Motorola 110502USM001 - Vanguard 60 Router User Manual page 48

Using Bridging Routers to Connect LANs and Networks
Case 2
Case 3
Limitation for
Bridging and
Routing in the
Same Node
2-32
In case 2, where the Data Link address is different from the Vanguard node's own
data link address, the packet can be forwarded by bridging (assuming the packet is
not destined for a host on the local network). The NAP node examines every packet
on the LAN for possible bridging as described here:
• The packet is passed to the bridge forwarder in the Vanguard internetworking
node. The routing forwarder is not used for this type of packet.
• If using Transparent Bridging (TB), the bridge accesses the MAC address
associated with the packet.
- The TB scans its forwarding table looking for a match with the MAC
address of the packet it is processing. The matching entry in the table
includes information as to where to forward the frame.
- The frame is forwarded along its spanning tree links so that a copy
eventually reaches the desired destination if a match is not found.
• If using Source Route Bridging, the bridge accesses the Routing Information
Field (RIF) in the packet frame.
- The frame is forwarded according to the RIF.
- The RIF indicates the specific next link to use to forward the packet frame.
It may also indicate either a spanning tree broadcast or an all route
broadcast of the packet within the bridge network.
In case 3, a Broadcast Data Link address is used as described here:
• The Vanguard node gives a copy of the packet frame to the bridge forwarder.
• If Transparent Bridging is used, the frame is forwarded along the spanning
tree links so that a copy eventually reaches all networks.
• If Source Route Bridging is used, the frame is forwarded according to the RIF
in the same way as for a specific MAC address as previously described.
• An additional copy of the packet is given to the routing forwarder for
processing.
Although bridging and routing for the same protocol is supported within the same
Vanguard node, there is a basic limitation that these functions cannot occur for end
stations both using the same LANs. If this is attempted, as was described previously,
packets can be duplicated and sent by both the bridge and the router between the
same end stations, which causes communication failures. To avoid such
circumstances, use the filters that can be configured for both bridges and routers to
block the type of traffic that you do not want forwarded.
The Bridged Protocols parameter in the Configure Bridge Parameter menu controls
whether IP, IPX, or both protocols are forwarded over bridge links. In most cases, it
is better to route IP and IPX rather than bridge them. Disabling the forwarding of IP
or IPX with the Bridged Protocols parameter is like filtering the protocol on all
bridge links.
Vanguard Routing Model