Figure 3: Spanning Tree and Redundant Connectivity You can also create redundant links between devices by using EtherChannel groups. Spanning-Tree Address Management IEEE 802.1D specifies 17 multicast addresses, ranging from 0x00180C2000000 to 0x0180C2000010, to be used by different bridge protocols. These addresses are static addresses that cannot be removed. Regardless of the spanning-tree state, each device in the stack receives but does not forward packets that are destined for addresses between 0x0180C2000000 and 0x0180C200000F. If spanning tree is enabled, the CPU on the switch or on each switch in the stack receives packets that are destined for 0x0180C2000000 and 0x0180C2000010.
If spanning tree is disabled, the switch or each switch in the stack forwards those packets as unknown multicast addresses. Accelerated Aging to Retain Connectivity The default for aging dynamic addresses is 5 minutes, the default setting of the mac address-table aging-time global configuration command. However, a spanning-tree reconfiguration can cause many station locations to change. Because these stations could be unreachable for 5 minutes or more during a reconfiguration, the address-aging time is accelerated so that station addresses can be dropped from the address table and then relearned.
The accelerated aging is the same as the forward-delay parameter value ( spanning-tree vlan vlan-id forward-time seconds global configuration command) when the spanning tree reconfigures. Because each VLAN is a separate spanning-tree instance, the switch accelerates aging on a per-VLAN basis. A spanning-tree reconfiguration on one VLAN can cause the dynamic addresses that are learned on that VLAN to be subject to accelerated aging. Dynamic addresses on other VLANs can be unaffected and remain subject to the aging interval entered for the switch. Spanning-Tree Modes and Protocols The device supports these spanning-tree modes and protocols:
• PVST+—This spanning-tree mode is based on the IEEE 802.1D standard and Cisco proprietary extensions. The PVST+ runs on each VLAN on the device up to the maximum supported, ensuring that each has a loop-free path through the network. The PVST+ provides Layer 2 load-balancing for the VLAN on which it runs. You can create different logical topologies by using the VLANs on your network to ensure that all of your links are used but that Configuring Spanning Tree Protocol Spanning-Tree Address Management Configuring Spanning Tree Protocol 8