Implement and Troubleshoot Switch Administration

Managing MAC Address Table

The MAC address table contains address information that a switch uses to forward traffic between ports

• MACs are associated with 1 or more ports
• Dynamic address
  • Switch learned addresses that will age out when not in use
• Static Addresses
  • Manually entered, does not age and not lost during switch reset

Disabling MAC Address Learning on Interface or VLAN

• Causes flooding on network
• Disabling on SVI, all IP packets are flooded in L2 domain
• No mac address-table learning [interface]
• Show vlan internal usage
• Show mac address-table learning [interface]
• Default mac address-table learning (Global config)

Errdisable Recovery

Software on switch that detects an error situation and diables the port

• Port is effectivly shutdown
• LED is set to amber
• Show interface

cat6knative#show interfaces gigabitethernet 4/1 status

PortName Status Vlan DuplexSpeed Type
Gi4/1err-disabled 100full 1000 1000BaseSX

• Syslog Message example (BPDU Guard)
  • %SPANTREE-SP-2-BLOCK_BPDUGUARD:
       Received BPDU on port GigabitEthernet4/1 with BPDU Guard enabled. Disabling port.
    %PM-SP-4-ERR_DISABLE:
       bpduguard error detected on Gi4/1, putting Gi4/1 in err-disable state
• Causes of error disable
  • Duplex mismatch
  • Port channel misconfiguration
  • BPDU guard violation
  • UniDirectional Link Detection (UDLD) condition
  • Late-collision detection
  • Link-flap detection
  • Security violation
  • Port Aggregation Protocol (PAgP) flap
  • Layer 2 Tunneling Protocol (L2TP) guard
  • DHCP snooping rate-limit
  • Incorrect GBIC / Small Form-Factor Pluggable (SFP) module or cable
  • Address Resolution Protocol (ARP) inspection
  • Inline power
• Error disable detection is enabled by default
  • Disable – no errdisable detect cause
  • Show errdisable detect

cat6knative#show errdisable recovery
ErrDisable ReasonTimer Status
-------------------------------
udld Enabled
bpduguardEnabled
security-violatioEnabled
channel-misconfigEnabled
pagp-flapEnabled
dtp-flap Enabled
link-flapEnabled
l2ptguardEnabled
psecure-violationEnabled
gbic-invalid Enabled
dhcp-rate-limitEnabled
mac-limitEnabled
unicast-floodEnabled
arp-inspection Enabled

Timer interval: 300 seconds

Interfaces that will be enabled at the next timeout:

InterfaceErrdisable reasonTime left(sec)
--------------------------------------------
Fa2/4bpduguard273

• Recover port from error disable
  • Identify root cause of errdisable
  • Correct the problem
  • Re enable the port
    • Shutdown , no shutdown on the interface
    • Cmd: Errdisable recovery

cat6knative#errdisable recovery cause ?
all Enable timer to recover from all causes
arp-inspectionEnable timer to recover from arp inspection error disable
state
bpduguard Enable timer to recover from BPDU Guard error disable
state
channel-misconfig Enable timer to recover from channel misconfig disable
state
dhcp-rate-limit Enable timer to recover from dhcp-rate-limit error
disable state
dtp-flapEnable timer to recover from dtp-flap error disable state
gbic-invalidEnable timer to recover from invalid GBIC error disable
state
l2ptguard Enable timer to recover from l2protocol-tunnel error
disable state
link-flap Enable timer to recover from link-flap error disable
state
mac-limit Enable timer to recover from mac limit disable state
pagp-flap Enable timer to recover from pagp-flap error disable
state
psecure-violation Enable timer to recover from psecure violation disable
state
security-violationEnable timer to recover from 802.1x violation disable
state
udldEnable timer to recover from udld error disable state
unicast-flood Enable timer to recover from unicast flood disable state

L2 MTU

The MTU on a switch may need to be adjusted based on protocols being used. Example: 802.1q tunnel requires extra header space, adjusting the MTU from 1500 to 1504 would prevent fragmentation.