At times, it is useful to have a multipoint network, such as Frame Relay, behave as if each connection
were a point-to-point link. The network example in Figure 29.6 has two connections
from one location (multipoint); this type of setup can lead to network problems if not thoroughly
understood. This multipoint configuration experiences problems primarily because of
the way it handles routing updates. In distance-vector routing protocols, there is a mechanism
known as split horizon. Split horizon states that it is never advantageous to send routing information
back out on the interface through which that information was learned. Or, simply put,
“Don’t tell me what I told you.” This is used to stop possible routing loop problems. Consider
the split-horizon implication of Figure 29.6.
In this figure, Router B sends a routing update to Router A, telling it about its directly connected
networks. Router A receives the routing information on its serial 0 interface and modifies
its routing table. Router A does not send the routing information back out serial 0 because of split
horizon. Because the routing information cannot be sent back out serial 0, Router C never learns
the networks off Router B. The networks are unreachable from Router C because Router C has
not heard of Router B’s directly connected networks.
The problem in Figure 29.6 is that there is only one physical interface and there are two virtual
circuits. The solution is to create a logical interface for each circuit, which solves the split
horizon issues. A subinterface is a logical interface within the router that is mapped to a particular
DLCI. When you set up subinterfaces, the interface previously configured for multipoint
will now appear as two point-to-point interfaces to the router. This would change the previous
example, as shown in Figure 29.7.
In this figure, Router A learns of Router B’s networks on the Serial 0.1 subinterface. Without
violating the split-horizon rule, Router A can send all the network information out on subinterface
Serial 0.2 to Router C.
To configure a subinterface on an interface, use the interface type.subinterface-number
[point-to-point | multipoint] command. For illustration purposes, configure Router A
with a subinterface. (The router commands are shown next.) Both types of subinterfaces that can
be configured appear in this example: point-to-point and multipoint. Point-to-point is used when
each PVC is a separate subnet. Multipoint is used when all PVCs use the same subnet.
FIGURE 2 9 . 7 Split-horizon issues with subinterfaces
PVC 16
PVC 17
1
2
Routing
update
A
B
C
Logical
interface
Physical
interface
Subnet 1
Subnet 2
Router A
Router B
Router C
Also notice in the following configuration that you can use any subinterface number, but
for administration purposes the DLCI number can be used. The subinterface number is only
locally significant:
RouterA(config)#interface serial0.?
<0-4294967295> Serial interface number
RouterA(config)#interface serial0.16 ?
multipoint Treat as a multipoint link
point-to-point Treat as a point-to-point link
RouterA(config)#interface serial0.16 point-to-point
RouterA(config-subif)#ip address 192.168.1.1 255.255.255.0
RouterA(config-subif)#frame-relay interface-dlci 16
RouterA(config-subif)#exit
RouterA(config)#interface serial0.17 multipoint
RouterA(config-subif)#ip address 192.168.2.2 255.255.255.0
RouterA(config-subif)#frame-relay map ip 192.168.2.1 17 broadcast
The configuration of Router A will now look like this:
RouterA#show running-config
Building configuration...
Current configuration:
!
version 11.3
!
hostname RouterA
!
interface Serial0
no ip address
encapsulation frame-relay
!
interface Serial0.16 point-to-point
ip address 192.168.1.1 255.255.255.0
frame-relay interface-dlci 16
!
interface Serial0.17 multipoint
ip address 192.168.2.2 255.255.255.0
frame-relay map ip 192.168.2.1 17 broadcast
!
end
RouterA#
This configuration specifies which DLCI is associated with which subinterface. This is necessary
because the router has no way of determining which particular DLCI should be associated
with which subinterface.
Note:
Many people find the point-to-point subinterface configuration easier and less
prone to routing errors than a physical multipoint configuration.