STP in the datacenter

STP logical interfaces maximum

  • RSTP / MST reach topology maximus. Switch specs logical / virtual interfaces.  (6500/ 6748)
    • Logical interfaces == ((trunk ports * vlans ) + non-trunk interfaces)
    • Virtual interfaces ( Per line card ) = (( trunk ports * vlans ))
    • verify with ‘show spantree sum total‘

Example logical interfaces:

6500 Chassis:

  • 120 VLANS
  • 49 Access layer switches
  • 2 connections to each switch ( double uplink but with etherchannel counts as 1 logical interface )
  • 1 Cross Connect to secondary Agg Switch.
  • 30 other devices connected
    • (( 120 * 50 ) + 30 ) = 6030 Logical interface (out of 10,000 max)

Example Virtual interfaces:

Cisco 6748 Linecard

  • 120 VLANS
  • 12 Access layer switches, 4 Etherchannel-bundled connections.
    • 12 x 4 = 48 ports  ( Virtual interfaces counts every interface )
      • ( 120 * 48 ) = 5760 Virtual Interfaces (out of 1800 max)

High Availability

Nic teaming options

  • Adapter fault tolerance (AFT):  active / standby  – Two nics  One switch
  • Switch fault tolerance (SFT):  active / standby  – Two nics Two switches
  • Adaptive Load Balancing (ALB):  active / active – One IP Two Macs
  • Etherchannel – LAG

Expectations and Failover Timers

OperationFailover time
OSPF / EIGRPSubsecond
RTSP1-2 Seconds
Etherchannel1 Second
HSRP Timers3 Seconds
Service Modules1-5 Seconds
Windows TCP Stack Tolerance9 Seconds