Logical Versus Physical

All interfaces in JUNOS and most other networking devices have two properties; logical and physical. Physical properties belong to an entire physical port, whereas logical properties correspond to the portion of the interface represented by a unit or channel number. Depending on the type of physical port installed, a large number of properties can be configured. We will detail some of the most common here:

• Clocking: Aligns bits as they are transmitted out of the interface. A clock signal can be supplied by a Service Provider or by the router.



• Encapsulation: Layer 2 encapsulation used on the interface. Examples include Frame Relay, Point to Point Protocol or PPP, and Cisco Highlevel Data Link Control or HDLC



• MTU: Maximum Transmission Unit, which is the maximum size of the frame transmitted from the interface.



• Keepalives: Used to verify the operation of the interface. Most encapsulations enable keepalives by default, but they can be disabled to aid troubleshooting.



• Layer 1/2 Options: Various bit and byte settings for the interface media such as framing, flow control, and source address filters

All router interfaces sending and recieving traffic or packets require a logical unit to be configured. This way, an interface may be divided into multiple logical interfaces (subinterfaces in Cisco) This division then could be used to create multiple Virtual Local Area Networks or VLANs with their own logical unit. Note: Many router vendors do not require a subinterface or logical unit on every physical interface but a Juniper router does. Even point-to-point interfaces and non VLAN tagged interfaces need a logical unit to be configured.

In JUNOS, unit numbers MUST be configured before any other logical configuration is applied. Here are some of the common logical properties that can be configured.

• Protocol Family refers to the Layer 3 protocols that can be sent and recieved on this interface. The most common is family inet. Other protocol amilies include IPv6, Multi Protocol Label Switching or MPLS, and ISO (Intermediate System to Intermediate System IS-IS)



• Protocol Address: Layer 3 family address such as family inet (IP address)



• Virtual Circuit Address: Circuit identifier use when an interface is divided logically. Logical interfaces include VLAN IDs, Frame Relay Data Link Connection Identifier or DLCI, or ATM virtual path/Virtual Connection Identifiers (VP/VCI)



• Logical unit numbers can range from 0-16,385



• The best practice is to keep circuit address the same as the unit number for ease of toubleshooting.



• Note: If you are configuring a point to point interface the unit number MUST be zero!

"More Interface Configuration Examples"

Let's demonstrate configuring a Fast Ethernet interface in JUNOS:

Note: Remember interfaces in JUNOS are 'automatically enabled' when the physical connection is wired. So, after cabling up, we check the status of our Fast Ethernet Interface:

1. root@routername> show interfaces terse fe-2/0/1


2. Once we determine that the interface is up, we enter configuration mode: root@routername#configure
   
3. We are brought to the [edit] hierarchy and now, to edit our interface: root@routername# edit interfaces fe-2/0/1


4. Establish the unit: root@routername#set unit 0 family inet address 10.10.20.122/24


5. JUNOS software requires a mask for every IP address in the classless CIDR notation (Classless Inter Domain Routing). This is denoted by the / (slash)


6. Execute a show command: root@routername#show to verify our newly created interface


7. Activate our new changes with commit: root@routername# commit-and-quit


8. Let's verify our newly created interface with ping: root@routername#ping 10.10.20.121 count 3

The reason for the count command with ping is, a Juniper router will send an endless number of pings unless a number of packets is specified or you use Ctrl-C