vPC Initialization Order of Operations

vPC Processes :

1.  vPC Processes Starts
2. IP/UDP 3200 Peer Keepalive connectivity established
3.  Peer-link adjacency form 
4. vPC Primary/ Secondary role election 
5. vPC consistency check performed 
6. Layer 3 SVIs move to UP/UP state
7. vPC member ports move to UP/UP state

vPC Consistency Check 

1.  vPC Peers sync control plane over peer link with Cisco Fabric Service (CFS)
2. Includes advertisement of "consistency parameters" that must match for vPC to form successfully e.g line card type (M or F), speed , duplex , trunking, LACP mode , STP configs etc

3 types of Consistency checks: 

Type 1 Global:- 

•  Mismatch results in vPC failing to form 
• E.g STP mode Rapid-PVST vs MST

Type 1 interface 

• Mismatch results in VLAN being suspended or vPC member 
• E.g STP port type network Vs. Normal

Type 2 

• Mismatch results in syslog message but not vPC Failure can results in failure in data plane
• E.g MTU Mismatch 

Peer Keepalive & Peer link Fate Sharing

• Keepalive configured via layer 3 SVI's
• SVI Vlan is allowed on peer link
• STP always prefers peer link
• Peer Link fails , but primary still up 
• No Layer 2 path for SVI exists or Secondary disables SVI
• Secondary Cannot ping primary
• Secondary promoted to operational primary 
• Split brain occurs

vPC Peer link failure detection 

• vPC Peer Link Fails (e.g line card outage)

vPC Secondary Pings Primary over peer keepalive 

• if vPC primary is alive 
• Disable vPC member ports on secondary 
• Disable SVI's on secondary 
• Goal is to force end host to forward via primary 

If vPC primary is dead 

• Promote vPC secondary to operational primary 
• Continue to forward traffic on new primary \

Peer Keepalive and peer link must not share fate in order to prevent split brain

• E.g Separate MGMT Switch , Separate Port Channels on Separate line cards.