MPLS and MAN as a backup.

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Mar 10th, 2009
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I need help understandind. Let's say we have to locations that have MAN connection and running EIGRP and iBGP between each other.No will connect those two to MPLS. We will be getting routes from 3rd location. So the traffic that is destined to 3rd location will go straight to that location (eBGP). My question is now: Let say we will advertise same subnets from those 2 locations(that have MAN and EIGRP on it)..second router will learn from MPLS cloud that same subnet that it learns throguh EIGRP is also in the cloud..and as eBGP AD is 20..it will try to use that path to reach subnets behind second location - not desirable-it should go through MAN connection choosing EIGRP. How to acomplish that? Am I right thinking this will work this way? Please help.

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Giuseppe Larosa Tue, 03/10/2009 - 11:19
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Hello Mateusz,

route summarization can help:


the most specific route to destination is used regardless of admin distance and metric.


So you need to advertise less specific routes over the eBGP sessions:


the command to use is


router bgp 100

aggregate-address + option summary-only


see


http://www.cisco.com/en/US/docs/ios/iproute/command/reference/irp_bgp1.html#wp1011467


in this way an eBGP route is used only when the most specific EIGRP route is missing


Hope to help

Giuseppe


Matt qomat Tue, 03/10/2009 - 14:22
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Yes. That is one thing I was thinking about and that is propably best approach to advertise more specifics from each of those locations. Let say now one of those routers will send specific route to the cloud..second router learns same IP from EIGRP on the MAN and eBGP..which path will it choose ? Looks like eBGP so it is still not preferred.

Giuseppe Larosa Tue, 03/10/2009 - 14:51
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Hello Mateusz,

the key point is that in the eBGP you need to advertise only aggregate routes.

the specific routes need to be confined in the EIGRP domain.


the aggregate address should work also on routes imported from EIGRP.


Hope to help

Giuseppe


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