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640 CHAPTER 44: CONFIGURING GRE
By default, no identification key word of the tunnel interface is configured.
Setting the Tunnel
Interface to Check with
Checksum
It is stipulated in RFC 1701 that if the checksum field of the GRE header is set, the
checksum is valid. The transmitting side calculates the checksums of GRE header
and payload. The receiving side calculates the checksum of the received message
and compares it with the checksum field in the message. If the two checksums are
identical, the message will be processed, otherwise it will be discarded.
If only one end of the tunnel is configured to check with the checksum, the
message will not be checked with checksum. Only when both ends of the tunnel
are configured to check the checksum, the message will be checked with the
checksum.
Perform the following tasks in the tunnel interface view.
Table 692 Set Tunnel Interface to Check with Checksum
By default, the tunnel interface to check with the field of checksum is disabled.
Settng the Tunnel
Interface to Synchronize
the Datagram Sequence
Number
It is stipulated in RFC 1701 that if the sequence-datagram in the GRE header is set,
both the receiving side and the transmitting side will synchronize the sequence
numbers. The synchronized message should be further processed, or it is
discarded.
With the sequence numbers, the message is unreliable but in order. The receiving
end establishes sequence numbers for the message, which is received by the local
end and successfully de-encapsulated. The sequence numbers are integers
between 0 and 2
32
–1 and the sequence number of the first packet is 0. After the
channel is established, the sequence numbers is accumulated and cyclically
counted. If the receiving end receives a message whose sequence number is less
than or equal to that of the message received the last time, the packet will be
considered illegal. If the receiving end receives an out-of-order message, the
packet will be discarded automatically.
Only when the synchronization mechanism to enable or disable sequence
numbers is established at both ends of the tunnel, the channel can be established.
Perform the following tasks in the tunnel interface view.
Table 693 Set the Tunnel to Synchronize Datagram Sequence Numbers
Set the identification key word of tunnel
interface.
gre key key-number
Cancel the identification key word of
tunnel interface.
undo gre key
Operation Command
Set tunnel interface to check with check
sum.
gre checksum
Disable tunnel interface to check with
check sum.
undo gre checksum
Operation Command
Set tunnel interface to synchronize
sequence numbers.
gre sequence-datagrams