Overview Of The Dhcp Snooping Database Agent - Cisco WS-SUP32-GE-3B - Supervisor Engine 32 Software Configuration Manual

Overview of DHCP Snooping
Figure 34-2
switch uses the packet formats when DHCP snooping is globally enabled and when the ip dhcp
snooping information option global configuration command is entered. For the circuit ID suboption,
the module field is the slot number of the module.
Figure 34-2
Circuit ID Suboption Frame Format
Suboption
type
1 byte
Remote ID Suboption Frame Format
Suboption
type
1 byte

Overview of the DHCP Snooping Database Agent

To retain the bindings across reloads, you must use the DHCP snooping database agent. Without this
agent, the bindings established by DHCP snooping are lost upon reload, and connectivity is lost as well.
The database agent stores the bindings in a file at a configured location. Upon reload, the switch reads
the file to build the database for the bindings. The switch keeps the file current by writing to the file as
the database changes.
The format of the file that contains the bindings is as follows:
<initial-checksum>
TYPE DHCP-SNOOPING
VERSION 1
BEGIN
<entry-1> <checksum-1>
<entry-2> <checksum-1-2>
...
...
<entry-n> <checksum-1-2-..-n>
END
Each entry in the file is tagged with a checksum that is used to validate the entries whenever the file is
read. The <initial-checksum> entry on the first line helps distinguish entries associated with the latest
write from entries that are associated with a previous write.
Catalyst Supervisor Engine 32 PISA Cisco IOS Software Configuration Guide, Release 12.2ZY
34-4
shows the packet formats for the remote ID suboption and the circuit ID suboption. The
Suboption Packet Formats
Circuit
ID type
Length Length
1 6 0 4
1 byte 1 byte 1 byte
Remote
ID type
Length Length
2 8 0 6
1 byte 1 byte 1 byte
VLAN Module Port
2 bytes 1 byte 1 byte
MAC address
6 bytes
Chapter 34 Configuring DHCP Snooping
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