Westermo RedFox Series - Manual (Page 522)

Westermo OS Management Guide
Version 4.17.0-0
23.1.4 Relay Agents in Switched Networks
The DHCP protocol uses layer-2 broadcast (Destination MAC: ff:ff:ff:ff:ff) for some
of its protocol messages. Therefore, a (broadcast) DHCP packet coming in to a
switch, will typically be flooded on all ports of the same LAN. This is illustrated in
fig. 23.4 a):
❼ A broadcast DHCP message comes in on port ”A” of the switch (step ”1a”).
❼ The message is broadcasted unmodified on all other ports within the LAN
(here ports ”B”-”F”), see step ”1b”.
❼ In this case, the switch is also running a DHCP relay service on the LAN.
The relay agent will process the incoming DHCP packet, and forwards it to
the configured DHCP server, which here happens to reside in the direction
of port ”E” (step ”2”). The packet in step ”2” is modified as compared to
the initial broadcast packet: It is sent as unicast to the DHCP server, and it
contains the relay agents IP address as giaddr. If the relay agent has DHCP
option 82 enabled, such information is also added.
Relay
Agent
2
Towards
DHCP
Server
Relay
Agent
2
Towards
DHCP
Server
1a 1b 1b 1b
1b
1b
1a
1b 1b
a) No DHCP snooping support b) DHCP snooping supported
A B C D
F E
Broadcast
DHCP
packet
A B C D
F E
Broadcast
DHCP
packet
Figure 23.4: Propagation of DHCP broadcast packets in switches running DHCP
relay agents. All ports are on the same (V)LAN. The switch in figure a) does not
support DHCP snooping, while the switch in figure b) supports DHCP snooping.
As seen in fig. 23.4 a), using (layer-2) switches as DHCP Relay Agents can result
in multiple versions of a DHCP message to be sent towards the DHCP server:
the original request being switched/broadcasted, and the one being relayed by
the relay agent process. This will not cause any problems if the DHCP server
is located on some remote network; then only the relayed packet will reach the
server. However, if the DHCP server is located within the same LAN, adequate
support is needed at the DHCP server to know which request to serve and which
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