Cisco Catalyst 9500 StackWise Virtual

A step-through lab for a distribution-layer StackWise Virtual pair: what happens when a member fails, when the StackWise Virtual Link (SVL) splits, and how each dual-active detection (DAD) method, Fast Hello and enhanced PAgP (ePAgP), removes the old active switch from the network.

Sources: Catalyst 9500 High Availability Configuration Guide, IOS XE 17.15 and 26.x; Catalyst 9000 StackWise Virtual white paper; TAC tech note 216537; C9550 data sheet. Acronyms are spelled out on first use.

8SVL members, all the same speed
4Fast Hello DAD links maximum
~50 msSSO (Stateful Switchover) takeover
126Usable MECs (Po127/128 reserved)
4094VLAN reserved for SVL control
64 BStackWise Virtual Header per frame
Topology: core above a StackWise Virtual distribution pair, access switch below CORE DISTRIBUTION ACCESS SV domain 10 · one logical switch Po10 MEC · LACP Po10 MEC · LACP SVL · 2 × 40G DAD · Fast Hello Po20 MEC · PAgP Po20 MEC · PAgP Users · APs · endpoints
Figure.
Normal

CLI


      
    

  




  

    
Release history on Catalyst 9500

    
From the feature history tables in the 17.15 and 26.x guides.

    
    
      
  1. 16.6.1StackWise Virtual introducedC9500-12Q, -16X, -24Q, -40X.
    2. 
      
  2. 16.10.1High Performance modelsC9500-32C, -32QC, -48Y4C, -24Y4C. Use 16.10 or later on C9500H; the SVL option was removed from 16.9.6 (CSCvt46115).
    3. 
      
  3. 16.11.1Recovery reload controlRecovery-mode switch reloads itself once the SVL returns; dual-active recovery-reload-disable turns that off.
    4. 
      
  4. 16.12.1Secure StackWise VirtualEncrypts SVL control traffic, FIPS 140-2 compliant, 128-bit key. Not applied to DAD links.
    5. 
      
  5. n/aBGP EVPN VXLAN on SVLListed in the feature history without a release number.
    6. 
      
  6. 17.2.1BUM traffic optimizationsvl l2bum optimization, on by default. Keeps flooded traffic off the SVL for VLANs built only from MECs.
    7. 
      
  7. 17.10.1C9500X-28C8DNew architecture: LACP replaces LMP, IS-IS replaces SDP, SVL and DAD links added or removed without a reload.
    8. 
      
  8. 26.2.1C9550 Smart SwitchesFive Silicon One models (E104, E100, K100) with StackWise Virtual front-side stacking on all ports, SSO and ISSU listed as available.
    9. 
      
  9. 26.2.1Recovery-mode prompt changeOn C9500H and C9500X the hostname reverts to Switch# in recovery mode instead of adding (recovery-mode). Confirm with show stackwise-virtual dual-active-detection.
    10. 
    

  


  

    
Limits and caveats

    
Platform restrictions and operational gotchas from the configuration guides and the TAC troubleshooting note. Check these before you cable anything.

    

    

      
Limits and restrictions

      

      

        
ItemC9500 / C9500HC9500XC9550 New

        
          
SVL membersUp to 8, all the same speedUp to 8Not yet published

          
SVL speeds10/40G (C9500); 10/25/40/100G (C9500H)50/100/400GFront-side, on all ports

          
Fast Hello DADUp to 4 direct links, never an SVL portUp to 4Not yet published

          
ePAgP DADTrusted PAgP MECs; no count statedNo count statedNot yet published

          
Adding SVL or DADWrite memory and reload the pairLive, no reloadNot yet published

          
Link health / rolesLMP / SDPLACP / IS-ISNot yet published

          
MECs126 (Po127, Po128 reserved)240 (Po241 reserved)Not yet published

          
Control VLAN4094 reserved network-wideNot applicableNot yet published

          
1G for SVLNot supportedNot yet published

          
Breakout portsData only; never SVL or DAD (4×10G breakout and QSA included on C9500)Not yet published

          
C9500-32CSVL and DAD on ports 1–16 only––

          
C9500-32QCNative 40G/100G ports only, not converted ports––

          
C9500-NM-2QCannot mix its 40G ports with fixed downlinks in one SVL––

          
OpticsCisco transceivers only. SFP-10G-T-X cannot be SVL or DAD. Dual-rate optics lock to their highest speed.Not yet published

          
Software16.6.1 (C9500), 16.10.1 (C9500H)17.10.1 (28C8D)IOS XE 26.2.1 minimum

          
PairingSame model, license (Network Advantage), software version and SDM template; directly connectedNot yet published

          
Secure SVLTwo-node front-side only; mutually exclusive with FIPS mode; not on DAD linksNot yet published

          
LACP MECMaximum 8 member portsNot yet published

        
      

      

      
C9550 values come from the public C9550 data sheet. Cisco has not yet published a C9550 StackWise Virtual configuration guide, so the blank cells stay unknown until it does.

    

    

      
Caveats that catch people

      
    
        
  • Config lives in ROMMON. stackwise-virtual link and stackwise-virtual dual-active-detection are stored in ROMMON variables and the running config, not the startup config. The DAD command does not even appear in show running-config after it is applied.
    • 
        
  • Recovery-reload-disable is not persistent. dual-active recovery-reload-disable is saved only to the running config, so plan for it to be gone after a reload.
    • 
        
  • Normal shut/no shut is blocked on SVL and DAD ports. Cisco's troubleshooting note uses test platform software nif_mgr port disable for lab tests only.
    • 
        
  • Early recovery mode. A switch can enter recovery mode with some SVL members still cabled if links flap or LMP/SDP exchanges fail. Partially seated transceivers are a known cause.
    • 
        
  • Management port conflict. The guide says management interfaces stay up in recovery mode; the white paper log shows Gi0/0 going down. Keep console access to both members.
    • 
        
  • ePAgP has hard dependencies. PAgP desirable MEC, a member on each switch, an ePAgP-capable Cisco neighbor and stack-mac persistent timer 0. LACP or mode on gives no detection.
    • 
        
  • SPAN cannot use SVL or Fast Hello DAD ports as source or destination.
    • 
        
  • Private VLANs: set the native VLAN on isolated trunk ports explicitly.
    • 
        
  • Custom SVI MACs: if one SVI or routed port uses mac-address and needs Layer 3 injected packets, all others must share the same first four bytes.
    • 
        
  • Multicast: carry it on MECs rather than ECMP so a switchover doesn't change multicast routes.
    • 
        
  • CLI or NETCONF replace of SVL settings lands in the CONFD database and only shows in IOSd after write memory and reload.
    • 
      

    

    

      
Troubleshooting commands

      

    

  

  


  

    
Configuration reference

    

    

      
Design rules from the sources

      
    
        
  • Build the SVL from at least two members, spread across ASICs and separate physical paths. A single member failing costs roughly 50–100 ms for flows hashed onto it; only losing the last member splits the pair.
    • 
        
  • Never use the same port for SVL and DAD. On C9500 the SVL and DAD configuration needs a reload to take effect (C9500X adds and removes links live).
    • 
        
  • Run both DAD methods where you can. Cisco states Fast Hello and ePAgP can be deployed together for higher resiliency.
    • 
        
  • ePAgP needs a PAgP MEC (mode desirable) with at least one member on each switch, terminating on a Cisco switch that supports ePAgP. An LACP MEC gives no dual-active detection.
    • 
        
  • Set stack-mac persistent timer 0 so the stack MAC never changes on switchover; Cisco requires it for ePAgP to work without PAgP flaps.
    • 
        
  • Dual-home everything with MEC. Single-homed ports force traffic across the SVL and lose service entirely if their switch goes into recovery.
    • 
        
  • Keep console or out-of-band access to both members. The switch in recovery mode shuts its front-panel ports, and the white paper's sample log also shows its management port Gi0/0 going down.
    • 
      

    

    

      
Reference configuration

      

    

    

      
Abbreviations

      

        
SVL
StackWise Virtual Link, the inter-chassis EtherChannel carrying control and data

        
DAD
Dual-Active Detection, finds a split where both switches think they are active

        
SSO
Stateful Switchover, the hot-standby takeover model

        
NSF
Nonstop Forwarding, keeps hardware forwarding during switchover

        
MEC
Multichassis EtherChannel, one port-channel with members on both switches

        
LACP
Link Aggregation Control Protocol (IEEE 802.3ad)

        
PAgP
Port Aggregation Protocol, Cisco's EtherChannel negotiation protocol

        
ePAgP
Enhanced PAgP, adds a TLV carrying the active switch ID

        
TLV
Type-Length-Value field inside a protocol message

        
LMP
Link Management Protocol, per-member SVL health and integrity

        
SDP
StackWise Discovery Protocol, resolves active and standby roles

        
SVH
StackWise Virtual Header, the 64-byte header on every SVL frame

        
GOLD
Generic Online Diagnostics, hardware fault detection

        
BUM
Broadcast, Unknown unicast and Multicast traffic

        
ROMMON
ROM Monitor, the boot environment that stores SVL and DAD settings

        
FIPS
Federal Information Processing Standards (140-2 crypto validation)

        
SPAN
Switched Port Analyzer, port mirroring

        
ECMP
Equal-Cost Multipath routing

        
ISSU
In-Service Software Upgrade

        
SDM
Switch Database Management template, the hardware table profile

      

      

        Sources
        Configuring Cisco StackWise Virtual, C9500 HA guide, IOS XE 17.15
        Configure Cisco StackWise Virtual, C9500 HA guide, IOS XE 26.x
        Cisco Catalyst 9000 StackWise Virtual white paper
        Cisco C9550 Series Smart Switches data sheet
        Troubleshoot SVL on Catalyst 9000 Switches (TAC tech note 216537)
      

    

  

  




Built from the Cisco Catalyst 9500 High Availability Configuration Guides (IOS XE 17.15 and 26.x), the Catalyst 9000 StackWise Virtual white paper and TAC tech note 216537. Behavior can differ by release and platform; verify against the guide and Bug Search Tool for your exact software before production use.