Nexus Spanning Tree

Covering some of the topics for both RPVST+ and MST in this post.

General
  • Both STP modes cannot run simultaneously in a VDC.
  • In order for spanning-tree to correctly account for paths 10G or greater, use the spanning-tree pathcost method long command.
RPVST+
I'm very familiar with all of the basic RPVST+ commands so I am not going to document any here.

  • Rapid Spanning-tree uses version two (2) BPDU's.  Legacy 802.1D uses version zero (0).
  • The Switch learns the link type during the proposal and agreement phase.  The type selected can be overridden with this command interface command - spanning-tree link-type
The default timers as as follows:
  • Hello - 2 seconds
  • Forward Delay - 15 seconds
  • Maximum Age - 20 seconds
Port Roles (from the L2 Switching Guide)



Influencing Link Selection

Nexus spanning-tree uses the same Port Cost and Port-Priority commands as IOS.  Use lower Costs select one path over another.  Use a lower Port-Priority for redundant path selection.

RPVST+ interoperates with MST. To disable this interoperation, us this command spanning-tree mst simulate pvst  global


When you connect two Cisco devices through 802.1Q trunks, the switches exchange spanning tree
BPDUs on each VLAN allowed on the trunks. The BPDUs on the native VLAN of the trunk are sent
untagged to the reserved 802.1D spanning tree multicast MAC address (01-80-C2-00-00-00). The BPDUs on all VLANs on the trunk are sent tagged to the reserved Cisco Shared Spanning Tree Protocol (SSTP) multicast MAC address (01-00-0c-cc-cc-cd).

 MST

MST maps multiple VLANs into a single spanning tree instance.  Each spanning tree instance is completely independent from the other spanning tree instances. MST is compatible with both 802.1D and RPVST+

Regions
Switch with the same MST configuration are called regions. Each region can have up to 65 MST instances.  Instance numbers can range from 1 to 4094 (0 is reserved).

IST, CIST and CST

  • An IST is a spanning tree that runs inside a region.  IST is the only STP that sends and receives BPDUs.
  • The CST interconnect regions
  • The CIST is a collection of the ISTs in each MST region.  The CIST is the same as an IST inside a region and is the same as a CST outside the region.
Here's a diagram from the L2 Switching Guide



MST Configuration
The MST configuration must be identical on all Switches in a MST region. The three parameters of the MST configuration are:
  1. Name
  2. VLAN to MST instance mapping
  3. Revision number.  The revision number must be set and also incremented manually each time the configuration is changed.
Sample configuration:

spanning-tree mode mst
spanning-tree domain 1
spanning-tree mst 1-4 priority 24576
spanning-tree mst configuration
  name TEST-MST
  revision 10
  instance 1 vlan 1,11,111
  instance 2 vlan 2,22,222
  instance 3 vlan 3,33,333
  instance 4 vlan 4,44,444

For private VLANs, all secondary VLANs must be in the same MSTI as the associated primary VLAN. Use the private-vlan synchronize command in the MST configuration mode.

Spanning Tree Extensions
I'm not going to list all the commads with descriptions.  I'm just including a sample configuration of most of the options:

spanning-tree port type edge bpduguard default
spanning-tree port type edge bpdufilter default
spanning-tree port type network default
spanning-tree loopguard default

interface Ethernet2/7
  shutdown
  switchport
  switchport mode private-vlan promiscuous
  spanning-tree link-type point-to-point
  spanning-tree bpduguard enable
  spanning-tree guard loop
  spanning-tree bpdufilter enable
  spanning-tree mst 1 cost 10
  spanning-tree vlan 1 cost 10
  switchport private-vlan mapping 20 30,40

interface Ethernet2/8
  shutdown
  switchport
  switchport mode trunk
  spanning-tree mst simulate pvst
  spanning-tree mst pre-standard
  spanning-tree guard root
  spanning-tree lc-issu disruptive
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