is to only have a 10 Mbps link and this link will also be used as a transit connection between the two locations.

Many thanks


3 Replies 3

Frequent Contributor
Frequent Contributor

Any ideas??

If it's not clear please let me know



I don't think it will be a big issue to take the bgp routing table over the 10m link. Unlike other routing protocols you will only get a large update when a circuit first comes up after a outage.

I will have to look at this a little more to give a opionion on default router. The issue you have is in addition to providing the default route to the internet you are using it to get between the 3600's.

Now if you can ensure that all prefixes from both BGP are identical or at least your default router always has the preferred ones then maybe you can make this work.

Lets for example say ISP1 give you a route to x.x.x.x/22 and isp2 gives you 4 /24 routes covering this same range. Your 2 BGP routers will agree that the /24 routes are preferred.

Now lets assume that your default route is on the the router that is receiving the /22. It will receive the traffic from the local network and attempt to send it to the other router since it has a IBGP route to the network. Assuming you have disribited the "connected" interfaces into EIGRP it will send the traffic to the 3600. Once it gets to the 3600 it does not have all the bgp routes and it will send it back to the default route... and you have a loop. If you do not redistribute connected interface I forget what BGP does when his nexthop is reachable via the default route but the the default route is the same router. I suspect it will drop the traffic.

The only way to fix this is to either redistribute the BGP into EIGRP or run bgp on the 3600 routers and between the 3600 routers. I forget but you may also need a route-reflector on one of the 3600's.

In either case you really don't want those huge routing tables in a 3600 router.

It would be much better if you could connect the 7200's together somehow. You could use tunnels or some kind of vlan bridging if the 3600's need to stay the way they are.

Frequent Contributor
Frequent Contributor

Thanks for the post.

Luckily my colleague has just purchased another 7200 VXR for the existing site giving him 2 x 7200's. When we connect this site to the new site he is going to purchase another 7200 so we are part way to having 4 x 7200's. The reason i added the 3600 routers into the topology is for redundancy as my fear was that if we had the 10 Mbps link connecting into the 7200's then if we lost one of the 7200 we effectively lost an entire site. However if we had two routers at each site (one connecting to a provider and one connecting to the other site) then if we lost either of the routers within the site then packets can either route to the internet via the sites own connection to the provider, or route to the internet via the other site provider over the 10 Mbps link.

Your advice has helped me come to a conclusion so thanks.



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