Dual-Active Detection (DAD) link on VSS

[uppermost]

DAD link prevents both supervisors from becoming active in event of a VSL link failure. 

My question is - do I need DAD link. if I have 4 x 100G ports configured as VSL links?

I need other ports for access switches.

My core switch - 9500-24Y4C

[MHM Cisco World]

It optional cisco recommends but not mandatory.

DAD dont carry any mgmt/control/data traffic  it only use for heartbeat.

[Flavio Miranda]

Hi @uppermost 

 I believe the terminology here might be important.

VSS, VSL and DAD X  StackWise Virtual, SVL

VSS, VSL and DAD is suiltible for IOS device like 6500, 6800, etc.

Stackwise Vitual and SVL is suitible to IOS-XE like 9500,9300,etc

As we are still handling both kind of device and many companies are migrating or preparing to migrate ( 6807-XL Last Date of Support: HW is 2027!) both concept are still present.

But, Stackwise vitual is a new way to perform VSS and it does not required DAD. The link beween devices is now called SVL and it is more like a tradditional stack.

Reading the document for StackWise Virtual we start to see afirmation like "only", "cannot", etc.

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

Restrictions for Cisco Catalyst 9500 Series High Performance Switches

• On C9500-32C switches, you can configure SVL and DAD only on interfaces numbered 1-16 on the front panel of the switch.

• On C9500-32QC, you can configure SVL and DAD only on native 100G and 40G interfaces (default configuration ports). You cannot configure SVL and DAD on converted 100G and 40G interfaces.

• In a Cisco StackWise Virtual solution, ports that support 4X10G breakout cables can be used only as data-only ports and cannot be used for configuring SVLs or DAD links.