What's the role of Router/Switch name number?

Question

I have a CGR1240 Router in Cisco Packet Tracer:

I have a question about the name of port.

I have no idea about the name number of a router/switch port.

why there is 2/* in CGR Router? such as FastEthernet2/3, GigabitEthernet2/1? and in somewhere I also see the FastEthernet0/0/1, there is three number.

is there any rule of the Router/Switch port name?

Answer 1

Cisco usually names their interfaces following the (internal) structure of the given device.

Generally, this follows the syntax

<InterfaceType><Chassis>/<ModuleOrSlot>/<Port>   or
<InterfaceType><Slot>/<Subslot>/<Port>    or
<InterfaceType><Module>/<Port>

So that makes FastEthernet2/0

Type:   Fast Ethernet (10 or 100Mbit)
Module: 2
Port:   0

Traditionally, "onboard" ports are in "Module 0". Hence the classic router models of the 2600, 2800, 2900 (and the 1800, 1900, 3800, 3900 and... and...) series came with 2 onboard ports:

FastEthernet 0/0 and FastEthernet 0/1  (for the 2600s)
GigabitEthernet0/0 and GigabitEthernet0/1 (for the 2800/2900 series)

They had (E)HWIC and NEM slots, and if you inserted e.g. a 2-port FastEthernet card, or a 2port serial card in Slot 2, these would become

FastEthernet1/0 and FastEthernet1/1
Serial2/0 and Serial2/1

Also, by some strange tradition, Cisco switches start numbering their ports at "1", while routers number their ports from "0". The chassis I can think of start numbering from "1", modules mostly from "0", for both switches and routers.

Now if your device happens to be of particular internal topology, it might very well be that all your ports reside on "Module 2".

Stackable/multichassis switches such as the 2960-S and X, or the 3750 series (and their younger siblings 3650 and 3850), and the VSS capable switches as well (6500 series, 4500-X), have three segments, where the first number represents the chassis, the second the module, and the third the Port. some models have modules/slots for additional ports, such as a 4x10G Module in the Cat3750/3850.

GigabitEthernet1/0/5
GigabitEthernet2/0/5
TenGigabitEthernet1/1/2
TenGigabitEthernet2/1/2

And then, some of the more sophisticated routers follow the <slot>/<subslot>/<port> scheme, for example: https://www.cisco.com/c/en/us/td/docs/interfaces_modules/shared_port_adapters/install_upgrade/ASR1000/asr_sip_spa_hw/ASRintro.html

This is all rather superficial - I think one could write books about Cisco interface numbering :-), and we haven't even come to the carrier grade devices yet.

Also, the interface type designation (Ethernet, FastEthernet, GigabitEthernet, TenGigabitEthernet) usually shows the maximum speed the interface is capable of, even if it runs at a lower rate (e.g a 1G Interface at 100Mbit) [1]. Caution applies: the interface speed alone does not imply that the device's forwarding engine is capable of such throughputs, especially with the (older) low end routers.

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[1] unless of course, you are on a Nexus platform, where a switchport of any speed (from 1G to 100G) is called ethernetX/Y