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| 3.1 |  | Spanning-Tree Protocol Operation |
| 3.1.2 |  | Broadcast loops |

Broadcasts and physical loops are a dangerous combination. Figure shows how broadcast loops are generated. Two switches, Cat-1 and Cat-2, have STP disabled. Host A and Host B are connected to Cat-1 and Cat-2 by a hub.

Basic STP Operation begins by sending a frame to the broadcast MAC address FF-FF-FF-FF-FF-FF, which travels to both Cat-1 and Cat-2. When the frame arrives at Port-1/1 on Cat-1, Cat-1 follows the standard transparent bridging algorithm and floods the frame out all the other ports, including Port 1/2. The frame exiting Port 1/2 travels to all nodes on the lower Ethernet segment, including Port1/2 on Cat-2. Cat-2 floods the broadcast frame out all the other ports, including Port 1/1, and, once again, the frame shows up at Port-1/1 on Cat-1. Oblivious to the loop, Cat-1 sends the frame out Port 1/2 for the second time and a broadcast loop is now in effect.

The discussion so far has not mentioned that not only would Cat-1 have received the initial broadcast from Host A, but Cat-2 as well. The same problem would propagate in the reverse direction as a result. The “feedback” loop would occur in both directions.

An important conclusion to be drawn is that bridging loops are much more dangerous than routing loops. Why is this? Suppose the initial broadcast frame were an Ethernet version II frame, as shown in Figure .

The Ethernet version II frame contains only two MAC addresses, a Typefield, a cyclic redundancy check (CRC), and the network-layer packet as data. In contrast, an IP header contains a Time To Live (TTL) field that is set by the source and is decremented at each router. By discarding packets that reach TTL = 0, routers prevent “runaway” datagrams. Unlike IP, Ethernet does not have a TTL field. Therefore, after a frame starts to loop in the network as just described, it continues until a switch is turned off or a link is broken. The network shown in Figure is extremely simple. Feedback loops of this type grow exponentially as additional switches and links are incorporated into the network. A single broadcast can render a network unusable, all because STP is disabled or configured incorrectly.