VPC - [Part 3] - INTERVIEW QUESTIONS

Question  - 1 Why VPC ? How VPC is working ?

• VPC is Cisco proprietary. Actually a traditional network uses spanning-tree protocol to prevent layer-2 loops. It does have some limitations. As per spanning tree process - to prevent loops, it blocks some links and uses other link as active.
• If active link will be fail, then blocked port will be ready to use.  In case of link failure STP uses the SPF (Shortest Path First) algorithm to calculate which link will be unblock, after that link goes through several states (Listening, Learning and forwarding) before it will be active.
• So blocked ports are unused, means wasting of bandwidth. Modern Spanning Tree (PVST) has some improvement regarding this. One link may be blocked for VLAN 10, and the same link may be active for VLAN 20. This is a type of load balancing in spanning tree. But what will happen if there is a lot of traffic on VLAN-10 and VLAN-20, in this situation traffic is not load balanced normally. 
• Spanning-tree also impact in sub-optimal paths. If traffic needs to travel from Switch A to Switch B, it will be pass through the Switch A.
• We can improve these things via Ether-Channels. It will allows many active up-links between two switches or devices. But it still has some limitations, Ether-Channel will be only between two devices. If any single switch will fail , it will cause an outage for the bundle of links.
• VPC is an extension to Ether-Channels. There are single method, which cross the limitations of spanning-tree and Ether-Channel. 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.
• All links in the VPC will be forward traffic actively. VPC is Multi-Chassis Ethernet Technology (MCEC), but are not a stack. VSS (Virtual Switching System) on the Catalyst platform is another multi-chassis technology.
• VPC also provide high-availability. Both set of links will be active, so it also have a fast convergence time. If a link or a group of links will fail, the other links already forwarding traffic. This is much faster than spanning-tree.

Question - 2 What is purpose of VPC ?

• 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.
• It can provide Layer 2 multi-pathing, basically Layer 2 multi-pathing allows to create redundancy by increasing bandwidth, enabling multiple parallel paths between nodes and load-balancing traffic where alternative paths exist.

Question - 3 Can we have more than one VPC domain ID on a single device ?

• We can have only one VPC domain ID on each device.

Question - 4 What are the advantages of VPC ?

• It allows a single device to use a Port Channel across two upstream devices
• Eliminates Spanning Tree Protocol blocked ports
• It provides a loop-free topology
• Bandwidth utilization, it uses all available up-link bandwidth
• When a link or device will be fail, then it will provide fast convergence.
• High availability

Question - 5 Explain each component of VPC.

• VPC - Basically VPC allows to the combined Port-channel between the VPC peer devices and the downstream device.
• VPC peer switch -  It is one of a pair of switches that are connected to the the VPC peer link. Where one switch will be selected as the primary device, and other will be the secondary device.
• VPC peer link - It is used to synchronize states between the VPC peer devices. It is responsible to control traffic between two VPC switches and also multicast, broadcast data traffic. In the situation of link failure, it carries uni-cast traffic. Peer links required at least two 10 Gigabit Ethernet interfaces.
• VPC domain - VPC domain including both VPC peer devices, VPC peer keep-alive link, VPC peer link, and all the Port-Channels in the vPC domain connected to the downstream device.
• VPC peer keep-alive link - 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.
• VPC member port - Member ports are interfaces that belong to the VPCs.
• Orphan Port - An orphan port is a port that is configured with a VPC VLAN (i.e a VLAN that is carried over the VPC peer link) and is not configured as a VPC member port.

Question - 6 Which configuration parameters must be identical in VPC ?

The below configuration parameters must be configured identically on both devices of the VPC peer link or the VPC will enter suspend mode.

• Port-Channel mode – On, off, or active
• Link speed per Port-Channel
• Duplex mode per Port-Channel
• Trunk mode per Port-Channel – Native VLAN
• Spanning Tree Protocol mode
• Spanning Tree Protocol region configuration for Multiple Spanning Tree (MST) Protocol
• Enable or disable state per VLAN
• Spanning Tree Protocol global settings

               – Bridge assurance setting
               – Port type setting (you should set all VPC interfaces as network ports)
               – Loop guard settings

• Spanning Tree Protocol interface settings

               – Port type setting
               – Loop guard
               – Root guard

• Quality of service (QoS) configuration and parameters

              – Maximum transmission unit (MTU)

Question - 7 Can we connect non-VPC device with VPC peer ?

Any device that supports layer-2 port-channels can connect by a VPC. The device does not need to be VPC aware. Devices include physical servers, firewalls, other switches, and load balancer.

Question - 8 Explain VPC Domain.

• VPC domain is the logical collection of all VPC components, it containing both VPC peer devices, VPC peer keep-alive link, VPC peer link, VPC member ports and all the Port-Channels in the VPC domain connected to the downstream device.
• There can only be one domain per switch. Every pair of VPC enabled switches should use a different domain ID.

Question - 9 Explain VPC Member Ports.

• Interfaces that belong to the VPCs are know as VPC member ports.
• Member ports are VPC, from which other devices connect to. Connected device have to configure its own port-channel.
• The configuration will be the same on both switches. We can use different ports numbers, as long as they match in duplex, speed etc.. 

Question - 10 Explain VPC Orphan Ports.

• Orphan port is a non-vPC port on a VPC enabled switch. It is any device that connects to only one of the VPC peer, not both peers. Example - Like server’s management port.
• It may carry any VLAN, even if it is present on a VPC member port or on the peer-link
• You can verify it by using “show vpc orphan-ports”.

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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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