Cisco 7609-S Software Configuration Guide - Page 557

Chapter 37 Configuring DHCP Snooping Understanding DHCP Snooping Figure 37-2 shows the packet formats for the remote ID suboption and the circuit ID suboption. The 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 Circuit type ID type Length Length 1604 VLAN Module Port 1 byte 1 byte 1 byte 1 byte 2 bytes 1 byte 1 byte Remote ID Suboption Frame Format Suboption Remote type ID type Length Length 2806 1 byte 1 byte 1 byte 1 byte MAC address 6 bytes 116300 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: TYPE DHCP-SNOOPING VERSION 1 BEGIN ... ... END Each entry in the file is tagged with a checksum that is used to validate the entries whenever the file is read. The 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 Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX 37-5