Cisco Catalyst 4500 Series, Руководство по настройке программного обеспечения, Страница: 708 / 2086

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Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS 15.2(5)Ex
Chapter 23 Configuring STP and MST
About STP
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Disabled—In this state, the Layer 2 interface does not participate in spanning tree and does not
forward frames.
MAC Address Allocation
The supervisor engine has a pool of 1024 MAC addresses that are used as the bridge IDs for the VLAN
spanning trees. Use use the
show module command to view the MAC address range (allocation range
for the supervisor) that the spanning tree uses for the algorithm.
MAC addresses for the Catalyst 4506 switch are allocated sequentially, with the first MAC address in
the range assigned to VLAN 1, the second MAC address in the range assigned to VLAN 2, and so forth.
For example, if the MAC address range is 00-e0-1e-9b-2e-00 to 00-e0-1e-9b-31-ff, the VLAN 1 bridge
ID is 00-e0-1e-9b-2e-00, the VLAN 2 bridge ID is 00-e0-1e-9b-2e-01, the VLAN 3 bridge ID is
00-e0-1e-9b-2e-02, and so on. On other Catalyst 4500 series platforms, all VLANS map to the same
MAC address rather than mapping to separate MAC addresses.
STP and IEEE 802.1Q Trunks
802.1Q VLAN trunks impose some limitations on the spanning tree strategy for a network. In a network
of Cisco switches connected through 802.1Q trunks, the switches maintain one instance of spanning tree
for each VLAN allowed on the trunks. However, non-Cisco 802.1Q switches maintain only one instance
of spanning tree for all VLANs allowed on the trunks.
When you connect a Cisco switch to a non-Cisco device (that supports 802.1Q) through an 802.1Q trunk,
the Cisco switch combines the spanning tree instance of the 802.1Q native VLAN of the trunk with the
spanning tree instance of the non-Cisco 802.1Q switch. However, all per-VLAN spanning tree
information is maintained by Cisco switches separated by a network of non-Cisco 802.1Q switches. The
non-Cisco 802.1Q network separating the Cisco switches is treated as a single trunk link between the
switches.
Note
For more information on 802.1Q trunks, see Chapter 19, “Configuring Layer 2 Ethernet Interfaces.”
Per-VLAN Rapid Spanning Tree
Beginning in Cisco IOS XE Release 3.8.0E and Cisco IOS Release 15.2(4)E, Per-VLAN Rapid Spanning
Tree (PVRST+) is the default STP mode on Cisco Catalyst 4500 series, Cisco Catalyst 4900M, Cisco
Catalyst 4948E and Cisco Catalyst 4948F switches.
Per-VLAN Rapid Spanning Tree is the same as PVST+, although PVRST+ utilizes a rapid STP based
on IEEE 802.1w rather than 802.1D to provide faster convergence. PVRST+ uses roughly the same
configuration as PVST+ and needs only minimal configuration. In PVRST+, dynamic CAM entries are
flushed immediately on a per-port basis when any topology change is made. UplinkFast and
BackboneFast are enabled but not active in this mode, because the functionality is built into the Rapid
STP. PVRST+ provides for rapid recovery of connectivity following the failure of a bridge, bridge port,
or LAN.
Like per-VLAN Spanning Tree (PVST+), per-VLAN Rapid Spanning Tree (PVRST+) instances are
equal to the number of vlans configured on the switch and can go up to a maximum of 4094 instances.
For enabling information, see “Enabling Per-VLAN Rapid Spanning Tree” on page 20.