300-135 TSHOOT Quick Study Notes: VLANs, VTP, STP, EtherChannel

Troubleshooting VLANS:
  • missing VLAN: the VLAN must exist in the VLAN database (show vlan brief)
  • incorrect port assigment: switchports need to be in correct VLANs (default: VLAN 1)
  • incorrect IP addressing: devices in the VLAN need to be in the same subnet

Troubleshooting trunks:
  • encapsulation mismatch (802.1Q/ISL)
  • incompatible trunking modes (access, trunk, dynamic auto/desirable)
  • native VLAN mismatch
  • allowed VLANs (note: VTP pruning)
  • check: show interfaces trunk

Troubleshooting VTP:
  • no trunk
  • domain name mismatch
  • version mismatch (VTPv1 [default], v2 and v3 [compatible])
  • wrong mode (server, client, transparent, off)
  • password mismatch
  • revision number (highest number overwrites the VLAN database)

STP basics:
  • based on the ability to compare BPDUs and find the better, or superior, BPDU
  • BPDUs are evaluated sequentially, looking for the lower value: root bridge ID > root path cost > sender bridge ID > sender port ID > receiver port ID (local; not in BPDU)

STP calculation:
  1. Elect the root switch. The switch with the lowest bridge ID.
  2. Choose root port (per switch; not root). The port with the best resulting BPDU from all received BPDUs.
  3. Choose designated port (per segment). The port with the best BPDU "locally" (on the segment).

EtherChannel troubleshooting:
  • mismatched port configurations: speed, duplex, port mode, [native] VLAN, allowed VLANs, and the port type (L2/L3) must be identical
  • mismatched port-channel configurations: devices must be configured with compatible modes (LACP/PAgP/on)

EtherChannel load balancing: