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- The interface serialx.y point-to-point global configuration command creates a logical
subinterface numbered y under physical interface serialx, and the frame-relay interface-dlci
{dlci} subinterface subcommand statically associates and maps a local DLCI with an IP address
on a subinterface. This ensures a router associate the correct PVC with the corresponding
subinterface when it receives LMI messages regarding a PVC (up / down).
- The subinterface and DLCI number do not have to be matched on both end routers of a PVC.
However, it is a best practice to assign a subinterface number that matches the DLCI value
assigned to the subinterface for easier administration and troubleshooting.
- Below shows the output of the show frame-relay map EXEC command on RT1:
RT1#sh frame-relay map
Serial0/0.102 (up): point-to-point dlci, dlci 102(0x66,0x1860), broadcast
status defined, active
Serial0/0.103 (up): point-to-point dlci, dlci 103(0x67,0x1870), broadcast
status defined, active
Serial0/0.104 (up): point-to-point dlci, dlci 104(0x68,0x1880), broadcast
status defined, active
RT1#
- The output is different from the output of the same command in previous example of full-mesh
network without subinterfaces – there is no corresponding Layer 3 address for each entry. This is
because these subinterfaces are point-to-point subinterfaces. Whenever a packet is being sent out
from a particular subinterface, the router would know the DLCI to use to encapsulate the packet
(similar to using an outgoing interface in static routing – a router sends data without knowing the
IP address of the router at the other end). Mapping with Inverse ARP or static mapping
configuration is only needed when more than one VC associated with a particular interface or
subinterface, as this is the time when a router would confuse about which DLCI to use.
- LMI messages are exchanged between a DTE (router) and DCE (Frame Relay switch) which
allows the DTE to detect whether it has connectivity to its local Frame Relay switch. The debug
frame-relay lmi privileged command displays the information about the exchange of LMI
messages. LMI keepalive messages (Status Enquiry messages) are originated from the DTE,
as indicated with the out keyword; and the DCE would response with Status Reply messages,
as indicated with the in keyword. LMI status messages are originated from the DCE to inform
the DTE regarding the status of virtual circuits. The no keepalive interface subcommand
disables the use of LMI messages, which include LMI keepalive and status messages.
RT1#debug frame-relay lmi
Frame Relay LMI debugging is on
Displaying all Frame Relay LMI data
RT1#
00:14:30: Serial0/0(out): StEnq, myseq 44, yourseen 43, DTE up
00:14:30: datagramstart = 0xE015DC, datagramsize = 13
00:14:30: FR encap = 0xFCF10309
00:14:30: 00 75 01 01 01 03 02 2C 2B
00:14:30:
00:14:30: Serial0/0(in): Status, myseq 44, pak size 13
00:14:30: RT IE 1, length 1, type 1
00:14:30: KA IE 3, length 2, yourseq 44, myseq 44
RT1#
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