Home > Cisco CCNA4 > Module 5 – Frame Relay

Module 5 – Frame Relay

June 18th, 2009 Tom GT

Index

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]

Categories: Cisco CCNA4 Tags: , ,
Comments are closed.