Hello Praveen,
Logical units allow greater flexibility, as the name suggests it allows us to to subdidivde a physical interface into multiple logical units.
Each logical unit can have its own properties with appropriate encapsulations configured. As an example, when vlan-tagging or flexible ethernet-services vlan-tagging enabled you can assign individual logical units to a specific vlan and configure the logical units with their own specific properties.
Here is an example :
ge-0/3/2 {
vlan-tagging;
encapsulation flexible-ethernet-services;
unit 1 {
vlan-id 1;
family inet {
mtu 1500;
address 10.1.1.1/30;
}
}
unit 2 {
vlan-id 2;
family inet {
mtu 1000;
address 10.1.2.1/30;
}
}
unit 513 {
encapsulation vlan-vpls;
vlan-id 513;
family vpls;
}
unit 514 {
encapsulation vlan-ccc;
vlan-id 514;
family ccc;
}
}
# run show interfaces ge-0/3/2
Physical interface: ge-0/3/2, Enabled, Physical link is Up
Interface index: 155, SNMP ifIndex: 507
Link-level type: Flexible-Ethernet, MTU: 1518, Speed: 1000mbps, BPDU Error: None, MAC-REWRITE Error: None, Loopback: Disabled, Source filtering: Disabled, Flow control: Enabled, Auto-negotiation: Enabled, Remote fault: Online
Device flags : Present Running
Interface flags: SNMP-Traps Internal: 0x4000
CoS queues : 4 supported, 4 maximum usable queues
Schedulers : 256
Current address: 00:1b:c0:89:2c:5f, Hardware address: 00:1b:c0:89:2c:5f
Last flapped : 2012-09-26 19:45:55 UTC (2w3d 15:29 ago)
Input rate : 0 bps (0 pps)
Output rate : 0 bps (0 pps)
Ingress rate at Packet Forwarding Engine : 0 bps (0 pps)
Ingress drop rate at Packet Forwarding Engine : 0 bps (0 pps)
Active alarms : None
Active defects : None
Interface transmit statistics: Disabled
Logical interface ge-0/3/2.1 (Index 120) (SNMP ifIndex 568)
Flags: SNMP-Traps 0x4000 VLAN-Tag [ 0x8100.1 ] Encapsulation: ENET2
Input packets : 0
Output packets: 1
Protocol inet, MTU: 1500
Flags: Sendbcast-pkt-to-re, User-MTU
Addresses, Flags: Is-Preferred Is-Primary
Destination: 10.1.1.0/30, Local: 10.1.1.1, Broadcast: 10.1.1.3
Logical interface ge-0/3/2.2 (Index 121) (SNMP ifIndex 569)
Flags: SNMP-Traps 0x4000 VLAN-Tag [ 0x8100.2 ] Encapsulation: ENET2
Input packets : 0
Output packets: 2
Protocol inet, MTU: 1000
Flags: Sendbcast-pkt-to-re, User-MTU
Addresses, Flags: Is-Preferred Is-Primary
Destination: 10.1.2.0/30, Local: 10.1.2.1, Broadcast: 10.1.2.3
Logical interface ge-0/3/2.513 (Index 122) (SNMP ifIndex 570)
Flags: SNMP-Traps 0x4000 VLAN-Tag [ 0x8100.513 ] Encapsulation: VLAN-VPLS
Input packets : 0
Output packets: 0
Protocol vpls, MTU: 1518
Flags: Is-Primary
Logical interface ge-0/3/2.514 (Index 123) (SNMP ifIndex 574)
Flags: SNMP-Traps 0x4000 VLAN-Tag [ 0x8100.514 ] Encapsulation: VLAN-CCC
Input packets : 0
Output packets: 0
Protocol ccc, MTU: 1518
Flags: Is-Primary
Disclaimer : The above snippet is just an emaple to help demonstrate topic discussed here.
Hope this helps.