Spanning tree limits

An “STP instance” is “a tree” in a spanning tree. So this only applies to PVST/PVST+ and similar. MST has a way lower limit (like 16), but you shouldn’t need more than a couple. But you know that if you’re using MST. So in most cases I would say this is the limit of the number of VLANs you can have since if you have 50+ VLANs then you’re probably running some form of spanning tree.

* From official documentation

Cisco 6500/7600

The 6500/7600 is a bit special in many ways, so it’s no surprise that it’s different here too. The 6500/7600 has a per-chassi limit of 10’000 “active logical ports” and a per line card limit of 1’800 “virtual ports” (more with MST).

Active logical ports (10’000 per chassi with PVST+, 50’000 with MST)

An active logical port is simply one for every VLAN on all ports combined. A trunk port carrying 100 VLANs is 100 logical ports, and a switchport mode access port is 1. 46 trunk ports carrying 120 VLANs plus 2 non-trunk ports therefore means 46*120+2=5’522 logical ports. Well within the 10’000 limit.

show spanning-tree summary total

Router#show spanning-tree summary total
Switch is in rapid-pvst mode
Root bridge for: [...]
EtherChannel misconfig guard is enabled
Extended system ID           is disabled
Portfast Default             is disabled
PortFast BPDU Guard Default  is disabled
Portfast BPDU Filter Default is disabled
Loopguard Default            is disabled
UplinkFast                   is disabled
BackboneFast                 is disabled
Pathcost method used         is short

Name                   Blocking Listening Learning Forwarding STP Active
---------------------- -------- --------- -------- ---------- ----------
116 vlans                    0         0        0        189        189

(189 active logical ports in this example)

12.2(33)SXI with Sup720 and later upgrade this limit to 100’000 for MST and 12’000/15’000 for RPVST+/PVST+.

Virtual ports (1’800 per line card with PVST+, 6’000 with MST)

Virtual ports are calculated much like logical ports, with two differences:

1. Virtual ports are calculated per line card, not per chassi
2. Virtual ports count for each interface part of a port-channel

So while you can have the 46 trunks (with 120 VLANs) + 2 accessports in one chassi, you can’t have them on the same line card. You can’t have 12 portchannel trunkports with 4 ports each either, because that’s 12*4*120=5'760 virtual ports. With MST that’s ok, but you are closing in on the 6’000 limit.

show vlan virtual-port slot X

Router#show vlan virtual-port slot 3
Slot 3
Port        Virtual-ports
-------------------------
Te3/1               1
Te3/2               116
Te3/3               2
Total virtual ports:119

(119 virtual ports on line card 3)

In Release 12.2(33)SXI1 and later releases, the virtual port limit does not apply to WS-X67xx and WS-X65xx switching modules.

Summary of 6500/7600

If you have many trunks, especially many trunks on the same line card, check to make sure that you’re not hitting these two distinct limits. You may have to move trunks around in order to keep down the number of virtual ports per line card.

Nexus 7k with NX-OS 5.x

Max VLANs: 4096 per VDC, 16,384 VLAN ports per system
Logical ports per system: 16,000 Rapid PVST+, 90,000 MST

Sources

• Cisco Service Delivery Center Infrastructure 2.1 Design Guide
• Data center networking - Architechture and design guidelines
• Release Notes for Cisco IOS Release 12.2(33)SXH and Later Releases

More?

If you have more information, please let me know and I will update this post.