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If multiple PVCs are required and are not in a PVC group, the interface is not
activated until all required PVCs are active.
If a required PVC belongs to a PVC group, the Frame Relay interface becomes
inactive when all PVCs in the PVC group are inactive. If at least one PVC in the
group is active, the interface becomes active following a successful exchange of
LMI frames between the router and the FR switch. If there are multiple PVC groups,
the interface does not become active until at least one PVC
in each group
is active.
A “required PVC group” is a group of circuits associated by name, where “name” is
the name of the required PVC group.
These features can be used with WAN Reroute so that an alternate link can be
brought up if all PVCs, required PVCs, or a group of PVCs become inactive on the
primary FR link.
Frame Relay Frame
An FR frame consists of a fixed size address field with variable sized encapsulated
user data. Figure 24 illustrates a Frame-Relay frame format.
HDLC Flags
Located in the first and last octet, these flags indicate the beginning and end of the
frame.
Data Link Connection Identifier (DLCI)
This 10-bit routing ID resides in bits 3 to 8 of octet 2 and bits 5 to 8 of octet three.
The DLCI is the MAC address of the circuit. The DLCI allows the user and network
management to identify the frame as being from a particular PVC. The DLCI
enables multiplexing of several PVCs over one physical link.
87654321
Octet ┌─────────────────────────────────────────────┐
1 │ HDLC Flag = 0x7e │
├───────────────────────────────────┬────┬────┤
2 │ Data Link MSB/LSB (DL) │C/R │ EA │
├─────────────────────────┬────┬────┼────┼────┤
3 │ Connection ID (CI) │FECN│BECN│ DE │ EA │
├─────────────────────────┴────┴────┴────┴────┤
││
. user .
. data .
..
││
├─────────────────────────────────────────────┤
│ Frame Check │
├─────────────────────────────────────────────┤
│ Sequence │
├─────────────────────────────────────────────┤
N │ HDLC Flag = 0x7e │
└─────────────────────────────────────────────┘
Figure 24. Frame-Relay Frame Format
Using Frame Relay
392
MRS V3.2 Software User’s Guide