Cisco 7609-S Software Configuration Guide - Page 286

Understanding How STP Works Chapter 20 Configuring STP and Prestandard IEEE 802.1s MST Disabled State A Layer 2 LAN port in the disabled state does not participate in frame forwarding or STP, as shown in Figure 20-7. A Layer 2 LAN port in the disabled state is virtually nonoperational. Figure 20-7 Interface 2 in Disabled State All segment frames Forwarding Station addresses Port 1 BPDUs Network management and data frames Filtering database System module Frame forwarding S5696 Data frames Port 2 Network management frames Disabled All segment frames A disabled Layer 2 LAN port performs as follows: • Discards frames received from the attached segment. • Discards frames switched from another port for forwarding. • Does not incorporate end station location into its address database. (There is no learning, so there is no address database update.) • Does not receive BPDUs. • Does not receive BPDUs for transmission from the system module. STP and IEEE 802.1Q Trunks 802.1Q trunks impose some limitations on the STP strategy for a network. In a network of Cisco network devices connected through 802.1Q trunks, the network devices maintain one instance of STP for each VLAN allowed on the trunks. However, non-Cisco 802.1Q network devices maintain only one instance of STP for all VLANs allowed on the trunks. When you connect a Cisco network device to a non-Cisco device through an 802.1Q trunk, the Cisco network device combines the STP instance of the 802.1Q VLAN of the trunk with the STP instance of the non-Cisco 802.1Q network device. However, all per-VLAN STP information is maintained by Cisco network devices separated by a cloud of non-Cisco 802.1Q network devices. The non-Cisco 802.1Q cloud separating the Cisco network devices is treated as a single trunk link between the network devices. 20-12 Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX OL-4266-08