for a DTE to send a frame that is replicated and delivered across more than one VC. However,
routers need to send broadcasts for several features to work. In particular, routing protocol
updates and SAP updates are broadcasts.
copies of the broadcasts across each VC that you instruct it to. Of course, if there are only a few
VCs, this is not a big problem. However, if hundreds of VCs terminate in one router, then for
each broadcast, hundreds of copies could be sent. The IOS can be configured to limit the
amount of bandwidth that is used for these replicated broadcasts.
solution. The router places these broadcasts into a different queue than user traffic so that the
user does not experience a large spike in delay each time some broadcast is replicated and sent
over each VC.
overhead, a short example can show the significance of this overhead. For example, if a router
knows 1000 routes, uses RIP, and has 50 VCs, then 1.072MB of RIP updates are sent every 30
seconds. That averages to 285kbps. (Math: 536-byte RIP packets, with 25 routes in each packet,
for 40 packets per update, with copies sent over 50 VCs: 536
the CCNA exam and is covered in the "Frame Relay Configuration" section later in this chapter.
The issues that relate to dealing with the volume of these updates is more likely a topic for the
CCNP and CCIE exams.
route onto the same interface in which the route was learned. (Refer to Chapter 6, "Routing
Protocols," for a full explanation.) However, split horizon could cause some problems with
Frame Relay. Thankfully, several configuration options help you deal with this issue. But first,
refer back to Figure 8-12. If split horizon was enabled on Atlanta, then Atlanta would learn
about 140.1.12.0/24 from Charlotte but would not advertise the route to 140.1.12.0/24 in its
updates to Nashville or Boston.
a different subinterface for each VC to the three remote sites. Split horizon is enabled on each