How Bridging Works - Intel ER8100STUS - Express 8100 Router Reference Manual

Bridge Spoofing
Loop-Back check

How Bridging Works

When are packets routed
and when are they
bridged?
Bridging TCP/IP and
Novell IPX
Bridging and routing
services
94
The Spanning Tree Algorithm is described in
108.
Within bridged networks which implement the Spanning Tree Algorithm, con-
figuration BPDU packets are transmitted between the bridges to communicate to-
pology and other changes. For financial reasons, it is desirable to avoid the
unnecessary transmission of configuration BPDU packets over WAN links.
A Bridge Spoofing feature is available in the Intel Express 8100 Router which
stops the transmission of configuration BPDU packets over a WAN link unless
they are important. This is known as BPDU Spoofing.
Another feature of Bridge Spoofing is that it stops the forwarding of Non-BPDU
packets over WAN links to ports which are in a Blocking State. A Blocking State
can be assigned to a bridges port by the Spanning Tree Algorithm and the bridge
port then discards all packets it receives. Forwarding packets to a bridge port
which is Blocking is therefore unnecessary and is a waste of WAN resources.
This is known as Non-BPDU Spoofing.
Bridge Spoofing is described in
A loop-back check is implemented in this router to ensure that packets transmit-
ted received back on the same link are discarded. This feature stops loop-backs
in the network segments, for example, if a WAN link is setup to loop back pack-
ets.
If a packet is received for forwarding by the router and the appropriate routing
protocol is enabled, the packet is routed. Otherwise the packet is bridged.
The Intel Express 8100 Router is normally set up to route TCP/IP and Novell
IPX. If routing of these protocols is disabled, the router automatically bridges
them if bridging services are enabled. The router cannot route a protocol to be-
tween two links and bridge them between others, a routing protocol must be dis-
abled completely before the protocol is bridged. However, IPX routing services
must be configured for one particular Ethernet packet type (SNAP, TYPE, 802.2
or 802.3). All other IPX Ethernet packets types are bridged if bridging services
are enabled.
Bridging Services operate at the MAC Layer (also known as Data Link Layer or
Layer 2). Bridging Services operate independently of Routing Protocols which
are Network layer (Layer 3) services. Bridging can therefore be used to forward
packets in any Network Layer protocol.
WAN Bridging Services
"Spanning Tree Algorithm", p.
"Bridge Spoofing", p. 114.
How Bridging Works