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Introduction:
The Cisco Virtual Switching System is a clustering technology that pools two Cisco Catalyst 4500-E Series Switches with Cisco Catalyst Supervisor Engine 7-E or 7-LE or two Catalyst 4500-X Series Switches into a single virtual switch. In a VSS, the data plane of both clustered switches is active at the same time in both chassis. VSS members are connected by virtual switch links (VSLs) using standard Gigabit or 10 Gigabit Ethernet connections between the VSS members. VSLs can carry regular user traffic in addition to the control plane communication between the VSS members.

Refer to https://supportforums.cisco.com/document/124626/virtual-switching-system-vss-configuration-cisco-4500-series-switches
Physical vs Logical Topology in a VSS Configuration
vss.jpg
This document describes how to configure a virtual switching system (VSS) for the Catalyst 4500 series switch (Supervisor Engine 7-E, Supervisor Engine 7L-E).
Some key point to be remembered for Cisco 4500 VSS:
1) Configuration/Capability Supported supervisors on Catalyst 4500-E: VSS support Supervisor Engine 7-E or 7-LE (identical pairs).For more information about hardware to support please refer 4500 VSS Hardware requirement
2) Software requirement: Cisco IOS XE 3.4.0SG and ROMMON IOS Version 15.0(1r) SG7 later released support VSS.(Also refer How to Upgrade Cisco 4500 SUP7-E & Sup7L-E ROMMON To support VSS).
3)license requirement : 
To know more on license requirement refer "Release Notes for the Catalyst 4500E Series Switch"
Feature
LAN Base
IP Base
Enterprise Service
  • Virtual Switching System (VSS



No
Yes
(SUP7E only)
Yes
  • Support for Layer 3 MEC—VSS with Layer 3 Multichassis EtherChannel (MEC) at the aggregation layer
  • Support for VSLP Fast Hello—With VSLP Fast Hello, the Catalyst 4500-X configured for VSS can now connect Access Switches that do not support the ePAgP protocol.
  • Support for VSL Encryption
  • Support for Asymmetrix chassis

No
Yes (SUP7E)
No (SUP7LE)
Yes (SUP7E)
Yes (SUP7LE)
4) Single-sup cross-chassis VSS support: Yes.
5) Quad-sup VSS configuration with in-chassis redundant sups: In-chassis redundant sups in rommon mode with active uplinks.
6) It also supports 10 Gigabit Ethernet Virtual Switch Link (VSL) and 1 Gigabit Ethernet VSL.
7) SSO and nonstop forwarding (NSF) must be configured on each switch. If a VSS does not meet the requirements for SSO redundancy; it will be incapable of establishing a relationship with the peer switch. Catalyst 4500/4500-X series switches' VSS does not support route processor redundancy (RPR) mode.
Prerequisite:
Before configuring VSS on Cisco 4500 please verify hardware and software requirement.
SW1#sh ver | in IOS
Cisco IOS Software, IOS-XE Software, Catalyst 4500 L3 Switch Software (cat4500e-UNIVERSAL-M), Version 03.04.00.SG RELEASE SOFTWARE (fc3)
Cisco IOS-XE software, Copyright (c) 2005-2010, 2012 by cisco Systems, Inc.
All rights reserved. Certain components of Cisco IOS-XE software are
or the applicable URL provided on the flyer accompanying the IOS-XE
documentation or "License Notice" file accompanying the IOS-XE software
SW1#sh ver | in ROM
ROM: 15.0(1r)SG7
System returned to ROM by power-on
SW1#sh license image levels
Module name       Image level Priority Configured Valid license
--------------------------------------------------------------------
WS-X45-SUP7-E     entservices  1         YES        entservices            
                  ipbase       2         NO         ipbase                
    Role           Current Level     Reboot Level ----------
                  lanbase      3         NO         lanbase     Module Nam e----------------------------------------------------------
entservices       entservices
WS-X45-SUP7-E  Active        
SW2#sh ver | in IOS
Cisco IOS Software, IOS-XE Software, Catalyst 4500 L3 Switch Software (cat4500e-UNIVERSAL-M), Version 03.04.00.SG RELEASE SOFTWARE (fc3)
Cisco IOS-XE software, Copyright (c) 2005-2010, 2012 by cisco Systems, Inc.
All rights reserved. Certain components of Cisco IOS-XE software are
or the applicable URL provided on the flyer accompanying the IOS-XE
documentation or "License Notice" file accompanying the IOS-XE softw
SW2#sh ver | in ROM
ROM: 15.0(1r)SG7
System returned to ROM by power-on
SW2#sh license image levels
Module name       Image level Priority Configured Valid license
--------------------------------------------------------------------
WS-X45-SUP7-E     entservices  1         YES        entservices            
                  ipbase       2         NO         ipbase                
  Role           Current Level     Reboot Level -----------
                  lanbase      3         NO         lanbase Module Name  ---------------------------------------------------------
 entservices       entservices
WS-X45-SUP7-E   Active      

Configuration Steps:

STEP1: Assigning Virtual Switch Domain and Switch Numbers
First you have to configure the same virtual switch domain number on both switches of the VSS. The virtual switch domain is a number between 1 and 255.After domain number you must configure one switch to be switch number 1 and the other switch to be switch number 2.
SW1#conf t
Enter configuration commands, one per line. End with CNTL/Z.
SW1(config)#switch virtual domain 10
Domain ID 10 config will take effect only
after the exec command 'switch convert mode virtual' is issued
SW1(config-vs-domain)#switch 1
SW1(config-vs-domain)#exit
SW1(config)#
SW2#conf t
Enter configuration commands, one per line. End with CNTL/Z.
SW2(config)#switch virtual domain 10
Domain ID 10 config will take effect only
after the exec command 'switch convert mode virtual' is issued
SW2(config-vs-domain)#switch 2
SW2(config-vs-domain)#exit
SW2(config)#
STEP2: Configuring VSL Port Channel:
Then you need to configure VSL with a unique port channel on each switch. During the conversion, the VSS configures both port channels on the VSS Active switch. If the VSS Standby switch VSL port channel number has been configured for another use, the VSS comes up in RPR mode. To avoid this situation, check that both port channel numbers are available on both of the switches.
SW1(config)#int port-channel 5
SW1(config-if)#switchport
ual link 1 SW1(config-if)#no shut SW
SW1(config-if)#switch vir t1(config-if)#exit
2: %SPANTREE-6-PORTDEL_ALL_VLANS: Port-channel5 deleted from all Vlans
*Jan 24 05:19:57.0 9

SW2(config)#int port-channel 10
SW2(config-if)#switchport
ual link 2 SW2(config-if)#no shut SW
SW2(config-if)#switch vir t2(config-if)#exit SW2(config)#
6-PORTDEL_ALL_VLANS: Port-channel10 deleted from all Vlans
*Jan 24 05:14:17.273: %SPANTREE -

STEP3: configure the VSL ports
You need to add the VSL physical ports to the port channel. In the following example, interfaces Gigabit Ethernet 7/3 and 7/4 on Switch 1 are connected to interfaces Gigabit Ethernet 4/45 and 4/46 on Switch 2.
SW1(config)#int range gig7/3 - 4
SW1(config-if-range)#switchport mode trunk
SW1(config-if-range)#channel-group 5 mode on
WARNING: Interface GigabitEthernet7/3 placed in restricted config mode. All extraneous configs removed!
WARNING: Interface GigabitEthernet7/4 placed in restricted config mode. All extraneous configs removed!
SW1(config-if-range)#exit

SW2(config)#int range gig4/45 - 46
SW2(config-if-range)#switchport mode trunk
SW2(config-if-range)#channel-group 10 mode on
WARNING: Interface GigabitEthernet4/45 placed in restricted config mode. All extraneous configs removed!
WARNING: Interface GigabitEthernet4/46 placed in restricted config mode. All extraneous configs removed!
SW2(config-if-range)#exit

Note: Once the interfaces are put into VSL port-channel with “channel-group" command, then the interfaces goes into “notconnect” status. Interface status will show UP, but the line protocol will be down. The interface will be in UP/down (not connect) status, till the switch is rebooted in step 4.
STEP4: Converting the Switch to Virtual Switch Mode:
You need to enter the “switch convert mode virtual” command on Switch 1 for Converting to Virtual Switch Mode .After you enter this command it will prompted to confirm the action. Enter yes. The system creates a converted configuration file, and saves the file to the bootflash:
SW1#switch convert mode virtual
his command will convert all interface names
Tto naming convention "interface-type switch-number/slot/port",
save the running config to startup-config and
s/no]: yes Convert
reload the switch. Do you want to proceed? [y eing interface names Building configuration...
3 bytes[OK] Saving converted configuration to bootflash: .
Compressed configuration from 6551 bytes to 28 9.. Destination filename [startup-config.converted_vs-20130124-062921]?
n (G) Signature verification PASSED Rommon
Please stand by while rebooting the system... Restarting system. Romm o (P) Signature verification PASSED
D
FPGA   (P) Signature verification PASS E

Similarly you need to enter the “switch convert mode virtual” command on Switch 2 for converting to Virtual Switch Mode.
SW2#switch convert mode virtual​
This command will convert all interface names
to naming convention "interface-type switch-number/slot/port",
save the running config to startup-config and
s/no]: yes Convert
reload the switch. Do you want to proceed? [y eing interface names Building configuration...
4 bytes[OK] Saving converted configuration to bootflash: .
Compressed configuration from 6027 bytes to 27 7.. Destination filename [startup-config.converted_vs-20130124-052526]?
n (G) Signature verification PASSED Rommon
Please stand by while rebooting the system... Restarting system. Romm o (P) Signature verification PASSED FPGA   (P) Signature verification PASSED
                                     * * Welcome to Rom Mon
************************************************************ *                    itor for   WS-X45-SUP7-E System.       * * Copyright (c) 2008-2012 by Cisco Systems, Inc.           *
                 * *****************************************
* All rights reserved.                                     * *                                        
*******************

After you confirm the above commands on both switches, the running configuration is automatically saved as the startup configuration and the switch reboots. After the reboot, the switch is in virtual switch mode, so you must specify interfaces with three identifiers (switch/module/port).
When switches are being converted to VSS, you should not set them to ignore startup-config. If done, the switch can be enabled to parse the startup-config at the rommon prompt. Ignoring startup-config in VSS mode causes a switch to boot in a semi-VSS mode, which can only be corrected by a reboot and by enabling the parsing of startup-config.
Verification:
1) To displays the virtual switch domain number, and the switch number and role for each of the switches you can use “show switch virtual”command.

SW1#sh switch virtual
xecuting the command on VSS member switch role = VSS Active, id = 1
E Switch mode                  : Virtual Switch
mber          : 1 Local switch op
Virtual switch domain number : 10 Local switch n uerational role: Virtual Switch Active Peer switch number           : 2
mmand on VSS member switch role = VSS Standby, id = 2
Peer switch operational role : Virtual Switch Standby Executing the c o Switch mode                  : Virtual Switch Virtual switch domain number : 10 Local switch number          : 2
Virtual Switch Active
Local switch operational role: Virtual Switch Standby Peer switch number           : 1 Peer switch operational role :

2) Once both switches cluster in single virtual switch, you will only have Active switch console and your Standby switch console appears as follow:

SW2-standby>
tandby console disabled
S

3) To displays the role, switch number, and priority for each of the switch in the VSS use “show switch virtual role” command.
SW1#sh switch virtual role
xecuting the command on VSS member switch role = VSS Active, id = 1
E RRP information for Instance 1
--------------------------------- Valid Flags   Peer     Preferred R
---------------------------------- -eserved                Count     Peer       Peer
--------------- TRUE   V       1           1         1 Switch Swi
---------------------------------------------------- -tch Status Preempt       Priority Role     Local   Remote
  SID ----------------------------------------------------------
       Number         Oper(Conf)   Oper(Conf)         SID  ---------- LOCAL   1     UP     FALSE(N )     100(100) ACTIVE   0       0
epresents the local switch Flags : V - Valid In dual-active re
REMOTE  2     UP     FALSE(N )     100(100) STANDBY 6834   6152 Peer 0 rcovery mode: No Executing the command on VSS member switch role = VSS Standby, id = 2 RRP information for Instance 2
Count     Peer       Peer ---------------------------------------
-------------------------------------------------------------------- Valid Flags   Peer     Preferred Reserved                ----------------------------- TRUE    V       1           1         1 Switch Switch Status Preempt       Priority Role     Local   Remote
2     UP     FALSE(N )     100(100) STANDBY  0       0 REMOTE  1
       Number         Oper(Conf)   Oper(Conf)         SID     SID -------------------------------------------------------------------- LOCAL       UP     FALSE(N )     100(100) ACTIVE   6152   6834 Peer 0 represents the local switch Flags : V - Valid
In dual-active recovery mode: N o

4) To Displays information about the VSL use “show switch virtual link”command.
SW1#sh switch virtual link
xecuting the command on VSS member switch role = VSS Active, id = 1
E VSL Status : UP VSL Uptime : 3 minutes VSL Control Link : Gi1/7/4
VSL Status : UP VSL Uptime : 3 minutes VSL Control Link : Gi2/4/45
Executing the command on VSS member switch role = VSS Standby, id = 2

5) You can also verify information about the VSL port channel using “show switch virtual link port-channel”command.

SW1#sh switch virtual link port-channel 
xecuting the command on VSS member switch role = VSS Active, id = 1
E Flags: D - down       P - bundled in port-channel
standby (LACP only)        R - Layer
       I - stand-alone s - suspended        H - Hot -3     S - Layer2        U - in use     N - not in use, no aggregation
e, no aggregation due to minimum links not met  
       f - failed to allocate aggregator           M - not in u s     m - not in use, port not aggregated due to minimum links not met        u - unsuitable for bundling
rts ------+-------------+
       d - default port     w - waiting to be aggregated Group Port-channel Protocol   P o-----------+------------------- 5     Po5(SU)          -       Gi1/7/3(P) Gi1/7/4(P)
member switch role = VSS Standby, id = 2 Flags: D -
10    Po10(SU)         -       Gi2/4/45(P) Gi2/4/46(P) Executing the command on VS Sdown       P - bundled in port-channel        I - stand-alone s - suspended        H - Hot-standby (LACP only)
ate aggregator          
       R - Layer3     S - Layer2        U - in use     N - not in use, no aggregation        f - failed to allo c  M - not in use, no aggregation due to minimum links not met        m - not in use, port not aggregated due to minimum links not met
ts ------+-------------+----------
       u - unsuitable for bundling        d - default port   w - waiting to be aggregated Group Port-channel Protocol   Po r-+------------------- 5     Po5(SU)          -       Gi1/7/3(P) Gi1/7/4(P) 10    Po10(SU)         -       Gi2/4/45(P) Gi2/4/46(P)
SW1 #

Reference:

Catalyst Confidence: VSS on the 4500 :