6.2.6 Routing Issues in a Transit
Autonomous System All routers in the path between IBGP
neighbors, known as the transit path, must also be running BGP,
as illustrated in Figure . In this example, routers A, B, E,
and F are the only ones running BGP. Router B has an EBGP
neighbor statement for router A and an IBGP neighbor statement
for router E. Router E has an EBGP neighbor statement for
router F and an IBGP neighbor statement for router B. Routers C
and D are not running BGP. Routers B, C, D, and E are running
OSPF as their IGP. Network 10.0.0.0 is owned by AS 65101 and is
advertised to router B via an EBGP session. Router B advertises
it to router E through an IBGP session. Routers C and D never
learn about this network, because it is not redistributed into
the local routing protocol (OSPF), and routers C and D are not
running BGP. If router E advertises this network to router F in
AS 65103, and router F starts forwarding packets to network
10.0.0.0 through AS 65102, router E would send the packets to
its BGP peer, router B. However, to get to router B, the
packets must go through router C or D, but those routers do not
have an entry in their routing tables for network 10.0.0.0.
Thus, when router E forwards packets with a destination address
in network 10.0.0.0 to either router C or D, those routers
discard the packets. Even if routers C and D have a default
route pointing to the exit points of the autonomous system
(routers B and E), there is a good chance that when router E
sends a packet for network 10.0.0.0 to router C or D, those
routers may send it back to router E, which forwards it again
to router C or D, causing a routing loop. To solve this
problem, BGP must be implemented on routers C and D. In other
words, all routers in the transit path within the autonomous
system must be running BGP, and the IBGP sessions must be fully
meshed.
Content 6.3 Configuring
BGP 6.3.1 Basic BGP Configuration The
syntax of basic BGP configuration commands is similar to the
syntax for configuring internal routing protocols. However,
there are significant differences in how BGP functions. Use the
router bgp autonomous-system command to identify
to the router that any subsequent subcommands belong to this
routing process. This command also identifies the local
autonomous system in which this router belongs. The router
needs to be informed of the autonomous system so that it can
determine whether the BGP neighbors to be configured next are
IBGP or EBGP neighbors. Figure displays the parameter for the
router bgp command. The router bgp command alone
does not activate BGP on a router. You must enter at least one
subcommand to activate the BGP process. Web Links
Configuring BGP
http://www.cisco.com/univercd/cc/td/doc/product/
software/ios122/122cgcr/fipr_c/ipcprt2/1cfbgp.htm
Content 6.3 Configuring BGP
6.3.2 Activate a BGP Session Use the
neighbor ip-address remote-as
autonomous-system command to activate a BGP session for
external and internal neighboring routers. This command
identifies a peer router with which the local router
establishes a session. Figure displays the parameters for this
command. Note
A peer group is a group of BGP
neighbors that all have the same update policies. Peer groups
are described later in this lesson. The address is the
destination address for all BGP packets going to this
neighboring router. The address must be reachable, because BGP
attempts to establish a TCP session and exchange BGP updates
with the device at this IP address. The autonomous system
number identifies whether this neighbor is an EBGP or IBGP
neighbor. If the number is the same as the autonomous system
number for this router, that neighbor is an IBGP neighbor, and
the IP address listed in the neighbor command does not
have to be directly connected. If the number is different, the
neighbor is an EBGP, and the address in the neighbor
command must be directly connected by default. In Figure ,
router A in AS 65101 has two neighbor statements. Router A
knows that router C (neighbor 192.168.1.1 remote-as 65102) is
an external neighbor, because AS 65102 in the neighbor
statement for router C does not match the autonomous system
number of router A, which is AS 65101. Router A can reach AS
65102 via 192.168.1.1, which is directly connected to router
A. Neighbor 10.2.2.2 (router B) is in the same autonomous
system as router A. The second neighbor statement on router A
defines router B as an IBGP neighbor. AS 65101 runs EIGRP
between all internal routers. Router A has an EIGRP path to
reach IP address 10.2.2.2. As an IBGP neighbor, router B can
be multiple routers away from router A.
Content
6.3 Configuring BGP
6.3.3 Shutting Down a BGP Neighbor Use the
neighbor ip-address shutdown command to
administratively shut down and re-enable a BGP neighbor. If you
implement major policy changes to a neighboring router and you
change multiple parameters, you must administratively shut down
the neighboring router, implement the changes, and then bring
the neighboring router back up with the no neighbor
ip-address shutdown command.
Content
6.3 Configuring BGP
6.3.4 BGP Configuration Considerations The BGP
neighbor statement informs the router of the destination IP
address for each update packet. The router must decide which IP
address to use as the source IP address in the BGP routing
update. When a router creates a BGP packet for a neighbor, it
checks the routing table for the destination network to reach
that neighbor. The IP address of the outbound interface, as the
routing table indicates, is used as the source IP address of
the BGP packet. This source IP address must match the address
in the corresponding neighbor statement on the other router.
Otherwise, the routers will not be BGP peers because they are
not able to establish a BGP session.
Content
6.3 Configuring BGP
6.3.5 IBGP Peering Issue To establish the IBGP
session between routers A and D, as shown in Figure , which
neighbor IP address should be used? The problem is as follows.
If router D uses neighbor 10.3.3.1 remote-as 65102, but
router A is sending the BGP packets to router D via router B,
the source IP address is 10.1.1.1. When router D receives this
BGP packet via router B, it does not recognize this BGP packet
because 10.1.1.1 was not configured as a neighbor of router D.
Therefore, the IBGP session between routers A and D cannot be
established. A solution is to establish an IBGP session using a
loopback interface when there are multiple paths between IBGP
neighbors.
Content 6.3
Configuring BGP 6.3.6 BGP neighbor
update-source Command The update-source option of
the neighbor command overrides the default source IP
address used for BGP packets. It is necessary to tell the
router which IP address to use as the source address for all
BGP packets if you want to use a loopback interface instead of
the physical interface. If you do not use the
update-source option, an announcement going to a
neighbor uses the IP address of the exiting interface as the
source address for a packet. When a router creates a packet,
whether it is a routing update, a ping, or any other type of IP