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broadcasts a DHCP Request packet, looking for DHCP servers. DHCP servers respond
to this packet with a DHCP Response packet. The client then chooses a server to obtain
TCP / IP configuration information, such as its own IP address.
Since DHCP uses a broadcast mechanism, a DHCP server and its client must
physically reside on the same subnet. However, it is not practical to have one DHCP
server on every subnet; in fact in many cases, DHCP / BOOTP clients and their
associated DHCP / BOOTP server(s) do not reside on the same IP network or subnet.
In such cases, a third-party agent is required to transfer BOOTP messages between
clients and servers.
BOOTP / DHCP Relay, described in RFC 1542, enables a host to use a BOOTP or
DHCP server to obtain basic TCP / IP configuration information, even if the servers do
not reside on the local subnet. When a BOOTP / DHCP Relay Agent receives a DHCP
Request packet destined for a BOOTP / DHCP server, it inserts its own IP address into
the DHCP Request packet so the server knows the subnet where the client is located.
Then, depending on the configuration setup, the switch either:
• Forwards the packet to a specific server as defined in the switch’s configuration using
unicast routing, or
• Broadcasts the DHCP Request again to another directly attached IP subnet specified
in the switch configuration for the receiving IP subnet.
When the DHCP server receives the DHCP request, it allocates a free IP address for
the DHCP client from its scope in the DHCP client’s subnet, and sends a DHCP
Response back to the DHCP Relay Agent. The DHCP Relay Agent then broadcasts this
DHCP Response packet received from the DHCP server to the appropriate client.
4.7.Security Features
The switch provides security features which allow you to control management access
and network access as described in the following sections.

4.7.1.SNMP Community Strings

Access to the switch using network management tools is controlled by SNMP
community strings. This switch supports up to five community strings. A character string
indicating the access rights of the management community must be provided whenever
you send an SNMP message to the switch. Each community has either read-only or
read / write access rights. A community that has read-only access can use only use
GET and GETNEXT commands to view the current configuration settings and status of
the switch. But a community with read / write access can use GET and GETNEXT
commands, as well as the SET command to configure the switch.