Virtual port channel (vPC) or Multichassis Etherchannel


Virtual Port Channel
A port channel combines multiple physical link into a single logical link provided that these all physical links must belong to same network switch. However, in this case if our switch goes down we lose connectivity to all the devices connected to it. This is where virtual port channel comes to rescue. A virtual port channel allows you to bundles physical links spread across two separate physical switches. Following figures show simple port channel and virtual port channel topologies. 


In this case Switch L runs simple port channel (LACP) because this switch has no idea that it is connecting to two different switches. But wait a minute how is that possible? With above topology it is not possible. We need little more on switch A and B side so that they can dodge Switch L to appear as single switch. We need to add extra physical links between Switch A and Switch B so that these switches can inform each other that they are both working on to fool Switch L. So our topology will look like this.

Cisco sees a little problem here. If link between Switch A and Switch B fails then how can Switch A and B communicate each other? Remember these switch need to communicate with each other in order to fool Switch L (Fool means present two switches as one switch to Switch L). In case of failures these peers also need to know whether the peer link is failed or peer itself is dead. This is why we connection Switch A and B with an extra physical link (It can be logical Layer 3 connectivity usually via management port for heart beat). Following figure shows full picture of VPC and their naming convention. 


Benefits of Using vPC
vPC provides the following benefits:
·       Allows connectivity from single device to multiple upstream switches
·       Provide loop free topology and no blocking STP ports.
·       Loadbalancing on all links. Optimal use of links.
·       Fault tolerant in case of link failures as long as there is one link up in vPC.

Cisco Fabric Services Protocol
Cisco Fabric Services protocol is reliable management protocols used by peer switches to synchronize control and data plane information via vPC peer link. Any port that is not part of the vPC is called orphan port. vPC ports which loss complete connectivity to one of the vPC peer are also called Orphan ports.

Loop avoidance in vPC:
vPC uses forwarding rules on hardware level to avoid loops. For example if a packet arrive at peer switch from vPC members port and is forwarded to another peer via peer link then this packet is not forwarded to any other vPC members port. It can be forwarded to any orphan port though.

Configuration of vPC

  1. Enable VPC feature on both switches (Switch A and Switch B) using following command.

Feature vpc

2.      Create vPC domain on both switches. It should be same on both switches.

vpc domain <domain_ID>

3.      Provide peer keep alive connectivity. A must do step before configuring Peer link.

peer-keepalive destination <remote peer IP> source <local IP> vrf mgmt.

           You can use any layer 3 connectivity in any vrf.

4.     Create Peer link. Peer link is nothing but regular trunked port channel with vpc peer-link command in it.

Interface port-channel <peer link number>
switchport mode trunk
vpc peer-link

5.     Create vPC. Again its configuration is same as port channel with additional command of vpc vpc_Number in it.
Interface port-channel <vPC number>
vpc <vPC number>
 

                               Configuration Example Topology


Switch A:

feature vpc

vpc domain 10

peer-keepalive destination 10.0.0.1 source 10.0.0.100 vrf default

interface Ethernet1/7

switchport

channel-group 1 mode active

interface Ethernet9/7

switchport

channel-group 1 mode active



interface port-channel1

switchport mode trunk

vpc peer-link



interface Ethernet1/8

switchport

channel-group 2 mode active

interface Ethernet9/8

switchport

channel-group 2 mode active



interface port-channel2

switchport mode trunk

vpc 2



Switch B:

feature vpc

vpc domain 10

peer-keepalive destination 10.0.0.100 source 10.0.0.1 vrf default

interface Ethernet1/7

switchport

channel-group 1 mode active

interface Ethernet9/7

switchport

channel-group 1 mode active



interface port-channel1

switchport mode trunk

vpc peer-link



interface Ethernet1/8

switchport

channel-group 2 mode active

interface Ethernet9/8

switchport

channel-group 2 mode active



interface port-channel2

switchport mode trunk

vpc 2


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