Understanding MSTP
Over time I was thinking of putting together the two blog posts made in the past about MSTP and adding more clarification for MSTP multi-region section. This new blog post recaps the information posted previously and provides more details this time. Additionally, it discusses some MSTP design-related questions. Both single-region and multiple-region MSTP configurations are reviewed in the post. The reader is assumed to have good understanding of classic STP and RSTP protocols as well as Cisco’s PVST/PVST+ implementations. Table of Contents
Historical Review Logical and Physical Topologies Implementing MSTP Caveats köksinredning in MSTP Design MSTP Single-Region Configuration Example Common and Internal Spanning Tree (CIST) Common Spanning köksinredning Tree (CST) Mapping MSTI s to CIST MSTP Multi Region Design Considerations Interoperating with PVST+ Scenario 1: CIST Root and CIST Regional Root Scenario 2: MSTIs and the Master Port Scenario 3: PVST+ and MSTP Interoperation Conclusions Further Reading
In the beginning, there was IEEE STP protocol, köksinredning which was preceded by DEC and IBM STP variants. All of them utilized the same logic originally proposed by Radia Perlman in 80s, while she was working in DEC. The IEEE version was adapted for use with multiple VLANs using 802.1q frames tagging. A shared spanning-tree, sometimes called Mono Spanning Tree (MST) by Cisco, or more often – Common köksinredning Spanning Tree (CST) was used to create a single loop-free topology. The drawback of this approach is inability to perform VLAN traffic köksinredning engineering across köksinredning redundant links: if a link is blocked, it is blocked for all VLANs. Another issue related to STP construction – more traffic is forwarded over the links closer to the root bridge, which puts higher demand on the root bridge resources – both in terms of CPU and links capacity utilization.
To overcome these limitations, Cisco introduced proprietary Per-VLAN köksinredning Spanning Tree Protocol (PVST), using separate STP instance per VLAN. Initially, PVST was created to be used with Cisco’s proprietary köksinredning ISL encapsulation only, but the later PVST+ version allowed for tunneling PVST BPDUs over 802.1q trunks and IEEE STP domains. PVST allowed köksinredning for using different logical topology with every VLAN, enhancing basic Layer 2 traffic engineering. Every VLAN may use its own root bridge and forwarding topology allowing for more fair resource köksinredning utilization. This method has some limitation as it does not deal with the actual network link capacities and utilization, but rather statistically multiplexes köksinredning VLANS to different topologies. However, this is the limitation inherent to any load-balancing method based on STP. The main problem of PVST was that with the number of VLANs growing, PVST becomes a waste of switch resources and management burden. This is because the number of different logical topologies is usually much smaller than the number of active VLANs.
With time, PVST adopted köksinredning fast-convergence properties köksinredning introduced by IEEE RSTP protocol, but the core feature of keeping a separate copy of STP per VLAN did not change. Seeing the problems associated with PVST approach, Cisco came with idea of decoupling the concepts of STP instances and VLANs. The initial implementation was called MISTP (Multiple Instances Spanning Tree) and later evolved köksinredning into IEEE 802.1s standard called MSTP (Multiple Spanning Trees Protocol). Logical and Physical Topologies
The core idea of MSTP is utilizing the fact that a redundant physical topology only has a small amount of different spanning-trees (logical topologies). The figure below shows a ring topology of three switches and three different spanning trees that may result from different root bridge placements.
Instead of running an STP instance for every VLAN, MSTP runs a number of VLAN-independent STP instances (representing logical topologies) and then administrator maps each VLAN to the most appropriate logical topology (STP instance). The number of STP instances is kept to minimum (saving switch resources), but the network capacity is utilized in more optimal fashion, by using all possible köksinredning paths for VLAN traffic.
The switch logic for VLAN traffic forwarding has changed a little bit. In order for a frame to be forwarded out of a port, two conditions must be met: first, VLAN must be active on this port (e.g. not filtered) and second, the STP instance the VLAN maps to, must be in non-discarding state for this port. The second property is normally enforced automatically, as MAC addresses are not learned on discarding ports. It is worth reminding that due to multiple logical topologies active on a port, the port could be blocking for one instance and forwarding for another (note that in (R)PVST+ a port is either forwarding or discarding for a VLAN ). The figure below demonstrates six VLANs using two MSTP instances, thus reducing the number of STP trees that would be required with PVST from 6 to 2.
The following is a s
Over time I was thinking of putting together the two blog posts made in the past about MSTP and adding more clarification for MSTP multi-region section. This new blog post recaps the information posted previously and provides more details this time. Additionally, it discusses some MSTP design-related questions. Both single-region and multiple-region MSTP configurations are reviewed in the post. The reader is assumed to have good understanding of classic STP and RSTP protocols as well as Cisco’s PVST/PVST+ implementations. Table of Contents
Historical Review Logical and Physical Topologies Implementing MSTP Caveats köksinredning in MSTP Design MSTP Single-Region Configuration Example Common and Internal Spanning Tree (CIST) Common Spanning köksinredning Tree (CST) Mapping MSTI s to CIST MSTP Multi Region Design Considerations Interoperating with PVST+ Scenario 1: CIST Root and CIST Regional Root Scenario 2: MSTIs and the Master Port Scenario 3: PVST+ and MSTP Interoperation Conclusions Further Reading
In the beginning, there was IEEE STP protocol, köksinredning which was preceded by DEC and IBM STP variants. All of them utilized the same logic originally proposed by Radia Perlman in 80s, while she was working in DEC. The IEEE version was adapted for use with multiple VLANs using 802.1q frames tagging. A shared spanning-tree, sometimes called Mono Spanning Tree (MST) by Cisco, or more often – Common köksinredning Spanning Tree (CST) was used to create a single loop-free topology. The drawback of this approach is inability to perform VLAN traffic köksinredning engineering across köksinredning redundant links: if a link is blocked, it is blocked for all VLANs. Another issue related to STP construction – more traffic is forwarded over the links closer to the root bridge, which puts higher demand on the root bridge resources – both in terms of CPU and links capacity utilization.
To overcome these limitations, Cisco introduced proprietary Per-VLAN köksinredning Spanning Tree Protocol (PVST), using separate STP instance per VLAN. Initially, PVST was created to be used with Cisco’s proprietary köksinredning ISL encapsulation only, but the later PVST+ version allowed for tunneling PVST BPDUs over 802.1q trunks and IEEE STP domains. PVST allowed köksinredning for using different logical topology with every VLAN, enhancing basic Layer 2 traffic engineering. Every VLAN may use its own root bridge and forwarding topology allowing for more fair resource köksinredning utilization. This method has some limitation as it does not deal with the actual network link capacities and utilization, but rather statistically multiplexes köksinredning VLANS to different topologies. However, this is the limitation inherent to any load-balancing method based on STP. The main problem of PVST was that with the number of VLANs growing, PVST becomes a waste of switch resources and management burden. This is because the number of different logical topologies is usually much smaller than the number of active VLANs.
With time, PVST adopted köksinredning fast-convergence properties köksinredning introduced by IEEE RSTP protocol, but the core feature of keeping a separate copy of STP per VLAN did not change. Seeing the problems associated with PVST approach, Cisco came with idea of decoupling the concepts of STP instances and VLANs. The initial implementation was called MISTP (Multiple Instances Spanning Tree) and later evolved köksinredning into IEEE 802.1s standard called MSTP (Multiple Spanning Trees Protocol). Logical and Physical Topologies
The core idea of MSTP is utilizing the fact that a redundant physical topology only has a small amount of different spanning-trees (logical topologies). The figure below shows a ring topology of three switches and three different spanning trees that may result from different root bridge placements.
Instead of running an STP instance for every VLAN, MSTP runs a number of VLAN-independent STP instances (representing logical topologies) and then administrator maps each VLAN to the most appropriate logical topology (STP instance). The number of STP instances is kept to minimum (saving switch resources), but the network capacity is utilized in more optimal fashion, by using all possible köksinredning paths for VLAN traffic.
The switch logic for VLAN traffic forwarding has changed a little bit. In order for a frame to be forwarded out of a port, two conditions must be met: first, VLAN must be active on this port (e.g. not filtered) and second, the STP instance the VLAN maps to, must be in non-discarding state for this port. The second property is normally enforced automatically, as MAC addresses are not learned on discarding ports. It is worth reminding that due to multiple logical topologies active on a port, the port could be blocking for one instance and forwarding for another (note that in (R)PVST+ a port is either forwarding or discarding for a VLAN ). The figure below demonstrates six VLANs using two MSTP instances, thus reducing the number of STP trees that would be required with PVST from 6 to 2.
The following is a s
No comments:
Post a Comment