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View Full Version : Challenge: Why the BGP Neighbor is not UP?


admin
06-15-2009, 12:28 AM
http://www.netbraintech.com/share/why_the_bgp_neighbor_is_not_up.gif
Routers R1 and R3 are configured as following to be BGP neighbor. And router R2 has two static routes so that R1 is able to ping the interface loopback0 of R3 and R3 is able to ping the loopback0 of R1.

R1
interface loopback0
ip address 1.1.1.1 255.255.255.255
!
interface fastethernet0/0
ip address 12.1.1.1 255.255.255.0
!
router bgp 65001
no synchronization
bgp log-neighbor-changes
network 1.1.1.1 mask 255.255.255.255
neighbor 3.3.3.3 remote-as 65001
no auto-summary
!
ip route 0.0.0.0 0.0.0.0 12.1.1.2



R3
interface loopback0
ip address 3.3.3.3 255.255.255.255
!
interface fastethernet0/0
ip address 23.1.1.3 255.255.255.0
!
router bgp 65001
no synchronization
bgp log-neighbor-changes
network 3.3.3.3 mask 255.255.255.255
neighbor 1.1.1.1 remote-as 65001
no auto-summary
!
ip route 0.0.0.0 0.0.0.0 23.1.1.2




The "show ip bgp neighbors" result of R1 is displayed as following:

The BGP neighbors are not successfully established. Please select TWO possible reasons:

A. Missed the ebgp multi-hop configuration.
B. Missed the update-source configuration.
C. The no synchronization command was configured.
D. Default route was used to access the BGP neighbor.

http://www.netbraintech.com/share/why_the_bgp_neighbor_is_not_up2.gif
To view the answer, please click Here (http://www.netbraintech.com/netbrain-solutions/one_skill_at_a_time/one_skill_at_a_time_no2.php)

Harrison
06-17-2009, 05:59 AM
The answer D looks interesting, I never heard before. After a experiment, I saw the result was really like this.

Jason Bricks
06-17-2009, 06:01 AM
Can the BGP neighbor be established if the hold time and keepaive of the two peers are mismatched ?

Mike Cotrone
06-17-2009, 06:04 AM
The iBGP Peers are using each other's loopback address for the source peering address. You have to use the "Update Source Loopback0" command on each iBGP peer to properly establish the peer and pass iBGP routes. Also you have to have valid routes through R2.

Wouldn't be A: This is not a eBGP Configuration but an iBGP configuration
Wouldn't be C: You usually want no sync so iBGP doesn't rely on the IGP RIB to pass iBGP routes

Chris
06-17-2009, 06:06 AM
What about D?

Andy Fields
06-17-2009, 06:07 AM
OPNET Sentinel or NetDoctor would automatically find and alert you on the issue! But maybe that is cheating for this academic exercise :)

jeevan
10-29-2011, 11:17 AM
because their is a use of default route ...
for neighborship to UP ,, there is a need to define a particular route / static route for that neighbor router IP ......... on any one of the neighbor :-)

bhupen1979@gmail.com
04-26-2012, 08:36 AM
Answer is B, update-source is not configured

docwavers
05-05-2012, 12:51 AM
B would be the best answer although D may also cause you issues, it should work with this, but you would be better off running an IGP to cover your Loopback and L3 links between devices.