Table 5: Gre Header Descriptions; Figure 5: Gre Header - Alcatel-Lucent 7705 Service Manual

GRE Header
The GRE header format is shown in
7705 SAR OS Services Guide
1
C
Field
C
Reserved0
Ver
Protocol Type
Checksum (optional)
Reserved1 (optional)
Figure 5 and described in

Figure 5: GRE Header

12 bits 3
Reserved0 Ver
Checksum (optional)
16 bits

Table 5: GRE Header Descriptions

Description
Specifies whether there is a checksum in the header
If set to 1, both the checksum and reserved1 fields must be
present
On the 7705 SAR, in the network egress (transmit) direction, the
C bit is always set to 0; therefore, the checksum and reserved1
fields are omitted from the header. The GRE header is therefore
always 4 bytes (32 bits) in the network egress direction.
In the network ingress direction, the C bit validity is checked. If
it is set to a non-zero value, the GRE packet is discarded and the
IP discards counter is increased.
Indicates whether the header contains optional fields
Not applicable to the 7705 SAR — first 5 bits of the field are
always set to 0 and bits 6 to 12 are reserved for future use and
also set to 0 by the 7705 SAR
Always set to 000 for GRE
At network ingress, if a GRE packet is received with the version
field set to any value other than 000, the packet is discarded and
the IP discards counter is increased
Specifies the protocol type of the original payload packet —
identical to Ethertype with the only supported option being
MPLS unicast (0x8847)
Not applicable
Not applicable
Services Overview
Table 5.
16 bits
Protocol Type
Reserved1 (optional)
16 bits
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