why iBGP peers not send external routes between each other....

[will]

Hi, I am wondering why one of two locally connected IBGP peers is not advertising an external route to the other. RTRA is sending 9.1.1.0 to RTRB, but RTRB is not sending to RTRA. Both routers are getting this route from their locally connected ISP bgp peer (2 different ISPs). RTRA is modifying the local preference to be 150 (on inbound). This is passing to RTRB. Does local preference dictate that a local ibgp peer should not advertise its own routes back to other ibgp peers if it sees them having a higher local preference? I thought ibgp rtr tables would contain all paths with different local preferences. The IBGP config is simple and no route maps exist between the ibgp peers.

Thx in advance! Will

RTRA# sh ip bgp 9.1.1.0

BGP routing table entry for 9.1.1.0/24, version 168079102

Paths: (1 available, best #1, table Default-IP-Routing-Table)

Advertised to update-groups:

     2        

4323 2828 31911 <-----FROM LOCALLY CONNECTED ISP

   206.169.157.189 from 206.169.157.189 (168.215.52.162)

     Origin IGP, localpref 150, valid, external, best

     Community: 4323:1035 4323:2503 4323:50000

RTRA#sh ip bgp | i 9.1.1.0

*> 9.1.1.0   206.169.157.189               150     0 4323 2828 31911 i

RTRA#sh ip bgp neighbors 10.33.10.14 advertised-routes | i 9.1.1.0

*> 9.1.1.0   206.169.157.189               150     0 4323 2828 31911 i

RTRA#sh ip bgp | b 9.1.1.0                                      

*> 9.1.1.0   206.169.157.189               150     0 4323 2828 31911 i

RTRA BGP PARTNER COMMANDS:

neighbor 10.33.10.14 remote-as 15555

neighbor 10.33.10.14 description iBGP session to RTRB

neighbor 10.33.10.14 next-hop-self

===============================

RTRB# sh ip bgp 9.1.1.0

BGP routing table entry for 9.1.1.0/24, version 69606387

Paths: (2 available, best #1, table Default-IP-Routing-Table)

Not advertised to any peer

4323 2828 31911     <-----FROM LOCALLY CONNECTED IBGP PEER

   10.33.10.13 from 10.33.10.13 (65.192.100.2)

     Origin IGP, metric 0, localpref 150, valid, internal, best

7385 3356 17054 31911   <-----FROM LOCALLY CONNECTED ISP

   67.138.183.1 from 67.138.183.1 (209.63.112.123)

     Origin IGP, localpref 100, valid, external

     Community: 7385:950 7385:3356

RTRB#sh ip bgp | i 9.1.1.0

*>i9.1.1.0   10.33.10.13            0   150     0 4323 2828 31911 i

RTRB#sh ip bgp neighbors 10.33.10.13 advertised-routes | i 9.1.1.0

RTRB# (NOTHING ADVERTISED)

RTRB#sh ip bgp | b 9.1.1.0                                      

*>i9.1.1.0   10.33.10.13             0   150     0 4323 2828 31911 i

*             67.138.183.1                           0 7385 3356 17054 31911 i

RTRB BGP PARTNER COMMANDS:

neighbor 10.33.10.13 remote-as 15555

neighbor 10.33.10.13 description iBGP session to RTRA

neighbor 10.33.10.13 next-hop-self

[milan.kulik · Accepted solution]

Hi,

note: If a BGP router receives the same prefix from multiple sources, it advertises only the best one to its neighbors.

So it this case:

RTRB  would advertise the prefix

*>9.1.1.0   10.33.10.13            0   150     0 4323 2828 31911

But another rule is: a BGP router never advertises a prefix to a neighbor whose IP address is the same as the next-hop attribute of the prefix advertised. (BGP "split horizon").

And  this is your case!

HTH,

Milan