Cisco Systems 4500 Identifying a Port with DHCP Option 82

34-4
Software Configuration Guide—Release 15.0(2)SG
OL-23818-01
Chapter 34 Configuring ANCP Client
Enabling and Configuring ANCP Client
Example 2
ANCP_Client# show ancp multicast interface Fa2/3 vlan 19
ANCP Multicast Streams
Interface FastEthernet2/3 VLAN 19: client ID 0x0106000700130203
Group Source Joined on
239.5.6.7 - 15:03:14 UTC Tue Aug 26 2008
ANCP_Client#
Note Specifying the show ancp multicast command without parameters or keywords lists everything.
Identifying a Port with DHCP Option 82
Note To use DHCP option 82, you need to enable DHCP and DHCP snooping (see
Chapter 45, “Configuring DHCP Snooping, IP Source Guard, and IPSG for Static Hosts”).
If you identify the port with DHCP option 82, you need to configure the Catalyst 4500 series switch as
a DHCP relay to insert the DHCP option 82. This action adds a tag in the DHCP packet from the DHCP
client so that the DHCP server knows the port connected to this specific DHCP client. The DHCP server
can then map the IP address it is providing to the client with the DHCP option 82 it received from the
switch. The DHCP server only needs to lookup the DHCP option 82 associated with a given IP address
and provide it to the ANCP server. This allows the ANCP client on the switch to identify the proper port
using an identifier the switch understands. The configure DHCP snooping on the Catalyst 4500 series
switch, use the following commands:
Switch(config)# ip dhcp snooping
Switch(config)# ip dhcp snooping vlan vlan-range
By default, DHCP option 82 is inserted when DHCP snooping is activated. Turning this default of f could
cause ANCP to function improperly with the DHCP circuit-id; it must remain active. To acti vate it , enter
this command:
Switch(config)# ip dhcp snooping information option
Note The DHCP option 82 circuit-ID is inserted in the Active-Flow report (when queried for all multicast
flows) even if a configured circuit-ID exists.
ANCP allows a remote server to request the list of active flows from the ANCP client (Catalyst 4500
series switch is the ANCP client). This list is very similar to the output from the show ancp multicast
command except that it follows the ANCP protocol packet format (see IETF.org). Observe that the
show ancp multicast command provides the flows that have been activated with the
ancp port client identifier command while the ANCP active flow request only reports the client ID in
DHCP option 82 circuit-ID format, regardless of the activation mechanism.
Refer to Chapter 45, “Configuring DHCP Snooping, IP Source Guard, and IPSG for Static Hosts” for
details on the CLI.