Cisco Cisco Nexus 5010 Switch デザインガイド
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MST
Rapid PVST+
Separation between VLAN creation and
topology definition
topology definition
Yes
No
Ease of implementation of per-VLAN load
balancing in spanning-tree topologies
balancing in spanning-tree topologies
Requires the definition of 2 instances and of the
VLAN-to-instance mapping
VLAN-to-instance mapping
(In vPC deployments, VLAN load balancing is
not needed any more.)
not needed any more.)
Easier to deploy for VLAN load balancing in
classic spanning-tree designs
classic spanning-tree designs
(In vPC deployments, VLAN load balancing is
not needed any more.)
not needed any more.)
Configuration complexity
More complex because of the need to maintain
the region configuration across all switches in
the Layer 2 domain; The network administrator
needs to manually ensure that vPC peers are
configured identically
the region configuration across all switches in
the Layer 2 domain; The network administrator
needs to manually ensure that vPC peers are
configured identically
Easier
Integration of large Layer 2 domains
Easier, due to the concept of regions
More convoluted because a single device
becomes the root for the whole Layer 2 domain;
no possibility to create Layer 2 regions
becomes the root for the whole Layer 2 domain;
no possibility to create Layer 2 regions
Rapid PVST+ offers slightly better flexibility for load balancing VLANs on a typically V-shape spanning-tree
topology. With the adoption of vPC, this benefit is marginal because topologies are becoming intrinsically loop free,
at which point the use of per-VLAN load balancing compared to per-instance load balancing is irrelevant (with vPC,
all links are forwarding in any case).
MST Concepts
With IEEE 802.1s, you can create an instance, which means a number, to represent the topology, and you can
map VLANs to this instance (Figure 1).
Figure 1. Traditional MST Design with Instance Load Balancing
Figure 1 shows the topology according to IEEE 802.1s, or MST. The instance that represents the topology at the
left is MST 1. VLAN A, B, C, and D are mapped to MST 1. At the right of the figure you can see the forwarding
topology for MST 2.
If you do show spanning-tree VLAN A from the access layer, you see that the root bridge is Agg1. If you type the
same command for VLAN E on the same switch, you see that the root bridge is Agg2. This difference occurs
because VLAN A and VLAN E are mapped to two different instances.