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:
- Cisco
- RFC 1490
- incompatible
- types:
; 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
Link management interface (LMI)
- 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
Datalink connection identifier (DLCI)
- 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