BGP - EIGRP redistribution problems

Unanswered Question
May 21st, 2007

I have the following topology:

ignore the ...'s. Had to put them there for formatting

ISPA............ISPB

|.....................|

RA ------------ RB

RA and ISPA are EBGP peers

RB and ISPB are EBGP peers

RA and RB are IBGP peers

ISPA - 172.16.1.1

RA - 172.16.1.2, 192.168.1.1

RB - 192.168.1.2, 10.1.1.2

ISPB 10.1.1.1

I want redistribute a default network, advertised from my ISP's, into my core. The core is running EIGRP.

I want clients that are closer to RA to exit via ISPA and clients closer to RB to exit via ISPB.

I want RA to redistribute the default from ISPA and RB to redistribute the default from ISPB. Then, my downstream clients can head toward the closest exit point.

Here is the problem:

ISPB is down.

RA learns the default route from the ebgp connection with ISPA and enters the route in it's routing table. It redistributes the default route into EIGRP.

RB leans the default route through EIGRP and enters the route into the routing table as an external EIGRP route.

Now, when ISPB come back up, RB shows the default route from ISPB in its BGP table, but RB will not replace the external EIGRP route currently in the routing table.

The default route RB learns from ISPB is from an EBGP connection and has an admin distance of 20 compared to the admin distance of 170 from the external EIGRP route it learns from RA. So, why won't RB update the routing table with the BGP route?

I have this problem too.
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sachinraja Mon, 05/21/2007 - 22:35

Hello

I'm really not sure if advertising the default routes via two different service providers, into your IGP, is a good option. for outgoing connections, it is always advicible to use the bgp policies like local preference and use route-maps which can selectively put the traffic on the respective outgoing routers... anyway, you can still have static default routes on your core and not redistribute it on your IGP.

Infact you can advertise defaults from your IGP routers inside your core and work on a hierarchical design and make sure the closest network reaches the appropriate routers.. I hope I'm inline with your problem.. if not pls correct me !!!!

Raj

loobitize Tue, 05/22/2007 - 05:40

Thanks for responding.

Why is it not advisable to advertise default routes from two different service providers into IGP?

There are firewalls between the core and the respective internet providors, so if I use a static default route on the core, and the firewall goes down, I would blackhole internet traffic. I actually have a pair of firewalls behind each provider, but lets assume that a pair goes down for some reason.

With regard to the heirarchical network, let me explain further how the network is layed out. ISPA is at data center A, ISPB is at data center B. All of the remote sites have equal cost paths to DCA and DCB. At the very least, I want the traffic orginating from the DC's to exit the closest point, though I may end up using route maps to force traffic from one remote site out DCA and traffic from another remote site out DCB.

Does that make sense?

vineetccnp Tue, 05/22/2007 - 06:11

Hi,

As far as i understand RA and RB are IBGP peers and from both the routers you are redistributing BGP default route in EIGRP network downstream.

My advice is make distribute list and stop accepting default routes in eigrp process, instead let router learn default route through IBGP.

Hope this will solve your problem.

sachinraja Tue, 05/22/2007 - 06:42

Hello David,

firewalls inbetween ? more complexity :) I was anyway mentioning that it is advisible to advertise default routes from the RA and RB router with default metrics isnt advisible. all the internal routers will learn the default with equal paths and there can be sub-optimal routing due to this. It is good if you can advertise the routes with different metrics into the IGP. eg, RA can inject default with metric 5 and RB with metric 30.

The Ebgp ruters which are connected to RA & RB can advertise defaults for your IBGP routers. hence, if the ISP A link goes down, it will stop advertizing the default and all the traffic can go via ISP B link. if the firewall fails, you will not get the BGP default routes nor any other routes from the ISP, which will make traffic flow through the second link..

Follow the URL given below:

http://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a008009487d.shtml

might help you. hope this helps.. all the best. rate replies if found useful..

Raj

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