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Spanning Tree Priority on Nexus vPC+ and Fabricpath
Submitted by admin on Wed, 09/05/2012 - 22:44
If you are familiar with Nexus vPC configuration, you might have been setting different STP priority on the primary and secondary switches so the primary is always a STP root, and have that lined up with, for example, HSRP active node. With vPC+ (ie. running vPC on a pair of switches that participate in fabricpath), the two Nexus switches appear as a single logical switch to both fabricpath cloud and upstream/downstream vPC switches, so it is actually crucial to make sure the upstream/downstream vPC switches receive consistent STP root priority regardless of which path is active. In this article, we demonstrate the importance of setting identical STP priority on the vPC+ peer switches, how the switches react when a superior BPDU is received, and other implications using Cisco Nexus 5000.
Below are a diagram, switch vPC configuration, and show-command outputs when everything is configured properly. Here the downstream switch ACESS-SW1 is connecting via port-channel 1 to the Nexus switches. Vlan 2 and 10 are in mode fabricpath, while vlan 20 and 21 are Classical Ethernet. A few things to note are:
- Both Nexus switches uses a virtual MAC address (c84c.75ec.8000) as their STP ID instead of the respective VLAN interface MAC address (547f.eeaf.78dc and 547f.eeaf.97ca) for VLAN that are in mode fabricpath
- Nexus switches continue to use the actual VLAN interface MAC address for the Classical Ethernet VLAN.
- Both Nexus switches claim to be STP root
- There is no STP running over peer-link as they are configured as ‘switchport mode fabricpath’, while Po1 still runs STP
- Both Nexus switches require identical fabricpath (virtual) switch ID, which is advertised to the fabricpath cloud along with their individual fabricpath switch ID
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************** Configuration ************** NEXUS-SW1# sh spann root Root Hello Max Fwd VLAN0010 Bridge ID Priority 8202 (priority 8192 sys-id-ext 10) Interface Role Sts Cost Prio.Nbr Type
!--------- NEXUS-SW2 -------- NEXUS-SW2# sh spann vl 10 VLAN0010 Bridge ID Priority 8202 (priority 8192 sys-id-ext 10) Interface Role Sts Cost Prio.Nbr Type |
Scenario 1: Better BPDU received from downstream vPC switch
When we configure ACCESS-SW1 with better STP priority on VLAN 10, the followings were observed.
- Both Nexus switches becomes blocking with error ‘*L2GW_Inc’ on interface Po1 VLAN 10
- After restoring the STP priority on ACCESS-SW1, the Nexus switches transition to ‘*LOOP_Inc’
- Po1 on the ACCESS-SW1 side needs to ‘shut’ and ‘no shut’ for VLAN 10 STP on the Nexus to return to forwarding state
This behavior is a result of the requirement that vPC+ peer switches need to be STP root on all VLAN that are in mode fabricpath.
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************** Configuration ************** VLAN0010 Bridge ID Priority 8202 (priority 8192 sys-id-ext 10) Interface Role Sts Cost Prio.Nbr Type NEXUS-SW1# sh spann vl 10 VLAN0010 Bridge ID Priority 8202 (priority 8192 sys-id-ext 10) Interface Role Sts Cost Prio.Nbr Type |
Scenario 2: Dual-homed downstream switch with no vPC
Here we have ACCESS-SW2 that is dual-connected to both Nexus but no running etherchannel. We then change the STP priority on NEXUS-SW2 to 16384. The followings were observed.
- Ethernet1/2 on NEXUS-SW2 changes STP state to blocking with error ‘*L2GW_Inc’
Because ACCESS-SW2 is not dual-connected with vPC, the BPDU it receives from NEXUS-SW1 was accepted on Te1/1/1 (as it has priority of 8192) and advertised out Te1/1/2 to NEXUS-SW2. Since NEXUS-SW2 is running vPC+, it would not accept any superior BPDU, hence putting the interface in STP blocking.
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************** Before Changes ************** VLAN0002 Bridge ID Priority 8194 (priority 8192 sys-id-ext 2) Interface Role Sts Cost Prio.Nbr Type !--------- NEXUS-SW2 --------- VLAN0002 Bridge ID Priority 8194 (priority 8192 sys-id-ext 2) Interface Role Sts Cost Prio.Nbr Type !---------- ACCESS-SW2 ---------------- VLAN0002 Bridge ID Priority 32770 (priority 32768 sys-id-ext 2) Interface Role Sts Cost Prio.Nbr Type ************** Configuration ************** VLAN0002 Bridge ID Priority 16386 (priority 16384 sys-id-ext 2) Interface Role Sts Cost Prio.Nbr Type |
Scenario 3: Controlling active link for dual-homed non-vpc switch
Now that the Nexus switches appear as a single switch, with everything in BPDU being equal, by default, ACCESS-SW2 would break the tie by STP port ID. If you want to have control over which link is forwarding, you can define STP port-priority on the NEXUS-SW1 or NEXUS-SW2 interface Ethernet1/2 and the one with lower priority will cause the corresponding interface on ACCESS-SW2 to forward.
Here, interface Te1/1/1 is forwarding by default. We will lower the port-priority on Ethernet 1/2 of NEXUS-SW2 down to 64 (default is 128). Te1/1/2 subsequently becomes forwarding and Te1/1/1 went into blocking.
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************** Configuration ************** VLAN0002 Bridge ID Priority 32770 (priority 32768 sys-id-ext 2) Interface Role Sts Cost Prio.Nbr Type |
References: Cisco Nexus 5000 Series NX-OS FabricPath Configuration Guide, Release 5.1(3)N1(1)
About Author
Metha Chiewanichakorn, CCIE#23585 (Ent. Infra, Sec, SP), is a Cisco networking enthusiast with years of experience in the industry. He is currently working as a consulting engineer for a Cisco partner. As a founder of and an instructor at labminutes.com, Metha enjoys learning and challenges himself with new technologies.