Chapter 5- STP Concepts
The Spanning Tree Algorithm
invented by Radia Perlman while working for Digital Equipment Corporation, and published in the 1985 paper "An Algorithm for Distributed Computation of a Spanning Tree in an Extended LAN." Her spanning tree algorithm (STA) creates a loop-free topology by selecting a single root bridge where all other switches determine a single least-cost path.
Learning
the switch port receives and processes BPDUs and prepares to participate in frame forwarding. It also begins to populate the MAC address table. However, in the learning state, user frames are not forwarded to the destination.
Elect a Root Port from Multiple Equal-Cost Paths
the switch will determine a port using the following criteria: 1.Lowest sender BID 2.Lowest sender port priority 3.Lowest sender port ID
STP builds a loop-free topology in a four-step process:
1. Elect the root bridge. 2. Elect the root ports. 3. Elect designated ports. 4. Elect alternate (blocked) ports.
Different types of STP
PVST+, RSTP, 802.1D-2004, Rapid PVST+, MSTP, MST
PVST+
Per-VLAN Spanning Tree (PVST+) is a Cisco enhancement of STP that provides a separate 802.1D spanning tree instance for each VLAN configured in the network. PVST+ supports PortFast, UplinkFast, BackboneFast, BPDU guard, BPDU filter, root guard, and loop guard.
RSTP
Rapid Spanning Tree Protocol (RSTP) or IEEE 802.1w is an evolution of STP that provides faster convergence than STP.
Loop-Free Topology NV
-A blocked port has the effect of making that link a non-forwarding link between the two switches -Creates a topology where each switch has only a single path to the root bridge, similar to branches on a tree that connect to the root of the tree.
Broadcast Storm
-A broadcast storm is an abnormally high number of broadcasts overwhelming the network during a specific amount of time. -Broadcast storms can disable a network within seconds by overwhelming switches and end devices. -To prevent these issues from occurring in a redundant network, some type of spanning tree must be enabled on the switches. Spanning tree is enabled, by default, on Cisco switches to prevent Layer 2 loops from occurring.
1. Elect the Root Bridge
-After a switch boots, it begins to send out BPDU frames every two seconds. -These BPDU frames contain the BID of the sending switch and the BID of the root bridge, known as the Root ID. -The switch with the lowest BID will become the root bridge.
Determining the Root Path Cost
-After electing Root bridge STA begins to process the Root path cost -The path information, known as the internal root path cost, is determined by the sum of all the individual port costs along the path from the switch to the root bridge. -The default port costs are defined by the speed at which the port operates.
Impact of Default BIDs
-Because the default BID is 32768, it is possible for two or more switches to have the same priority. -the switch with the lowest MAC address will become the root bridge.
Different types of Designated ports
-Designated Ports on Root Bridge: All ports on the root bridge are designated ports -Designated Port When There is a Root Port: If one end of a segment is a root port, then the other end is a designated port. -Designated Port When There is No Root Port:This leaves only segments between two switches where neither of the switches is the root bridge. In this case, the port on the switch with the least-cost path to the root bridge is the designated port for the segment.
2. Elect the Root Ports
-Every non-root switch will select one root port. The root port is the port closest to the root bridge in terms of overall cost (best path) to the root bridge. This overall cost is known as the internal root path cost. -The internal root path cost is equal to the sum of all the port costs along the path to the root bridge.
3. Elect the Designated Port
-Every segment between two switches will have one designated port. The designated port is a port on the segment (with two switches) that has the internal root path cost to the root bridge. In other words, the designated port has the best path to receive traffic leading to the root bridge. -What is not a root port or a designated port becomes an alternate or blocked port. The end result is a single path from every switch to the root bridge.
Issues with Redundant Switch Links *NV
-Path redundancy provides multiple network services by eliminating the possibility of a single point of failure. When multiple paths exist between two devices on an Ethernet network, and there is no spanning tree implementation on the switches, a Layer 2 loop occurs. -A Layer 2 loop can result in MAC address table instability, link saturation, and high CPU utilization on switches and end-devices, resulting in the network becoming unusable. -STP was developed specifically as a loop prevention mechanism for Layer 2 Ethernet.
Redundancy in Layer 2 Switched Networks *NV
-Redundant networks require the addition of physical paths, but logical redundancy must also be part of the design. -Having alternate physical paths for data to traverse the network makes it possible for users to access network resources, despite path disruption. -However, redundant paths in a switched Ethernet network may cause both physical and logical Layer 2 loops.
Block Redundant Paths NV
-STP ensures that there is only one logical path between all destinations on the network by intentionally blocking redundant paths that could cause a loop -When a port is blocked, user data is prevented from entering or leaving that port. Blocking the redundant paths is critical to preventing loops on the network.
Link Failure Causes Recalculation -NV
-The physical paths still exist to provide redundancy, but these paths are disabled to prevent the loops from occurring. -If the path is ever needed to compensate for a network cable or switch failure, STP recalculates the paths and unblocks the necessary ports to allow the redundant path to become active. -STP recalculations can also occur any time a new switch or new inter-switch link is added to the network.
Select the Root Bridge NV
-begins by selecting a single root bridge. -Each switch will determine a single, least cost path from itself to the root bridge. NOTE: The STA and STP refers to switches as bridges. This is because in the early days of Ethernet, switches were referred to as bridges.
Layer 2 Loops
-can form, causing broadcast, multicast and unknown unicast frames to loop endlessly. -For example, broadcast frames, such as an ARP Request are forwarded out all of the switch ports, except the original ingress port. This ensures that all devices in a broadcast domain are able to receive the frame. If there is more than one path for the frame to be forwarded out of, an endless loop can result. -When a loop occurs, the MAC address table on a switch will constantly change with the updates from the broadcast frames, which results in MAC database instability.
Spanning Tree Protocol (STP)
-is a loop-prevention network protocol that allows for redundancy while creating a loop-free Layer 2 topology. -IEEE 802.1D is the original IEEE MAC Bridging standard for STP. -STP Compensates for Network Failure
During STA and STP functions NV
-switches use Bridge Protocol Data Units (BPDUs), are used to elect the root bridge, root ports, designated ports, and alternate ports. -BPDU contains a bridge ID (BID) that identifies which switch sent the BPDU. -BID contains a priority value, an extended system ID, and the MAC address of the switch. The lowest BID value is determined by the combination of these three fields.
Steps for an STA Topology NV
1.Select the Root Bridge 2.Block Redundant Paths 3.Loop-Free Topology 4.Link Failure Causes Recalculation
Bridge Priority
A customizable value that can be used to influence which switch becomes the root bridge. -default priority value for all Cisco switches is the decimal value 32768. -The range is 0 to 61440 in increments of 4096. -A lower bridge priority is preferable. A bridge priority of 0 takes precedence over all other bridge priorities.
Disabled
A switch port in the disabled state does not participate in spanning tree and does not forward frames. The disabled state is set when the switch port is administratively disabled.
Different Port States
Blocking, Listening, Learning, Forwarding, Disabled
Forwarding
In the forwarding state, a switch port is considered part of the active topology. The switch port forwards user traffic and sends and receives BPDU frames.
Extended System ID
Constitutes 12 bits of the 8-byte BID and contains the ID of the VLAN with which an STP BPDU is associated. The presence of the extended system ID results in bridge priority values incrementing in multiples of 4096.
STP convergence requires three timers
Hello Timer -the interval between BPDUs. The default is 2 seconds but can be modified to between 1 and 10 seconds. -Forward Delay Timer -the time that is spent in the listening and learning state. The default is 15 seconds but can be modified to between 4 and 30 seconds. -Max Age Timer -the maximum length of time that a switch waits before attempting to change the STP topology. The default is 20 seconds but be modified to between 6 and 40 seconds. -Note: Times can be changed on the root bridge, which dictates the value of these timers for the STP domain.
4. Elect Alter (Blocked) Ports
If a port is not a root port or a designated port, then it becomes an alternate (or backup) port. Alternate ports and backup ports are in discarding or blocking state to prevent loops.
MST
Multiple Spanning Tree (MST) is the Cisco implementation of MSTP, which provides up to 16 instances of RSTP and combines many VLANs with the same physical and logical topology into a common RSTP instance. Each instance supports PortFast, BPDU guard, BPDU filter, root guard, and loop guard.
MSTP
Multiple Spanning Tree Protocol (MSTP) is an IEEE standard inspired by the earlier Cisco proprietary Multiple Instance STP (MISTP) implementation. MSTP maps multiple VLANs into the same spanning tree instance.
Blocking
The port is an alternate port and does not participate in frame forwarding. Max age timer of secounds
Listening
The port receives BPDUs to determine the path to the root. The switch port also transmits its own BPDU frames and informs adjacent switches that the switch port is preparing to participate in the active topology.
Rapid PVST+
This is a Cisco enhancement of RSTP that uses PVST+ and provides a separate instance of 802.1w per VLAN. Each separate instance supports PortFast, BPDU guard, BPDU filter, root guard, and loop guard.
802.1D-2004
This is an updated version of the STP standard, incorporating IEEE 802.1w.
STP
This is the original IEEE 802.1D version (802.1D-1998 and earlier) that provides a loop-free topology in a network with redundant links. Also called Common Spanning Tree (CST), it assumes one spanning tree instance for the entire bridged network, regardless of the number of VLANs.
MAC Address
When two switches are configured with the same priority and have the same extended system ID, the switch having the MAC address with the lowest value, expressed in hexadecimal, will have the lower BID.
