Juniper® JNCIA Exam Cram Notes : Interface Types And Properties

4. Junos Configuration Basics

4.4 Interface Types and Properties

Juniper Networks platform has primarily two types of interface. These are:

1. Permanent Interfaces: These are always present in the router and Transient interfaces, these can be inserted or removed from the router by user.

Each router has two permanent interfaces. These are:

  • Management Ethernet interface: This interface enables us to access the router using ssh, and telnet. The interface uses out-of-band connectivity, and does not provide packet forwarding capabilities for the transit data packets.
  • Internal Ethernet interface: Connects the Routing Engine (running the JUNOS Internet software) to 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. The Internal Ethernet interface is configured automatically when the JUNOS software boots.

2. Transient Interfaces: Transient Interfaces are the interfaces that receive user's data packets from the network and transmit the packets to the network. These interfaces are physically located on a Physical Interface Card. They can be inserted and removed at any time.

These interface need to be configured before using it. We can also configure the interfaces that are not in the chassis. When the JUNOS software activates the router's configuration it finds out the interfaces that are present and activates only those interfaces.

The maximum transmission unit (MTU) of the physical interface can be changed. Each interface has a different default value. In juniper, the possible MTU range is 256 to 9192 bytes.

In addition, each router has two serial ports, labeled console and auxiliary. Console port can be used to connect tty-type terminals to the router. The auxiliary port can connect to a modem.

The state of the interface are Down, BDR and DR. There are also other states of the interface other than this. BDR and DR are the election carried out in OSPF network.

fxp interfaces are used for managing our juniper devices. The fxp interfaces are the only current interface types that do not follow the two-letter designator format. These interfaces are special in their function.

Each Juniper Networks router contains the fxp0 and fxp1 permanent interfaces. The fxp0 interface performs the management functionality. This provides the out-of-band method used while connecting the router. The operation of a Juniper Networks platform itself relies on the fxp1.The fpc slot locates the interface which begin at 0 on Juniper Networks router.

The fxp1 interface connects the Routing Engine to the Packet Forwarding Engine. This communications link is how routing protocol packets reach the Routing Engine to update

The media type 'lo' indicate the loopback interface. These are the virtual interface that we can create on our juniper routers. These interfaces are mostly created for testing purposes.

Deactivate is the command used to deactivate the interface on junos devices. This command disables the interface from both receiving and transmitting the data.

Each interface has two types of properties assigned to it: physical properties and logical properties. Physical properties are tied to the entire physical port, whereas logical properties affect only that logical portion of the interface represented by unit numbers or channel numbers.

A physical property should always be configured before any logical identifier, such as a unit number.

All router interfaces that will send and receive transit traffic require a logical unit to be configured. This logical unit creates a division of the physical interface into multiple parts. For instance, an Ethernet interface can be subdivided into multiple virtual LANs (VLANs), each requiring its own logical unit.

Link-mode is used to specify full duplex, half duplex, or autonegotiation.


link-mode mode (automatic | full-duplex | half-duplex);

Note that protocol family and lo0 are configured at logical interface.

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