Port Fast Uplink Fast and Backbone Fast

The Cisco-proprietary PortFast, UplinkFast, and BackboneFast features each solve specific STP problems. Table 3-5 summarizes when each is most useful, and the short version of how they improve convergence time.

Table 3-5 PortFast, UplinkFast, and BackboneFast

KEY POINT

Feature

Requirements for Use

How Convergence Is Optimized

PortF ast

Used on access ports that are not connected to other switches or hubs

Immediately puts the port into forwarding state once the port is physically working

UplinkFast

Used on access layer switches that have multiple uplinks to distribution/core switches

Immediately replaces a lost RP with an alternate RP, immediately forwards on the RP, and triggers updates of all switches' CAMs

BackboneFast

Used to detect indirect link failures, typically in the network core

Avoids waiting for Maxage to expire when its RP ceases to receive Hellos; does so by querying the switch attached to its RP

PortFast

PortFast optimizes convergence by simply ignoring listening and learning states on ports. In effect, convergence happens instantly on ports with PortFast enabled. Of course, if another switch is connected to a port on which PortFast is enabled, loops may occur. So, PortFast is intended for access links attached to single end-user devices. To be safe, you should also enable the BPDU Guard and Root Guard features when using PortFast, as covered later in this chapter.

UplinkFast

UplinkFast optimizes convergence when an uplink fails on an access layer switch. For good STP design, access layer switches should not become root or become transit switches. (A transit switch is a switch that forwards frames between other switches.) Figure 3-7 shows the actions taken when UplinkFast is enabled on a switch, and then when the Root Port fails.

Figure 3-7 UplinkFast Operations

Access Design

Root Switch Non-Root Switch

Priority 8192 Priority (Default) 32768

Access Design

Root Switch Non-Root Switch

Priority 8192 Priority (Default) 32768

Uplinkfast

Upon enabling UplinkFast globally in a switch, the switch takes three actions:

■ Increases the root priority to 49,152

■ Sets the post costs to 3000

■ Tracks alternate RPs, which are ports in which root Hellos are being received

As a result of these steps, SW3 can become root if necessary, but it is unlikely to do so given the large root priority value. Also, the very large costs on each link make the switch unlikely to be used as a transit switch. When the RP port does fail, SW3 can fail over to an alternate uplink as the new RP and forward immediately.

The final step in Uplink Fast logic causes the switches to time-out the correct entries in their CAMs, but it does not use the TCN process. Instead, the access switch finds all the MAC addresses of local devices and sends one multicast frame with each local MAC address as the source MAC— causing all the other switches to update their CAMs. The access switch also clears out the rest of the entries in its own CAM.

BackboneFast

BackboneFast optimizes convergence for any generalized topological case, improving convergence when an indirect failure occurs. When some direct failures occur (for instance, a switch's RP interface fails), the switch does not have to wait for maxage to expire. However, when another switch's direct link fails, resulting in lost Hellos for other switches, the downstream switches indirectly learn of the failure because they cease to receive Hellos. Any time a switch learns of an STP failure indirectly, the switch must wait for maxage to expire before trying to change the STP topology.

BackboneFast simply causes switches that indirectly learn of a potential STP failure to ask their upstream neighbors if they know about the failure. To do so, when the first Hello goes missing, a BackboneFast switch sends a Root Link Query (RLQ) BPDU out the port in which the missing Hello should have arrived. The RLQ asks the neighboring switch if that neighboring switch is still receiving Hellos from the root. If that neighboring switch had a direct link failure, it can tell the original switch (via another RLQ) that this path to the root is lost. Once known, the switch experiencing the indirect link failure can go ahead and converge without waiting for maxage to expire.

NOTE All switches must have BackboneFast configured for it to work correctly.

PortFast, UplinkFast, and BackboneFast Configuration

Configuration of these three STP optimizing tools is relatively easy, as summarized in Table 3-6.

Table 3-6 PortFast, UplinkFast, and BackboneFast Configuration

Feature

Configuration Command

PortF ast

spanning-tree portfast (interface subcommand) spanning-tree portfast default (global)

UplinkFast

spanning-tree uplinkfast [max-update-rate rate] (global)

BackboneFast

spanning-tree backbonefast (global)

+9 -6

Responses

  • gerardina
    How to Enabling port fast, uplink fast, fast convergence?
    1 year ago
  • mungo
    What is uplinkfast and backbonefast?
    11 months ago
  • Madihah Yonas
    What is uplink fast and backbone fast?
    8 months ago

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