Like TCP, BGP is connection-oriented. An underlying connection between two BGP speakers is established before any routing information is exchanged. This connection takes place on TCP port 179 . As with EIGRP and OSPF, keepalive messages are sent out by the BGP speakers in order to keep this relationship alive.
Once the connection is established, the BGP speakers exchange routes and synchronize their tables. After this initial exchange, a BGP speaker will only send further updates upon a change in the network topology.
The IGP protocols that use Autonomous Systems, IGRP and EIGRP, require prospective neighbors to be in the same AS. This is not true with BGP. Routers can be in different Autonomous Systems and still exchange routes. The BGP neighbors do not have to be directly connected, and often are not, but do need to be able to reach the IP addresses they use in their neighbor statements.
A BGP peer that is in the same AS is referred to as an Internal BGP (iBGP) Peer, where a BGP peer in another AS is an External BGP (eBGP) Peer.
A sample iBGP configuration:
Router bgp 100
Neighbor 10.1.1.2 remote-as 100
A sample eBGP configuration:
Router bgp 100
Neighbor 10.1.1.2 remote-as 200
Cisco recommends that eBGP peers be directly connected, where iBGP peers generally will not be.
Before we get too much farther into BGP theory, let’s get a configuration started. You’ll use the router bgp command to configure a router as a BGP speaker. Right after that, the neighbor command will be used to identify this BGP speaker’s potential neighbors. (The terms "peer" and "neighbor" are interchangeable in BGP, but it's the neighbor statement that is used to statically define neighbors. BGP is not capable of discovering neighbors dynamically.)
While almost all of the neighbor options are just that -- optional -- you do have to specify the BGP AS of the remote router . BGP has no mechanism to dynamically discover neighbors. Remember, 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.
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!
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.
You'll learn as your progress through your CCNP and CCIE studies that BGP has a lot of options for the neighbor command. We'll look at these options in the next part of this Cisco CCNP BGP tutorial!
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage, home of free CCNP and CCNA tutorials, The Ultimate CCNA Study Package, and Ultimate CCNP Study Packages.
You can also join his RSS feed and visit his blog, which is updated several times daily with new Cisco certification articles, free tutorials, and daily CCNA / CCNP exam questions! Details are on the website.
For a FREE copy of his latest e-books, “How To Pass The CCNA" and “How To Pass The CCNP", just visit the website! You can also get FREE CCNA and CCNP exam questions every day! Pass the CCNP exam with The Bryant Advantage!
Was this article helpful to you? Leave a Public Comment or
Question:
This Article has been viewed 775
times.
Article added to SearchWarp.com on Tuesday, May 09, 2006 View other articles written by Chris Bryant CCIE 12933(13,693)
If you found this article interesting, you may want to check out:
Disclaimer: All information on this site is provided for informational purposes only! By no means is any
information presented herein intended to substitute for the advice provided to you by any health care or other professional
or organization.
