Uplink Fast
Sections:
Overview:
- UplinkFast is designed to speed up the convergence of STP in scenarios where a root port or primary link goes down
- Normally, STP takes time to reconverge when there's a failure in the network topology
- UplinkFast is built into the RSTP/RPVST+ or MST standard by default
How it Works:
- By configuring Uplink-Fast, the switch keeps track of alternative (backup) paths to the root bridge
- These backup paths are pre-calculated and ready to be used in case the primary path fails.
- When the root port fails, the switch immediately transitions to the best available backup path.
- If enabled in 802.1D STP standard, the priority value of the switch will become least preferred automatically by increasing the value to '49,152'
When to use:
- Uplink-Fast is commonly utilized at the edge access of the network where there are redundant uplinks (backup root ports)
- Not recommended for core switches because of the potential complexity and scale of the network in those roles
- UplinkFast is typically manually configured with older versions of STP (802.1D STP/PVST)
Effects of Spanning Tree without Uplink-Fast
In this lab example, lets demonstrate a root port failover scenario on HQ-Access-SW1 by effectively shutting down its Gi2/0 interface
After the effects of shutting down port Gi2/0 on the access switch, the failover root port Gi0/2 has to follow the forward delay convergence timers of the 802.1D Spanning Tree standard before fully transitioning over as the new root port
Effects of Spanning Tree with Uplink-Fast
With a Uplink-Fast configuration, there are three things that occur:
- The convergence timers for the root port failover are almost immediate granting the original Spanning Tree standard the benefits of Rapid Spanning Tree or RPVST+
- The priority value of the Uplink-Fast switch in 802.1D increases to '49172' to prevent the switch from becoming the Root Bridge and to prevent STP looping during future root port failover
- The port path cost of all links of the switch increase considerably to prevent the switch from becoming the Root Bridge of the Spanning Tree domain
