Cisco WS-X6K-SUP1A-2GE-RF Software Guide - Page 117

Chapter 8 Configuring Spanning Tree Understanding How Spanning Tree Protocols Work Calculating the Port Cost Using the Long Method 802.1t assigns 32-bit (long) default port cost values to each port using a formula that is based on the bandwidth of the port. You can also manually assign port costs between 1 and 200,000,000. The formula for obtaining default 32-bit port costs is to divide the bandwidth of the port by 200,000,000. Table 8-1 shows the default port cost values that are assigned by the switch and the recommended cost values and ranges for each type of port when you use the long method to calculate port cost. Table 8-2 Default Port Cost Values Using the Long Method Port Speed ≤100 kbps 1 Mbps 10 Mbps 100 Mbps 1 Gbps 10 Gbps Recommended Value 200000000 20000000 2000000 200000 20000 2000 Recommended Range 20000000 to 200000000 2000000 to 200000000 200000 to 20000000 20000 to 2000000 2000 to 200000 200 to 20000 Available Range 1 to 200000000 1 to 200000000 1 to 200000000 1 to 200000000 1 to 200000000 1 to 200000000 Calculating the Port Cost for Aggregate Links • As individual links are added or removed from an aggregate link (port bundle), the bandwidth of the aggregate link increases or decreases. These changes in bandwidth lead to recalculation of the default port cost for the aggregated port. Changes to the default port cost or changes resulting from links that autonegotiate their bandwidth could lead to recalculation of the spanning tree topology which may not be desirable, especially if the added or removed link is of little consequence to the bandwidth of the aggregate link (for example, if a 10-Mbps link were removed from a 10-Gbps aggregate link). Because of the limitations presented by automatically recalculating the topology, 802.1t states that changes in bandwidth will not result in changes to the cost of the port concerned. The aggregated port will therefore use the same port cost parameters as a stand alone port. Spanning Tree Port States Topology changes can take place in a switched network due to a link coming up or a link going down (failing). When a switch port transitions directly from nonparticipation in the topology to the forwarding state, it can create temporary data loops. Ports must wait for new topology information to propagate through the switches in the LAN before they can start forwarding frames. Also, they must allow the frame lifetime to expire for frames that have been forwarded using the old topology. Note With IOS Release 12.1.(1)E or later releases on the Multilayer Switch Feature Card (MSFC), the Address Resolution Protocol (ARP) on the STP Topology Change Notification feature ensures that excessive flooding does not occur when the MSFC receives a topology change notification (TCN) from the supervisor engine. The feature causes the MSFC to send ARP requests for all the ARP entries belonging to the VLAN interface where the TCN is received. When the ARP replies come back, the Policy Feature Card (PFC) learns the MAC entries, which were lost as a result of the topology change. Learning the entries immediately following a topology change prevents excessive flooding later. There is no configuration required on the MSFC. This feature works with supervisor engine software release 5.4(2) or later releases. 78-13315-02 Catalyst 6000 Family Software Configuration Guide-Releases 6.3 and 6.4 8-5