Cisco HDLC
As initially specified, HDLC did not include multiprotocol support or link quality monitoring and was not useful for anything other than frame description. These shortcomings were addressed by Cisco’s extensions to HDLC (cHDLC).[15] cHDLC is an HDLC frame with an Ethernet type code to identify the network protocol carried in the frame. The basic frame structure is shown in Figure 8-6. Framing components are identical to standard HDLC, with the opening and closing flags and FCS.
Figure 8-6. cHDLC frame format
Addressing in cHDLC is simple. The address is either 0x0F for unicast packets or 0x8F for broadcast packets. Labeling a frame as broadcast or multicast has very little significance because IOS does not support multidrop links. Broadcast is simply a reflection of higher-level protocols designating a frame as a broadcast. Control information is set to zero; cHDLC does not support the HDLC window. To distinguish between packets, the third and fourth bytes contain a type code, which is usually the Ethernet type code for the higher-layer protocol. Table 8-2 shows common type-code assignments. After the type code, the higher-level packet is encapsulated as it would be on an Ethernet network. After the data, a frame check sequence and flag close out the frame.
Table 8-2. cHDLC protocol codes
|
Protocol |
cHDLC protocol code (hexadecimal) |
|---|---|
|
IP |
0x0800 |
|
Cisco SLARP (not ... |
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