BGP 4-byte AS Numbers

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Nov 4th, 2009
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Ok, stupid question I know, but I want to be 100% sure before I commit to anything...

1. If I'm going to Dual home ISPs, I will NEED to have an ASN correct?

2. On my router, if I do router bgp ? I get the following options: Router(config)#router bgp ?

<1-65535> Autonomous system number

<1.0-XX.YY> 4 Octets Autonomous system number

Does this mean this router supports a 4-byte ASN? What does <1.0-XX.YY> mean? Are the 4-byte ASNs entered in a different format? Thanks!

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Peter Paluch Wed, 11/04/2009 - 14:19
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Hi Ryan,

I also suggest having a look at the great Jeff Doyle's technical blog. In the following article, he was discussing the issues of running 4B ASN numbers in BGP, and as always, his writing is crisp and clear - a pleasure to read.

Best regards,



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