Wave NCBSLXE2, Installation And Operation User Manual

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SPEEDLAN Installation and Operation User Guide
Bridging Setup 6-5
Advanced Features Button
Clicking Advanced displays this dialog box. Select the appropriate check box for your network. The
check boxes are described below:
•
Pass Bad Ethernet Source
The standard Ethernet bridges we have tested will pass Ethernet packets with a broadcast or
multicast address as their source (i.e., packets with their first bit set to 1). The Ethernet
specification for Transparent (i.e. Non-Source-Routing) bridges does not allow these types
of packets, which are considered bad packets. Our studies have shown that a common
failure mode of many Ethernet interfaces and networking software is to transmit packets like
these. If you do not need to permit Source-Routing packets, we suggest that you deny these
packets. The default setting is selected to permit these packets.
•
Pass Unseen Ethernet Source
Standard Ethernet bridges will always forward packets with destination addresses that have
not been learned (i.e., have not previously been seen as a source address of a packet). This
characteristic is needed in order for the Ethernet bridge to operate correctly. The downside
to this, as our studies have shown, is the failure mode of many Ethernet interface cards will
send out erroneous packets with good CRCs but with random Ethernet destination and
source addresses. Standard bridges will permit these erroneous packets because they have
not "learned" the random destination, and then add this packet's random source address to
their finite learned table. This situation is not uncommon and can greatly hinder the
operation of standard bridges. If you choose to deny unlearned packets, the brouter will not
forward unicast packets to Ethernet addresses that have not already been seen as a source
address. This scheme works for most protocols because it relies on the characteristics of
most upper-layer protocols to transmit ARP requests or hello packets.
After careful testing
and consideration, only qualified network engineers should select the Deny option.
The default value for this setting is selected.