Motorola 68241 User Guide - Page 47

Using Bridging Routers to Connect LANs and Networks Using Bridging Routers to Connect LANs and Networks Introduction A bridging router is a device in which both bridge and router software run concurrently. The Vanguard internetworking systems provide bridging router functions. Note For additional information on Bridging, refer to the Bridging Manual (Part Number T0100-02). Operation of Bridging Router Bridging and Routing in the Vanguard Node A bridging router handles packets as follows: • Routes packets if a specific routing protocol is globally enabled. • Filters packets if you configure specific protocol filters. • Bridges packets if they are not routed or filtered. In this case, they are forwarded according to their destination MAC address. Note that Vanguard support both bridging and routing within the same node for the same protocol. When using this feature, it is important to configure filters that disable either routing or bridging between the two end points to prevent packet duplication. Bridges work at the Data Link layer providing connectionless service. Routers operate at the Network layer and, therefore, offer some flexibility in choosing several network level services. How Vanguard Bridging Routers Work Vanguard bridging routers examine every packet on a network. The source host builds a packet from the Application layer downward to the Physical layer and passes the packet over the network. There can be several different values for the Data Link (MAC) destination address used with the packet. The Data Link address may be: • Addressed to the Motorola internetworking node itself (case 1) • Addressed to some other node (case 2) • Addressed as a broadcast packet (case 3) Case 1 In case 1, the frame is addressed to the Vanguard internetworking node, as described here: • A host sends a frame to the MAC address of the Vanguard node, itself. • The packet is given to the routing forwarder within the node. The bridging forwarder is not used in this case. • The routing forwarder accesses the Network layer that contains the destination address of the packet. • The forwarder compares the Network layer destination address to the addresses in the routing table to obtain the best match. - If the best match is made, the router rebuilds the Data Link layer and passes the packet to the next hop. - If no match is made, the router discards the packet. For IP, an unreachable packet is transmitted back to the sender. Vanguard Routing Model T0100-01, Revision B 2-31 Release 5.4