Cisco Certification Exam Review:
Frame Relay
By Chris Bryant, CCIE #12933
Frame map statements always reference the local router's DLCI and the remote router's IP address.
Rather that use a dedicated path, Frame Relay uses virtual circuits to get data from one endpoint to the other.
Packet-switching technologies such as Frame Relay are considered more efficient than circuit-switching technologies such as ISDN.
Frame Relay is a shared-bandwidth technology, which makes it more affordable than leased lines (dedicated lines between Point A and Point B).
Cisco routers are DTEs, while the frame switches that make up a frame cloud are DCEs.
To allow broadcast traffic such as RIP and OSPF hellos to be transported across a frame relay cloud, the broadcast keyword must be added to the frame map statement.
DLCIs are locally significant and are not advertised to other routers.
Switched Virtual Circuits use a four-step process: Call Setup, Data Transfer, Idle, and Call Termination.
Dynamic frame mappings are created by inverse ARP; to prevent these from forming, run no frame inverse-arp under the appropriate Serial interface.
The fields of a frame header are the Flag, Address, Data, Frame Check Sequence, and another Flag field. The Address field contains the DLCI.
The LMI type must be agreed upon by the DTE and the closest DCE. When there's an LMI mismatch, the line protocol will go down.
debug frame lmi can reveal one of four status messages:
0x0 indicates that the switch knows of this DLCI, but it's not usable.
0x2 indicates that everything is fine and connectivity via this DLCI exists.
0x3 indicates that the DLCI is active, but congestion exists.
0x4 indicates that the switch doesn't know about this particular DLCI, but did at one time.
LMI Autosense allows a DTE to detect the proper LMI type to exchange with the DCE. To do so, the DTE will send Full Status Requests to the DCE, and the DCE answers with the sole LMI type it's running. The DTE then sends LMI messages using that particular LMI type.
The LMI Virtual Circuit Status Message extension is required; the LMI Multicasting, Simple Flow Control, and Global Addressing extensions are optional.
IETF and Cisco encapsulation can be configured on a per-PVC or per-interface basis. IETF must be used when one endpoint is not a Cisco router.
If a DLCI shows as deleted on the local router, the problem is most likely on the local router; if the DLCI shows as inactive, the problem is most likely on the remote router or with the frame relay switch(es).
Using subinterfaces with frame relay can help you circumvent the rule of split horizon.
On multipoint frame interfaces, use frame map statements; on point-to-point frame interfaces, use the frame interface-dlci command.
To your success,
Chris Bryant
CCIE #12933
chris@thebryantadvantage.com
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