Configuration Guide User guide

1014 FastIron Configuration Guide
53-1002494-02
Configuring IP parameters – Layer 3 Switches
DHCP introduces the concept of a lease on an IP address. Refer to “How DHCP Client-Based
Auto-Configuration and Flash image update works” on page 1030. The DHCP server can allocate
an IP address for a specified amount of time, or can extend a lease for an indefinite amount of
time. DHCP provides greater control of address distribution within a subnet. This feature is crucial if
the subnet has more devices than available IP address. In contrast to BOOTP, which has two types
of messages that can be used for leased negotiation, DHCP provides 7 types of messages. Refer to
“Supported Options for DHCP Servers” on page 1033.
DHCP allocates temporary or permanent network IP addresses to clients. When a client requests
the use of an address for a time interval, the DHCP server guarantees not to reallocate that
address within the requested time and tries to return the same network address each time the
client makes a request. The period of time for which a network address is allocated to a client is
called a lease. The client may extend the lease through subsequent requests. When the client is
done with the address, they can release the address back to the server. By asking for an indefinite
lease, clients may receive a permanent assignment.
In some environments, it may be necessary to reassign network addresses due to exhaustion of the
available address pool. In this case, the allocation mechanism reuses addresses with expired
leases.
Configuration Notes for configuring DHCP servers
• DHCP server is supported in the Layer 2, base Layer 3, edge Layer 3, and full Layer 3 software
images.
• In the event of a controlled or forced switchover, a DHCP client will request from the DHCP
server the same IP address and lease assignment that it had before the switchover. After the
switchover, the DHCP Server feature will be automatically re-initialized on the new active
controller or management module.
• For DHCP client hitless support in an IronStack, the stack mac command must be used to
configure the IronStack MAC address, so that the MAC address does not change in the event of
a switchover or failover. If stack mac is not configured, the MAC address/IP address pair
assigned to a DHCP client will not match after a switchover or failover. Furthermore, in the
Layer 3 router image, if the stack mac configuration is changed or removed and the
management port has a dynamic IP address, when a DHCP client tries to renew its lease from
the DHCP server, the DHCP server will assign a different IP address.
• If any address from the configured DHCP pool is used, for example by the DHCP server, TFTP
server, etc., you must exclude the address from the network pool. For configuration
instructions, refer to “Specifying addresses to exclude from the address pool” on page 1023.
DHCP Option 82 support
The DHCP relay agent information option (DHCP option 82) enables a DHCP relay agent to include
information about itself when forwarding client-originated DHCP packets to a DHCP server. The
DHCP server uses this information to implement IP address or other parameter-assignment
policies.
In a metropolitan Ethernet-access environment, the DHCP server can centrally manage IP address
assignments for a large number of subscribers. If DHCP option 82 is disabled, a DHCP policy can
only be applied per subnet, rather than per physical port. When DCHP option 82 is enabled, a
subscriber is identified by the physical port through which it connects to the network.