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Redundant Serial Links for Router Configuration

Hi any one please help out;

I have company A with a leased line to ISP_A and another company B which side by side and with a leased line to ISP_B.

Both company LANs can be linked up by a fiber optic cable.

Intentions: If leased line of company A fails, traffic need to be routed to 2610B and vice-versa.

Company A:


s0/0 : - 202.188.31.x

e0/0 : - 202.188.6.x

Company B:


s0/0 : - 202.189.3.x

e0/0 : - 202.189.10.x


(1) How do I configure the router 2610A and 2610B for such scenerio ?

(2) How do change the route of ISP routers during failure of any of the links ?

(Does dynamic routing can solve this problem ?)

(3) How to make sure the incoming traffic can reach to the internal servers ?

(4) Is there any sample configuration for such scenerio ?

Please help out...


Re: Redundant Serial Links for Router Configuration

I assume you aren't using BGP. So, have each router point it's default gateway to the connected ISP. Then redistribute that static route into a routing protocol (eg OSPF or EIGRP). That route will be exchanged between the two routers but the metric will be worse for the remote default route, so it will only be used in case of failure of the local ISP. Local traffic will be sent out the local router to the internet as their gateway is the local router. Inter-company traffic can be sent over the private link as the routes can be learned via EIGRP.

Hope it helps.



Re: Redundant Serial Links for Router Configuration

The solution to this challenge will depend upon whether you have an ASN and are using BGP or if you need to work without BGP. The latter can work, but only if you do not need to support providing access to the IP addresses of either client from the Internet. That is, you can continue surfing, but web and email will be down.

There is a white paper on my web site which goes over the many options available and some of the strengths and weaknesses of each. There is more detail in chapter 8 of my book, take a look at listings 8-25 and 8-26 (also on my web site) for how NAT can be used to support multiple real address ranges, or listings 8-28 and 8-29 for a typical BGP dual router configuration.

Good luck and have fun!

Vincent C Jones

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