VPC - [Part 4] - INTERVIEW QUESTIONS

Question - 1 What is VPC Roles ?

• VPC role is non-preemptive, it means a device can be operate as VPC secondary peer device  which is configured as VPC primary. It can happen in the below scenario:

1. When the original device goes down or fail, in this situation VPC secondary peer device will become the new VPC primary peer device. 
2. When the system is recovering itself, then previously primary peer device will now the secondary device and previously secondary peer device will be the primary device. 

• VPC role defines which one VPC peer device will process BPDUs and will respond to ARP requests.
• If VPC peer-link goes down, then in this situation VPC role also defines which actions should be taken by primary VPC peer device and secondary VPC peer device. 

Question - 2 What happen if VPC peer link goes down ?

• If VPC peer-link goes down but VPC peer-keep-alive link is still up, then in this situation VPC secondary peer device will perform the following operations:

1. Suspends its VPC member ports
2. Shut down the SVI (Switch Virtual Interface) associated to the VPC VLAN

• Above actions will use to move complete traffic to the VPC primary peer device
• If VPC peer-link will down, then both VPC peer devices cannot synchronize with each other, so above process will isolate one of the peer device.

Question - 3 What is VPC role priority ?

• You can set role priority manually via command "role priority" to influence VPC election process.
• Range of priority values is - 1 to 65636, and default value is 32667. 
• Always lower value will be preferable,  switch with lower priority value  has a better chance of being the primary VPC.
• If you will change the priority of the VPC peer devices in live network , it can cause the interfaces in network to go up and down.
• If you want to set the role priority to make one VPC peer device the primary device, then you will have to set role priority on both peer device,  lower priority value on primary VPC device and higher priority value on secondary VPC device. After that shutdown the VPC peer link on both devices via shutdown command, and re-enable the port channel on both devices via no shutdown command.

Question - 4 What is VPC Hitless Role Change ?

• VPC Hitless role change is basically used to switch VPC roles between VPC peers without impacting traffic flows. 
• VPC role switching is done based on the VPC role priority value of the  VPC peer device under the VPC domain. 
• A VPC peer device with lower role priority will be elected as the primary VPC device when the VPC role preempt command is executed.

Question - 5 Where can we use VPC Hitless Role Change feature in VPC ?

You can use Hitless VPC role change feature in below scenarios:

• Request for role change - If you want to change the roles of the VPC peer devices in a VPC domain, then you can use Hitless VPC role change feature.
• Primary switch reboot - If device comes up after reload and roles are defined already, then you can use hitless VPC role change feature to recover the roles, if after reload primary device takes the role of operational secondary and secondary device takes the role of primary operational, then you can use VPC role preempt command to restore the previous defined role. Before using the VPC role preempt command, always check the current device role priority and put the command no port-channel limit under the VPC domain.

Question - 6 What is VPC Master Sticky Bit ?

• It is a Programmed Protection Mechanism used to avoid unnecessary role swapping because role swapping causing disruption on the network. 
• When Primary Switch will reloaded unfortunately,  then VPC Master Sticky Bit will allow the live switch “sticks” to its primary role when a down switch is coming up or when isolated switch is being integrated back into the VPC domain.

Question - 7 When should Master Sticky Bit value be true or false ? 

•  VPC Master Sticky Bit value will be set to TRUE in below situation:

1. When exist VPC Primary peer device will reload and VPC enabled switch will switch its role from VPC Secondary to VPC Operational Primary. 
2. If the role switches from VPC Operational Secondary to VPC Primary, then sticky bit will not be set
3. When VPC peer switch switches its role from none establish to VPC Primary when reload restore timer (240 sec by default) expires 

• Sticky Bit is set to FALSE by default, VPC Master Sticky Bit value will be set to FALSE in the below situation:

1. When a vPC-enabled switch will be reload )
2. When VPC role priority will be changed or re-entered. You can check VPC Sticky master bit value via this NX-OS exec mode command

Nexus7K# show system internal vpcm info global | include ignore-case sticky

        Sticky Master: TRUE

Question - 8 Explain VPC delay restore.

• After a reload when the peer adjacency is already established, VPC delay restore feature used to delay VPC from coming up on the restored VPC peer device
• You can set use VPC delay restore value via command "delay restore time ". 
• To revert to set the default delay value, you can use the command "no delay restore".
• VPC delay restore default timer is 30 seconds. 
• You can set VPC delay restore from 1 to 3600 seconds
• VPC Delay Restore Interface Vlan - It is used to delay the VLAN interfaces on the restored VPC peer device from coming up.
• For VPC Delay Restore Interface Vlan, You can use the "delay restore interface-vlan time" .
• Default timer for VPC Delay Restore Interface Vlan is10 seconds.

Question - 9 Explain VPC election process.

In VPC one device will be selected as Primary VPC peer device and another one will be selected as secondary VPC peer device based on the below parameter.

1. VPC master sticky-bit set to 0 or 1 - Switch bit with true will become primary and switch with false bit will become secondary.
2. If master sticky-bit is same, then the switch with lowest priority value will become primary and switch with higher priority value will be become secondary.
3. If priority value is same, then switch with lowest mac will become primary and switch with higher mac address will become secondary.

Question - 10 Explain VPC Auto-Recovery.

• When both VPC peer switches go down due to power outage, if only one peer switch is restored, then this switch will become Primary VPC peer switch due auto-recovery feature.
• And the VPC links will come up after the auto-recovery time period.
• The default auto-recovery period is 240 seconds.

Question - 11 What is the role of primary and secondary in VPC ?

• Both peer switches forward traffic in the data plane. 
• But the control plane functions will handle by only the primary switch. 
• One of these will send BPDU frames, and response to ARP

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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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