Q. What happens if all VSL connections between the virtual switch members are lost?
A. VSLs can be configured with up to eight links between the two switches across any combination of line cards or supervisor ports to provide a high level of redundancy. If for some rare reason all VSL connections are lost between the virtual switch members leaving both the virtual switch members up, the VSS will transition to the dual active recovery mode.
The dual active state is detected rapidly (subsecond) by any of the following three methods:
Enhancement to PagP used in MEC with connecting Cisco switches
L3 Bidirectional Forwarding Detection (BFD) configuration on a directly connected link (besides VSL) between virtual switch members or through an L2 link through an access layer switch L2 Fast-Hello Dual-Active Detection configuration on a directly connected link (besides VSL) between virtual switch members (supported with 12.2(33)SXI) In the dual active recovery mode, all interfaces except the VSL interfaces are in an operationally shut down state in the formerly active virtual switch member. The new active virtual switch continues to forward traffic on all links.
Reference: http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps9336/ prod_qas0900aecd806ed74b.html

https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipmulti_pim/configuration/xe-16/imc-pim-xe-16-book/imc-tech-oview.html Bidir-PIM is designed to be used for many-to-many applications within individual PIM domains. Multicast groups in bidirectional PIM mode can scale to an arbitrary number of sources without incurring overhead due to the number of sources.

Explanation
For example:
Site 0001
Data:10.0.0.0/24
Voice: 10.0.1.0/24
summary route : 10.0.0.0/23
Site 0002
Data:10.0.2.0/24
Voice: 10.0.3.0/24
summary route: 10.0.2.0/23
cont...
site 0129
Data:10.1.0.0/24
Voice: 10.1.1.0/24
summary route: 10.1.0.0/23
site 0130
Data:10.1.2.0/24
Voice: 10.1.3.0/24
summary route: 10.1.2.0/23
so 3rd octet is odd number assigned to voice, and even number assigned to data; for security team to recongize voice prefix, use an ACL with wildcast to filter odd number on third octet, started from 10.0.1.0 0.0.254.255, 10.1.1.0 0.0.254.255......, 10.1.1.0 0.0.254.255 etc; for 10.0.1.0 0.0.254.255, any IP in binary that started with 00001010.00000000.xxxxxxx1.xxxxxxxx will be matched (x = either 0 or 1), covert 3rd octet into dec, for example, 10000001 = 129 which is a voice VLAN.

Stub No Type 5 AS-external LSA allowed, accepts only 1,2,3 but no 4 totaly stubby No Type 3, 4 or 5 LSAs allowed except the default summary route NSS, accepts only 1,2 and route 0.0.0.0 NSSA No Type 5 AS-external LSAs allowed, but ABR converts type 7 to type 5. It accepts type 3 and 4 totaly NSSA No Type 3, 4 or 5 LSAs except the default summary route, but ABR converts type 7 to type 5.

ENSLD 300-420 cert guide page 117. When creating a backbone there should never be L1 routers between (L2 only, or) L1/L2 routers.