Best Practice: 2routers, 2isp, 1AS


   Want to ask out anybody on the list about a "best practice" of the
setup below:

- 2 ISP's (A & B)
- 2 Routers (A & B)

I want Router-A for ISP-A, Router-B for ISP-B and have Router-A &
Router-B talk and be able to pass routes on each side in an event of a
physical failure on one of the Routers.

I was planning at first to setup a multi-home BGP, but I want to have
physical redundancy as well.


L |
N |

Any recommendation would awesomely appreciated.


There are alot more questions that need to be asked. Like how much address space do you have to announce? What routes are you getting from each ISP?

Assuming you are an end user, and knowing the very limited information I know at this point, I would make sure that these two routers LAN interfaces are in some sort of transit vlan/subnet with my downstream router, which would also be participating in iBGP. Alternately you could have that router do VRRP/HSRP with your two border routers, but I prefer iBGP.

I would then setup both routers using OER (Optimized Edge Routing, i think now known as Performance Based Routing), to handle outbound. You could just announce your /24 out each provider (assuming that's what you had) to handle inbound, or if you have larger than that you could announce the aggregate out both and more specifics out each to do some type of balancing.

Its hard to say there is a best practice here, as there are so many scenarios. I will say that I like OeR/PfR for edge customers who are dual homed. BGP is very arbitrary, and its nice to have some real metrics that mean something to play with :slight_smile:


thanks for the reply brian. :slight_smile:

sorry for a bit lack on the info, I was thinking of using VRRP. but my
2 links are running on different interface-types isp1 runs via
ethernet while the other is on an ATM interface. I only have 1 router
that has an ATM interface. setting it to VRRP would cause me problems
if it was a physical failure. I have a small /24 to advertise on my
AS. I'll go and check on the "Performance Based Routing" you


You can still use vrrp in the inside. We have a similar configuration to what you have defined. Two routers, 4 ISPs, BGP annoucing 2 /24's. We get partial routes and prepend on 3 of the isps to only use our primary. Our primary is delivered via fiber and the backup isps are delivered via copper ethernet. We use interface tracking with reachability to determine if we are having a problem with one of our downstreams. This way, if we still have a link light, but no traffic flow we can detect and adjust accordingly.


Could also just push default into OSPF from both ends (assuming you
have the iBGP between both borders) if your goal is redundancy.

-Jack Carrozzo

   We did discuss this when designing our solution. In then end we decided not to introduce another routing protocol to our enviornment. We rely exclusivly on BGP and EIGRP. It may not be the best solution, but servicability is also important.


I'll do some digging on "interface tracking" for cisco gear.