PerVLAN Spanning Tree and STP over Trunks
If only one instance of STP was used for a switched network with redundant links but with multiple VLANs, several ports would be in a blocking state, unused under stable conditions. The redundant links would essentially be used for backup purposes.
The Cisco Per VLAN Spanning Tree Plus (PVST+) feature creates an STP instance for each VLAN. By tuning STP configuration per VLAN, each STP instance can use a different root switch and have different interfaces block. As a result, the traffic load can be balanced across the available links. For instance, in the common building design with distribution and access links in Figure 3-4, focus on the left side of the figure. In this case, the access layer switches block on different ports on VLANs 1 and 2, with different root switches.
Figure 3-4 Operation of PVST+ for Better Load Balancing
With different root switches and with default port costs, the access layer switches end up sending VLAN1 traffic over one uplink and VLAN2 traffic over another uplink.
Using 802.1Q with STP requires some extra thought as to how it works. 802.1Q does not support PVST+ natively; however, Cisco switches do support PVST+ over 802.1Q trunks. So, with all Cisco switches, and PVST+ (which is enabled by default), PVST+ works fine.
When using 802.1Q with non-Cisco switches, the switches must follow the IEEE standard completely, so the trunks support only a Common Spanning Tree (CST). With standard 802.1Q, only one instance of STP runs only over the native VLAN, and that one STP topology is used for all VLANs. Although using only one STP instance reduces the STP messaging overhead, it does not allow load balancing by using multiple STP instances, as was shown with PVST+ in Figure 3-4.
When building networks using a mix of Cisco and non-Cisco switches, along with 802.1Q trunking, you can still take advantage of multiple STP instances in the Cisco portion of the network. Figure 3-5 shows two general options in which two CST regions of non-Cisco switches connect to two regions of Cisco PVST+ supporting switches.
Figure 3-5 Combining Standard IEEE 802.1Q and CST with PVST+
Figure 3-5 Combining Standard IEEE 802.1Q and CST with PVST+
The left side of Figure 3-5 shows an example in which the CST region is not used for transit between multiple PVST+ regions. In this case, none of the PVST+ per-VLAN STP information needs to pass over the CST region. The PVST+ region maps the single CST instance to each of the PVST+ STP instances.
The rest of Figure 3-5 shows two PVST+ regions, separated by a single CST region (CST Region 2). In this case, the PVST+ per-VLAN STP information needs to pass through the CST region. To do so, PVST+ treats the CST region as a single link and tunnels the PVST+ BPDUs across the CST region. The tunnel is created by sending the BPDUs using a multicast destination MAC of 0100.0CCC.CCCD, with the BPDUs being VLAN tagged with the correct VLAN ID. As a result, the non-Cisco switches forward the BPDUs as a multicast, and do not interpret the frames as BPDUs. When a forwarded BPDU reaches the first Cisco PVST+ switch in the other PVST+ region, the switch, listening for multicasts to 0100.0CCC.CCCD, reads and interprets the BPDU.
NOTE 802.1Q, along with 802.1s Multiple-instance Spanning Tree (MST), allows 802.1Q trunks for support multiple STP instances. MST is covered later in this chapter.
Continue reading here: STP Configuration and Analysis
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