|
In the example depicted by the figure,
RTB speaks EBGP to RTA, which is a different AS, and IBGP to RTC,
which resides in the same AS. To start the EBGP process with RTA,
use the following commands:
RTB(config)#router bgp 200
RTB(config-router)#neighbor 10.1.1.2 remote-as 100
Note that the neighbor
command's remote-as
value, 100, is different from the AS number specified by the router
bgp command (200). Because
the two AS numbers are different, BGP will start an EBGP connection
with RTA. Communication will occur between autonomous systems.
The commands to configure IBGP are
essentially the same as those to configure EBGP, except for the
possible addition of the update-source
interface keyword.
RTB(config)#router bgp 200
RTB(config-router)#neighbor 172.16.1.2 remote-as 200
RTB(config-router)#neighbor 172.16.1.2 update-source loopback 0
The remote-as
value (200) is the same as RTB's BGP AS number, so BGP recognizes
that this connection will occur within AS 200. It attempts to
establish an IBGP session. In reality, AS 200 is not a remote AS at
all; it is the local AS because both routers reside there. For
simplicity, the keyword remote-as
is used when configuring both EBGP and IBGP sessions.
Note also the second neighbor
command, which is used to assign an optional parameter to be used
when communicating with that neighbor. It is typical to use multiple
neighbor commands for the same BGP neighbor, each specifying a
particular BGP option.
In this example, the option specified
is update-source loopback 0.
If multiple pathways to the neighbor exist, then the router can use
any IP interface to speak BGP with that neighbor. The update-source
loopback 0 command
is used to instruct the router to use interface loopback 0 for TCP
connections. This command is typically used in all IBGP
configurations. Without this command, BGP routers can use only the
closest IP interface to the peer. The capability to use any
operational interface provides BGP with robustness in case the link
to the closet interface fails. Because EBGP sessions are typically
point-to-point, there is no need to use this command with EBGP.
Returning to the example
configuration, assume that the following route appears in RTB's
table:
O 192.168.1.0/24 [110/74] via
10.2.2.1, 00:31:34, Serial2
RTB learned this route via an IGP
(OSPF). AS 200 uses OSPF internally to exchange route
information. Can RTB advertise this network via BGP? Certainly.
Redistributing OSPF into BGP will work, but the BGP network
command will do the same thing:
RTB(config-router)#network
172.16.1.0 mask 255.255.255.252
RTB(config-router)#network 10.1.1.0 mask 255.255.255.252
RTB(config-router)#network 192.168.1.0
The first two network
commands above include the mask
keyword, so only a particular subnet is specified. The third network
command results in the OSPF
route being advertised by BGP without redistribution. Remember that
the BGP network
command works differently than the IGP network
command.
 |
 |
Interactive
Lab Activity
(Flash,
372 kB) |
| |
The
purpose of this activity is to configure both
IBGP and EBGP sessions on the ISP1 router. All
other routers have already been configured for
BGP. |
|
|
|
|