VPC - [Part 2] - VSS vs VPC

Both VSS (Virtual Switching System) and VPC (Virtual Port Channel) support Multi-Chassis Ether-Channel, and used to create a Port-Channel, whose one end has one device (Switch A) and another end has physically connected to two different physical devices (Switch B and Switch C), and these two devices will logically appears as one devices.

VPC (Virtual Port Channel)

• Nexus supports VPC feature and we can use VPC for Data Center environment

• In VPC, when you will configure VPC in Nexus Switches, then still their control plane are separate. Both switches will be controlled by their own control plane.

• In VPC, Switches will be manage separately. Separate IP will be used to access, monitor and manage the switches. Virtually both switches will appear as single logical switch to downstream device. Actually VPC allows the links which are physically connected to two different Nexus devices to appear as a single Port-Channel to a third device which can be a switch, server, or any other networking device.

• Because, in VPC, Switches will be manage separately. Separate IP will be used to access, monitor and manage the switches. So we need to configure gateway redundancy.

• But, when we will put both switches (Switch A and Switch B) in VPC, then they will not be accessed with single logical name. In VPC Separate IP will be used to access, monitor and manage the switches. So they will be accessed and managed separately. Means, interfaces will be view in separate CLI, Switch A will have its own port only and Switch B have its own.

• In VPC, separate control plane instances of STP, IGP, BGP will be used, like they are being used in two different switches.

• VPC is used for Layer-2 Port-Channels only.

• VPC only supports LACP.

• In VPC, Control Messages are carried by CFS (Cisco Fabric Services) over Peer Link and a Peer keep-alive link sends periodic keep-alive messages between VPC peer devices. It can be a management interface or Switched Virtual Interface (SVI). It sends the messages between two VPC peer device, VPC peer sends the keep-alive message to other peer to inform, originating switch is operating and running VPC.

VSS (Virtual Switching System)

• Cisco 6500 and 4500 series switches support VSS, we can use VSS for Campus Environment.

• In VSS, when you will configure switches, they will be merged logically, it will be become one logically switch, means a single control plane will control both switches in Active Standby manner.

• In VSS, there is only one logical switch will be manage and configure by administrator, so single IP will be used to access the switch. They will not manage as separate switch. And you will have to put configuration on Active Switch like Stacking. 

• Because, in VSS, when you will configure switches, they will be merged logically, there is only one logical switch will be manage and configure by administrator, and single IP will be used to access the switch. So we cannot use HSRP active and Standby in VSS. Only one single IP will be assign to L3 interface and it can be used as gateway for the devices in that particular Vlan, but still there is redundancy because same IP is using for two switches , if one switch fail or went down, another can take over.

• Check the below diagram. When we will put both switches (Switch X and Switch Y) in VSS, then they will be access by Single logical name A.

• In VSS if all are TenGigabitEthernet ports then interfaces will be view in single CLI as TenGigabitEthernet1/1/1, TenGigabitEthernet1/1/2, TenGigabitEthernet1/1/3........ for Switch A and TenGigabitEthernet2/1/1, TenGigabitEthernet2/1/2, TenGigabitEthernet2/1/3....... for Switch B. Check below picture

• In VSS same instances of STP, IGP, BGP etc will be used.

• VSS can support Layer-3 Port-Channels. 

• VSS supports both type Ether-Channel PAgP and LACP

• In VSS Control Messages and Data Frames flow between active and standby via VSL (Virtual Switch Link).

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Anubhav Upadhyay

Hello and welcome to networktopic Blog. My name is Anubhav. I am a Senior Network Egineer. I have created this blog specially to serve interview questions and answer on Network Routing and Switching, I will try my best to serve correct and updated networking knowledge for you as per my corporate experience.

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