Frame-Relay Notes

This is part of the Cisco Notes series on Mike’s World News.

Frame Relay is a Common WAN protocol which exists at layer 2 only, therefore frames are transmitted. The frames are a variable sized.

can transmit upto 1.54 mb.

Your router is the DTE. They receive the clocking speed from the DCE, inside the frame-relay cloud.

CIR is the committed information rate. The provider guarantee you will get this bandwidth. The higher the CIR, the more the price will be.

The Virtual Circuit (VC) is the link between your router and the destination
PVC = Permanent Virtual Circuit. Similar to a lease line.

SVC = Switched Virtual Cicuit.  (rare, most Teleco’s don’t offer it.)

PVC is easy to setup.

The connection is referred to as the DLCI (a layer 2 address), which maps to a layer 3 address.

DLCI is only locally important/signficant. It only needs to be unique on the local router. The same DLCI number can be used multiple times in the entire network to identify different devices.

you map local DLCI to remote IP address, or use Inverse ARP to automatically map it.

Uses Local Managment Interface (LMI) to manage the connection and report status between router and provider switch.

3 LMI standards: Cisco, ANSI, and ITU/Q933a.
Cisco is most common.

if you are not connecting to a Cisco device, the LMI type you use will most likely be IETF

IOS version 12 and later will auto detect the LMI in use.

If congestion is detected
FECN is sent to notify that packets will be slowed down due to congrestion.
BECN will be sent transmitting device to tell it to slow down
DE = discard eligable. 1st packets to be dropped if congression is detected.

Frame Relay switches perform error checking, but not error correction.

Frame Relay can be configured either by Inverse ARP, map, or by sub interface.

Inverse ARP is the default, but doesn’t work if you more than one connection over the same serial connection.

Frame-Relay map {remote IP-address} {local DLCI} broadcast

serial 0/0.1
frame-relay interface-DLCI {local DLCI}

Frame Relay is simple to setup because a lot of it is done automatically (Inverse ARP, LMI, etc).

show frame-relay map
will display the contents of the frame-relay map cache.

show frame-relay pvc
will show the DLCI stats and info

Frame-Relay is a non-broadcast multi access environment

Split Horizon causes routing update issues in a multi location environment. To get around that, we use subinterfaces.

To troubleshoot Frame-Relay:

show frame-relay map
show frame-relay pvc
show frame-relay lmi

if frame-relay is up and down, then ensure that you are using the correct LMI type

if
pvc status = active
then connection is good.
if
pvc status = static
then LMI has been disabled.
if
pvc status = deleted
then we have lost connection between the 2 devices.

to view the DLCI numbers, you can either:
show run
or
show frame-relay pvc

to view encapsulation method, either:
show run
or
show interface

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