3Com 10014299 manual Bpdu Forwarding Mechanism, Spanning tree topology

296CHAPTER 19: CONFIGURING BRIDGE

Figure 110 Spanning tree topology

Root Bridge/

Designed Bridge

DP

DP

DP

Bridge 1 DP

RP

DP Designated

Bridge

DPBridge 2 DP

	DP	DP
RP

Bridge 3Designated DP

Bridge

RP DP

Bridge 4Designated DP

Bridge

RP		DP
Bridge 5		DP
	Designated
	Bridge

Hub

Hub

RP = Root Port

DP= Designated Port

BPDU Forwarding Mechanism

Upon the initiation of the network, all the bridges assume themselves as the root bridge. The designated interface of the bridge regularly sends its BPDU once a Hello Time. If it is the root port receives the BPDU, it will increase the Message Age carried in the BPDU and enable the timer to time this BPDU. If a path fails, the root port on this path will not receive new BPDUs any more and old BPDUs will be discarded due to timeout, which will result in the spanning tree recompilation. A new path will thus be generated to replace the failed one.

However, the recomputed new BPDU will not be propagated throughout the network right away, so the old root port and designated ports that have not detected the topology changes will still forward the data through the old path. If the newly elected root port and designated ports begin to forward data immediately, a temporary loop may be introduced. In STP, a transitional state mechanism is thus adopted. Specifically, the root port and the designated ports will undergo a transitional state for an interval of forward delay to enter the forwarding state to resume the data forwarding. Such a delay ensures that the new BPDU has already been propagated throughout the network before the data frames are forwarded according to the latest topology.

Multi-Protocol Router Generally, a router is called multi-protocol router when it can implement the routed protocols like IP and IPX, as well as the bridging protocol. For a multi-protocol router, the bridging protocol can be either enabled or disabled. However, if both the routed protocols and the bridging protocols are enabled on a router, the router will be taken as a multi-protocol router. In this case, whether a packet should be routed through IP or IPX or forwarded via the bridge will depend on the protocol type of the packet. For example, bridging protocol and IP are concurrently enabled on a router. If the packet to be processed is an IP packet, it