二层交换技术-初级一.ppt

Slide 1 of 1 Purpose: Emphasize: Slide 1 of 1 Purpose: Emphasize: A looped topology is often desired to provide redundancy, but looped traffic is undesirable. The Spanning Tree protocol was originally designed for bridges. Today, it is also applied to LAN switches and routers operating as a bridge. STP ensures all bridged segments are reachable but any points where loops occur will be blocked. Slide 1 of 1 Purpose: Emphasize: The three general rules when dealing with STP: 1. One root bridge per network. The root is the bridge with the lowest Bridge ID. All the ports on the root bridge are designated ports (forwarding). 2. For every non root bridge, there is a root port (forwarding). The root port is the port with the lowest accumulated path cost to the root bridge. 3. For every segment, there is only one designated port. The designated port forwards traffic for the segment. The designated port has the lowest accumulated path cost to the root bridge. Slide 1 of 1 Purpose: Emphasize: By default, the switch with the lowest MAC address will be the Root bridge. Note: The Catalyst switches support an instance of Spanning tree per vlan. Each vlan will use an unique mac address for Spanning tree purpose. On the Cat 1900, the address it uses for Spanning tree are the Mac address on the various ports. VLAN is discussed in the next chapter. The IEEE 802.1d spec specify for a 16 bit priority field. The Cat 1900 switch only supports the 802.1d stp. The default priority on the Cat 1900 is 32,768 in decimal or 8000 in hex, the midrange value. BPDU contain the following fields: Protocol ID version Message type Flags Root ID Cost of path Bridge ID Port ID Message age Max age Hello time Forward delay Slide 1 of 1 Purpose: Emphasize: RPs and DPs are normally in the forwarding state. There is only one DP per segment. Slide 1 of 1 Purpose: Emphasize: There are two cost calculation method. The Cat1900 uses the older method. The new method is design