selection of ebgp and ibgp

[anitachoi3]

Dear expert

I study the BGP behaviour from the cisco document as below.

http://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a00800945bf.shtml#conf1

I have one question about the section "Verification of Inbound Traffic from AS 10 to AS 11", below please find the result of "show" command.

===== result output ===

R103# show ip bgp

BGP table version is 4, local router ID is 10.10.34.3

Status codes: s suppressed, d damped, h history, * valid, > best, i - internal

Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path

*> 10.10.34.0/24    0.0.0.0                  0         32768 i

*> 192.168.11.0     10.10.13.1               0             0 11 i            

!--- The next hop is R101.

*  192.168.12.0     10.10.13.1               0             0 11 11 11 11 i  

*>i                 10.10.34.4               0    100      0 11 i            

!--- The next hop is R104.

R103# show ip route

!--- Output suppressed.

Gateway of last resort is not set

B    192.168.12.0/24 [200/0] via 10.10.34.4, 00:04:46 

!--- The next hop is R104.

B    192.168.11.0/24 [20/0] via 10.10.13.1, 00:04:46  

!--- The next hop is R101.

======== end ========

The question is that why the next hop 10.10.34.4 to be selected (it should be ibgp, the AD is 200); the next hop 10.10.13.1 not to be selected (it should be ebgp, the AD is 20 but longer as path only). Please advice.

rdgs

[Thotsaphon Lueangwattanaphong · Accepted solution]

Hi,

  BGP has a long length decistion to select best path to be installed in RIB. In regarding your question,R3 should think of selecting 192.168.12.0 network as follow:

1. R3 can reach the next-hop 10.10.13.1 and 10.10.34.4.  Sure it's all connected network.

2. R3 compares a highest weight value for both paths. Well they are  0 so it's a tie.

3. R3 compares a highest local preference value for both paths. Well they are 100. so it a tie

4. Route originated by local router? no 192.168.12.0/24 is not originated by R3. Can't use this as a condition

5. R3 compares a shortest AS path. R3 routes 192.168.12.0/24 via R4 AS-Path =1  and via R1 AS-Path =4. So R4 wins.

  You can check values associated with the prefix by using "show ip bgp 192.168.12.0 255.255.255.0" on R3 and compare it.

  Comparing iBGP and EBGP  is a condition as well but not a first condition.

  Just check out this link: http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094431.shtml

HTH,

Toshi