Services Overview
7705 SAR OS Services Guide Page 51
GRE Header
The GRE header format is shown in Figure 5 and described in Table 5.
Figure 5: GRE Header
Table 5: GRE Header Descriptions
Field Description
C 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.
Reserved0 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
Ver 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
Protocol Type Specifies the protocol type of the original payload packet —
identical to Ethertype with the only supported option being
MPLS unicast (0x8847)
Checksum (optional) Not applicable
Reserved1 (optional) Not applicable
19874
Checksum (optional)
C
1
Ver Protocol TypeReserved0
Reserved1 (optional)
12 bits
16 bits
16 bits
3 16 bits
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