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Module 24
Multiple Spanning Tree (MST)
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# 📘 CCNA 200-301 - Video 24: Multiple Spanning Tree (MST) ## Deep Study Notes --- ## 📌 Learning Objectives By the end of this video, you should understand: - Why MST is needed (limitations of PVST+) - MST concepts (regions, instances, mapping) - MST configuration - MST verification and troubleshooting - MST vs. PVST+ comparison --- ## 🧠 Core Concepts ### 1. Limitations of PVST+ (Per-VLAN Spanning Tree) **PVST+ (Per-VLAN Spanning Tree Plus):** - Runs a separate STP instance for each VLAN - Default on Cisco switches - Provides load balancing across VLANs **The Problem: Resource Consumption** ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ PVST+ RESOURCE CONSUMPTION │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ With 1000 VLANs: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ • 1000 separate STP instances │ │ │ │ • 1000 BPDUs sent every 2 seconds (2000 BPDUs/sec total) │ │ │ │ • Significant CPU and memory usage on switches │ │ │ │ • Slow convergence for all VLANs │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ Problem Illustration: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ │ │ │ │ Switch A ───────────────────────────── Switch B │ │ │ │ │ │ │ │ │ │ │ VLAN 10: BPDUs (Hello 2s) │ │ │ │ │ │ VLAN 20: BPDUs (Hello 2s) │ │ │ │ │ │ VLAN 30: BPDUs (Hello 2s) │ │ │ │ │ │ ... │ │ │ │ │ │ VLAN 1000: BPDUs (Hello 2s) │ │ │ │ │ │ │ │ │ │ │ └─────────────────────────────────┘ │ │ │ │ │ │ │ │ Total BPDUs per second = (Number of VLANs × 1) / 2 seconds │ │ │ │ For 1000 VLANs: 500 BPDUs per second │ │ │ │ │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` --- ### 2. What is MST (Multiple Spanning Tree)? **Definition:** Multiple Spanning Tree Protocol (MSTP) is an IEEE standard (802.1s) that maps multiple VLANs to a single spanning tree instance, reducing resource consumption while still providing load balancing. **Analogy:** Think of MST like a subway system. Instead of building a separate track for every train line (PVST+), you group lines that take similar routes into the same tunnel (MST instance). This uses fewer resources but still gets trains where they need to go. ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ MST CONCEPT │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ PVST+ (One instance per VLAN): │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ │ │ │ │ Instance 10 (VLAN 10) ──────┐ │ │ │ │ Instance 20 (VLAN 20) ──────┼─── 1000 instances │ │ │ │ Instance 30 (VLAN 30) ──────┤ │ │ │ │ ... │ │ │ │ │ Instance 1000 (VLAN 1000) ───┘ │ │ │ │ │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ MST (Multiple VLANs per instance): │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ │ │ │ │ Instance 1: VLANs 10, 20, 30 ────┐ │ │ │ │ Instance 2: VLANs 40, 50, 60 ────┼─── 10-15 instances │ │ │ │ Instance 3: VLANs 70, 80, 90 ────┤ │ │ │ │ ... │ │ │ │ │ Instance 15: VLANs 910-1000 ─────┘ │ │ │ │ │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` --- ### 3. MST Terminology | Term | Description | |------|-------------| | **MST Region** | Group of switches with identical MST configuration | | **MST Instance (MSTI)** | A spanning tree instance within a region | | **IST (Internal Spanning Tree)** | Instance 0; carries BPDUs and maps VLANs not assigned to other instances | | **CIST (Common and Internal Spanning Tree)** | Single spanning tree connecting all MST regions and PVST+ switches | | **MST Configuration** | Region name, revision number, VLAN-to-instance mapping | ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ MST ARCHITECTURE │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ CIST (Global) │ │ │ │ Connects all regions and PVST+ switches │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ │ ┌────────────────────────┼────────────────────────┐ │ │ │ │ │ │ │ ▼ ▼ ▼ │ │ ┌─────────────────┐ ┌─────────────────┐ ┌─────────────────┐ │ │ │ MST Region A │ │ MST Region B │ │ PVST+ Switch │ │ │ │ │ │ │ │ │ │ │ │ Region Name: │ │ Region Name: │ │ │ │ │ │ "Campus" │ │ "Branch" │ │ │ │ │ │ │ │ │ │ │ │ │ │ IST (Instance0)───────│ IST (Instance0) │ │ │ │ │ Instance 1: │ │ Instance 1: │ │ │ │ │ │ VLANs 10-20 │ │ VLANs 10-20 │ │ │ │ │ │ Instance 2: │ │ Instance 2: │ │ │ │ │ │ VLANs 30-40 │ │ VLANs 30-40 │ │ │ │ │ └─────────────────┘ └─────────────────┘ └─────────────────┘ │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` --- ### 4. MST Regions **What Makes a Region?** All switches in an MST region must have identical: | Parameter | Description | |-----------|-------------| | **Region Name** | Identifies the region (case-sensitive) | | **Revision Number** | Version number (1-65535) | | **VLAN-to-Instance Mapping** | Same mapping for all VLANs | ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ MST REGION BOUNDARIES │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ WITHIN REGION (Same Configuration): │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ │ │ │ │ Switch A ──────────────── Switch B ──────────────── Switch C │ │ │ │ Region: CAMPUS Region: CAMPUS Region: CAMPUS │ │ │ │ Revision: 1 Revision: 1 Revision: 1 │ │ │ │ Map: V10→Inst1 Map: V10→Inst1 Map: V10→Inst1 │ │ │ │ │ │ │ │ Result: All switches are in same region │ │ │ │ MST BPDUs exchanged within region │ │ │ │ │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ BETWEEN REGIONS (Different Configuration): │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ │ │ │ │ Switch A ───────────────────────────── Switch D │ │ │ │ Region: CAMPUS Region: BRANCH │ │ │ │ Revision: 1 Revision: 1 │ │ │ │ Map: V10→Inst1 Map: V10→Inst2 │ │ │ │ │ │ │ │ Result: Different regions → Boundary between regions │ │ │ │ CIST BPDUs exchanged between regions │ │ │ │ │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` --- ### 5. MST Instances **Instance 0 (IST - Internal Spanning Tree):** - Always exists (cannot be deleted) - Carries BPDUs for the entire region - Maps VLANs not assigned to other instances - Determines region root **Instance 1-4094 (MSTI - Multiple Spanning Tree Instances):** - Configured by administrator - Each instance has its own root bridge - Each instance can have different topology for load balancing ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ MST INSTANCE EXAMPLE │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ Configuration: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ Instance 0 (IST): VLANs 1-9, 100-4094 (default) │ │ │ │ Instance 1: VLANs 10, 20, 30 (Engineering VLANs) │ │ │ │ Instance 2: VLANs 40, 50, 60 (Sales VLANs) │ │ │ │ Instance 3: VLANs 70, 80, 90 (IT VLANs) │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ Topology: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ │ │ │ │ Instance 1 (VLANs 10,20,30) │ │ │ │ ┌─────────┐ │ │ │ │ │ Root A │ │ │ │ │ └────┬────┘ │ │ │ │ │ │ │ │ │ ┌────▼────┐ │ │ │ │ │ Switch B│ │ │ │ │ └─────────┘ │ │ │ │ │ │ │ │ Instance 2 (VLANs 40,50,60) │ │ │ │ ┌─────────┐ │ │ │ │ │ Switch B│ ← Different root for load balancing │ │ │ │ └────┬────┘ │ │ │ │ │ │ │ │ │ ┌────▼────┐ │ │ │ │ │ Root A │ │ │ │ │ └─────────┘ │ │ │ │ │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` --- ### 6. MST Configuration **Step-by-Step MST Configuration:** ```cisco ! Step 1: Enter MST configuration mode Switch(config)# spanning-tree mst configuration ! Step 2: Set region name Switch(config-mst)# name CAMPUS ! Step 3: Set revision number Switch(config-mst)# revision 1 ! Step 4: Map VLANs to instances Switch(config-mst)# instance 1 vlan 10,20,30 Switch(config-mst)# instance 2 vlan 40,50,60 Switch(config-mst)# instance 3 vlan 70,80,90 ! Step 5: Verify configuration Switch(config-mst)# show pending Switch(config-mst)# exit ! Step 6: Configure instance root bridges Switch(config)# spanning-tree mst 1 root primary Switch(config)# spanning-tree mst 2 root secondary ! Step 7: Configure instance priority Switch(config)# spanning-tree mst 1 priority 4096 ``` **Complete MST Configuration Example:** ```cisco hostname SwitchA ! ! Configure MST spanning-tree mode mst ! spanning-tree mst configuration name CAMPUS revision 1 instance 1 vlan 10,20,30 instance 2 vlan 40,50,60 instance 3 vlan 70,80,90 ! ! Set root bridges spanning-tree mst 1 root primary spanning-tree mst 2 root secondary ! ! Configure interfaces interface GigabitEthernet0/1 switchport mode trunk ! end ``` --- ### 7. MST Port Roles **MST Port Roles (Same as RSTP):** | Port Role | Description | |-----------|-------------| | **Root Port** | Best path to root of instance | | **Designated Port** | Port on segment with best path to root | | **Alternate Port** | Backup to root port | | **Backup Port** | Backup to designated port | | **Boundary Port** | Port connecting to different MST region or PVST+ | ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ MST PORT ROLES │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ WITHIN MST REGION: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ │ │ │ │ Switch A (Root) │ │ │ │ │ │ │ │ │ │ Designated Port │ │ │ │ ▼ │ │ │ │ Switch B │ │ │ │ │ │ │ │ │ │ Root Port │ │ │ │ ▼ │ │ │ │ Switch C │ │ │ │ │ │ │ │ Alternate Port (blocked) on Switch C → Switch A link │ │ │ │ │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ BOUNDARY PORT (to PVST+ region): │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ │ │ │ │ MST Region PVST+ Region │ │ │ │ ┌─────────┐ ┌─────────┐ │ │ │ │ │ Switch A│══════════════════│ Switch D│ │ │ │ │ │ │ Boundary Port │ │ │ │ │ │ └─────────┘ └─────────┘ │ │ │ │ │ │ │ │ Switch A sends PVST+ BPDUs on boundary port │ │ │ │ │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` --- ### 8. MST Verification Commands | Command | Purpose | |---------|---------| | `show spanning-tree mst configuration` | Display MST region configuration | | `show spanning-tree mst [instance]` | Display MST instance details | | `show spanning-tree mst interface [int]` | Display port status for MST | | `show spanning-tree mst summary` | Display MST summary | | `show spanning-tree mst detail` | Detailed MST information | | `show spanning-tree mst boundary` | Show boundary ports | **Example Outputs:** ```cisco Switch# show spanning-tree mst configuration Name [CAMPUS] Revision 1 Instances configured 4 Instance Vlans mapped -------- ------------------------------------------------------- 0 1-9,100-4094 1 10,20,30 2 40,50,60 3 70,80,90 ---------------------------------------------------------------- Switch# show spanning-tree mst 1 ###### MST01 vlans mapped: 10,20,30 Bridge address 001a.2b3c.4d5e priority 4096 (4096 sysid 0) Root address 001a.2b3c.4d5e priority 4096 (4096 sysid 0) Root port this switch is root path cost 0 Regional root this switch is root Operational hello time 2 , forward delay 15, max age 20, txholdcount 6 Configured hello time 2 , forward delay 15, max age 20, max hops 20 Interface Role Sts Cost Prio.Nbr Type ---------------- ---- --- --------- -------- -------------------------------- Gi0/1 Desg FWD 2000 128.1 P2p Gi0/2 Desg FWD 2000 128.2 P2p Switch# show spanning-tree mst interface gigabitEthernet 0/1 GigabitEthernet0/1 of MST00 is designated forwarding Edge port: no (default) port guard: none (default) Link type: point-to-point (auto) bound to: none Internal path cost: 2000 Internal port priority: 128 Port role: designated Port id: 128.1 Port cost: 2000 State: forwarding ``` --- ### 9. MST vs. PVST+ Comparison | Feature | PVST+ | MST | |---------|-------|-----| | **Standard** | Cisco proprietary | IEEE 802.1s | | **Instances** | 1 per VLAN (up to 1005) | Up to 4094 instances (typically 10-15) | | **BPDU Count** | 1 per VLAN per link | 1 per instance per link | | **CPU/Memory** | High (with many VLANs) | Low | | **Convergence** | Per-VLAN, slower | Faster | | **Load Balancing** | Yes (per VLAN) | Yes (per instance) | | **Interoperability** | Cisco only | Multi-vendor | | **Complexity** | Low | Medium | | **Best For** | Small networks (< 100 VLANs) | Large networks (> 100 VLANs) | **BPDU Comparison:** | Scenario | PVST+ | MST | |----------|-------|-----| | **100 VLANs, 10 links** | 1000 BPDUs (100 × 10) | 10-20 BPDUs (instances × links) | | **CPU Load** | High (process 1000 BPDUs) | Low (process 10-20 BPDUs) | | **Bandwidth** | High BPDU overhead | Low BPDU overhead | --- ### 10. MST with PVST+ Interoperability **Problem:** MST region must work with legacy PVST+ switches. **Solution:** MST boundary ports send PVST+ BPDUs to PVST+ switches. ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ MST-PVST+ INTEROPERABILITY │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ MST Region (CAMPUS) PVST+ Switch │ │ ┌─────────────────┐ ┌─────────────────┐ │ │ │ │ │ │ │ │ │ Switch A │ │ Switch D │ │ │ │ (MST) │ Trunk │ (PVST+) │ │ │ │ │══════════════════│ │ │ │ │ Boundary Port │ │ │ │ │ └─────────────────┘ └─────────────────┘ │ │ │ │ What happens: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ • Switch A sends PVST+ BPDUs to Switch D (one per VLAN) │ │ │ │ • Switch D sees normal PVST+ neighbor │ │ │ │ • All VLANs appear to PVST+ as being in the same spanning tree │ │ │ │ • Switch A translates MST topology to PVST+ │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ Important: │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ • MST region appears as one switch to PVST+ │ │ │ │ • PVST+ sees MST region as having the same root for all VLANs │ │ │ │ • VLANs within MST region can still load balance │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` **Configuration for Interoperability:** ```cisco ! On MST switch connecting to PVST+ switch Switch(config)# interface gigabitEthernet 0/1 Switch(config-if)# spanning-tree mst pre-standard ! For older PVST+ Switch(config-if)# exit ! On PVST+ switch (default behavior) Switch(config)# spanning-tree mode pvst ! No special configuration needed ``` --- ### 11. MST Best Practices ``` ┌─────────────────────────────────────────────────────────────────────────────┐ │ MST BEST PRACTICES │ ├─────────────────────────────────────────────────────────────────────────────┤ │ │ │ 1. PLAN INSTANCE MAPPING │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ • Group VLANs with similar traffic patterns │ │ │ │ • Keep number of instances low (10-15) │ │ │ │ • Use instance 0 for default VLANs │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ 2. CONSISTENT CONFIGURATION │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ • Same region name on all switches in region │ │ │ │ • Same revision number │ │ │ │ • Same VLAN-to-instance mapping │ │ │ │ • Verify with `show spanning-tree mst configuration` │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ 3. ROOT BRIDGE PLACEMENT │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ • Configure root bridges for each instance │ │ │ │ • Use different roots for different instances for load balancing │ │ │ │ • `spanning-tree mst [instance] root primary` │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ 4. MAX HOPS │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ • Default max hops: 20 │ │ │ │ • Adjust if region is large │ │ │ │ • `spanning-tree mst max-hops [value]` │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ │ 5. MIGRATION STRATEGY │ │ ┌─────────────────────────────────────────────────────────────────────┐ │ │ │ • Start with switches in PVST+ mode │ │ │ │ • Configure MST but don't enable yet │ │ │ │ • Change one switch at a time to MST │ │ │ │ • Verify interoperability with PVST+ switches │ │ │ └─────────────────────────────────────────────────────────────────────┘ │ │ │ └─────────────────────────────────────────────────────────────────────────────┘ ``` --- ## 🔧 Complete Configuration Examples ### Lab 1: Basic MST Configuration with 3 Instances **Topology:** ``` Switch A (Root for Inst1) Switch B (Root for Inst2) │ │ └──────────────┬───────────────┘ │ Switch C (Access) ``` **Switch A Configuration:** ```cisco hostname SwitchA ! ! Enable MST mode spanning-tree mode mst ! ! Configure MST region spanning-tree mst configuration name CAMPUS revision 1 instance 1 vlan 10,20,30 instance 2 vlan 40,50,60 ! ! Set root bridges spanning-tree mst 1 root primary spanning-tree mst 2 root secondary ! ! Interfaces interface range GigabitEthernet0/1-2 switchport mode trunk ! end ``` **Switch B Configuration:** ```cisco hostname SwitchB ! spanning-tree mode mst ! spanning-tree mst configuration name CAMPUS revision 1 instance 1 vlan 10,20,30 instance 2 vlan 40,50,60 ! spanning-tree mst 1 root secondary spanning-tree mst 2 root primary ! interface range GigabitEthernet0/1-2 switchport mode trunk ! end ``` **Switch C Configuration:** ```cisco hostname SwitchC ! spanning-tree mode mst ! spanning-tree mst configuration name CAMPUS revision 1 instance 1 vlan 10,20,30 instance 2 vlan 40,50,60 ! interface range GigabitEthernet0/1-2 switchport mode trunk ! ! Access ports interface FastEthernet0/1 switchport mode access switchport access vlan 10 spanning-tree portfast ! interface FastEthernet0/2 switchport mode access switchport access vlan 40 spanning-tree portfast ! end ``` --- ### Lab 2: MST with Load Balancing **Topology:** ``` Switch A (Core) / \ / \ Switch B Switch C (Dist) (Dist) | | Switch D Switch E (Access) (Access) ``` **Configuration (All Switches):** ```cisco ! Common MST configuration on all switches spanning-tree mode mst ! spanning-tree mst configuration name CAMPUS revision 2 instance 1 vlan 10,20,30 instance 2 vlan 40,50,60 instance 3 vlan 70,80,90 ! ! Load balancing: Different roots for different instances ! On Switch A: spanning-tree mst 1 root primary spanning-tree mst 2 root secondary spanning-tree mst 3 root secondary ! ! On Switch B: spanning-tree mst 1 root secondary spanning-tree mst 2 root primary spanning-tree mst 3 root secondary ! ! On Switch C: spanning-tree mst 1 root secondary spanning-tree mst 2 root secondary spanning-tree mst 3 root primary ``` --- ### Lab 3: MST to PVST+ Migration **Step 1: Verify Current PVST+ Configuration** ```cisco Switch# show spanning-tree summary Switch# show vlan brief ``` **Step 2: Configure MST (but don't enable yet)** ```cisco Switch(config)# spanning-tree mst configuration Switch(config-mst)# name CAMPUS Switch(config-mst)# revision 1 Switch(config-mst)# instance 1 vlan 10,20,30 Switch(config-mst)# show pending Switch(config-mst)# exit ``` **Step 3: Change Mode to MST (one switch at a time)** ```cisco Switch(config)# spanning-tree mode mst ! Verify Switch# show spanning-tree summary ``` **Step 4: Verify Interoperability** ```cisco Switch# show spanning-tree mst boundary ! Check boundary ports connecting to PVST+ switches Switch# show spanning-tree mst 1 ! Verify root bridges are as expected ``` --- ## ✅ Exam Tips (For CCNA 200-301) | Topic | What Cisco Tests | |-------|------------------| | **MST Purpose** | Reduce STP instances, save resources | | **MST Region** | Same name, revision, mapping required | | **IST (Instance 0)** | Always exists; maps unassigned VLANs | | **MSTI** | User-configured instances (1-4094) | | **Boundary Port** | Connects to different MST region or PVST+ | | **Max Hops** | Default 20; limits MST region size | ### Common Exam Scenarios: **Scenario 1:** "A network has 500 VLANs and switches are experiencing high CPU usage. Which STP mode should be implemented?" - **Answer:** MST (Multiple Spanning Tree) **Scenario 2:** "Two switches have the same MST region name but different revision numbers. Are they in the same region?" - **Answer:** No, revision numbers must match to be in same region **Scenario 3:** "VLANs 100-200 are not assigned to any MST instance. Which instance carries them?" - **Answer:** Instance 0 (IST) ### Mnemonics: **MST Configuration Requirements:** **"3 R's" - Region, Revision, Mapping** - **R**egion name (same) - **R**evision number (same) - **M**apping (VLAN-to-instance same) **MST vs. PVST+ Decision:** **"More than 100, MST is the one"** - >100 VLANs → Use MST - <100 VLANs → PVST+ may be fine --- ## 📝 Summary (1-Minute Revision) ``` MST (Multiple Spanning Tree - IEEE 802.1s): PURPOSE: ├── Reduce STP instances (group VLANs together) ├── Save CPU and memory resources ├── Reduce BPDU overhead └── Provide load balancing across instances MST COMPONENTS: ├── MST Region: Switches with identical config ├── IST (Instance 0): Default instance ├── MSTI: User-configured instances (1-4094) ├── CIST: Global spanning tree connecting regions └── Boundary Port: Port connecting to different region/PVST+ CONFIGURATION: ├── spanning-tree mode mst ├── spanning-tree mst configuration ├── name [region-name] ├── revision [number] ├── instance [id] vlan [list] └── show spanning-tree mst configuration VERIFICATION: ├── show spanning-tree mst configuration ├── show spanning-tree mst [instance] ├── show spanning-tree mst interface └── show spanning-tree mst boundary MST vs. PVST+: ├── PVST+: 1 instance per VLAN (resource intensive) ├── MST: Multiple VLANs per instance (efficient) └── Best for large networks (100+ VLANs) KEY COMMANDS: ├── spanning-tree mst [instance] root primary ├── spanning-tree mst [instance] priority [value] └── spanning-tree mst max-hops [value] ``` --- ## 🧪 Practice Questions **1. Which IEEE standard defines Multiple Spanning Tree Protocol?** - A) 802.1D - B) 802.1Q - C) 802.1s - D) 802.1w <details> <summary>Answer</summary> <b>C) 802.1s</b> - MST is defined in IEEE 802.1s (now integrated into 802.1Q). </details> **2. What is the default MST instance that carries VLANs not mapped to other instances?** - A) Instance 1 - B) Instance 0 (IST) - C) Instance 4094 - D) No default instance <details> <summary>Answer</summary> <b>B) Instance 0 (IST)</b> - The Internal Spanning Tree (Instance 0) carries all VLANs not mapped to other instances. </details> **3. Which of the following must match for switches to be in the same MST region? (Select three)** - A) Region name - B) Revision number - C) VLAN-to-instance mapping - D) Switch model <details> <summary>Answer</summary> <b>A, B, C</b> - Region name, revision number, and VLAN-to-instance mapping must match exactly. </details> **4. What is the default maximum hops for MST?** - A) 10 - B) 20 - C) 30 - D) 50 <details> <summary>Answer</summary> <b>B) 20</b> - Default max hops in MST is 20. </details> **5. Which command enters MST configuration mode?** - A) `spanning-tree mst` - B) `spanning-tree mst configuration` - C) `mst configuration` - D) `vtp mst` <details> <summary>Answer</summary> <b>B) `spanning-tree mst configuration`</b> - This enters MST configuration mode. </details> **6. What is a boundary port in MST?** - A) Port connected to another switch in same region - B) Port connected to different MST region or PVST+ switch - C) Port in errdisable state - D) Port with Root Guard enabled <details> <summary>Answer</summary> <b>B) Port connected to different MST region or PVST+ switch</b> - Boundary ports connect to different MST configurations. </details> **7. Which MST feature allows different VLAN groups to use different root bridges?** - A) PortFast - B) UplinkFast - C) Load balancing - D) BPDU Guard <details> <summary>Answer</summary> <b>C) Load balancing</b> - Different MST instances can have different root bridges for load balancing. </details> **8. A switch has MST configuration with revision 1. Another switch has same region name but revision 2. Are they in same region?** - A) Yes, region name matches - B) No, revision numbers must match - C) Yes, as long as VLAN mapping is same - D) Depends on switch model <details> <summary>Answer</summary> <b>B) No, revision numbers must match</b> - All three parameters (name, revision, mapping) must match. </details> **9. Which command displays the MST region configuration?** - A) `show spanning-tree` - B) `show spanning-tree mst` - C) `show spanning-tree mst configuration` - D) `show mst region` <details> <summary>Answer</summary> <b>C) `show spanning-tree mst configuration`</b> - This shows the region name, revision, and VLAN mapping. </details> **10. What is the primary advantage of MST over PVST+?** - A) Faster convergence - B) Better security - C) Reduced BPDU overhead with many VLANs - D) Easier configuration <details> <summary>Answer</summary> <b>C) Reduced BPDU overhead with many VLANs</b> - MST significantly reduces BPDU traffic and CPU usage in large VLAN environments. </details> **11. Which VLANs are mapped to instance 0 by default?** - A) VLANs 1-1005 - B) VLANs 1-4094 - C) VLANs not assigned to other instances - D) No VLANs <details> <summary>Answer</summary> <b>C) VLANs not assigned to other instances</b> - Instance 0 carries all VLANs not explicitly mapped to other instances. </details> **12. How many MST instances can be configured?** - A) 64 - B) 256 - C) 1005 - D) 4094 <details> <summary>Answer</summary> <b>D) 4094</b> - Up to 4094 MST instances can be configured (though typically only 10-15 are used). </details> --- ## 📌 Next Steps After completing Video 24, you should be ready for: - **Video 25:** Switch Stacking and Chassis Aggregation - **Video 26:** Route Summarization **Lab Practice:** 1. Configure MST with 3 instances on 3 switches 2. Verify all switches are in same region with `show spanning-tree mst configuration` 3. Configure different root bridges for different instances 4. Test load balancing by checking which links are blocked per instance 5. Add a PVST+ switch and observe boundary ports 6. Migrate from PVST+ to MST --- **Ready for Video 25?** Share the link or say "next" and I'll continue with Switch Stacking and Chassis Aggregation. I'll continue with **Video 25: Switch Stacking and Chassis Aggregation** based on the standard CCNA 200-301 curriculum. ---