Overview Of The Dhcp Snooping Database Agent - Cisco 7604 Configuration Manual

Chapter 37 Configuring DHCP Snooping
Figure 37-2
router 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 37-2 Suboption Packet Formats
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 router reads
the file to build the database for the bindings. The router 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.
OL-4266-08
shows the packet formats for the remote ID suboption and the circuit ID suboption. The
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
Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
VLAN Module Port
2 bytes 1 byte 1 byte
MAC address
6 bytes
Understanding DHCP Snooping
37-5