Catalyst 9500 DAD

[faizal_vi]

Hi ,

Looking for few clarification on DAD link on Catalyst 9500 switches.

1. is DAD link a must to form Stackwise Virtual?

2. Can DAD be formed on a 1 G copper or 1 G Fibre links?

Regards

[pieterh · Accepted solution]

1) dual-active-detection is not mandatory but strongly recommended
   why would you want HA, but take the risk of dual-active state ?

2) The two switches periodically exchange special dual-active hello messages containing information about the switch state.
    if only for those hello messages 1G link could be enough, but selection depends on expected switch load
    and how fast you want the detection to take place
   NB! the example below shows a fast interface (40G !) selected

High Availability Configuration Guide, Cisco IOS XE Fuji 16.9.x (Catalyst 9500 Switches) - Configuring Cisco StackWise Virtual [Support] - Cisco

Step 3: Configure StackWise Virtual Dual-Active-Detection Link
If a StackWise Virtual link fails, the StackWise Virtual standby switch cannot determine the state of the StackWise
Virtual active switch. To ensure that switchover occurs without delay, the StackWise Virtual standby switch assumes
that the StackWise Virtual active switch has failed and initiates switchover to take over the Cisco StackWise Virtual
active role.
In order to avoid Dual Active situation, we will configure Dual-Active Detection link with fast hello method as below.

[Joe Price]

Why can't the 40g stack-wise connection do the work of the DAD also?  Why do you need a second (possibly costly 40g connection) when less expensive 9200 / 9300 /2960x / 3750 series don't have such links and the dual "active" senario doesn't exist.

[pieterh]

read carefully
Step 3: Configure StackWise Virtual Dual-Active-Detection Link
If a StackWise Virtual link fails, the StackWise Virtual standby switch cannot determine the state of the StackWise
Virtual active switch.

and why ?, because you feel stability of your network is important.
If you did not think it's important , then you may totally stay away from  redundancy/HA 

that's why it is strongly preferred (not manadatory) to configure a separate link for HA and DAD.
as a consequence you need some extra hardware (here interface) to realize this.

did you read the document @balaji.bandi linked ?
https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst9500/software/release/16-11/configuration_guide/ha/b_1611_ha_9500_cg/configuring_cisco_stackwise_virtual.html

here is the IOS-XE 17.10 version
High Availability Configuration Guide, Cisco IOS XE Dublin 17.10.x (Catalyst 9500 Switches) - Configuring Cisco StackWise Virtual [Support] - Cisco

an extract from this document (but I suggest to read more):

Dual-Active Detection

If the standby switch detects a complete loss of the SVL, it assumes the active switch has failed and will take over as the active switch. However, if the original Cisco StackWise Virtual active switch is still operational, both the switches will now be Cisco StackWise Virtual active switches. This situation is called a dual-active scenario. This scenario can have adverse effects on network stability because both the switches use the same IP addresses, SSH keys, and STP bridge IDs. Cisco StackWise Virtual detects a dual-active scenario and takes recovery action. DAD link is the dedicated link used to mitigate this.

[balaji.bandi]

1. is DAD link a must to form Stackwise Virtual?

Dual Active detect ( DAD) to detect if the SVL become split brain and reboot one of the device, so network traffic will work as expected - so if you looking really high availability - I must stress to add link - not sure why you do not like to do it ?

2. Can DAD be formed on 1 G copper or 1 G Fibre links?

Not matter Copper or Fibre.

[faizal_vi]

Thank you for your answers. I have one more clarifications to add to that. Can we use the OOB MGMT port for the DAD link.

[balaji.bandi · Accepted solution]

personally i would say NO, Cat 9500 MGMT ports works in different way like for MGMT OOB purpose only

look at the DAD link guidance :

https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst9500/software/release/16-11/configuration_guide/ha/b_1611_ha_9500_cg/configuring_cisco_stackwise_virtual.html