Atlanta via one subinterface, and because routing updates to Nashville and Boston exit two
other subinterfaces, then advertising 140.1.12.0/24 to Nashville and Boston is allowed.
problem exists. However, if the VC from Mount Pilot to Raleigh went down, then split horizon
on Mayberry would be harmful. Mount Pilot will advertise routes to 199.1.11.0, and Mayberry
will receive that information in a routing update. However, because no subinterfaces are used,
Mayberry would not advertise 199.1.11.0 to Raleigh with split horizon enabled.
reasons described for Figure 8-11.
using multipoint subinterfaces. The IOS defaults to disable split horizon on Frame Relay
interfaces in all cases except for point-to-point subinterfaces. Table 8-16 summarizes these
settings and shows that the current default settings work around the split horizon issues
described in the last few paragraphs.
configuration command can be used to enable split horizon. Similarly, the no ip split horizon
interface configuration command disables split horizon on that interface.
router. However, Cisco requires that CCNAs understand Frame Relay address mapping, for two
main reasons. First, static mapping is just the kind of nasty question that is likely to crop up on
the exam. Second, understanding mapping offers a good opportunity to review the concepts
behind routing. As with most features implemented dynamically and by default, you can ignore
mapping most of the time.
ARP process dynamically builds a mapping between an IP address and a LAN address. This
section discusses the basics behind why mapping is needed for LAN connections and Frame
Relay, with a focus on Frame Relay.