Data Communications Networking Devices - 4th Ed.pdf

5.1 BRIDGES _____________________________________________________________________ 555Once path costs are assigned to each path in a network, each bridge will have oneor more costs associated with different paths to the root bridge. One of those costs islower than all other path costs. That cost is known as the bridge's root path cost andthe port used to provide the least path cost towards the root bridge is known as theroot port.Designated bridgeAs previously discussed, the spanning tree algorithm does not permit active loopsin an interconnected network. To prevent this situation from occurring, only onebridge linking two networks can be in a forwarding state at any particular time.That bridge is known as the designated bridge, while all other bridges linking twonetworks will not forward frames and will be in a blocking state of operation.Constructing the spanning treeThe spanning tree algorithm employs a three-step process to develop an activetopology. First, the root bridge is identi®ed. In Figure 5.8b) we will assume bridge1 B1) was selected as the root bridge. Next, the path cost from each bridge to theroot bridge is determined and the minimum cost from each bridge becomes the rootpath cost. The port in the direction of the least path cost to the root bridge, knownas the root port, is then determined for each bridge. If the root path cost is the samefor two or more bridges linking LANs, then the bridge with the highest prioritywill be selected to furnish the minimum path cost. Once the paths are selected, thedesignated ports are activated.In examining Figure 5.8a), let us now use the cost entries assigned to eachbridge. Let us assume bridge 1 was selected as the root bridge as we expect a largeamount of traf®c to ¯ow between Token-Ring 1 and Ethernet 1 networks.Therefore, bridge 1 will become the designated bridge between Token-Ring 1 andEthernet 1 networks.In examining the path costs to the root bridge, note that the path through bridge2 was assigned a cost of 10, while the path through bridge 3 was assigned a cost of15. Thus, the path from Token-Ring 2 via bridge 2 to Token-Ring 1 becomes thedesignated bridge between those two networks. Hence, Figure 5.8b) shows bridge3 inactive by the omission of a connection to the Token-Ring 2 network. Similarly,the path cost for connecting the Ethernet 3 network to the root bridge is lower byrouting through the Token-Ring 2 and Token-Ring 1 networks. Thus, bridge 5becomes the designated bridge for the Ethernet 3 and Token-Ring 2 networks.Bridge protocol data unitOne question that is probably in readers' minds by now is how does each bridgeknow whether or not to participate in a spanned tree topology? Bridges obtaintopology information by the use of Bridge Protocol Data Unit BPDU) frames.The root bridge is responsible for periodically transmitting a `HELLO' BPDUframe to all networks to which it is connected. According to the spanning treeprotocol, HELLO frames must be transmitted every 1 to 10 seconds. The BPDU