
Configuring Line Services
10-8 115391-A Rev. A
Datagram Encapsulation
Before transmitting data across the link, PPP encapsulates data in a frame similar
to a high-level data link control (HDLC) frame (F
igure 10-3).
Figure 10-3. PPP-Encapsulated Frame
The parts of the PPP frame function as follows:
• The Flag field marks the beginning and end of a frame. Peers on synchronous
lines exchange flags continuously when there are no frames to transmit.
• The Address field indicates which device originated the frame.
• The Control field shows the frame type (information or administrative).
• The Protocol field indicates the operative network layer protocol.
• The Data field contains the data one link sends to the other. Its length is less
than or equal to the MTU line size. The default maximum length is 1594
bytes; LCP negotiations determine the actual length.
• The Frame Check Sequence (FCS) field shows the sequence order of the
frame; router hardware computes the FCS. A 16- or 32-bit cyclic redundancy
check (CRC) is at the end of each frame.
PPP0003A
PPP frame
Flag
Address Control Protocol
Data
FCS Flag
1 byte 1 byte 1 byte1 byte 2 bytes Variable 2 or 4
bytes
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