Ask the device what it believes with show commands

short · 45 min · Objective 5.5

Task

The show commands are how you ask a device what it believes to be true, and the answer is frequently different from what you assumed. You will run the Linux equivalents of each one on your lab hosts, so the command-to-question mapping the exam tests becomes something your fingers know.

Steps

  1. show interface. Run ip -s link show eth0 and sudo ethtool eth0. This is the workhorse: status, speed, duplex and the error counters — CRC, runts, giants, drops. Note that this is where you confirm whether errors are increasing.
  2. show mac-address-table. On the bridge host, bridge fdb show lists which MAC addresses were learned on which ports. Use it to find where a device physically is, and to spot a MAC flapping between ports, which signals a loop or a duplicate.
  3. show route. ip route show. Read it with the selection order from domain 2 — longest prefix first, then administrative distance, then metric — to see which route will actually be used and whether a sane default is present.
  4. show arp. ip neigh show. The device's own IP-to-MAC map: it confirms layer 2 reachability, exposes a duplicate address as one IP against two MACs, and reveals ARP poisoning.
  5. show vlan. bridge vlan show. Which VLANs exist and which ports belong to each — the first command to run for an incorrect-VLAN-assignment fault, because it shows immediately whether a port is where you believed or sitting in VLAN 1 by default.

Verify

ip -s link show eth0
ip route show
ip neigh show
bridge fdb show 2>/dev/null | head; bridge vlan show 2>/dev/null | head

Each command answers one diagnostic question with real output: the interface counters say whether the port is erroring, the route table says which path a packet will take, the neighbour table says who is reachable at layer 2, and the bridge tables say where a device is and which VLAN it is in. Running them is the point — the mapping is only useful once the output is familiar.

Notes

Two device commands have no clean Linux equivalent and are worth stating for the exam. show config is the running configuration — what the device is actually doing, as opposed to what the documentation claims — and comparing it against the golden configuration is how configuration drift is found. show power reports the PoE budget and its allocation, and it is the command that diagnoses the "devices boot and then reset" fault by showing available budget against draw.

The instrument side of this lesson cannot be practised in a VM, which is exactly why CompTIA examines it: a toner and probe identifies which cable is which; a cable tester checks wiring correctness; a TDR reports the distance to a fault; a visual fault locator shows a broken fibre by the light escaping; and a Wi-Fi analyzer shows channel overlap and signal strength. Knowing which instrument answers which question is the whole objective — and the applied lab turns that into a decision you can test.