7.9 Chapter Summary

The interface is the point at which a router sends data to another device. Since the mission of a router is to forward data packets through networks, the interface is the sending and receiving component that allows that forwarding to take place.

Properly configuring an interface is the first step to the network configuration of a router. This configuration encompasses the lowest three layers of the OSI model. Physical, data link, and network configurations all take place at the interface and logical unit hierarchies.

Logical units allow the router to be connected to many more networks than if just the physical interfaces were to be used. Thousands of network links can be configured on one router through the use of logical units, which Juniper Networks calls logical units. These units are where the three family addresses are enabled. Even if an interface is not going to have multiple logical units configured, the single interface has to be configured under a unit.

These interfaces are grouped together in a PIC mounted in a FPC. The FPC can have different types of PIC interfaces installed, but individual PICs have the same type of interface. There is a system to identify these FPCs with PICs housing interfaces with logical units. Juniper Networks has a naming convention that identifies them in a specific order from FPC to logical unit. The two larger routers (M40 and M160) have vertical FPCs, whereas the M5, M10, and M20 have horizontal ones. The naming convention for the three small chassis is actually the same as that for the larger two routers turned on their right side (the administrator's right side facing the interfaces).

There are two main components to configuring the interfaces: the physical and the logical. The physical interface has properties and parameters that affect the entire port and all of the logical units on that port. This can affect timing, signaling, framing, and the like. The logical interface gets the NBMA and IP addressing assigned, along with any other family-associated configuration.

Different interface types with different features that can be implemented have been discussed. Ethernet and the associated speeds of Ethernet allow VRRP and MAC address filtering. ATM offers a QoS class that can be assigned to a logical unit. SONET (ATM, POS, and nonconcatenated) offers APS link fail-over that can even be configured between two different routers. Ethernet and SONET allow links to be aggregated to form a larger virtual link when more bandwidth is required between two routers. SONET interfaces were originally formed to multiplex data stream circuits, which can also be concatenated or not.

Tunnel and loopback interfaces were also discussed; both interfaces have no physical components. These interfaces reside purely in the processes of the router; they have specific and important functions though. The tunnel interface allows a virtual connection to be set up through a network to another tunnel interface. This allows the two networks connected through the tunnel to think they are directly connected. The loopback interface is a placeholder for an independent IP address that will not fail unless the router does. This function allows the router to keep a stable identification.

Some of the configuration in this chapter is required for basic communication just to set up a network. Some of the features described provide more stability and scalability in the network once it is up. The router is now able to communicate with another device that is directly connected either physically or virtually. How does that help pass data through a network and what communications does this enable? That is the subject of Chapter 8.