Fast Convergence Components Of Stp; Portfast; Uplinkfast - Cisco RJ-45-to-AUX Brochure

capability. The channel is then added to the spanning tree as a bridge port.
Warning Dynamic VLAN ports can force a VLAN change; as a result, PAgP cannot be used to form a bundle
on ports that are configured for dynamic VLANs. The VLANs must be static VLANs, meaning that
the port on the switch must be assigned to a VLAN. PAgP also requires that all ports in the channel
belong to the same VLAN or be configured as trunk ports.
If you have a pre−existing EtherChannel bundle, and a VLAN of a port contained in the bundle is modified,
all ports in the bundle are modified to match the VLAN configuration. PAgP will not group ports running at
different speeds or duplex. PAgP will change the port speed and duplex for all ports in the bundle.
All ports in a Fast EtherChannel bundle should be assigned to the same VLAN or be configured as trunked
ports. You must also configure both ends of the link with the same trunking mode.
Tip You can configure the broadcast limits by percentage limit or by packets−per−second.
Packets−per−second allows unicast packets to be dropped when the broadcast limit is exceeded.

Fast Convergence Components of STP

There are a number of protocols for STP that allow for fast convergence, including PortFast, UplinkFast, and
BackboneFast. In the following sections I'll discuss the functions of these protocols and components.

PortFast

PortFast is a switch function that can be used on ports where a single server or workstation is connected, to
allow a port to enter the forwarding mode almost immediately. Doing so prevents the port from entering the
listening and learning states.
As we've discussed, when a switch using STP is powered up, the ports running STP go through four states
before forwarding frames through each port. In order to get to the forwarding state, the STA makes each port
wait up to 50 seconds before data is allowed to be forwarded. This delay may cause problems with certain
protocols and applications. By implementing PortFast, you can avoid these delays.

UplinkFast

One of the most important factors in a network is the convergence time when a link fails. By implementing
the UplinkFast function, you can reduce the time it takes for the network to converge by optimizing
convergence times. Because of the convergence time of STP, some end stations become inaccessible,
depending on the current state of each switch port.
By decreasing convergence time, you reduce the length of the disruption. UplinkFast allows a port in a
blocked state on a switch to almost immediately begin forwarding when the switch detects a link failure or a
topology change. However, UplinkFast must have direct knowledge of the link failure in order to move a
blocked port into a forwarding state.
Note An Uplink Group is a root port that is in a forwarding state and a set of blocked ports that does not
include self−looping ports. The Uplink Group is the alternate path when the currently forwarding link
fails.
The UplinkFast feature should be placed only on Access layer switches that are connected to the end−user
nodes. In order to utilize UplinkFast, several criteria must be met:
UplinkFast must be enabled on the switch.
211