Virtual Switch System
From Internetworkpro
The primary benefit of this mode allows Multichassis EtherChannel (MEC) to occur. Additional benefits include a common configuration across two aggregation switches, NSF/SSO failover between chassis and the ability to avoid layer 3 in the access (desirable in a datacentre deployment)
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[edit] Architecture
At the heart of the Virtual Switching System, there is the Virtual Switch Link or VSL. The VSL is a dedicated 10GE link (or bundle of links) that are used to carry control plane traffic between VSS members.
The VSL essentially acts as an extension to the backplane, allowing control traffic to travel between chassis. As it uses existing X2 optics, the distance limitations of the VSS is limited only by supported 10GE ethernet standards.
When a VSS system is configured, the two chassis share a common configuration (see below for how this works). The devices also share a common MAC address and can truly load balance. In many ways, it is similar to how a Catalyst 3750 operates.
A common application is datacentre or high-end distribution where layer-2 links can etherchannel in to two chassis for high availability.
[edit] Hardware Requirements
The system must be running the Supervisor 720 10GE running PFC3C or PFC3CXL. In addition, any 10GE cards used for the VSL must be:
- 6708 (8-port 10GE line card) or Supervisor (Sup720-10GE) uplinks
- Using the DFC3C or DFC3CXL in conjunction with th PFC3C or PFC3CXL Supervisor
A caveat is that any DFCs in the system must also be upgraded to DFC3C or DFC3CXL. DFC3 and DFC3B and their XL equivalents are not supported. All line cards must also be 67xx cards (CEF or dCEF 720)
[edit] Dual Active
In the worst case failure of a VSS (the link between the two switches goes down) we end up with a dual active situation. This is because the standby switch assumes the active has gone down and thus takes over.
[edit] Configuration
The configuration is summarised in to the following steps:
- Configure the Virtual Switch Domain on both devices and designate each switch as either Switch 1 (primary) or Switch 2 (secondary)
- (Optional) Configure switch priority settings
- Configure the virtual switch links
- Run the conversion (causing switches to reload)
- Reconfigure the standby switches VSL on the active switch to complete configuration
[edit] Configuring the Virtual Switch Domain
The Virtual Switch Domain defines the grouping for the switches within the VSS system. The domain itself is an ID between 1 and 255 and should be unique for its layer 2 domain.
On Switch 1:
router-1# conf t router-1(config)# switch virtual domain <domain> router-1(config-vs-domain)# switch 1
On Switch 2:
router-2# conf t router-2(config)# switch virtual domain <domain> router-2(config-vs-domain)# switch 2
[edit] Configuring Switch Priorities (Optional)
VSS priorities are similar in nature to HSRP priorities. The highest priority node will assume to be active and the lowest standby. Both switches must have the same priority configuration settings to form a VSS system. By default, switch 1 will assume active (primary) and switch 2 will assume it is secondary.
To make switch 2 the primary switch, do the following on both switches:
router(config-vs-domain)# switch 1 priority 100 router(config-vs-domain)# switch 2 priority 110
[edit] Configure the Virtual Switching Link
The VSL link is a special link that carries control plane data between the two chassis. This must be configured on a 10GE port from either the Sup720-10GE or a 6708 line card.
In a deployment scenario the individual interfaces will differ. This article assumes the supervisor ports will be used to create the VSL link.
On Switch 1:
router-1(config)# interface port-channel 1 router-1(config-if)# no shut router-1(config-if)# switch virtual link 1 router-1(config-if)# exit router-1(config)# interface range tenGigabitEthernet 1/4 - 5 router-1(config-if-range)# no shut router-1(config-if-range)# channel-group 1 mode on router-1(config-if-range)# end
On Switch 2:
router-2(config)# interface port-channel 2 router-2(config-if)# no shut router-2(config-if)# switch virtual link 2 router-2(config-if)# exit router-2(config)# interface range tenGigabitEthernet 1/4 - 5 router-2(config-if-range)# no shut router-2(config-if-range)# channel-group 2 mode on router-2(config-if-range)# end
[edit] Executing the Conversion
This step creates a VSS system. This will convert all interface names in to a three mode notation,
chassis/slot/port. Executing this mode will require a reload to merge both switches configurations, renumber all ports and to negotiation NSF/SSO etc between chassis and supervisors.
On both switches, issue:
router# switch convert mode virtual
You should select "yes" to reload the switch.
It's a good idea to reload both switches once they initially convert, and make sure the VSL link comes up properly. It's been reported on #cisco that VSL conversions frequently fail due to configuration differences related to prior VSL port-channel options. It may be necessary to manually edit "standby" VSS nodes before they turn on for the first time to prevent initial dual-active issues.
[edit] Finalising the Conversion
The conversion must be finalised by reconfiguring the port channel on the secondary switch.
On the active switch (probably Switch 1 unless you have set priority) enter the following:
router(config)# interface port-channel 2 router(config-if)# no shut router(config-if)# switch virtual link 2 router(config-if)# exit router(config)# interface range tenGigabitEthernet 2/1/4 - 5 router(config-if)# channel-group 2 mode on router(config-if)# no shut router(config-if)# end
The system should now be a virtual switch! At this point, you should save your config and verify the system. You can do this via the show switch virtual command.
router# sh switch virtual Switch mode : Virtual Switch Local switch number : 1 Local switch operational role: Virtual Switch Active Peer switch number : 2 Peer switch operational role : Virtual Switch Standby
[edit] Comparison with Nortel's Split MLT
Nortel have a similar technology labelled Split Multilink Trunk (Split MLT). The table below compares Split MLT to Cisco's VSS.
| Feature | Cisco VSS | Nortel Split MLT |
|---|---|---|
| Number of Configurations | 1 | 2 |
| Max ports (GE) | 771 | 384 per switch (768 total) |
| Max ports (10GE) | 132 | 24 per switch (48 total) |
| VLANs | 4094 | 1980 |
| STP | Yes | Disabled |
Split MLT's primary disadvantage over Cisco's VSS is Nortel's requirement to disable spanning tree, the vastly decreased number of active vlans and the requirement to maintain two configurations. Note: You do not have to eliminate STP. SMLT is designed to remove the requirement for STP.
[edit] Future Developments
VSS has been architechted to support more than just two chassis, so pending market demand, this may become a supported configuration in the future.
