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Thursday, August 29, 2013

BGP Protocol Configuration and Missing Prefix

when configuring protocol BGP - one of the boarders, I do not see prefixes that advertises my other Border ... In order ... 

There are two boarders are included in the AS78:
Point1 - announcement, received full-view;
Point2 - announcement, received the default.
Each of the boarders has two BGP session c:
ISP1, AS70;
ISP2, AS32.
Between Point1 and Point2 not connected via iBGP, all communication must be through the external connections.
On Point1 received full-view:
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up / Down State / PfxRcd 4 32 2632424 44442 43720971 0 0 4w0d 462061 4 70 1701047 36822 43720971 0 0 3w2d 464547
But I do not see in this table prefix
Point1 # sh ip bgp
% Network not in table
Communicated with specialists ISP, they have successfully received the prefix of Point2 and advertised to Point1.
It seems that he Point1, somehow excludes him from the table, the command "# sh ip bgp neighbors received-routes" I do not see that the prefix is received from a neighbor.
Maybe this is normal behavior and something needs tweaking to prefixes taken from their neighbors have the same AS, but as long as the documentation did not find anything about this situation. 

The reason point1 does not see is that this prefix is reject on point1 because of the AS path loop detection. This prefix is originated by AS78 and on reception point1 checks the AS path, sees its own AS in the path and drops it. This is not an issue if point1 have a default route coming from ISP1 and ISP2 in addition to the full view. The other way to fix it is to allow point1 to receive prefixes originated by AS78 using the "allowas-in" command. This command should be used carefully as it disabled the AS path loop detection. It should not be a problem with the right filtering in place.

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