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

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 129 th 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 Implementing MST on a Large Campus spanning-tree mode mst spanning-tree mst configuration name InfoTech-CN-01 revision 1

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 Implementing MST on a Large Campus 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

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 You Rich Ingram rni@umn.edu