implementing mst on a large campus implementing mst in a large campus environment february 13, 2007...
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Implementing MST on a Large Campus
Implementing MSTin a Large Campus Environment
February 13, 2007Rich Ingram
Overview
• Introduction• Multiple Spanning Tree Protocol• Our Implementation
Implementing MST on a Large Campus
Introduction
Implementing MST on a Large Campus
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
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
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
Multiple Spanning Tree Protocol
Implementing MST on a Large Campus
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
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
Lesson Learned #1
• Know your hardware• MSTP can help you
Implementing MST on a Large Campus
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
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
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
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
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
Our Implementation
Implementing MST on a Large Campus
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
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
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
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
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
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
Lesson Learned #4
• How many Roots did you say you have?
• Make sure your MST configurations match
Implementing MST on a Large Campus
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
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
Our Implementation – Other
• VTP auto pruning• UDLD• no keepalive
Implementing MST on a Large Campus
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
Testing
• Don’t forget to test this stuff!• Good test plans are hard to write• Do it anyway
Implementing MST on a Large Campus
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
