Junos OS

i am from cisco background and working with them for past 8 yrs, i just moved to junos products,and while taking e-learning flash course, i foud that interfaces are configured with the word " unit 0 " what is the meaning and reason of using this command ?

since once we define an interface we can configure it , but why unit  0 is always reqd ?

plz clarify  "team"

thanks.

Unit is the designation that Junos uses to allow multiple logical interfaces to be configured on a single physical interface.  The interface is for physical configurations and unit is used for logical configuration parameters.  As you saw in the e-learning module these are then designated after the decimal point in the interface name.

Rather than adding a unit designation for those interfaces that have multiple logical interfaces, Junos requires the unit designation on all interfaces with a default first unit number of zero.  This requirement makes all interface names consistent regardless of whether or not there are multiple logical interfaces.  The naming convention is consistent for all possible interface configurations.

Since Junos is a hierarchical and not a flat structure this also makes all interface detail configuration appear at the same level when reading a configuration.   This also makes adding logical interfaces to interfaces intially configured as just a single one easier and more consistent.

The only configurations then that occur at the interface level are physical configurations that apply to the physical interface.  Any logical configuration occur at the unit level.

Can i configure the interface without unit 0?? is it possible.

The unit number is required but it does not have to be 0, this is only the default first unit number.

Just to add to what theothers have said, the unit # is the equivalent of CISCOs subinterface. There are some circumstances where only a unit 0 can be used, for example when configuring the family "ethernet-switching', only unit 0 can be used. So its kind of like taking one physical interface and dividing it in multiple interfaces by using the logical unit number.  Sometimes i use the example of creating multiple partitions on a harddrive and so you can have C:\, D:\, G:\, U;\ and so forth and they look like can be managed like separate drives even though it is one physical drive.