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These blocked links are placed in a standby condition, and may be automatically enabled
to repair the network if another link is lost. The Spanning Tree Algorithm maintains a
single path between all nodes in a network, by forming a tree-like structure. The Bridge
Priority determines where the node sits in the tree. A Bridge with the lowest priority
configured (0) will become the root node in the network, and will direct traffic between
each of its branches. The root node is typically the unit that handles the majority of traffic
in the network. As a low bandwidth radio device, the RM-90 is configured with a Bridge
Priority of (255) by default. The intention is reduce traffic that the RM-90 must handle, by
placing it at the branch level in the network tree. As a branch, the RM-90 needs only pass
traffic to devices that are its “leaves”.
There is some overhead in maintaining a network utilizing the Spanning Tree Algorithm.
Users wishing to increase their throughput, at the expense of redundancy should disable
Spanning Tree.
3.11 Wireless Message Filtering
When configured as a Bridge, the RM-90 will transmit all broadcast messages appear-
ing at its wired Ethernet port. When the RM-90 is configured as a Router, this does not
occur.
In many cases, the intended recipient of the broadcast traffic does not lie at the opposite
end of a proposed radio link. Reducing unnecessary broadcast traffic sent over the radio
link, will increase available bandwidth for data. The RM-90 has a filtering feature to help
reduce unnecessary wireless transmissions and enhance security.
The RM-90 may be configured to reject or accept messages to and from certain
Addresses. To accept wireless messages from particular devices a “Whitelist” of
Addresses must be made. Alternatively to reject messages from particular devices, a
“Blacklist” of Addresses must be made. Filtering applies only to messages appearing at the
wired Ethernet port of the configured RM-90.
The Filter comprises of two lists: one of MAC Addresses and another listing IP protocol
details. Each list may be set as either a blacklist (to block traffic for listed devices and
protocols), or as a whitelist (to allow traffic for listed devices and protocols). The Filter
operates on two rules listed below.
1. A Blacklist has priority over a whitelist. Traffic matching detail in a blacklist will be dis-
carded if it also appears in a whitelist.
2. When one or both lists are whitelists, traffic must have matching detail in at least one
of the whitelists for it to be passed. Note that, as this must agree with rule 1 above,
the traffic detail must not match anything in a blacklist, if present, for it to be passed.
When configuring a Whitelist it is important to add the Addresses of all devices connect-
ed to the RM-90 wired Ethernet port, that communicate over the wireless link. It is par-
ticularly important to add the Address of the configuration PC to the Whitelist. Failure
to add this address will prevent the configuration PC from making any further changes
to configuration. Design of the filter may be simplified by monitoring network traffic and
forming a profile of traffic on the wired network. Network Analysis software, such as the
freely available Ethereal program, will list broadcast traffic sent on the network.
