Information Technology Reference
In-Depth Information
Frame Relay LAPF Frame Format
Figure 5-7
Number of bytes:
8
16
Variable
16
8
Flag
Address
Data
FCS
Flag
Address sub-fields:
Bits:
8 7 6 5 4 3
2
1
8 7 6 5
4
3
2
1
DLCI
C/R
EA
DLCI
FECN
BECN
DE
EA
The 10-bit DLCI provides the PVC identifier, which has local significance between the router
and the Frame Relay switch. The C/R bit is not used. The EA (extended address) bit is a mechanism
that extends the address field an additional 2 bytes. There are two EA bits in a 16-bit address
field. The eighth bit of every byte in the address field indicates the extended address. If the bit
is set to 0, the containing byte is not the last byte of the address. If the bit is set to 1, the byte is
the last byte of the address. Implementations that use the nonextended DLCI addresses set the
first EA bit to 0 and the second EA bit to 1.
The control field has the three bits: FECN, BECN, and DE, which were discussed in the
previous section.
Table 5-1 displays the Frame Relay LAPF frame address field.
Frame Relay LAPF Frame Address Field
Table 5-1
Bit
Description
DLCI
10-bit PVC identifier
C/R
Not used
EA
Extended Address; indicates if the containing byte is the last in the addressing field;
a mechanism to extend DLCI length beyond 10 bits
FECN
Bit set in frames traveling to the destination, when congestion is on traffic from
source to destination
BECN
Bit set in frames traveling to the source, when congestion is on traffic from
source to destination
DE
Bit set to indicate that the frame is eligible for discard
Frame Relay LMI Frame Relay Format
The LMI frame format is shown in Figure 5-8.
 
Search WWH ::




Custom Search