[j-nsp] Only announce BGP learned networks
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
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