Cisco 9500 Stackwise Deployment? 1 physical link for SVL?

[Lost & Found]

Hi All,
I was tasked to migrate our client's core switch from their Catalyst 4500 to the new 9500 series. The current setup is they have 2 physical switches installed on different Building (A & B), they also have an existing fiber link between building a & b.

On the access switches they do have dual-homed and singled-homed connection while connection to external networks (internet/branch sites) is in Building A. Refer to below topology.

Topology: 

Now, they have already bought 2 9500 series devices and I am now preparing for deployment.
Question:

1. Since my client only has 2 fiber links between building A & B, Would it be possible to use a single link for Stackwise link and another 1 for the Dual-Active Detection link? 

2. If Q#1 is possible then what would be the down side of this design ? This is a single point of failure right ? since if the Stackwise Link(1physical link) goes down then we cannot utilize the Dual-Active Detection link to forward the data traffic? 

3. The following SFP modules installed on 4500 can be used on C9500 series ? 
   NAME: "TenGigabitEthernet1/2/1", DESCR: "SFP-10Gbase-LR"
   PID: SFP-10G-LR       
   NAME: "TenGigabitEthernet1/2/2", DESCR: "SFP-10Gbase-LR"
   PID: SFP-10G-LR        
   NAME: "TenGigabitEthernet2/2/1", DESCR: "SFP-10Gbase-LR"
   PID: SFP-10G-LR        
   NAME: "TenGigabitEthernet2/2/2", DESCR: "SFP-10Gbase-LR"
   PID: SFP-10G-LR       

4. If we cannot fully utilize the available physical link and have a redundancy in terms of core switches interconnection (building a & b) what would be the best approach for this ? What pops in my mind is using the Traditional deployment (HSRP/VRRP with PO and trunk links)? 
   Thank you

[balaji.bandi]

Since my client only has 2 fiber links between building A & B, Would it be possible to use a single link for Stackwise link and another 1 for the Dual-Active Detection link? 

No i would not advise, you need Dual links for , 1 DAD this is consider as good design, if you compromise on this you will have later more and more down times, due to split brains of Stackwise link failure.

[Lost & Found]

@balaji.bandi Thanks for your response, So for this one, we have option like to use a traditional setup HRPS/Trunk links or Routing protocol between Core1 and 2... Or just add additional link to we can have 2 physical link for SVL and 1 for DAD at least. 

[balaji.bandi]

yes, if you do not need it SVL, you can go traditional Trunk, any way if the devices is single home, i do not see any advantage here, VSS or SVL.

Couple of question for Long term :

1. Do you going to have dual home devices (i do see Operation / CTRL rooms) ? at any time future, where your access layer connected dual uplinks

2. If you need dual home - then SVL is good to go for Long term solution. since you have Operation , CTRL dual home connected.

[Lost & Found]

@balaji.bandi ,That's correct, They have a couple of access switches which have dual uplink. 

Indeed, with Stackwise virtual they can have an active/active setup so that traffic or communication which is within Building B will be processed by Core SW#2 rather than going to Core Sw#1(in building A active VIP and Root bridge) then to another network which is located in Building B (traditional setup). 

Lastly, What if the SVL goes down and we still have active DAD link... Does it disable the secondary functionality like data forwarding?  In that case single homed devices connected to secondary will be affected. 

[Reza Sharifi]

Lastly, What if the SVL goes down and we still have active DAD link... Does it disable the secondary functionality like data forwarding?  In that case single homed devices connected to secondary will be affected. 

When SVL goes down, the primary switch will reboot because it detects a split-brain on the network. That is the reason you should always have 2 links for SVL. This way if one link goes down you still have a second one and no split-brain. VSS or StackWise Virtual is not a good solution for single-homed devices. 

HTH

[balaji.bandi]

Indeed, with Stackwise virtual they can have an active/active setup so that traffic or communication which is within Building B will be processed by Core SW#2 rather than going to Core Sw#1(in building A active VIP and Root bridge) then to another network which is located in Building B (traditional setup). 

SVL Physically 2 but logically 1 device. if you have Dual uplink the traffic will be passing on both the links (technically).

yes orphan device can be connected to single device there is no harm, but if that connected parent device go down, network not reachable (that is is the Limitation) - not see advantage here of SVL or VSS.

Lastly, What if the SVL goes down and we still have active DAD link... Does it disable the secondary functionality like data forwarding?  In that case single homed devices connected to secondary will be affected.

Things go split brain you have different complicated issues around the network to fix. that time Dual Home device act funny.