Implementing MST in a Large Campus Environment February 13, 2007 Rich Ingram rni@umn.edu

1. Implementing MSTin a Large Campus EnvironmentFebruary 13, 2007Rich Ingramrni@umn.edu Implementing MST on a Large Campus

2. Overview • Introduction • Multiple Spanning Tree Protocol • Our Implementation Implementing MST on a Large Campus

3. Introduction Implementing MST on a Large Campus

4. U of M Twin Cities Campus Network • 23 Cisco 6509s • 4,323 Cisco 3750s • 1,133 Switch Stacks • 74,414 Switchports • Redundant 10-Gigabit Backbone • Topology: 18 layer-2 switched domains interconnected by a layer-3 MPLS-VPN backbone Implementing MST on a Large Campus

5. Spanning Tree • Finds and blocks loops • Don’t turn it off !!! • Cisco’s default is PVST+ • If you don’t need MSTP, run Rapid-PVST+ Implementing MST on a Large Campus

6. Alphabet Soup – The Standards • 802.1D – defines Bridges, including STP • 802.1Q – defines VLANs, assumes a single Spanning Tree • 802.1W – defines RSTP, rolled into 802.1D • 802.1S – defines MSTP, rolled into 802.1Q • Cisco options: PVST+, Rapid-PVST+, MST Implementing MST on a Large Campus

7. Multiple Spanning Tree Protocol Implementing MST on a Large Campus

8. What is MSTP? • Protocol to map VLANs to Spanning Trees • First, you create Multiple Spanning Trees called “Instances” • You then map VLANs to those Instances • All VLANs belong to an Instance Implementing MST on a Large Campus

9. Why Run MSTP? • Answer #1: Your hardware makes you • The 3750 can only run 128 Spanning Trees • Spanning Tree doesn’t run on the 129th VLAN • (interesting default) • Any loops on that VLAN and you’re toast Implementing MST on a Large Campus

11. Lesson Learned #1 • Know your hardware • MSTP can help you Implementing MST on a Large Campus

12. Why Run MSTP? • Answer #2: You have too many VLANs • Running Spanning Tree on every VLAN may be a problem with CPU load on some smaller switches Implementing MST on a Large Campus

13. MST0 – That Special Instance • By default, all VLANs are in MST0 • MST0 is where the BPDUs are • BPDUs are sent untagged (on the native VLAN) • BPDUs contain “M-Records” • M-Records hold topology information for the other MST Instances Implementing MST on a Large Campus

14. Other MSTP Instances • Maximum of 65 Instances • Each Instance has its own Topology and Root • Topology parameters are configurable per-Instance (root priority, port cost, etc.) • BPDU parameters are configurable only for MST0 (timers, hop count, etc.) Implementing MST on a Large Campus

15. MSTP Region • A Region contains one or more Instances • Defined by the MST configuration • Name • Revision Number • VLAN-to-Instance Mapping • If the MST configuration is different, the Region is different. Implementing MST on a Large Campus

16. Multiple MSTP Regions • Multiple Regions are interconnected by a single Common Spanning Tree (CST) • There is one Root for the CST • The CST interconnects all the MST0 Instances of all the Regions Implementing MST on a Large Campus

17. Our Implementation Implementing MST on a Large Campus

18. Our Implementation – Guidelines • Pre-define the MST configuration everywhere • Pre-define root priorities everywhere • Use Portfast to define Edge Ports • Make the Spanning Tree topology match the layer-3 topology (i.e. HSRP) Implementing MST on a Large Campus

19. Lesson Learned #2 • It’s 10 p.m. on Saturday night, do you know where your root is? • Answer #1: It’s right where I put it. • Answer #2: It’s not where I put it, but it’s in a good backup spot. • Answer #3: No, but it’s going to be in a lot of trouble when it gets back. Implementing MST on a Large Campus

20. Our Implementation –Hierarchical Root Priorities Range: 0-61440 (lower is better) • CN/DC: 4096 • RA: 8192 • BA/DA: 20480 • EN/DE: 28672 • Default (not used): 32768 Implementing MST on a Large Campus

21. Our Implementation – Core Nodes • No need for multiple topologies • All VLANs are in MST0 spanning-tree mode mst spanning-tree mst configuration name InfoTech-CN-01 revision 1 Implementing MST on a Large Campus

22. Our Implementation – Data Center • Two topologies (mostly) and HSRP • VLANs are split between MST1 and MST2 • In order to utilize redundant links • Needed a different topology for a special case Implementing MST on a Large Campus

23. Lesson Learned #3 • Spanning Tree topology != VLAN pruning • Manual pruning is dangerous • It’s easy to prune yourself off the active topology Implementing MST on a Large Campus

24. Lesson Learned #4 • How many Roots did you say you have? • Make sure your MST configurations match Implementing MST on a Large Campus

25. Our Implementation – Data Center spanning-tree mode mst spanning-tree mst configuration name DataCenter revision 1 instance 1 vlan 10-19, 30-39, 50-59, 70-79, 90-99, 110-119, 130-139, 150-159 instance 1 vlan 170-179, 190-199, 210-219, 230-239, 250-259, 270-279, 290-299 instance 1 vlan 310-319, 330-339, 350-359, 370-379, 390-399, 410-419, 430-439 instance 1 vlan 450-459, 470-479, 490-499, 510-519, 530-539, 550-559, 570-579 instance 1 vlan 590-599, 610-619, 630-639, 650-659, 670-679, 690-699, 710-719 instance 1 vlan 730-739, 750-759, 770-779, 790-799, 810-819, 830-839, 850-859 instance 1 vlan 870-879, 890-899, 910-919, 930-939, 950-959, 970-979, 990-999 instance 1 ... instance 2 vlan 2-9, 20-29, 40-49, 60-69, 80-89, 100-109, 120-129, 140-149 instance 2 vlan 160-169, 180-189, 200-209, 220-229, 240-249, 260-269, 280-289 instance 2 vlan 300-309, 320-329, 340-349, 360-369, 380-389, 400-409, 420-429 instance 2 vlan 440-449, 460-469, 480-489, 500-509, 520-529, 540-549, 560-569 instance 2 vlan 580-589, 600-609, 620-629, 640-649, 660-669, 680-689, 700-709 instance 2 vlan 720-729, 740-749, 760-769, 780-789, 800-809, 820-829, 840-849 instance 2 vlan 860-869, 880-889, 900-909, 920-929, 940-949, 960-969, 980-989 instance 2 ... instance 2 vlan 4000 instance 3 vlan 4001 Implementing MST on a Large Campus

26. Our Implementation –Edge Parameters • Global commands: • spanning-tree portfast bpdufilter default • Interface commands: • spanning-tree portfast [trunk] • spanning-tree guard root Implementing MST on a Large Campus

27. Our Implementation – Other • VTP auto pruning • UDLD • no keepalive Implementing MST on a Large Campus

28. Monitoring • It’s important to watch this stuff! • Most Spanning Tree events show up as SNMP Traps and not syslog • Pay attention to traps • Check manually every now and then (better yet, regularly) Implementing MST on a Large Campus

29. Testing • Don’t forget to test this stuff! • Good test plans are hard to write • Do it anyway Implementing MST on a Large Campus

30. Links IEEE 802.1 Standards • http://standards.ieee.org/getieee802/802.1.html Cisco 802.1W and 802.1S SRND • http://www.cisco.com/application/pdf/en/us/guest/tech/tk621/c1501/ccmigration_09186a0080174993.pdf Cisco Understanding Multiple Spanning-Tree Protocol White Paper • http://cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfc.shtml Cisco Understanding Rapid Spanning-Tree Protocol White Paper • http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfa.shtml Cisco 12.2SX Configuration Guide (see sections on MST) • http://cisco.com/en/US/products/hw/switches/ps708/products_configuration_guide_book09186a00801609ea.html Cisco MST Migration Configuration Example (includes configuration guidelines) • http://cisco.com/en/US/products/hw/switches/ps708/products_configuration_example09186a00807b075f.shtml Implementing MST on a Large Campus

31. Thank YouRich Ingramrni@umn.edu Implementing MST on a Large Campus