cisco 7600 ip community-list 4-byte ASN

[Tyler Adams]

Hi,

I'm trying to get an ip community-list to accept a community with a 4-byte ASN. I'm running image c7600s72033-adventerprisek9-mz.152-4.S4a.bin on a cisco 7603 router.

When i try to input ip community-list standard NAMEDLIST permit 0:XXXXXX with the x's being my 4byte asn number, it says command not recognized. 

It did take my  neighbor Y.Y.Y.Y remote-as XXXXXX statement though.

I tried searching the feature list for the image as well as searching google and could not find much information on the subjet. Does any on know how I can fit a 4-byte ASN number in the ip community-list command or is it just not possible with this image?

[Peter Paluch · Accepted solution]

Tyler,

One more thing. Apart from the need to move to extended BGP communities, there may indeed be a problem in practically applying this solution as of yet. RFC 5668 introduces the 4-Octet AS Specific BGP Communities that are to be used similarly to standard BGP communities. However, these extended communities have a specific type that must be supported by IOS to actually allow you to configure and set them. Toying around on my Dynamips running 15.3(3)XB12 Advanced IP Services IOS, this type of extended community does not yet seem to be supported:

R1(config)# route-map SetComm
R1(config-route-map)# set extcommunity ?
  cost               Cost extended community
  rt                 Route Target extended community
  soo                Site-of-Origin extended community
  vpn-distinguisher  VPN Distinguisher

R1(config-route-map)# set extcommunity

Testing the IOSv 15.5(2)T from my VIRL does not seem to support this type of extended community, either. So I believe that momentarily, going with 2B communities is the only sensible workaround, and you should definitely raise a TAC support ticket requesting addition of the RFC 5668 extended communities to IOS features.

Best regards,
Peter

[Peter Paluch]

Hi Tyler,

I believe that what you request is principially impossible to do with standard BGP communities.

Standard BGP communities are defined in RFC 1997 as 4B-long quantities, traditionally expressed as AA:NN where AA was supposed to be the autonomous system number and NN was the internal policy selector. Note, however, that a 4B-long ASN (the AA part) would by itself consume the entire value of a standard BGP community and there would be no space left for the internal policy selector (the NN part). With the advent of 4B-long ASNs, the standard communities can no longer be used in the same way they have been used with classic 2B ASNs. RFC 6793 that describes 4B ASNs puts it down quite nicely in Section 5:

As specified in [RFC1997], when the high-order two octets of the community attribute is neither 0x0000 nor 0xffff, these two octets encode the AS number. Quite clearly, this would not work for a NEW BGP speaker with a non-mappable four-octet AS number. Such BGP speakers should use four-octet AS specific extended communities [RFC5668] instead.

So in order to use communities with 4B ASNs, you will need to migrate to extended communities instead.

Best regards,
Peter