Understanding the Spanning Tree protocol family

Switch Keeper

Multiple Spanning Tree Protocol

Because RSTP is a Cisco proprietary protocol, switches from other vendors cannot enjoy the speedy convergence provided by RSTP. Luckily, an open standard called Multiple Spanning Tree Protocol (MSTP, or MST in a Cisco configuration) [4] serves a similar role. MSTP inherits the fast convergence features of RSTP and includes features to reduce resource usage.

RSTP works very well if there are only a small number of VLANs. However, when the number of VLANs increases, switches need to handle a lot of network topologies simultaneously. CPU resources and bandwidth utilization are used for STA calculations and protocol communication. Moreover, most of these topologies are actually the same! It does not make sense to spend resources to maintain lots of identical topologies for different VLANs. Figure 5 shows 100 VLANs from VLAN 101 to VLAN 200. A total of 100 Spanning Tree topologies are running on the switches, but the topologies of VLAN 101-150 are actually identical, as are VLANs 151-200.

Figure 5: RSTP becomes inefficient with a large number of VLANs.

MSTP can group the VLANs into instances using a mapping table. Each instance will have its own network topology that will apply to all VLANs that are mapped to the instance. For example, for the network in Figure 5, VLAN 101 to 150 can be grouped into instance 1 and VLAN 151 to 200 can be grouped into instance 2 (Figure 6). The total number of network topologies is then decreased from 100 to 2.

Figure 6: MSTP improves performance by grouping identical topologies.

Listing 2 shows the MSTP mapping table configuration, which should be repeated on all switches.

Listing 2

MSTP Mapping

SW1(config)#spanning-tree mst configuration
SW1(config-mst)#name Region1
SW1(config-mst)#revision 1
SW1(config-mst)#instance 1 vlan 101-150
SW1(config-mst)#instance 2 vlan 151-200

Then, enable MSTP on all switches using the command:

SW1(config)#spanning-tree mode mst

Next, set SW1 to be the root switch of instance 1 and set SW2 to be the root switch of instance 2:

SW1(config)#spanning-tree mst 1 root primary
SW2(config)#spanning-tree mst 2 root primary

Finally, tune the spanning tree cost on SW3, as shown in Listing 3.

Listing 3

Tuning the Spanning Tree Cost

SW3(config)#interface fastEthernet 0/1
SW3(config-if)#spanning-tree mst 2 cost 999
SW3(config-if)#interface fastEthernet 0/2
SW3(config-if)#spanning-tree mst 1 cost 999

Region

The concept of a region is another revolution of MSTP. You can split the entire network topology into different regions for easy management. Each region has its own instance topologies and does not influence other regions. The interconnection of regions is handled by the Common and Internal Spanning Tree (CIST). In CIST's view, each region is treated as a big virtual switch. The STA will be run on those big virtual switches to determine the state of the interregion connected ports.

In Figure 7, the network is grouped into three regions. The concept is similar to the autonomous system (AS) number used with the border gateway protocol (BGP). Each region has its own network topology. When the topology changes in a region, the change only occurs inside that region and does not influence other regions.

Figure 7: Regions improve stability: A change in topology is only relevant within the region.

Conclusion

Modern networks don't just transmit data but also provide voice service and even support emergency and security components such as CCTV, access control, and fire protection. High availability and resiliency are essential in today's network infrastructure. Even small intervals of downtime can create big problems. If you find some switches on your network that are still running conventional STP or PVST, it is a good idea to check the feasibility of migrating to RSTP or MSTP.

The Author

Jan Ho is a network engineer living in Hong Kong. He writes network tutorials at http://jannet.hk.

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