CCNA Complete Guide 2nd Edition.pdf - Cisco Learning Home

RT1
RT2 RT3
Figure 23-7A: RT1 with
Multipoint (Sub)interface
Figure 23-7: Split Horizon in Frame Relay Networks
- Split horizon defines that a routing update received from an interface cannot be readvertised back
to that same interface, hence this prevents RT1 (with a multipoint [sub]interface) from
forwarding the routing updates received from RT2 to RT3 and vice versa. Migration to point-topoint
subinterfaces configuration is recommended to overcome this problem. By having each
subinterface in a separate subnet, routing updates received from a subinterface can be propagated
to other subinterfaces, even though they belong to the same physical interface.
- Another possible solution is disable split horizon on a particular multipoint [sub]interface.
The no ip split-horizon eigrp {as-num} interface subcommand disables split horizon for
EIGRP on a particular multipoint [sub]interface.
Note: Changing the split horizon configuration of an interface would reset EIGRP neighbor
relationships with other routers connected to the interface.
- Note: EIGRP is categorized as a distance-vector routing protocol hence it inherits the split
horizon feature of DV routing protocols.
- Frame Relay does require Layer 3 broadcast support, eg: routing updates. However, Frame Relay
cannot simply send a single frame to a Frame Relay network and get forwarded to multiple
destinations as like LAN broadcasts; it sends copies of the broadcast to each VC. Broadcast
overhead exists when sending routing updates in networks with large number of routes and VCs.
The issues regarding Frame Relay broadcasts are out of the scope of CCNA.
- ATM are normally being used to build the core of the Frame Relay networks due to the
advantages of ATM. Frame Relay Network and Service Interworking refers to the use of
ATM between 2 Frame Relay switches. Both types use ATM as a transport media.
FRF.5 Defines how a Frame Relay switch can convert a Frame Relay VC to an ATM VC and
back to a Frame Relay VC. The result is transparent to both the end devices (routers).
FRF.8 Defines how 2 end devices (routers) can communicate when a router is connected to a
Frame Relay network and another one is connected to an ATM network.
162
RT1
RT2 RT3
Figure 23-7B: RT1 with
Point-to-Point Subinterfaces
00:04:45: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 100: Neighbor 200.1.1.2 (Serial0/0)
is down: split horizon changed
00:05:40: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 100: Neighbor 200.1.1.2 (Serial0/0)
is up: new adjacency
Copyright © 2008 Yap Chin Hoong
yapchinhoong@hotmail.com