Please note that candidates who fail an exam attempt must wait five 5 calendar days, beginning the day after the failed attempt, before they may retest for the same exam. Achieving or recertifying any of the certifications above automatically extends your active Associate and Professional level certification s up to the point of expiration of the last certification achieved. For more information, access the Cisco About Recertification page. In this setup, users connect to Switch A, and the network administrator typically seeks to achieve load balancing on the access switch Uplinks based on even or odd VLANs, or any other scheme deemed appropriate.
These sections are example cases where different types of STP are used on this setup:. These statements are true for this configuration:.
One spanning tree instance for each VLAN is maintained, which means instances for only two different final logical topologies. The original IEEE If the CST is applied to the topology of this diagram , the result resembles the diagram shown here:.
Note: The Cisco implementation enhances the This feature behaves exactly as the PVST in this example. The idea is that several VLANs can be mapped to a reduced number of spanning tree instances because most networks do not need more than a few logical topologies. In the topology described in the first diagram, there are only two different final logical topologies, so only two spanning tree instances are really necessary. There is no need to run instances. If you map half of the VLANs to a different spanning tree instance, as shown in this diagram, these statements are true:. The desired load balancing scheme can still be achieved, because half of the VLANs follow one separate instance.
From a technical standpoint, MST is the best solution. From an end-user's perspective, the main drawbacks associated with a migration to MST are:. The protocol is more complex than the usual spanning tree and requires additional training of the staff. Interaction with legacy bridges can be a challenge.
As previously mentioned, the main enhancement introduced by MST is that several VLANs can be mapped to a single spanning tree instance. This raises the problem of how to determine which VLAN is to be associated with which instance. The issue is irrelevant in the case of the If by error, two switches were misconfigured and had a different range of VLANs associated to the same instance, it was difficult for the protocol to recover properly from this situation.
The IEEE Think of a region as the equivalent of Border Gateway Protocol BGP Autonomous Systems, which is a group of switches placed under a common administration. A element table that associates each of the potential VLANs supported on the chassis to a given instance. In order to be part of a common MST region, a group of switches must share the same configuration attributes. It is up to the network administrator to properly propagate the configuration throughout the region. Other methods can be envisioned, as the IEEE specification does not explicitly mention how to accomplish that step.
Note: If for any reason two switches differ on one or more configuration attribute, the switches are part of different regions. For more information refer to the Region Boundary section of this document. In order to ensure consistent VLAN-to-instance mapping, it is necessary for the protocol to be able to exactly identify the boundaries of the regions. For that purpose, the characteristics of the region are included in the BPDUs.
The exact VLANs-to-instance mapping is not propagated in the BPDU, because the switches only need to know whether they are in the same region as a neighbor. Therefore, only a digest of the VLANs-to-instance mapping table is sent, along with the revision number and the name. Once a switch receives a BPDU, the switch extracts the digest a numerical value derived from the VLAN-to-instance mapping table through a mathematical function and compares this digest with its own computed digest.
If the digests differ, the port on which the BPDU was received is at the boundary of a region. In generic terms, a port is at the boundary of a region if the designated bridge on its segment is in a different region or if it receives legacy In this diagram, the port on B1 is at the boundary of region A, whereas the ports on B2 and B3 are internal to region B:.
According to the IEEE The terminology continues to evolve, as It is likely these names will change in the final release of Therefore, MST must be able to interact with For These are two functionally equivalent diagrams. Notice the location of the different blocked ports. In a typically bridged network, you expect to see a blocked port between Switches M and B.
Instead of blocking on D, you expect to have the second loop broken by a blocked port somewhere in the middle of the MST region. This makes it possible to understand that the virtual bridge blocks an alternate port on B. Also, that virtual bridge is on the C to D segment and leads Switch D to block its port.
The exact mechanism that makes the region appear as one virtual CST bridge is beyond the scope of this document, but is amply described in the IEEE However, if you keep this virtual bridge property of the MST region in mind, the interaction with the outside world is much easier to understand. These instances run the RSTP automatically by default, without any extra configuration work. Remember that MST only runs one spanning tree outside of the region, so except for the IST instance, regular instances inside of the region have no outside counterpart.
However, the network administrator does not have to map VLANs onto instance 0, and therefore this is not a source of concern. Unlike regular converged spanning tree topology, both ends of a link can send and receive BPDUs simultaneously. This is because, as shown in this diagram, each bridge can be designated for one or more instances and needs to transmit BPDUs. The MRecord contains enough information mostly root bridge and sender bridge priority parameters for the corresponding instance to calculate its final topology.
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The MRecord does not need any timer-related parameters such as hello time, forward delay, and max age that are typically found in a regular IEEE The only instance in the MST region to use these parameters is the IST; the hello time determines how frequently BPDUs are sent, and the forward delay parameter is mainly used when rapid transition is not possible remember that rapid transitions do not occur on shared links. The independence between instance and VLAN is a new concept that implies you must carefully plan your configuration.
This configuration results in pcA 's inability to send frames to pcB. Individual instances do not send individual BPDUs. It is clear that Switch B has to block one of its ports in order to avoid a loop. Remember that VLAN no longer means spanning tree instance. The topology is determined by the instance, regardless of the VLANs mapped to it. Suppose that VLANs 10 and 20 are both mapped to the same instance instance 1.
Shortly after this is completed, the network administrator notices that users in VLAN 20 have lost connectivity to the network. This is a typical misconfiguration problem.
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