How should a router that is being used in a Frame Relay network be configured to avoid split
horizon issues from preventing routing updates?
A.
Configure many sub-interfaces on the same subnet.
B.
Configure a single sub-interface to establish multiple PVC connections to multiple remote router
interfaces.
C.
Configure a separate sub-interface for each PVC with a unique DLCI and subnet assigned to
the sub-interface.
D.
Configure each Frame Relay circuit as a point-to-point line to support multicast and broadcast
traffic.
Explanation:
Point-To-Point Subinterfaces:
The concept of subinterfaces was originally created in order to better handle issues caused by
split-horizon over Non-Broadcast Multiple Access (NBMA) networks (e.g. frame relay, X.25) and
distance-vector based routing protocols (e.g. IPX RIP/SAP, AppleTalk). Split-horizon dictates that
a routing update received on an interface cannot be retransmitted out onto the same interface.
This rule holds even if the routing update was received on one frame relay PVC and destined to
retransmit out onto another frame relay PVC. Referring to figure 2, this would mean that sites B
and C can exchange routing information with site A, but would not be able to exchange routing
information with each other. Split-horizon does not allow Site A to send routing updates received
from Site B on to Site C and vice versa.
Note: For TCP/IP, Cisco routers can disable split-horizon limitations on all frame relay interfaces
and multipoint subinterfaces and do this by default. However, split-horizon cannot be disabled for
other protocols like IPX and AppleTalk. These other protocols must use subinterfaces if dynamic
routing is desired.
By dividing the partially-meshed frame relay network into a number of virtual, point-to-point
networks using subinterfaces, the split-horizon problem can be overcome. Each new point-to-point
subnetwork is assigned its own network number. To the routed protocol, each subnetwork now
appears to be located on separate interfaces (Figure 3). Routing updates received from Site B on
one logical point-to-point subinterface can be forwarded to site C on a separate logical interface
without violating split horizon.
