Wednesday, May 11, 2011

Frame Relay DLCIs

DLCIs are 10 bits long. Below shows the range of Frame Relay DLCI. Practically, it allows up to 992 PVCs per Frame Relay physical link.

DLCI Range Usage
0 LMI for ANSI Annex D or CCITT/ITU-T Q.933A type LMIs
1 – 15 Reserved for future use
16 – 1007 User traffic
1008 – 1022 Reserved for future use
1023 Reserved for LMI or and CLLM messages for Cisco-type LMI
Note: CLLM is referred to as Consolidated Link Layer Management.

The LMI DLCI in the show interface EXEC command defines the type of LMI being used.

ANSI LMI DLCI 0 LMI type is ANSI Annex D frame relay DTE
Cisco LMI DLCI 1023 LMI type is CISCO frame relay DTE
CCITT/ITU-T Q.933A LMI DLCI 0 LMI type is CCITT frame relay DTE

Router#sh int s0/0
Serial0/0 is up, line protocol is up
  Hardware is M4T
  Internet address is
  MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation FRAME-RELAY, crc 16, loopback not set
  Keepalive set (10 sec)
  LMI enq sent  75, LMI stat recvd 76, LMI upd recvd 0, DTE LMI up
  LMI enq recvd 0, LMI stat sent  0, LMI upd sent  0
  LMI DLCI 0  LMI type is ANSI Annex D  frame relay DTE
--- output omitted ---

The ITU Telecommunication Standardization Sector (ITU-T) coordinates standards for telecommunications on behalf of International Telecommunication Union (ITU). The Comité Consultatif International Téléphonique et Télégraphique (CCITT) or International Telegraph and Telephone Consultative Committee was created in 1956. It was renamed to ITU-T in 1993.

Multicasting support is an extension of the LMI specification that allows efficient distribution of routing information and ARP requests across a Frame Relay network. Multicasting uses the reserved DLCIs from 1019 to 1022.

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