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Frame Relay Address Mapping
- It is an important Frame Relay concept for mapping a L3 address with its corresponding L2
address, which is similar to the ARP process used for L3-to-L2 address mapping to figure out the
MAC address of the destination device on the same LAN before data transmission. Mapping is
only needed on multiaccess networks (point-to-point networks do not require mapping).
- Below shows the output of some Frame Relay address mapping commands on RT1:
RT1#sh frame-relay pvc
PVC Statistics for interface Serial0 (Frame Relay DTE)
Active Inactive Deleted Static
Local 2 0 0 0
Switched 0 0 0 0
Unused 0 0 0 0
DLCI = 102, DLCI USAGE = LOCAL, PVC STATUS = ACTIVE, INTERFACE = Serial0/0
input pkts 55 output pkts 52 in bytes 4907
out bytes 4845 dropped pkts 0 in FECN pkts 0
in BECN pkts 0 out FECN pkts 0 out BECN pkts 0
in DE pkts 0 out DE pkts 0
out bcast pkts 8 out bcast bytes 482
pvc create time 00:10:51, last time pvc status changed 00:09:41
DLCI = 103, DLCI USAGE = LOCAL, PVC STATUS = ACTIVE, INTERFACE = Serial0/0
input pkts 39 output pkts 18 in bytes 2564
out bytes 4845 dropped pkts 0 in FECN pkts 0
in BECN pkts 0 out FECN pkts 0 out BECN pkts 0
in DE pkts 0 out DE pkts 0
out bcast pkts 8 out bcast bytes 482
pvc create time 00:10:51, last time pvc status changed 00:05:30
RT1#
RT1#sh frame-relay map
Serial0/0 (up): ip 200.1.1.2 dlci 102(0x66,0x1860), dynamic,
broadcast,, status defined, active
Serial0/0 (up): ip 200.1.1.3 dlci 103(0x67,0x1870), dynamic,
broadcast,, status defined, active
RT1#
- The show frame-relay pvc EXEC command can also be used to display the information of
Frame Relay congestion control, eg: the number of FECN and BECN packets.
- There is no DLCIs address mapping configuration on all routers. So how the routers know the
appropriate DLCI numbers to use in the Frame Relay LAPF headers when forwarding the
encapsulated packets to each other?
- The DLCIs address mapping can be built with either static configuration, or via a dynamic
process called Inverse ARP. Inverse ARP resolves an IP address from a DLCI.
- The Inverse ARP process differs from the ARP process on LANs. As soon as a VC is up,
the Inverse ARP process starts with the learning the DLCI via LMI messages, and the router then
sends an Inverse ARP Request message to announce its network layer address over the VC,
hence allow its neighbor router to build a mapping between its L3 and L2 addresses (DLCI).
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Copyright © 2008 Yap Chin Hoong
yapchinhoong@hotmail.com