Say I have AS100 and want to connect to two ISPs say AS200 and AS300. I want to use two routers for redundancy (run HSRP to give my local stuff a default gateway).
Now if my local BGP routers are router A and B (both AS100) and the ISP ones are C (AS200) and D (AS300) can I have my router A peer with both C and D and my router B peer with C and D as well ?. I am presuming that we would be doing EBGP multihop ie my routers A and B are not directly connected to ISP routers C and D rather there are a couple of dumb non BGP routers providing the physical connections.
The idea here is that normally my outgoing traffic goes via my router A the to C or D whichever is the best route. If A fails my outgoing routing switches to B because of HSRP and still goes out the best route.
I agree with White. IBGP i think would be your best option. If you do this no matter how you do it your traffic will go over whichever ISP has the best path as you said. IF you want to loadbalnace outbound you'll need to use Local Pref. Every setup i've seen for redundancy for 2 routers connected to 2 ISP's uses IBGP not the setup you suggested.
I have 2 routers connected to 2 ISP's and run IBGP. and my failover works awesome. I also use AS path pre pending and Local Pref to load balance inbound and outbound.
I'll go one step further and say the question does not make sense unless the links to the ISPs are not directly connected to either of his two AS100 routers.
A little thought will quickly show that if router A has the link to X, running BGP from B to X is useless. If the link or router A or router X goes down, router A will cease to advertise reachability. Running BGP from B to X is a waste of time, that is what IBGP or your IGP are for.
If the ISP access is via a LAN link (as might be the case in a coloc setup), then careful consideration and discussion with the service providers is required to ensure that a consistent and robust design is implemented. If the link is via DSL or cable modem, the availability of BGP is big news.
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