CCNA Cisco Certified Network Associate Study Guide - FTP Server

492 Chapter 10 � Wide Area Networking Protocols
non-Cisco device with Frame Relay. So before choosing an encapsulation
type, check with your ISP and find out which one they use. (If they don’t
know, hook up with a different ISP!)
Data Link Connection Identifiers (DLCIs)
Frame Relay virtual circuits (PVCs) are identified by DLCIs. A Frame Relay
service provider, such as the telephone company, typically assigns DLCI values,
which are used by Frame Relay to distinguish between different virtual circuits
on the network. Because many virtual circuits can be terminated on one
multipoint Frame Relay interface, many DLCIs are often affiliated with it.
For the IP devices at each end of a virtual circuit to communicate, their IP
addresses need to be mapped to DLCIs. This mapping can function as a multipoint
device—one that can identify to the Frame Relay network the appropriate
destination virtual circuit for each packet that is sent over the single
physical interface. The mappings can be done dynamically through IARP or
manually through the Frame Relay map command.
Frame Relay uses DLCIs the same way that X.25 uses X.121 addresses,
and every DLCI number can be given either global or local meaning everywhere
within the Frame Relay network.
Sometimes a provider can give a site a DLCI that is advertised to all
remote sites as the same PVC. This PVC is said to have a global significance.
For example, a corporate office might have a DLCI of 20. All remote sites
would know that the corporate office is DLCI 20 and use this PVC to communicate
to the corporate office. However, the customary implementation is
to give each DLCI local meaning. What does this mean? It means that DLCI
numbers do not necessarily need to be unique. Two DLCI numbers can be
the same on different sides of a link because Frame Relay maps a local DLCI
number to a virtual circuit on each interface of the switch. Each remote office
can have its own DLCI number and communicate with the corporate office
using unique DLCI numbers.
DLCI numbers, used to identify a PVC, are typically assigned by the provider
and start at 16. Configuring a DLCI number to be applied to an interface
is shown below:
RouterA(config-if)#frame-relay interface-dlci ?
Define a DLCI as part of the current
subinterface
RouterA(config-if)#frame-relay interface-dlci 16