Routing – Netstat -nr Returning ‘0/1’ Meaning

You have hit what I have always considered a pain point in the naming here. Let me start with a small correction to what you said: technically speaking, the MTU is the maximum size of an L3PDU inside an Ethernet frame. The L3PDU is made up of IP headers and payload. So payload =/= MTU.

Now, when you talk about this informally, you often just say "payload", without clarifying whether you are talking about the payload of the Ethernet frame, or the effective payload of the L3PDU inside it. And this brings me to the issue: the confusion comes from the fact that some of the names actually overlap. When you talk about TCP MTU, you are effectively talking about the maximum packet size, which is a value that is independent from the physical Ethernet MTU, which remains 1500 bytes.

In summary, the real MTU is the physical one, i.e. the maximum size of the IP packet inside an Ethernet frame. When you talk about MTU with regards to other network layers, you are effectively talking about the maximum packet size, which doesn't necessarily have the same value of the physical MTU.

This article talks about this in a bit more detail, but here is an excerpt that is relevant to this question:

Higher-level network protocols like TCP/IP can be configured with a maximum packet size, a parameter independent of the physical layer MTU over which TCP/IP runs. Unfortunately, many network devices use the terms interchangeably. On both home broadband routers and Xbox Live enabled game consoles, for example, the parameter called MTU is in fact the maximum TCP packet size and not the physical MTU.

@3iron's answer gave me the hint that I needed. The notation is denoting that there are multiple NIC interfaces on the router in the diagram and it's making reference to this.

• F0/0 - interface #1
• F0/1 - interface #2

NOTE: The F indicates that the NIC/port is most likely a Fast Ethernet type of connection.

This diagram from the Cisco website shows them as well. Here's the referenced as Fa 0/1, which I believe is the more typical notation.

The other interesting take away is that the notation may sometimes include a 3rd digit to denote that the interface being referenced is not built into the Motherboard of the networking gear. The above diagram shows some examples of this, ie. T1 1/0/1. NOTE: The 3rd digit prefixes, and indicates which "slot" on the Motherboard a port is coming from.

Again the Cisco reference describes these slots like so:

I was also able to dig up this reference: Interface Nomenclature Guide of Router and Switch which describes the reference of F0/0 like so. Take note that there are 2 types of situations:

• Fixed interface series router
• Modular interface series router

This notation was originally used for fixed types of routers:

In fixed interface series router, the interface nomenclature is type slot_#/ port_#.

But with the advent of modular routers, that did not denote which addon card (WIC) was being referenced. So the notation was expanded:

• New naming convention only for WIC slots (WAN Interface Card) is type slot_#/subslot_#/ port_#.
• Ports installed directly on chassis still use classic convention that is type slot_#/ port_#.

Incidentally that same site had this table which describes the F0/0 notation like this:

Additional searches did turn up this URL: Standard Router Ports which had these types of ports listed:

• Aux port: This auxiliary port is used to connect a modem to the router, which can then be used to remotely modify the configuration on the router.
• Attachment Unit Interface (AUI) port: Before the WIC became a standard for providing expansion through an add-on port, the AUI allowed transceivers to be used, providing you with the ability to add various types of network connections, such as fiber or copper Ethernet connections.
• Serial: Connects a modem or other serial device to allow a WAN network interface to be used on the router.
• Ethernet/Fast Ethernet/Gigabit Ethernet: Standard network interfaces used to connect different network segments.
• Console: Serial configuration port for command-line access to router management and configuration. Refer to Figure 3-1 to see the console port.
• WAN Interface Card (WIC) port: Because a wide variety of WAN connectivity options are available (for example, T1, ISDN, ADSL), you can use this port to add different interfaces to a standard router.
• Hardware WAN Interface Card (HWIC) port: With the integration of services into routers, the WIC interface became too limiting. The HWIC interface was created to support a wider variety of hardware expansion options, such as switches and service cards. This port is backward compatible with most older WIC hardware.

As well as this URL: Different types of interfaces in a Cisco Router:

• Ethernet - Ethernet is typically Ethernet IEEE 802.3 standard based physical interface, which operates at 10 Mbps speed. The media standard used is 10BaseT.
• Fast Ethernet - Fast Ethernet is typically Ethernet IEEE 802.3u standard based physical interface which operates at 100 Mbps speed. The media standard used is 100BaseT.
• Gigabit Ethernet - Gigabit Ethernet is typically Ethernet IEEE 802.3ab standard based physical interface which operates at 1000 Mbps speed. The media standard used is 1000BASE-T
• Serial - Serial interfaces are typically used for WAN connections from ISP (Internet Service Providers) for connectivity types like Frame Relay, T1, T3, etc.
• FDDI Fiber Distributed Data Interface - FDDI networks operates at 100 Mbps speed and uses a token-passing mechanism to prevent collisions.
• Token Ring - Token Ring interfaces can operate at either 4 Mbps or 16 Mbps. In Token Ring networks a token is passed around the network (configured in ring topology), allowing the owner of the token to transmit a frame, to avoid collision.Token Ring networks vanished from networking industry long way back. New Cisco routers do not have a Token Ring interface.

Note: Only 10Mbps Ethernet interface has a name "Ethernet" in a Cisco Router. A 100Mbps Ethernet interface is called a "FastEthernet" interface and a 1000Mbps Ethernet interface is called a "GigabitEthernet" interface.

That last bit is a bit more telling. Since Ethernet would historically been used for 10MBps interfaces, the "F" in my "F0/0" & "F0/1" diagram would seem to imply that the interface is a FastEthernet capable connection (10/100MBps).

However, more recently, Ethernet can also be used interchangeably on Gigabit Ethernet as well as 10G Ethernet ports, so you need to pay special attention to the actual equipment that's been referenced by a diagram.

References

• Interface and Line Numbers in Cisco 1800, 2800 and 3800 Series Routers
• Standard Router Ports
• Different types of interfaces in a Cisco Router