The Switch's Domain

Understanding Loopguard

• Stops loops caused by unidirection link failures
• Example:
  • A is root bridge
  • Link between B and C is blocked (at B)
  • Link between B and C goes unidirectional
  • B releases the block, one-way loop occurs
• Loopguard should be enabled (per-port) on all switch uplinks

Switch(config-if)#spanning-tree guard loop
Switch(Config)#spanning-tree loopguard default

Understanding UDLD

• Different from loopguard, but from the same goal
• Designed specifically for fiber
• Sends a "Layer 2 ping" between neighbors
• Supports normal and aggressive (preferred) mode
• Cisco recommends use both!

switch(config)#udld <enable/aggressive>
switch(config-if)#udld port <aggressive>
switch#udld reset

Supervisor Redundancy Mechanisms

• Stateful Switchover (SSO)
  • Lightning quick failover between supervisor engines
  • Sync's startup, running, L2 (MAC) and L3 (FIB) tables, ACLs
• Cisco Non-Stop Forwarding (NSF)
  • Forwards traffic with CEF
  • Supports with EIGRP, OSPF, IS-IS, and BGP

Designing the Network

• Use local VLANs
• Set up HSRP intuitively for active VLANs
• Match STP root to active HSRP gateway