Every bridge and associated port is in one of the following spanning tree states: • Disabled—The port is not participating in spanning tree and is not active. • Listening—The port has received data from the interface and will listen for frames. In this state, the bridge receives only data and does not forward any frames to the interface or to other ports. • Learning—In this state, the bridge still discards incoming frames. The source address associated w..
Cisco IOS devices have the capability to log messages about system activity. These log messages can be useful in tracking system activity, errors, and notifications. Logging uses eight levels of notification messages, as summarized in Table 7-1. Table 7-1. Cisco IOS Logging Messages Level Description Level 0—Emergencies The system has become unusable. Level 1—Alerts Immediate action is needed to restore system stabili..
When congestion occurs, queues begin to fill, and in some cases, frames must be tail-dropped from the queues. Switches can (but are not required to) examine the FR Discard Eligibility (DE) bit when frames need to be discarded, and purposefully discard frames with DE set instead of frames without DE set. Both routers and switches can set the DE bit. Typically, a router makes the decision about setting the DE bit for certain frames, because the network engineer that controls ..
The link-state request packet, OSPF packet type 3, is sent in response to a router during the database exchange process. This request is sent when a router detects that it is missing parts of the database or when the router has a copy of LSA older than the one it received during the database exchange process. Figure 9-5 shows fields in the link-state request packet. The request packet contains each LSA specified by its LS type, link-state ID, and advertising router. This uniq..
Segmenting LANs with Repeaters
Legacy Ethernet systems such as 10Base5, 10Base2, and 10BaseT have distance limitations for segments as described in Chapter 1, "Desktop Technologies." Whenever you desire to extend the distance, you can use an internetworking device like a repeater. Repeaters operate at Layer 1 of the OSI model and appear as an extension to the cable segment. Workstations have no knowledge of the presence of a repeater which is completely transparent to the attached devices. A repeater attac..
