CISCO: Border Gateway Protocol for Interdomain Routing
This section presents fundamental information about BGP, including the following topics:
•
Internal BGP
•External BGP
•BGP and Route Maps
•Advertising Networks
Routers that belong to the same AS and exchange BGP updates are said to be running internal BGP (IBGP), and routers that belong to different ASs and exchange BGP updates are said to be running external BGP (EBGP). With the exception of the neighbor ebgp-multihop router configuration command (described in the section "External BGP" later in this chapter), the commands for configuring EBGP and IBGP are the same. This case study uses the terms EBGP and IBGP as a reminder that, for any particular context, routing updates are being exchanged between ASs (EBGP) or within an AS (IBGP).
shows a network that demonstrates the difference between EBGP and IBGP.
Figure 12-1 EBGP, IBGP, and Multiple ASs
Before it exchanges information with an external AS, BGP ensures that networks within the AS are reachable. This is done by a combination of internal BGP peering among routers within the AS and by redistributing BGP routing information to Interior Gateway Protocols (IGPs) that run within the AS, such as Interior Gateway Routing Protocol (IGRP), Intermediate System-to-Intermediate System (IS-IS), Routing Information Protocol (RIP), and Open Shortest Path First (OSPF).
BGP uses the Transmission Control Protocol (TCP) as its transport protocol (specifically port 179). Any two routers that have opened a TCP connection to each other for the purpose of exchanging routing information are known as peers or neighbors. In , Routers A and B are BGP peers, as are Routers B and C, and Routers C and D. The routing information consists of a series of AS numbers that describe the full path to the destination network. BGP uses this information to construct a loop-free map of ASs. Note that within an AS, BGP peers do not have to be directly connected.
BGP peers initially exchange their full BGP routing tables. Thereafter, BGP peers send incremental updates only. BGP peers also exchange keepalive messages (to ensure that the connection is up) and notification messages (in response to errors or special conditions).
In , the following commands configure BGP on Router A:
router bgp 100
neighbor 129.213.1.1 remote-as 200
The following commands configure BGP on Router B:
router bgp 200
neighbor 129.213.1.2 remote-as 100
neighbor 175.220.1.2 remote-as 200
The following commands configure BGP on Router C:
router bgp 200
neighbor 175.220.212.1 remote-as 200
neighbor 192.208.10.1 remote-as 300
The following commands configure BGP on Router D:
router bgp 300
neighbor 192.208.10.2 remote-as 200
The router bgp global configuration command enables a BGP routing process and assigns to it an AS number.
The neighbor remote-as router configuration command adds an entry to the BGP neighbor table specifying that the peer identified by a particular IP address belongs to the specified AS. For routers that run EBGP, neighbors are usually directly connected, and the IP address is usually the IP address of the interface at the other end of the connection. (For the exception to this rule, see the section "EBGP Multihop," later in this chapter.) For routers that run IBGP, the IP address can be the IP address of any of the router’s interfaces.
Note the following about the ASs shown in :
•Routers A and B are running EBGP, and Routers B and C are running IBGP. Note that the EBGP peers are directly connected and that the IBGP peers are not. As long as there is an IGP running that allows the two neighbors to reach one another, IBGP peers do not have to be directly connected.
•All BGP speakers within an AS must establish a peer relationship with each other. That is, the BGP speakers within an AS must be fully meshed logically. BGP4 provides two techniques that alleviate the requirement for a logical full mesh: confederations and route reflectors. For information about these techniques, see the sections "Confederations" and "Route Reflectors," later in this chapter.
•AS 200 is a transit AS for AS 100 and AS 300—that is, AS 200 is used to transfer packets between AS 100 and AS 300.
To verify that BGP peers are up, use the show ip bgp neighbors EXEC command. Following is the output of this command on Router A:
RouterA# show ip bgp neighbors
BGP neighbor is 129.213.1.1, remote AS 200, external link
BGP version 4, remote router ID 175.220.212.1
BGP state = established, table version = 3, up for 0:10:59
Last read 0:00:29, hold time is 180, keepalive interval is 60 seconds
Minimum time between advertisement runs is 30 seconds
Received 2828 messages, 0 notifications, 0 in queue
Sent 2826 messages, 0 notifications, 0 in queue
Connections established 11; dropped 10
Anything other than state = established indicates that the peers are not up. The remote router ID is the highest IP address on that router (or the highest loopback interface, if there is one). Notice the table version number: each time the table is updated by new incoming information, the table version number increments. A table version number that continually increments is an indication that a route is flapping, thereby causing routes to be updated continually.
