Logically the differences are STP - i.e. with SVI interfaces there must be a Layer-2 VLAN, with Sub-Interfaces there is no VLAN just a tagged logical port. However I am pretty certain the 2nd configuration you list won't work on a 3560 as you would need to define the VLAN Tag on each subinterface with the command 'encapsulation dot1q x'. I think you can do this on the Catalyst 6500 but not on any of the 'lesser' switches.
The SVI will remain up as long as at least one Layer-2 port is active & forwarding in the VLAN. This is a common problem on a lot of networks. The only way I can think you could achieve HSRP failover would be with Object Tracking - maybe track reachability of a next-hop on the SP's network.
Are you sure you can actually configure what you posted on a 3560? I just tried it on one here and it doesn't even let me create the subinterfaces.
We are pleased to announce availability of Beta software for 16.6.3.
16.6.3 will be the second rebuild on the 16.6 release train targeted
towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are
looking for early feedback from customers befor...
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