Implementing MST in a Large Campus Environment

Transcription

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

2 Overview Introduction Multiple Spanning Tree Protocol Our Implementation

3 Introduction

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

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+

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

7 Multiple Spanning Tree Protocol

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

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

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11 Lesson Learned #1 Know your hardware MSTP can help you

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

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

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.)

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.

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

17 Our Implementation

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)

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.

20 Our Implementation Hierarchical Root Priorities Range: (lower is better) CN/DC: 4096 RA: 8192 BA/DA: EN/DE: Default (not used): 32768

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

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

23 Lesson Learned #3 Spanning Tree topology!= VLAN pruning Manual pruning is dangerous It s easy to prune yourself off the active topology

24 Lesson Learned #4 How many Roots did you say you have? Make sure your MST configurations match

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, , , instance 1 vlan , , , , , , instance 1 vlan , , , , , , instance 1 vlan , , , , , , instance 1 vlan , , , , , , instance 1 vlan , , , , , , instance 1 vlan , , , , , , instance 1... instance 2 vlan 2-9, 20-29, 40-49, 60-69, 80-89, , , instance 2 vlan , , , , , , instance 2 vlan , , , , , , instance 2 vlan , , , , , , instance 2 vlan , , , , , , instance 2 vlan , , , , , , instance 2 vlan , , , , , , instance 2... instance 2 vlan 4000 instance 3 vlan 4001

26 Global commands: Our Implementation Edge Parameters spanning-tree portfast bpdufilter default Interface commands: spanning-tree portfast [trunk] spanning-tree guard root

27 Our Implementation Other VTP auto pruning UDLD no keepalive

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)

29 Testing Don t forget to test this stuff! Good test plans are hard to write Do it anyway

30 Links IEEE Standards Cisco 802.1W and 802.1S SRND Cisco Understanding Multiple Spanning-Tree Protocol White Paper Cisco Understanding Rapid Spanning-Tree Protocol White Paper Cisco 12.2SX Configuration Guide (see sections on MST) Cisco MST Migration Configuration Example (includes configuration guidelines)

31 Thank You Rich Ingram