JUNOS Interface Terminology

The router has two types of interfaces, permanent interfaces, which are always present in the router, and transient interfaces, which can be inserted into and removed from the router depending on your network configuration needs.

Each router has two permanent interfaces. The management Ethernet interface provides an out-of- band method for connecting to the router. You can connect to the management interface over the network using utilities such as ssh and Telnet. SNMP can use the management interface to gather statistics from the router. The internal Ethernet interface connects the Routing Engine (the portion of the router running the JUNOS Internet routing software) to the System Control Board (SCB), the System and Switch Board (SSB), the Forwarding Engine Board (FEB), or the System and Forwarding Module (SFM), depending on router model, which is part of the Packet Forwarding Engine. The router uses this interface as the main communications link between the JUNOS software and the components of the Packet Forwarding Engine.

Each router also has two serial ports, labeled console and auxiliary, for connecting tty-type terminals to the router using standard PC-type tty cables. Although these ports are not network interfaces, they do provide access to the router.

The router contains slots for installing FPC boards , and almost all FPCs can accommodate up to four PICs, which provide the actual physical interfaces to the network. These physical interfaces are the router's transient interfaces. They are referred to as transient because you can hot-swap an FPC or its PICs at any time. You can insert any FPC into any of the router's slots, and you can generally place any combination of PICs in any location on an FPC. You must configure each of the transient interfaces based on the slot in which the FPC is installed, the location in which the PIC is installed, and for some PICs, the port to which you are connecting. You can configure the interfaces on PICs that are already installed in the router as well as interfaces on PICs that you plan to install later. The JUNOS software detects which interfaces are actually present, so when the software activates its configuration, it activates only present interfaces and retains the configuration information for the interfaces that are not present. When the JUNOS software detects that an FPC containing PICs has been inserted into the router, the software activates the configuration for those interfaces.

Table 6.1 lists the interface types supported by the JUNOS software and each interface's software name .

Table 6.1. Interface Types Supported by the JUNOS Software

Interface Groups	Interface Type	Interface Name in Configuration
ATM	ATM	at
Channelized	Channelized DS-3 to DS-0 Channelized DS-3 to DS-1 Channelized E1 Channelized OC3 to T1 Channelized OC-12 to DS-3 Channelized STM-1 to E1
DS-3, T1, T3, E1, E3	DS-3 E1 3 T1 T3	ds e1 e3 t1 t3
Ethernet	Aggregated Ethernet Fast Ethernet Gigabit Ethernet 10 Gigabit Ethernet Internal Ethernet Management Ethernet	ae fe ge ge fxp fxp
Multilink	Frame Relay PPP	ml ml
SONET/SDH	Aggregated SONET/SDH SONET/SDH	as so
Other	Encryption GRE tunnel IP-IP tunnel Loopback	es gr ip lo

When you configure an interface, you give it a name that effectively specifies the properties for a physical interface descriptor. In most cases, the physical interface descriptor corresponds to a single physical device and consists of the interface name, which defines the media type, the slot in which the FPC is located, the location on the FPC in which the PIC is installed, the PIC port, and, optionally , the interface's channel and logical unit numbers . The interface name is represented by a physical part, a logical part, and a channel part in the following format:

  physical<:channel>.logical  

The physical part of an interface name identifies the physical device, which corresponds to a single physical network connector. This part of the interface name has the following format:

  type-fpc/pic/port  

fpc identifies the number of the FPC card on which the physical interface is located. pic identifies the number of the PIC card on which the physical interface is located. port identifies a specific port on a PIC.

Each physical interface descriptor can contain one or more logical interface descriptors. These allow you to map one or more logical (or virtual) interfaces to a single physical device. The logical unit part of the interface name corresponds to the logical unit number, which can be a number in the range 0 through 16,384.

The channel identifier part of the interface name is required only on channelized interfaces. Channel 0 identifies the first channelized interface. A nonconcatenated (that is, channelized) SONET/SDH OC-48 interface has four OC-12 channels, numbered 0 through 3. A Channelized OC-12 interface has twelve DS-3 channels, numbered 0 through 11.

There is a separator of some kind between each element of an interface name. In the physical part of the name, a hyphen (-) separates the media type from the FPC number, and a slash separates the FPC, PIC, and port numbers. In the virtual part of the name, a period (.) separates the channel and logical unit numbers. A colon (:) separates the physical and virtual parts of the interface name.