ProCurve 2900 Betriebsanweisung PDF herunterladen (Seite 90)

IP Routing Features

Configuring DHCP Relay

■ Where multiple DHCP servers are assigned to a VLAN, a DHCP client

request cannot be directed to a specific server. Thus, where a given VLAN

is configured for multiple DHCP servers, all of these servers should be

configured with the same IP addressing policy.

■ Where routing switch “A” is configured to insert its MAC address as the

Remote ID in the Option 82 fields appended to DHCP client requests, and

upstream DHCP servers use that MAC address as a policy boundary for

assigning an IP addressing policy, then replacing switch “A” makes it

necessary to reconfigure the upstream DHCP server(s) to recognize the

MAC address of the replacement switch. This does not apply in the case

where an upstream relay agent “B” is configured with option 82 replace,

which removes the Option 82 field originally inserted by switch “A”.

■ Relay agents without Option 82 can exist in the path between Option 82

relay agents and an Option 82 server. The agents without Option 82 will

forward client requests and server responses without any effect on Option

82 fields in the packets.

■ If the routing switch is not able to add an Option 82 field to a client’s DHCP

request due to the message size exceeding the MTU (Maximum Transmis-

sion Unit) size, then the request is forwarded to the DHCP server without

Option 82 information and an e rror message is logged in the switch’s Event

Log.

DHCP Packet Forwarding

The DHCP relay agent on the routing switch forwards DHCP client packets to

all DHCP servers that are configured in the table administrated for each VLAN.

Unicast Forwarding

The packets are forwarded using unicast forwarding if the IP address of the

DHCP server is a specific host address. The DHCP relay agent sets the

destination IP address of the packet to the IP address of the DHCP server and

forwards the message.

Broadcast Forwarding

The packets are forwarded using broadcast forwarding if the IP address of the

DHCP server is a subnet address or IP broadcast address (255.255.255.255).

The D HC P relay ag ent sets the DHCP server I P address to br oadcast IP ad dr ess

and will be forwarded to all VLANs with configured IP interfaces (except the

source VLAN).

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ProCurve 2900 Betriebsanweisung Seite 90