Frame Relay

  1. Frame Relay
    1. FR frame format
    2. Link management interface (LMI)
    3. Virtual circuit (VC)
    4. Datalink connection identifier (DLCI)
  2. QoS
  3. Inverse ARP

Frame Relay

  • switching frames: reuse timeslots
  • not limited to P2P topologies
  • if there is 11111 bit sequence in data, 0 is appended on transmission and discarded on receiving
  • NBMA
  • LAPF – link access procedure for Frame Relay
  • L3 protocol indication
    • types:
      1. Cisco
      2. RFC 1490
    • incompatible
; multilink FR
(config)# interface mfr <M>

; PPPoFR
(config)# interface virtual-template <VT>

(config)# frame-relay switching
(config)# connect <NAME> <INTF_SRC> <DLCI> <INTF_DST> <DLCI>

(config)# map-class frame-relay <MAP>
(config-map-class)# frame-relay end-to-end keepalive mode <MODE>
(config-map-class)# frame-relay end-to-end keepalive recv|send <sec>
(config-map-class)# frame-relay end-to-end keepalive event-window <N>
(config-map-class)# frame-relay end-to-end keepalive error-threshold <N>
; default – Cisco encap, for all DLCI
(config-if)# encapsulation frame-relay [ietf] [mfr <M>]

; DTE by default
(config-if)# frame-relay intf-type dte|dce

; default – Cisco
(config-if)# frame-relay interface-dlci <DLCI> [ietf]

; Cisco by default
(config-if)# frame-relay map ip <IP> <DLCI> [ietf]

; per interface
(config-if)# frame-relay class <MAP>

(config-if)# frame-relay ip rtp|tcp header-compression
; per DLCI
(config-fr-dlci)# class <MAP>

; PPPoFR
(config-fr-dlci)# ppp virtual-template <VT>
# show frame-relay pvc
# show frame-relay map
# show frame-relay end-to-end keepalive
# show frame-relay ip tcp|rtp header

FR frame format

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
                +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                |     Flag FD   |    DLCI   |C|E| DLCI  |F|B|D|E| – LAPF header
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/                                                               /
\                              Data                             \
/                                                               /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|              FCS              |    Flag FD    |                 – LAPF trailer
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Flag FD: 0x7e
C: 0 ≡ command, 1 ≡ response
E: 0 ≡ address end, 1 ≡ next byte continues the address
F: forward explicit congestion notification (FECN)
B: backward explicit congestion notification (BECN)
D: discard eligible

  • DCE passes info to DTE
    • synchronous LMI ≡ DCE responds after DTE request
  • timers
    • keepalive: 10s default
    • hold: 30s default
  • version must match on adjacent devices
  • A-bit:
    • active end-to-end
    • notifies endpoints about failure in transit
    • requires asyncronous LMI

Virtual circuit (VC)

  • types:
    • switched VC (SVC): established dynamically
    • permanent VC (PVC): manual assignment
  • 10 bits
  • values are locally significant
  • type must match on adjacent devices
  • 10 bits, 1000 valid values (first 16 are reserved)

QoS

  • FECN:
    • set after congestion
    • means that some ingress data over DLCI might be lost
  • BECN:
    • set by recipient after receiving FECN
    • means that some egress data over DLCI might be lost
  • DE:
    • whether the data can be dropped on congestion
  • FECN/BECN = 1 ≡ possible data loss; if transmission speed is decreased, data loss should be mitigated
  • CIR = Bc/T
    • CIR: committed information rate
    • EIR: exceeded information rate
    • Bc: burst commit
    • T: time interval

Inverse ARP

  • Cisco: only IPv4 is implemented
  • endpoint sends unsolicited update with own IP address
  • within VC, no mcast