Tag Archives: 2.1i

2.1.i Describe spanning-tree concepts

2.1.i [ii] STP dispute, STP bridge assurance

When a designated port detects a conflict, it keeps its role, but reverts to a discarding state because disrupting connectivity in case of inconsistency is preferable to starting a bridging loop.

The figure below illustrates a unidirectional link failure that typically creates a bridging loop. Switch A is the root bridge, and its BPDUs are lost on the link leading to switch B. The 802.1w-standard BPDUs include the role and state of the sending port. With this information, switch A can detect that switch B does not react to the superior BPDUs that it sends and that switch B is the designated, not root port. As a result, switch A blocks (or keeps blocking ) its port, which prevents the bridging loop.


You can use Bridge Assurance to protect against certain problems that can cause bridging loops in the network. Specifically, you can use Bridge Assurance to protect against a unidirectional link failure and a device that continues to forward data traffic when it is no longer running the spanning tree algorithm. Bridge Assurance is enabled by default and can only be disabled globally. In addition to that, Bridge Assurance can only be enabled on spanning tree network ports that are point-to-point links. Both ends of the link must have Bridge Assurance enabled. With Bridge Assurance enabled, BPDUs are sent out on all operational network ports in both directions, including alternate and backup ports, for each hello time period. If the port does not receive a BPDU for a specified period, the port moves into the blocking state and is not used in the root port calculation. Once that port receives a BPDU, it resumes the normal spanning tree transitions.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 1929-1936).  . Kindle Edition.



2.1.i Describe spanning-tree concepts

2.1.i [i] Compatibility between MST and RSTP

Both RSTP and MSTP improve the operation of the spanning tree while maintaining backward compatibility with equipment that is based on the (original) 802.1D spanning tree, with existing Cisco per-VLAN spanning tree (PVST), and with the existing Cisco-proprietary Multiple Instance STP (MISTP). By default, RSTP uses topology change TC flag. It also doesn’t use separate TCN BPDU when interoperating with another switching running 802.1D.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 1915-1918).  . Kindle Edition.


2.1.i Describe spanning-tree concepts

Describe spanning-tree concepts

The basic function of STP is to provide a loop free switched network; this is done by creating a topology of all participating STP switches. The best loop free path through the switched network is then determined from this topology information. The initial step taken by each STP is to elect a root switch; the root switch is used as a central point in a switched network to determine the best route through the switched network . Initially , all switches act as if they are the root switch and do this until they receive traffic from another superior switch (as determined by switch priority); this is referred to as a root switch election.

Another thing that must be understood is that multiple root switches can exist in the network depending on what STP mode is being used. By default, on Cisco switching equipment, each VLAN has its own STP instance and a root switch is elected for each VLAN; this mode is called Per VLAN Spanning Tree Plus (PVST +). If implementing RSTP, Rapid PVST + is used.

Once the root switch is elected, each of the ports is given a role depending on its place within the STP topology; the available port roles when using 802.1D spanning tree are shown below:

● Root—The port given this role is the selected best path to reach the root switch

● Designated—The port given this role is selected with the best path to a specific switched segment; there is only one designated port per switched segment.

● Alternate—The port given this role is selected as a backup to the root port; if the root port should have a problem, this port would take over the root port role.

● Backup—The port given this role is selected as a back to the designated port; if the designated port should have a problem this port would take over the designated port role.

Once the best path is calculated and each of the ports has been given a role , all ports with the alternate or backup STP roles will be blocked to prevent loops.