01
1.4 Interface & cable troubleshooting
- show interfaces counters: CRC/input errors → cable/noise; runts/giants → framing; late collisions → duplex mismatch.
- Fix duplex mismatch with auto/auto on BOTH ends. "show ip interface brief" gives up/up status fast.
02
2.1 VLANs & access ports
- A VLAN is a separate broadcast domain. Normal range 1–1005. Default VLAN is 1.
- Access port carries ONE data VLAN (plus optionally a voice VLAN). Inter-VLAN traffic needs a router or L3 switch.
- config: switchport mode access ; switchport access vlan 10.
03
2.2 Trunking (802.1Q)
- 802.1Q inserts a 4-byte tag with the VLAN ID into the frame on trunk links.
- The NATIVE VLAN (default 1) is sent UNTAGGED and must match on both ends.
- Hardcode "switchport mode trunk" to disable DTP and reduce VLAN-hopping risk.
04
2.3 Discovery protocols (CDP/LLDP)
- CDP = Cisco proprietary, ON by default. LLDP = 802.1AB open standard, OFF by default (needs "lldp run").
- "show cdp neighbors detail" reveals neighbor IP, platform and IOS - directly connected devices only.
05
2.4 EtherChannel (LACP)
- Bundles 2–8 physical links into one logical port; STP treats it as a single link (no blocking).
- LACP (802.3ad) negotiation: active/passive - at least one side must be active. PAgP is Cisco. "on" = no negotiation.
- All member ports must match speed, duplex, and VLAN/trunk config. L3 EtherChannel = "no switchport" + IP on the port-channel.
06
2.5 Spanning Tree (Rapid PVST+)
- STP prevents L2 loops by blocking redundant paths. Rapid PVST+ runs a separate RSTP (802.1w) instance per VLAN.
- Root bridge = lowest bridge priority (default 32768, set in multiples of 4096), tie-break lowest MAC. The root has NO root ports - all its ports are designated.
- RSTP roles: root, designated, alternate (blocked backup to root), backup. States: discarding, learning, forwarding.
- PortFast on END-DEVICE access ports only (skips listening/learning). Never on a switch-to-switch link.
STP protection (v1.1 additions) | Feature | Does | Where |
|---|
| BPDU guard | err-disables a PortFast port that receives a BPDU | access ports |
| Root guard | blocks a port that would become root | toward access/other domains |
| Loop guard | guards against unidirectional-link loops | non-designated ports |
| BPDU filter | stops sending/processing BPDUs | use with care |