Understanding Cross Stack Uplink Fast

Cross-stack UplinkFast (CSUF) provides a fast spanning-tree transition (fast convergence in less than 1 second under normal network conditions) across a stack of switches that use the GigaStack GBICs connected in a shared cascaded configuration (multidrop backbone). During the fast transition, an alternate redundant link on the stack of switches is placed in the forwarding state without causing temporary spanning-tree loops or loss of connectivity to the backbone. With this feature, you can have a redundant and resilient network in some configurations. You enable CSUF by using the spanning-tree stack-port interface configuration command. The CSUF feature is supported only when the switch is running PVST.

CSUF might not provide a fast transition all the time; in these cases, the normal spanning-tree transition occurs, completing in 30 to 40 seconds. For more information, see the "Events that Cause Fast Convergence" section on page 12-7.

How CSUF Works

CSUF ensures that one link in the stack is elected as the path to the root. As shown in Figure 12-5, Switches A, B, and C are cascaded through the GigaStack GBIC to form a multidrop backbone, which communicates control and data traffic across the switches at the access layer. The switches in the stack use their stack ports to communicate with each other and to connect to the stack backbone; stack ports are always in the spanning-tree forwarding state. The stack-root port on Switch A provides the path to the root of the spanning tree; the alternate stack-root ports on Switches B and C can provide an alternate path to the spanning-tree root if the current stack-root switch fails or if its link to the spanning-tree root fails.

Link A, the root link, is in the spanning-tree forwarding state; Links B and C are alternate redundant links that are in the spanning-tree blocking state. If Switch A fails, if its stack-root port fails, or if Link A fails, CSUF selects either the Switch B or Switch C alternate stack-root port and puts it into the forwarding state in less than 1 second.

Figure 12-5 Cross-Stack UplinkFast Topology


Spanning-tree root



Link A (Root link)

100 or 1000 Mbps

Stack-root port

Switch A

Stack port

Link B (Alternate redundant link)

100 or 1000 Mbps

¡Alternate stack-root port

Switch B

Stack port

Link C (Alternate redundant link)

100 or 1000 Mbps i Alternate stack-root port

Switch C Stack port

Multidrop backbone (GigaStack GBIC connections)

CSUF implements the Stack Membership Discovery Protocol and the Fast Uplink Transition Protocol. Using the Stack Membership Discovery Protocol, all stack switches build a neighbor list of stack members through the receipt of discovery hello packets. When certain link loss or spanning-tree events occur (described in "Events that Cause Fast Convergence" section on page 12-7), the Fast Uplink Transition Protocol uses the neighbor list to send fast-transition requests on the stack port to stack members.

The switch sending the fast-transition request needs to do a fast transition to the forwarding state of a port that it has chosen as the root port, and it must obtain an acknowledgement from each stack switch before performing the fast transition.

Each switch in the stack determines if the sending switch is a better choice than itself to be the stack root of this spanning-tree instance by comparing the root, cost, and bridge ID. If the sending switch is the best choice as the stack root, each switch in the stack returns an acknowledgement; otherwise, it does not respond to the sending switch (drops the packet). The sending switch then has not received acknowledgements from all stack switches.

When acknowledgements are received from all stack switches, the Fast Uplink Transition Protocol on the sending switch immediately transitions its alternate stack-root port to the forwarding state. If acknowledgements from all stack switches are not obtained by the sending switch, the normal spanning-tree transitions (blocking, listening, learning, and forwarding) take place, and the spanning-tree topology converges at its normal rate (2 * forward-delay time + max-age time).

The Fast Uplink Transition Protocol is implemented on a per-VLAN basis and affects only one spanning-tree instance at a time.

Events that Cause Fast Convergence

Depending on the network event or failure, the CSUF fast convergence might or might not occur.

Fast convergence (less than 1 second under normal network conditions) occurs under these circumstances:

♦ The stack-root port link fails.

If two switches in the stack have alternate paths to the root, only one of the switches performs the fast transition.

♦ The failed link, which connects the stack root to the spanning-tree root, recovers.

♦ A network reconfiguration causes a new stack-root switch to be selected.

♦ A network reconfiguration causes a new port on the current stack-root switch to be chosen as the stack-root port.

Note The fast transition might not occur if multiple events occur simultaneously. For example, if a stack member switch is powered off, and at the same time, the link connecting the stack root to the spanning-tree root comes back up, the normal spanning-tree convergence occurs.

Normal spanning-tree convergence (30 to 40 seconds) occurs under these conditions:

♦ The stack-root switch is powered off, or the software failed.

♦ The stack-root switch, which was powered off or failed, is powered on.

♦ A new switch, which might become the stack root, is added to the stack.

♦ A switch other than the stack root is powered off or failed.

♦ A link fails between stack ports on the multidrop backbone.


These limitations apply to CSUF:

♦ CSUF uses the GigaStack GBIC and runs on all Catalyst 3550 switches, all Catalyst 3500 XL switches, Catalyst 2950 switches with GBIC module slots, and only on modular Catalyst 2900 XL switches that have the 1000BASE-X module installed.

♦ Up to nine stack switches can be connected through their stack ports to the multidrop backbone. Only one stack port per switch is supported.

♦ Each stack switch can be connected to the spanning-tree backbone through one uplink.

♦ If the stack consists of a mixture of Catalyst 3550, Catalyst 3500 XL, Catalyst 2950, and Catalyst 2900 XL switches, up to 64 VLANs with spanning tree enabled are supported. If the stack consists of only Catalyst 3550 switches, up to 128 VLANs with spanning tree enabled are supported.

Connecting the Stack Ports

A fast transition occurs across the stack of switches if the multidrop backbone connections are a continuous link from one GigaStack GBIC to another as shown in the top half of Figure 12-6. The bottom half of Figure 12-6 shows how to connect the GigaStack GBIC to achieve a normal convergence time.

You should follow these guidelines:

♦ A switch supports only one stack port.

♦ Do not connect alternate stack-root ports to stack ports.

♦ Connect all stack ports on the switch stack to the multidrop backbone.

♦ You can connect the open ports on the top and bottom GigaStack GBICs within the same stack to form a redundant link.

Figure 12-6 GigaStack GBIC Connections and Spanning-Tree Convergence

GigaStack GBIC connection for fast convergence

y Catalyst 3550-12T

Figure 12-6 GigaStack GBIC Connections and Spanning-Tree Convergence

Catalyst 2950G-24

Catalyst 2950G-48

Catalyst 2950G-24

Catalyst 2950G-48

GigaStack GBIC connection for normal convergence Catalyst 2950G-12


"SCatalyst 2950b** 1


. 1 1 wini


Catalyst 2950G-24

Catalyst 2950G-24

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  • teresa
    What is cross stack uplinkfast?
    4 years ago

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