Redundancy with Intersite BGP

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Aug 14th, 2007
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Hi All,

Please have a look at the attachment before you start reading the requirement.



1. We have two sites LAN-1 & LAN-2 both connecting to two different ISP's 1 & 2.

2. Both the Sites are inter linked using STM-1.

3. Both the Sites has got a different set of Routable IP Networks.

4. Both the Sites will have a Catalyst 65XX on the core i.e. LAN-1 LAN-2.



1. In the Event of an ISP Failure in any of the Sites the Other Site completeley fall back on the other Site whic is working.

2. In the Event of any Hardware i.e. Router Running the Border Gateway fails the reachability should still go through the Other Site.



1. Will the Physical setup depicted in the diagram is good enough?

2. What should we be doing on the Border Gateways to make this work.

3. What the ISPs should be doing on thier Border Gateways to make this work.

Thanks in Advance,

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thomas.anthony Tue, 08/14/2007 - 06:30
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Need more info -

Is MAN link (STM-1) is used for interoffice traffic also or just used only for back up?

Are you planning to leak your route to ISP?

You want the entire route (internet route) from your ISP to inject in to your routing table,

What is the router model you have?

subra4u Tue, 08/14/2007 - 06:55
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1. The STM-1 Link is converted to a Fast Ethernet (for the routers at either side it is just a L2 connectivity).

2. We will have Inter Site Traffic as well.

3. We have 7200 and 3825 for the Border Gateways. 3640's for the Inter Site link.

4. We are loooking for ISP redundancy meaning our Servers are available on the Internet even when there is a link or Router failure.

Thanks for your Support & response

Danilo Dy Tue, 08/14/2007 - 07:41
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Can the inter-site link be able to connect directly to eBGP routers? If not, leave the 3640 for inter-site link.

You need to run direct iBGP between eBGP routers (7200 and 3825) thru inter-site link which can be achieved using GRE.

Do you really need to run BGP in 6500? For faster convergence in the LAN and between sites, you can try using OSPF.



subra4u Tue, 08/14/2007 - 08:56
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Thanks for the Response.

We are adding additional 3640's Routers just to build resilency even when there is a Border Gateway Router Failures. We can run OSPF but part of our other network run EIGRP.

But how do we get the ISP redundancy to work in the event of a Border Gateway Router Hardware Failure with OSPF or EIGRP.

Thanks in Advance,

Pavel Bykov Tue, 08/14/2007 - 09:10
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1. If you are using EIGRP, do not use OSPF. Use what you have already have implemented. He meant some IGP that has fast convergence. When you redistribute default information from BGP to IGP (to EIGRP in your example) it will find the closest exit for all the unknown addresses and if one side fails, it will automatically converge to the other side.

2. To leak your routes to ISPs you can use conditional advertising. In case of failure, the other side can leak the routes that were on the failed side. Check out the following document:


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