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Understanding DHCP Features
Figure 20-3
Circuit ID Suboption Frame Format (for user-configured string):
Suboption
type
1 byte
Remote ID Suboption Frame Format (for user-configured string):
Suboption
type
1 byte
Cisco IOS DHCP Server Database
During the DHCP-based autoconfiguration process, the designated DHCP server uses the Cisco IOS
DHCP server database. It has IP addresses, address bindings, and configuration parameters, such as the
boot file.
An address binding is a mapping between an IP address and a MAC address of a host in the Cisco IOS
DHCP server database. You can manually assign the client IP address, or the DHCP server can allocate
an IP address from a DHCP address pool. For more information about manual and automatic address
bindings, see the "Configuring DHCP" chapter of the Cisco IOS IP Configuration Guide,
Release 12.2.
DHCP Snooping Binding Database
When DHCP snooping is enabled, the switch uses the DHCP snooping binding database to store
information about untrusted interfaces. The database can have up to 8192 bindings.
Each database entry (binding) has an IP address, an associated MAC address, the lease time (in
hexadecimal format), the interface to which the binding applies, and the VLAN to which the interface
belongs. The database agent stores the bindings in a file at a configured location. At the end of each entry
is a checksum that accounts for all the bytes from the start of the file through all the bytes associated
with the entry. Each entry is 72 bytes, followed by a space and then the checksum value.
To keep the bindings when the switch reloads, you must use the DHCP snooping database agent. If the
agent is disabled, dynamic ARP inspection or IP source guard is enabled, and the DHCP snooping
binding database has dynamic bindings, the switch loses its connectivity. If the agent is disabled and only
DHCP snooping is enabled, the switch does not lose its connectivity, but DHCP snooping might not
prevent DHCP spoofing attacks.
Cisco Catalyst Blade Switch 3020 for HP Software Configuration Guide
20-6
User-Configured Suboption Packet Formats
Circuit
ID type
Length
Length
1
N+2
1
N
ASCII Circuit ID string
1 byte 1 byte 1 byte
Remote
ID type
Length
Length
2
N+2
1
N
ASCII Remote ID string or hostname
1 byte 1 byte 1 byte
Chapter 20
N bytes
(N = 3-63)
N bytes
(N = 1-63)
Configuring DHCP Features and IP Source Guard
OL-8915-03
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