BGP Community additive out of order?

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May 2nd, 2010
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From cli output:

13237 3356 3549 22047 from (
      Origin IGP, localpref 100, valid, external
      Community: 3356:2 3356:22 3356:86 3356:501 3356:666 3356:2065 3549:4852 3549:34152 13237:44049 13237:46067
      Last update: Fri Apr 30 06:28:19 2010

the community items sequence doesn't match the as-path( 3356 - 3549 - 13237 vs. 13237 3356 3549... )

if they are using additive community features, what would cause this?


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Giuseppe Larosa Sun, 05/02/2010 - 14:12
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Hello Gviewer,

the set of communities is converted in a string to create the output for the show, the order does not count and it is not guaranteed as you have noticed

for this reason is difficult to match on extended community list using regular expression

Hope to help


gviewer123 Sun, 05/02/2010 - 19:11
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Hi giuslar,

     Thanks for your reply.

     I am not trying to use some complex filters here, just wondering what would lead to this. Looks like

some routers ( I tested several IOS version from 11.3T to 12.x ) follows the "append" rule, if the aspath

is "3 2 1", then the communtiy string would looks like "1"xxx 2:yyy 3:zzz", but in Routing TCP/IP vol2

(2001 Edition)example 3-141, the order is inverted, new additive community is "prepended". So, I am

thinking is it because some differences in IOS code ? But why bother to change it?

Giuseppe Larosa Mon, 05/03/2010 - 13:03
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Hello Gviewer,

in example 3-141 of TCP/IP vol.II is shown a sh ip bgp from a router internal to AS 100 named colorado.

AS path attribute is 2000 because AS 100 is not added to AS path in an iBGP session within AS 100 (it will be added when sending the update to another AS)

so community 2000:xx is set by eBGP peer (austria)  and later BGP community 100:yy is set by iBGP peer (Idaho) of the router in which the sh ip bgp is been performed (colorado)

to be noted the eBGP peer of the router of example 3-141 will see an AS path of  100 2000 with BGP community 100:yy 2000:xx and this would be more similar to what you have seen in your tests.

The key point is that 100:xx is added by an iBGP peer.

Hope to help


gviewer123 Mon, 05/03/2010 - 20:00
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That is my question, when I repeat the example on my routers, the aspath sending to next AS is "100 2000", but the communtiy

is "2000:100 100:2000", the order is inverted, I change my IOS from 11.3T , 12.1t ... 12.3, it's still the same, nothing appears

in the order of that example.

The community order is not that import, yet I am curious, and try to find out in which scenario or configuration the order is changed.


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