11.3 Spanning Tree Protocol (STP) Flashcards
Describe the Spanning Tree Protocol (STP)
also referred to as the spanning tree algorithm (STA), assigns a designated bridge (or switch) for each route. Only the designated bridge can forward packets. Redundant bridges (and switches) are assigned as backups.
Describe Switching forward logic
The source VLAN of the frame is determined, using the access interface’s VLAN or the frame’s trunking header.
The source MAC address, source interface ID, and the VLAN ID are added to the MAC address table.
The MAC address table is searched for the destination MAC address of the frame.
The frame is forwarded to the interface of the matched address entry. If the MAC address is not found in the MAC address table, the frame is flooded out to other access ports in the same VLAN.
Describe the Forwarding state
Criteria for putting an interface into a forwarding state is as follows:
STP elects a root switch and all interfaces on the root switch are in a forwarding state.
On non-root switches, the port with the least administrative cost between the switch and the root switch is elected to the forwarding state.
The switch with the lowest root cost, compared with other switches, is also placed in the forwarding state.
Describe Bridge ID
The bridge ID (BID) is composed of the priority number of the host and its MAC address. A hello packet is the most common Bridge Protocol Data Unit (BPDU). Hello packets contain the root bridge ID, the sender’s bridge ID, the cost to the root, and the timers
Describe the bridge port election process
Every switch sends out a hello packet listing itself as the root bridge ID with a cost of zero.
As each switch receives a hello packet, it compares its root bridge ID to the root bridge ID listed in the hello packets.
If the listed BID is lower, the switch elects that as the root and forwards the hello packet listing itself as the sender.
Once the root bridge has been elected, its ports enter a forwarding state and continue to send hello packets.
The other switches examine the hello packets and determine which port has the lowest cost to the root. The port with the lowest cost to the root becomes the root port.
The port on each LAN segment with the lowest cost to the root becomes the designated port for that segment.
When the age timer expires, STP starts a new election process.
What is a Bridge loop/ switching loop
providing redundant paths between segments causes packets to be passed between the redundant paths endlessly
What are the benefits of STP
Eliminates bridge loops.
Provides redundant paths between devices.
Enables dynamic role configuration.
Recovers automatically from a topology change or device failure.
Identifies the optimal path between any two network devices.
Describe Bridge roles
The bridge role determines how the device functions in relation to other devices and whether the device forwards traffic to other segments.
Describe a Root Bridge
The root bridge is the master or controlling bridge.
Describe the Designated Bridge
A designated bridge is any other device that participates in forwarding packets through the network.
Describe Backup Bridge
All redundant devices are classified as backup bridges.
Describe Port state Disabled
A port in the disabled state is powered on but does not participate in forwarding or listening to network messages. A bridge must be manually placed in the disabled state.
Describe the Blocking Port state
When a device is first powered on, its ports are in the blocking state. In addition, backup bridge ports are always in the blocking state. Ports in a blocking state receive packets and BPDUs sent to all bridges, but will not process any other packets.
Describe the Listening port state
The listening state is a transitory state between blocking and learning. The port remains in the listening state for a specific period of time. This time period allows network traffic to settle down after a change has occurred. For example, if a bridge goes down, all other bridges go to the listening state for a period of time. During this time the bridges redefine their roles.
Describe the Learning port state
A port in the learning state is receiving packets and building the bridge database by associating MAC addresses with ports. A timer is also associated with this state. The port goes to the forwarding state after the timer expires.
Describe the Forwarding port state
The root bridge and designated bridges are in the forwarding state when they can receive and forward packets. A port in the forwarding state can both learn and forward. All ports of the root switch are in forwarding mode.
Describe the Root port type
The root port is the port on the designated switch with the lowest port cost back to the root bridge.
Describe the designated port type
One port on each segment is identified as the designated port. The designated port identifies the port on the segment that is allowed to send and receive frames onto that segment.
Describe the blocking port type
A blocking port is any port that is not a root or a designated port. A blocking port is in blocking state.
Describe PortFast
allows ports without any attached switches to transition immediately to the forwarding state. This transition is possible because bridging loops are eliminated on ports that do not have switches attached.
Descrieb Uplink Fast
enables a switch to maintain an alternate path back to the root bridge. If the root port or link goes down, the alternate port can be quickly used to re-establish communication with the root bridge.
Describe Ether Channel
EtherChannel provides a high degree of redundancy for network links by combining multiple parallel segments between two switches into a single link. To do this, the segments must have the same link speed. You can combine eight separate segments into one Ether Channel. STP convergence starts only if all of the links within the channel go down.
Describe Port Fast
A 30-second delay is created by the default 15-second listening state and 15-second learning state that are required to transition from blocking to forwarding. PortFast allows a port to skip these states and go from a blocking state to a forwarding state immediately. However, PortFast can easily cause switching loops. You should enable it only on ports connected to non-STP devices like workstations, printers, and servers.
Describe BPDU Guard
Bridge Protocol Data Unit Guard (BPDU Guard) prevents certain switch ports from connecting to other switches in order to prevent switching loops and unauthorized connections. Because only switches send BPDUs, BPDU Guard knows that a port that receives a BPDU is linked to another switch. BPDU Guard will disable ports as appropriate.
Describe Rapid STP
Rapid Spanning Tree Protocol (Rapid STP) is a variation of the standard STP specification. Rapid STP improves convergence performance by actively confirming when a switch port is ready to transition to a forwarding state. This eliminates the need for the listening and learning states, which can cause a 50-second, or more delay. Rapid STP reduces convergence time to about 10 seconds.
Describe Port Aggregation Protocol (PAGP)
Port Aggregation Protocol is a management function that checks the parameter consistency at either end of the link and assists the channel in adapting to link failure or addition. PAgP prevents loops and packet loss due to misconfigured channels. It facilitates network reliability.
Describe Link Aggregation Control Protocol (LACP)
Link Aggregation Control Protocol is based on the 802.3ad standard and has similar functions to PAgP. LACP should be used when configuring EtherChannel between Cisco switches and non-Cisco switches that support 802.3ad.
Switch(config-if)#channel-protocol lacp
Switch(config-if)#channel-protocol pagp
Selects the EtherChannel protocol on the interface.
Switch(config-if)#channel-group [1-8] mode auto
Switch(config-if)#channel-group [1-8] mode desirable
Selects the PAgP mode on the interface.
Switch(config-if)#channel-group [1-8] mode active
Switch(config-if)#channel-group [1-8] mode passive
Selects the LACP mode on the interface.
Switch(config-if)#channel-group [1-8] mode on
Enables the on mode and forces a port to join an EtherChannel without PAgP or LACP negotiations.
Switch(config-if)#no channel-group [1-8]
Disables EtherChannel on the interface.
Switch#show etherchannel
Displays EtherChannel details on the switch.
Switch#show etherchannel summary
Displays EtherChannel information for a channel with a one-line summary per channel group.
