To pass the BSCI exam, earn your CCNP certification, and
become an outstanding networker, you've got to master the
many details of BGP - and trust me, there are a lot of details
to master! Before you get into the more advanced features
of BGP, you should have the fundamentals down cold, and one
of those fundamentals is knowing the BGP adjacency states.
This will allow you to successfully analyze and troubleshoot
BGP peer relationships.
In the following example, a BGP peering is being created
between R1 and R3. R1(config-router)#neighbor 172.12.123.3
remote-as 200
BGP speakers do not have to be in the same AS to become peers.
To verify that the remote BGP speaker has become a peer, run
show ip bgp neighbor.
R1#show ip bgp neighbor
BGP neighbor is 172.12.123.3, remote AS 200, external link
BGP version 4, remote router ID 0.0.0.0
BGP state = Active
Last read 00:01:39, hold time is 180, keepalive interval
is 60 seconds
Received 0 messages, 0 notifications, 0 in queue
Sent 0 messages, 0 notifications, 0 in queue
Route refresh request: received 0, sent 0
Default minimum time between advertisement runs is 30 seconds
The output here can be a little misleading the first time
you read it. The first highlighted line shows 172.12.123.3
is a BGP neighbor, is located in AS 200, and is an external
link, indicating that the neighbor is in another AS entirely.
The second highlighted line shows the BGP state as Active.
This sounds great, but it actually means that a BGP peer connection
does not yet exist with the prospective neighbor. Before we
continue with this example, let’s look at the different
BGP states:
Idle is the initial state of a BGP connection. The BGP speaker
is waiting for a start event, generally either the establishment
of a TCP connection or the re-establishment of a previous
connection. Once the connection is established, BGP moves
to the next state.
Connect is the next state. If the TCP connection completes,
BGP will move to the OpenSent stage if the connection does
not complete, BGP goes to Active.
Active indicates that the BGP speaker is continuing to create
a peer relationship with the remote router. If this is successful,
the BGP state goes to OpenSent. You’ll occasionally see
a BGP connection flap between Active and Connect. This indicates
an issue with the physical cable itself, or with the configuration.
OpenSent indicates that the BGP speaker has received an Open
message from the peer. BGP will determine whether the peer
is in the same AS (iBGP) or a different AS (eBGP) in this
state.
In OpenConfirm state, the BGP speaker is waiting for a keepalive
message. If one is received, the state moves to Established,
and the neighbor relationship is complete. It is in the Established
state that update packets are actually exchanged.
So even though the show ip bgp neighbor output indicated
that this is an Active neighbor relationship, that’s
not as good as it sounds. Of course, the reason the peer relationship
hasn’t been established is that we haven’t configured
R3 yet!
R3(config)#router bgp 200
R3(config-router)#neighbor 172.12.123.1 remote-as 100
Verify the peer establishment with show ip bgp neighbor:
R3#show ip bgp neighbor
BGP neighbor is 172.12.123.1, remote AS 100, external link
BGP version 4, remote router ID 172.12.123.1
BGP state = Established, up for 00:01:18
Last read 00:00:17, hold time is 180, keepalive interval
is 60 seconds
Neighbor capabilities:
Route refresh: advertised and received(old & new)
Address family IPv4 Unicast: advertised and received
Received 5 messages, 0 notifications, 0 in queue
Sent 5 messages, 0 notifications, 0 in queue
Route refresh request: received 0, sent 0
Default minimum time between advertisement runs is 30 seconds
Local host: 172.12.123.3, Local port: 179 (BGP uses TCP Port
179)
Foreign host: 172.12.123.1, Foreign port: 11007
The peer relationship between R1 and R3 has been established!
About the Author:
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage,
home of free CCNA and CCNP tutorials, The Ultimate CCNA Study
Package, and Ultimate CCNP Study Packages.
For a FREE copy of his latest e-books, “How To Pass
The CCNA” and “How To Pass The CCNP”, visit
the website and download your free copies. You can also get
FREE CCNA and CCNP exam questions every day! Get your CCNA
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