Saturday, 19 March 2011

Frame Relay

Uses the DLCI (Data Link Connection Identifier) which also identify the type of circuit (PVS or SVC) in order to allow two internet devices to
communicate end-to-end through the frame relay cloud.
CIR (Committed Information Rate): Metric used when purchasing bandwidth, determining the guaranteed flow of traffic by the service provider. 
Traffic flow exceeding the CIR will not be guaranteed and retransmissions may occur.
DE (Discard Eligibility): When a Frame Relay router detects congestion on the network, the DE bit is turned on in.  A congested switch will first
discard these packets.  A CIR of zero will have the DE bit always turned on.

FECN (Forward-Explicit Congestion Notification): When the Frame Relay network detects congestion in the cloud, this bit is turned on and the
destination DTE  (Data Terminal Equipment) is informed the path traversed is congested.
BECN (Backward-Explicit Congestion Notification): When the switch detects congestion in the network, the BECN bit is set and the packet sent to
the source router, telling it to slow down the transmitting rate.

To enable Frame Relay.  IARP (Inverse-ARP) or static frame-relay maps must also be defined for the Frame Relay devices to talk together::
config t
int s0
encapsulation frame-relay [ietf]: Cisco encapsulation is default, use ietf (Internet Engineering Task Force) to connect to non-Cisco devices.
frame-relay interface-dlci number: number is between 16-1007.
frame-relay lmi-type cisco/ansi/q933a: Local Management Interface. used only between router and service provider’s switch. Provides information
about the local or global significance of the DLCI value and the status of virtual circuits.  cisco is default.

To create subinterfaces once the interface is configured:
int s0.subintnumber multipoint/point-to-point

To create maps which provide the DLCI to IP address conversion, if inverse-arp cannot be used, static maps must be defined:
no inverse-arp: inverse-arp, a different and dynamic method for converting DLCI to IP addresses.  It must be turned off to enable frame relay maps.
frame-relay map ip ip dlci [ietf/cisco] [broadcast]: frame relay maps is the only way to mix both cisco and ietf encapsulation types.
 

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