• Frame Relay Concepts
  • General Concepts
  • Bandwidth and Flow
  • Address Mapping and Topology
  • LMI
• Frame Relay Configuration
  • Basic Configuration
  • Static Map
  • Subinterfaces

Frame Relay Concepts

General Concepts

• Customer end is the DTE
• Frame Relay switch is the DCE
• The connection between two DTEs is called a Virtual Circuit (VC)
• Dynamic VCs are known as Switched VCs (SVCs)
• Static VCs are known as Permanent VCs (PVCs), which are the most common.
• Frame Relay has no error recovery, corrupt frames are discarded without notification. It is up to the upper layers to detect missing frames and request retransmission
• Several VCs can operate on one physical link
• DLCIs are used locally to identify where a packet is going, and is put in the address field of an outgoing frame.

Bandwidth and flow

• Typical port speed is 64kbps to 4Mbps, though some providers offer up to 45Mbps
• Each VC has a Commited Information Rate (CIR), which is the guaranteed bandwidth.
• As it is unlikely for all VCs to need to transmit at the same time, lines are commonly over-allocated by up to 2-3x the CIR
• Packets that are being sent over the CIR are marked as Discard Eligible, and will be discarded should a link become congested.
• The difference between the port speed and the CIR is the Excess Information Rate (EIR)
• The load average is worked out by dividing the data transferred by the committed time (Tc)
• Idle time can not be saved up for use at a later time of load
• The FECN bits are set on all frames downstream of a congested link
• The BECN bits are set on all frames upstream of a congested link
• When a DTE receives a frame with ECN bits set, it is expected to reduce the amount of traffic it creates

Address Mapping and Topology

• DLCIs are mapped to the IP address of the remote router, either with RARP or statically
• Frame Relay can be used in a full or partial mesh topology, point to point links are possible, but are usually not cost effective
• There can only be <100 VCs per physical interface, so full mesh topologies are not usually cost effective for larger networks

LMI

• Link Management Interface (LMI) is used on Frame Relay to provide status updates
• Some DLCIs are reserved for use by LMI
• There are three types of LMI supported by Cisco routers:
  • Cisco – original LMI extensions
  • ANSI – ANSI standard T1.617 Annex D
  • Q933a – ITU standard Q933 Annex A

Frame Relay Configuration

Basic Configuration

Router(config)# interface [interface]
Router(config-if)# ip address [address] [mask]
Router(config-if)# encapsulation frame-relay [cisco | ietf]

Static Map

Router(config)# frame-relay map [protocol] [address] [dlci] ['broadcast']

Subinterfaces

Router(config) interface [interface].[sub-if no.] [point-to-point | multipoint]
Router(config-if) ip address [address] [mask]
Router(config-if) frame-relay interface-dlci [dlci]