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STP
Problem: Consequently IT administrators have to implement redundancy in their hierarchical networks.
However adding extra links to switches and routers in the network introduces traffic loops that need to be managed in a dynamic way; when a switch connection is lost, another link needs to quickly take its place without introducing new traffic loops.
Solution: Spanning Tree Protocol
Layer 2 redundancy improves the availability of the network by implementing alternate network paths by adding equipment and cabling. Having multiple paths for data to traverse the network allows for a single path to be disrupted without impacting the connectivity of devices on the network.
In a hierarchical design, redundancy is achieved at the distribution and core layers through additional hardware and alternate paths through the additional hardware.
Ethernet frames do not have a time to live (TTL) like IP packets traversing routers. As a result, if they are not terminated properly on a switched network, they continue to bounce from switch to switch endlessly or until a link is disrupted and breaks the loop.
Broadcast Storms
A broadcast storm occurs when there are so many broadcast frames caught in a Layer 2 loop that all available bandwidth is consumed. Consequently, no bandwidth is available bandwidth for legitimate traffic, and the network becomes unavailable for data communication.
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. A port is considered blocked when network traffic is prevented from entering or leaving that port.
This does not include bridge protocol data unit (BPDU) frames that are used by STP to prevent loops. Y
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 Algorithm
STP uses the Spanning Tree Algorithm (STA) to determine which switch ports on a network need to be configured for blocking to prevent loops from occurring.
The STA designates:
- a single switch as the root bridge and uses it as the reference point for all path calculations. All switches participating in STP exchange BPDU frames to determine which switch has the lowest bridge ID (BID) on the network. The switch with the lowest BID automatically becomes the root bridge for the STA calculations.
The BPDU is the message frame exchanged by switches for STP. Each BPDU contains a BID that identifies the switch that sent the BPDU.
The BID contains:
- a priority value,
- the MAC address of the sending switch,
- and an optional extended system ID.
The lowest BID value is determined by the combination of these three fields.
The STA designates Port States:
Root ports - Switch ports closest to the root bridge. In the example, the root port on switch S2 is F0/1 configured for the trunk link between switch S2 and switch S1. The root port on switch S3 is F0/1, configured for the trunk link between switch S3 and switch S1.
Designated ports - All non-root ports that are still permitted to forward traffic on the network. In the example, switch ports F0/1 and F0/2 on switch S1 are designated ports. Switch S2 also has its port F0/2 configured as a designated port.
Non-designated ports - All ports configured to be in a blocking state to prevent loops. In the example, the STA configured port F0/2 on switch S3 in the non-designated role. Port F0/2 on switch S3 is in the blocking state.
Bridge Priority
The bridge priority is a customizable value that you can use to influence which switch becomes the root bridge. The switch with the lowest priority, which means lowest BID, becomes the root bridge (the lower the priority value, the higher the priority). For example, to ensure that a specific switch is always the root bridge, you set the priority to a lower value than the rest of the switches on the network. The default value for the priority of all Cisco switches is 32768. The priority range is between 1 and 65536; therefore, 1 is the highest priority.
Designated Port
The designated port exists on root and non-root bridges. For root bridges, all switch ports are designated ports. For non-root bridges, a designated port is the switch port that receives and forwards frames toward the root bridge as needed. Only one designated port is allowed per segment. If multiple switches exist on the same segment, an election process determines the designated switch, and the corresponding switch port begins forwarding frames for the segment. Designated ports are capable of populating the MAC table.
Each segment in a switched network can have only one designated port. When two non-root port switch ports are connected on the same LAN segment, a competition for port roles occurs. The two switches exchange BPDU frames to sort out which switch port is designated and which one is non-designated.
Non-designated Port
The non-designated port is a switch port that is blocked, so it is not forwarding data frames and not populating the MAC address table with source addresses. A non-designated port is not a root port or a designated port. For some variants of STP, the non-designated port is called an alternate port.
Disabled Port
The disabled port is a switch port that is administratively shut down. A disabled port does not function in the spanning-tree process. There are no disabled ports in the example.
The steps of a Port:
Blocking - The port is a non-designated port and does not participate in frame forwarding. The port receives BPDU frames to determine the location and root ID of the root bridge switch and what port roles each switch port should assume in the final active STP topology.
Listening - STP has determined that the port can participate in frame forwarding according to the BPDU frames that the switch has received thus far. At this point, the switch port is not only receiving BPDU frames, it is also transmitting its own BPDU frames and informing adjacent switches that the switch port is preparing to participate in the active topology.
Learning - The port prepares to participate in frame forwarding and begins to populate the MAC address table.
Forwarding - The port is considered part of the active topology and forwards frames and also sends and receives BPDU frames.
Disabled - The Layer 2 port does not participate in spanning tree and does not forward frames. The disabled state is set when the switch port is administratively disabled.
Cisco PortFast Technology
PortFast is a Cisco technology. When a switch port configured with PortFast is configured as an access port, that port transitions from blocking to forwarding state immediately, bypassing the typical STP listening and learning states.
You can use PortFast on access ports, which are connected to a single workstation or to a server, to allow those devices to connect to the network immediately rather than waiting for spanning tree to converge.
!!!!!!! If an interface configured with PortFast receives a BPDU frame, spanning tree can put the port into the blocking state using a feature called BPDU guard.
Note: Cisco PortFast technology can be used to support DHCP. Without PortFast, a PC can send a DHCP request before the port is in forwarding state, denying the host from getting a usable IP address and other information. Because PortFast immediately changes the state to forwarding, the PC always gets a usable IP address.
STP Convergence Steps
Convergence is an important aspect of the spanning-tree process. Convergence is the time it takes for the network to determine which switch is going to assume the role of the root bridge, go through all the different port states, and set all switch ports to their final spanning-tree port roles where all potential loops are eliminated. The convergence process takes time to complete because of the different timers used to coordinate the process.
To understand the convergence process more thoroughly, it has been broken down into three distinct steps:
Step 1. Elect a root bridge
Step 2. Elect root ports
Step 3. Elect designated and non-designated ports
BPDU Timers
The amount of time that a port stays in the various port states depends on the BPDU timers. Only the switch in the role of root bridge may send information through the tree to adjust the timers. The following timers determine STP performance and state changes:
- Hello time
- Forward delay
- Maximum age
Default Port Id for Fe/1 → 128.1 for Fe/2 128.2 etc
Default Bridge Id 32769.00AA4612AB00
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