| 5.5 | Redundant Links | ||
| 5.5.6 | Configuring UplinkFast |
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STP ensures that a loop-free topology is
maintained even in the face of topology changes. However, the
convergence to a loop-free topology can be time-consuming.
Implementing UplinkFast is one method to reduce this time.
During the time it takes for STP to converge, some end stations may become inaccessible, depending on the STP state of the switch port to which the station is attached. This represents a major disruption in network connectivity. Thus, decreasing convergence time and reducing the length of the disruption is a major factor in deciding to deploy redundancy. UplinkFast was developed in response to the need for fast convergence. UplinkFast allows a blocked port on a switch to almost immediately begin forwarding when the switch detects the failure of the forwarding link. UplinkFast must have direct knowledge of the link failure in order to move a blocked port into a forwarding state. Specifically, an uplink group consists of the root port, which is forwarding, and a set of blocked ports, except for self-looping ports. The uplink group provides an alternate path in case the currently forwarding link fails. The STP UplinkFast feature has been designed specifically for use with access-layer switches. This feature is not designed for use within the core layer of the network. In this example, the access switch has two uplinks to the distribution layer as follows:
One of the uplinks is redundant and is blocked by STP. As soon as a switch detects a link-down condition on the currently active link, UplinkFast moves the blocked port to the forwarding state. UplinkFast changes the port without passing through the listening and learning phases. Reconfiguration occurs within 3 to 4 seconds without going through the usual convergence time. The conditions for UplinkFast to trigger a fast reconfiguration are as follows:
UplinkFast initiates the reconfiguration as soon as the switch detects a link down on the root port. This process allows convergence to begin immediately without waiting for the MaxAge timer to expire. It is important to note that UplinkFast should be configured only on access-layer switches, because quick transitions of the root port to its nondesignated backup(s) can be deterministically achieved only at the end leaf-node switches in a Spanning-Tree topology. If UplinkFast is not being used, using the switch default settings is strongly recommended. The UplinkFast feature checks if a switch is itself the root bridge and if so, disables this feature. To enable UplinkFast on a set command-based switch, enter the following command in privileged mode:
To enable UplinkFast on a Cisco IOS command-based switch, enter the following command in global configuration mode:
The set spantree uplinkfast enable or spanning-tree uplink-fast command increases the path cost of all ports on the switch, making it unlikely that the switch will become the root switch. The station_update_rate value represents the number of multicast packets transmitted per 100 milliseconds (the default is 15 packets per millisecond). Note: When you enable the set spantree uplinkfast command, it affects all VLANs on the switch. You cannot configure UplinkFast on an individual VLAN.
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