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Frame-Relay interface states

January 26, 2009

Frame-Relay tends to create confusion in many areas from the network types, to routing protocols.

One typical confusion I’d like to point out is the behavior in states of physical interfaces compared to sub-interfaces when running frame-relay.

First let me explain two definitions: LMI and DLCI

Local Management Interface (LMI) is a set of enhancements to the basic Frame-Relay specification. LMI virtual circuit status messages provide communication and synchronization between a Frame Relay DTE (your router) and a DCE device (typically the provider’s frame-relay switches). These messages are used to periodically report on the status of Permanent Virtual Circuits (PVCs), which prevents data from being sent into black holes (that is, over PVCs that no longer exist). In short, LMI takes care of the communication between the DCE and DTE.

Datalink connection identifiers (DLCIs) uniquely identifies the Virtual circuits that provide a bidirectional communication path from one DTE device to another.


With Frame-Relay, a physical interface connecting to a Frame switch will be UP/UP, if it receives LMI from that frame switch, regardless of the DLCI it is learning or not learning. This means that a physical interface can be UP/UP, even though you do not have Layer2 communication,  because your DLCI’s are down. See the interface below is UP/UP, even though all the DLCI’s are inactive:
Serial0/1/0 is up, line protocol is up
...
Encapsulation FRAME-RELAY, loopback not set
Keepalive set (10 sec)
LMI enq sent 168, LMI stat recvd 169, DTE LMI up

R5#sh frame map
Serial0/1/0 (up): ip 123.1.0.1 dlci 501, static,
broadcast,CISCO, status defined, inactive
Serial0/1/0 (up): ip 123.1.0.3 dlci 503, static,
broadcast,CISCO, status defined, inactive
Serial0/1/0 (up): ip 123.1.0.4 dlci 504, static,
broadcast,CISCO, status defined, inactive

.

When you have a Multi-Point sub-interface with multiple DLCI’s defined, and all DLCI’s are inactive, the interface will show DOWN/DOWN comparatively:
R5#sh int Serial0/1/0.69
Serial0/1/0.69 is down, line protocol is down
R5#sh frame map
Serial0/1/0.69 (down): ip 123.1.0.1 dlci 501, static,
broadcast,CISCO, status defined, inactive
Serial0/1/0.69 (down): ip 123.1.0.3 dlci 503, static,
broadcast,CISCO, status defined, inactive
Serial0/1/0.69 (down): ip 123.1.0.4 dlci 504, static,
broadcast,CISCO, status defined, inactive

But with a Multi-Point sub-interface, when you have one single DLCI up, the interface will show UP/UP, so be careful of this configuration:
R5#sh int Serial0/1/0.69
Serial0/1/0.69 is up, line protocol is up
R5#sh frame map
Serial0/1/0.69 (up): ip 123.1.0.1 dlci 50, static,
broadcast,CISCO, status defined, active
Serial0/1/0.69 (up): ip 123.1.0.3 dlci 503, static,
broadcast,CISCO, status defined, inactive
Serial0/1/0.69 (up): ip 123.1.0.4 dlci 504, static,
broadcast,CISCO, status defined, inactive

.

The preferred way to configure frame-relay interface is as Point-to-Point sub-interfaces, because the sub-interface will only show UP/UP, if LMI was received and the DLCI is active:
R2#sh int Serial1/0.2
Serial1/0.2 is up, line protocol is up
R2#sh frame map
Serial1/0.2 (up): point-to-point dlci, dlci 205, broadcast
status defined, active

else the interface will be DOWN/DOWN:
R2#sh int Serial1/0.2
Serial1/0.2 is down, line protocol is down
R2#sh frame map
Serial1/0.2 (down): point-to-point dlci, dlci 205, broadcast
status defined, inactive

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One comment

  1. […] Frame-Relay interface states. by ruhann from routing bits […]



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