3Com 10014299 manual Preliminary examination state of bridging loops

294CHAPTER 19: CONFIGURING BRIDGE

Figure 108 Preliminary examination state of bridging loops

Broadcast address	Bridge Y
FFFFFFFFFFFF

Ethernet segment 1 Ethernet segment 2

Bridge X	Bridge Z
	Ethernet segment 3

As shown in the following figure, the broadcast frame is forwarded over Ethernet segment 2 and Ethernet segment 3 that are connected with Bridge Z. Upon detecting two copies of this frame on two different ports, Bridge Z forwards them to Ethernet segment 3 and Ethernet segment 2 again. Thus, Ethernet segment 2 and Ethernet segment 3 receive a copy of this frame for the second time. Like this, the frame is repeatedly forwarded over the network, which is called bridging loop.

Figure 109 Bridging loop

					Forwarding broadcast frame
						FFFFFFFFFFFF
Broadcast frame
							FFFFFFFFFFFF
	FFFFFFFFFFFF		Bridge Y
				Forwarding broadcast frame again
Ethernet segment 1
								Ethernet segment 2
Bridge X		Forwarding broadcast frame						Bridge Z
			FFFFFFFFFFFF
								Ethernet segment 3
			FFFFFFFFFFFF
		Forwarding broadcast frame again

In practice, if there are hundreds of physical segments, bridging loops will cause a sharp decline to the network performance. After the location where loops occur is detected, the only solution is to cut off all connections. It is obvious that eliminating loops is an essential requirement for ensuring the bridge working normally. Therefore, the third function of bridge is to locate loops and block redundant ports.

Spanning Tree Protocol Spanning Tree Protocol (STP) is used to prevent redundant paths through certain algorithms. A loop network is thus pruned to be a loop-free tree network so as to avoid the infinite cycling of data frames in the loop network.

STP transmits a type of special data frame called Bridge Protocol Data Unit (BPDU) between bridges. The overall network will compute a minimum spanning tree describing the distribution of bridges in the network. This minimum spanning tree