Westermo RedFox Series - Manual (Page 523)

Westermo OS Management Guide
Version 4.17.0-0
to ignore (see section 22.1.4.2 in the DHCP server chapter for more information).
The number of ”copies/versions” of a DHCP request can increase further if a
LAN consists of several switches with DHCP relay agents (discussed later on, see
fig. 23.5 ).
To mitigate multiplication of broadcast DHCP messages, some switches support
DHCP snooping (see also
section 23.1.5 for an alternative approach). With DHCP
snooping enabled on an Ethernet/DSL port, all DHCP packets will pass through
the DHCP relay agent – this includes broadcast and unicast DHCP packets, both
DHCP requests (to server) or DHCP responses (from server) coming in on that
port.
Fig. 23.4 b) shows the result when a broadcast DHCP packet comes in on a
port with DHCP snooping enabled.
When configuring a WeOS relay agent on a VLAN interface, all ports on that VLAN
will have DHCP snooping enabled
- the exception is products lacking hardware support for DHCP snooping 3 . More
fine-grain control to enable/disable DHCP snooping per port may be supported in
later WeOS versions.
DHCP relay service can be disabled on a per port basis. If DHCP relaying is dis-
abled on an Ethernet/DSL port, incoming DHCP packets will be switched as other
layer-2 packets (no DHCP snooping), and the DHCP relay agent on the switch will
ignore DHCP requests entering the switch on that port.
Fig. 23.5 presents an example where multiple relays are located within the same
VLAN – port 1-6 on all RA units are in the same VLAN, while port 7 on RA1 and
RA2 are associated with another VLAN used and used as upstreams interface. The
topology in
fig. 23.5 utilise several WeOS features to achieve a robust network:
FRNT ( chapter 14 ) is used to handle single link failures within the local network.
VRRP ( chapter 30 ) is used to handle router redundancy (RA1 and RA2). A second
DHCP server to protect against DHCP server failure 4 .
The relay agents (RA1-RA5) server DHCP clients connecting to the local access
ports (ports 1-4), and will relay each request (unicast) to the configured DHCP
server(s). Below a sample DHCP relay configuration is shown, which would be
suitable for all relay agents in
fig. 23.5 .
3 In WeOS products, DHCP Snooping is supported on all Ethernet/DSL ports, except for ports of
switchcore(s): MV88E6095, MV88E6185 and MV88E6046. Please see Detailed System Overview
page in the Web (
section 4.4.2 ) or use the ”show system-information” in the CLI ( section 7.3.2 )
to find information about what switchcore(s) is used in your product.
4 As of WeOS v4.17.0, the WeOS DHCP server ( chapter 22 ) does not provide dedicated DHCP
server failover support, but you can achieve redundancy using ”static” address assignment (no
address pools) with the same configuration at both DHCP servers.
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