Configure the Spanning-Tree portfast Setting Any port that is directly connected to an end station cannot create bridging loops. In switch A, run the following commands as port 20 is connected to a host. Enter the configuration mode for the interface. Shut down the interface. In this short post I will explore how this feature behaves, how it works and how to configure it on a cisco switch. The portfast feature brought the port into a forwarding state as soon as it came up enabling the PC to obtain a DHCP address.
The feature was then standardised and is now called the Edge port. An interface that has portfast configured on it will skip the listening and learning phases of spanning tree but only when the interface comes up. If portfast is configured on a port and it receives a BPDU the port will disable the portfast feature.
Another mis-conception is that when portfast is enabled on a port that it will stop sending BPDUs, this is not true the only thing that enabling portfast on a cisco switch does is to disable the listening and learning phases, all other elements are still there. To configure an interface with portfast, under the interface configuration simply enter the command spanning-tree portfast you will receive a warning which I will explain later. These two features enhance network reliability, manageability, and security for the existing L2 STP feature.
The PortFast feature is introduced to avoid network connectivity issues. These issues are caused by delays in STP enabled ports moving from blocking-state to forwarding-state after transitioning from the listening and learning states. STP enabled ports that are connected to devices such as a single switch, workstation, or a server can access the network only after passing all these STP states.
When the Forward Delay timer expires a second time, the port is transitioned to the forwarding or blocking state. When you enable PortFast on a switch or trunk port, the port is immediately transitioned to the spanning tree forwarding state.
To prevent loops from occurring in a network, the PortFast mode is supported only on nontrunking access ports because these ports typically do not transmit or receive BPDUs. The most secure implementation of PortFast is to enable it only on ports that connect end stations to switches. Because PortFast can be enabled on nontrunking ports connecting two switches, spanning tree loops can occur because BPDUs are still being transmitted and received on those ports.
BPDU guard provides a secure response to invalid configurations because the administrator must manually put the interface back in service. When you enable PortFast on the switch, spanning tree places ports in the forwarding state immediately, instead of going through the listening, learning, and forwarding states.
UplinkFast provides fast convergence using uplink groups in the network access layer after a spanning tree topology change. An uplink group is a set of ports per VLAN , only one of which is forwarding at any given time.
Specifically, an uplink group consists of the root port which is forwarding and a set of blocked ports not including self-looped ports. The uplink group provides an alternate path in case the currently forwarding link fails. This enhancement might not be useful for other types of applications and should not be enabled on backbone or distribution layer switches. Figure shows an example UplinkFast network topology. If Switch C detects a link failure on the currently active link L2 a direct link failure , UplinkFast unblocks the blocked port on Switch C and transitions it to the forwarding state immediately, without transitioning the port through the listening and learning states as shown in Figure This switchover takes approximately 1 to 5 seconds.
As soon as the switch transitions the alternate port to the forwarding state, the switch begins transmitting dummy multicast frames on that port, one for each entry in the local Enhanced Address Recognition Logic EARL table except those entries that are associated with the failed root port. By default, approximately 15 dummy multicast frames are transmitted per ms. Switches receiving these dummy multicast frames immediately update their EARL table entries for each source MAC address to use the new port, allowing the switches to begin using the new path almost immediately.
If connectivity on the original root port is restored, the switch waits for a period equal to twice the forward delay time plus 5 seconds before transitioning the port to the forwarding state. This situation allows the neighbor port enough time to transition through the listening and learning states to the forwarding state.
BackboneFast provides fast convergence in the network backbone after a spanning tree topology change occurs. A switch detects an indirect link failure the failure of a link to which the switch is not directly connected when the switch receives inferior BPDUs from its designated bridge on its root port or blocked ports. These inferior BPDUs indicate that the designated bridge has lost its connection to the root bridge. An inferior BPDU identifies a single switch as both the root bridge and the designated bridge.
Under normal spanning tree rules, the switch ignores inferior BPDUs for the configured maximum aging time specified by the set spantree maxage command. The switch tries to determine if it has an alternate path to the root bridge.
If the inferior BPDU arrives on a blocked port, the root port and other blocked ports on the switch become alternate paths to the root bridge. If the inferior BPDU arrives on the root port, all blocked ports become alternate paths to the root bridge. If the inferior BPDU arrives on the root port and there are no blocked ports, the switch assumes that it has lost connectivity to the root bridge, causes the maximum aging time on the root to expire, and becomes the root switch according to normal spanning tree rules.
If the switch has alternate paths to the root bridge, it uses these alternate paths to transmit a new kind of protocol data unit PDU called the Root Link Query PDU out all alternate paths to the root bridge. If the switch determines that it still has an alternate path to the root, it causes the maximum aging time on the ports on which it received the inferior BPDU to expire.
If all the alternate paths to the root bridge indicate that the switch has lost connectivity to the root bridge, the switch causes the maximum aging times on the ports on which it received an inferior BPDU to expire.
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