[j-nsp] Only announce BGP learned networks

Patrick Okui pokui at psg.com
Wed Feb 22 09:25:59 EST 2012

On Wed Feb 22 16:18:18 2012, Phil Mayers wrote:
>> Recommendations in the industry have been to equalize these
>> values to avoid issues.
> Interesting. I'd not heard that one before. What's the rationale?

Well, apart from l3vpns you'll typically want to have your 
infrastructure addresses in your IGP and "internet/customer" addresses 
in BGP. Default AD of 20 for eBGP in IOS means you'll believe an 
advertisement from an external AS before say an OSPF or ISIS one for 
the same exact prefix.[*]

Also, IGPs mark "external routes" as those received from outside the 
protocol's routing domain. In that case it does make sense to have 
different ADs for internal and external routes (helps somewhat with 
redistribution). BGP is different - iBGP is just a session with someone 
in your AS. Says absolutely nothing about the origin of the advertised 
route. As such different ADs for iBGP and eBGP don't make sense.

Put those two together and there's good reason to set the AD for both 
iBGP and eBGP to say 200 (the default AD for iBGP and higher than any 
IGP). IMHO Juniper's default preference settings make more sense.

[*] yes I know, in practice you'll probably also filter out stuff 
you're originating.

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