[nsp] ospf default routes and bgp injection

[Steve Lim]

Here's an easy reference (text fishing at Cisco.com is annoying):

Book: CCNP Self Study: BSCI by Paquet, Chp 8Advertising Networks into 
BGP, page 511-513.

The segment referenced above simply declares that it's a bad idea. But 
the paragraph doesn't give exampls.

Redistributing IGP into BGP has bad repercussions specifically if your 
IGP carries non-aggregated routes. The problem becomes acute when your 
link-state protocol detects flapping links. This causes the IGP to 
recalculate SPF (or appropriate algorithm) and announce this into BGP. 
BGP in turn adhusts it's tables. For Service Provider type networks, 
5-10 flapping links can occur at any one time. Each subsequent flapping 
link causes an exponential (not incremental) change in IGP and BGP 
response. A prolonged non-converged network results in routing loops.

Hope my simplified explanation helps.

Nick Kraal wrote:

> Following up on this one. We all know that the IGP of choice used should
> carry only infrastructure networks and kept as small as possible. The rest
> should be announced via iBGP. Is there a document that explains this
> rationale clear --having some difficulty convincing some folks.
> 
> Thanks in advance,
> 
> -nick/
> 
> ----- Original Message ----- 
> From: "Gert Doering" <gert at greenie.muc.de>
> To: "Christopher J. Wolff" <chris at bblabs.com>
> Cc: <cisco-nsp at puck.nether.net>
> Sent: Thursday, April 01, 2004 5:49 AM
> Subject: Re: [nsp] ospf default routes and bgp injection
> 
> 
> 
>>Hi,
>>
>>On Wed, Mar 31, 2004 at 09:58:12AM -0700, Christopher J. Wolff wrote:
>>
>>>I've considered ibgp as an end-to-end solution however never went so far
> 
> as
> 
>>>to implement it.
>>
>>Completely overkill in this network.  If there's no BGP speaker "down the
>>road", there is not much use in having all those routers actually carry
>>BGP information.
>>
>>
>>>My concern is making sure that the entire routing table
>>>isn't propagated all the way to the edge device, which could be
> 
> something
> 
>>>minimal like a 2611XM.  Any thoughts?  Thank you for your advice.
>>
>>Have the BGP speakers distribute an OSPF (or EIGRP, or even RIP :) )
>>default route to the smaller boxes.
>>
>>gert
>>-- 
>>USENET is *not* the non-clickable part of WWW!
>>
> 
> //www.muc.de/~gert/
> 
>>Gert Doering - Munich, Germany
> 
> gert at greenie.muc.de
> 
>>fax: +49-89-35655025
> 
> gert at net.informatik.tu-muenchen.de
> 
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>>
> 
> 
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-- 
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Steve Lim - Network Engineer (Michigan)
Corecomm -An ATX Communications Company
How does a fool and his money get together?