Command by command
| Cisco IOS | Linux | What it gives you |
|---|---|---|
show ip interface brief | ip -brief addr show | One line per interface |
show interfaces | ip link show | Layer 2 state, MTU, MAC |
show interfaces ge0 | ip addr show dev ge0 | A single interface |
show interfaces status | ip -brief link show | Link state without addresses |
What the output looks like
$ ip -brief addr show
lo UNKNOWN 127.0.0.1/8
ge0 UP 10.0.1.1/24
ge1 UP 10.0.2.1/24Why the two differ
IOS shows two status columns — Status for the physical line and Protocol for line protocol — which is why up/down means a cable is present but keepalives are failing. Linux splits the same idea differently: an interface has an administrative flag (whether you have brought it up) and an operational state (whether it is actually carrying traffic). ip link show prints both, as <BROADCAST,MULTICAST,UP,LOWER_UP> plus state UP. UP in the flags is administrative; LOWER_UP is the physical carrier. An interface with UP but no LOWER_UP is the Linux version of Cisco's up/down.
The trap
The loopback shows UNKNOWN rather than UP, and that is correct, not a fault. Virtual interfaces that have no carrier concept report UNKNOWN operational state because there is no physical layer to query. Do not go looking for a problem there.
Common questions
Why does ip -brief addr show not show the MAC address?
Because it shows layer 3. MAC addresses belong to the link layer, so they come from ip link show or ip -brief link show. Cisco's show ip interface brief makes the same split — it also omits MACs, and you use show interfaces to get them.
What is the equivalent of show interfaces description?
There is no direct one. Linux interfaces have an alias field that can hold a description, set with ip link set dev ge0 alias "uplink to core" and shown by ip link show on an alias line under the interface, but it is rarely used.
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