> For the complete documentation index, see [llms.txt](https://jncia-workbook.gitbook.io/workbook/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://jncia-workbook.gitbook.io/workbook/junos-basics/17-interface-configuration.md).
# 17) Interface Configuration
### Pre-Work
Load the config called **interface.config.cfg** using the **load\_config\_on\_nodes.py** script.
```
python3 load_config_on_nodes.py --lab_dir three-routers --config_filename interface.config.cfg
```
### Lab
Connect to R1, and configure ge-0/0/0 as follows:
* IP address `10.1.2.1/24`
* IPv6 address `2001:db8:1:2::1/64`
* L2 MTU `9000`
* Description `"Connection to R2"`
Configure the loopback as follows:
* IP address `1.1.1.1/32`
* IPv6 address `2001:db8::1/128`
### Answer
Expand to reveal
```
set interfaces ge-0/0/0 description "Connection to R2"
set interfaces ge-0/0/0 mtu 9000
set interfaces ge-0/0/0 unit 0 family inet address 10.1.2.1/24
set interfaces ge-0/0/0 unit 0 family inet6 address 2001:db8:1:2::1/64
set interfaces lo0 unit 0 family inet address 1.1.1.1/32
set interfaces lo0 unit 0 family inet6 address 2001:db8::1/128
```
### Explanation
Expand to reveal
In Junos, interface configuration is split into physical configuration and logical configuration. Items such as the description and L2 MTU are configured on the physical interface.
```
set interfaces ge-0/0/0 description "Connection to R2"
set interfaces ge-0/0/0 mtu 9000
```
\
L3 items such as the IPv4/IPv6 address are configured on the logical interface. (This should intuitively make sense - physical config goes on the physical interface, and logical config goes on the logical interface/unit). In Junos, the logical interface must have a `unit` number, which is similar to a subinterface in Cisco. Typically you will use `unit 0` when there is no VLAN encapsulation. Additionally, logical configuration goes under the corresponding `family`, for example `family inet` for IPv4 and `family inet6` for IPv6.
```
set interfaces ge-0/0/0 unit 0 family inet address 10.1.2.1/24
set interfaces ge-0/0/0 unit 0 family inet6 address 2001:db8:1:2::1/64
```
\
Also note that as a shorthand, you can do `ge-0/0/0.0` instead of `ge-0/0/0 unit 0`.
```
set interfaces ge-0/0/0.0 family inet address 10.1.2.1/24
set interfaces ge-0/0/0.0 family inet6 address 2001:db8:1:2::1/64
```
\
These same principles apply to the loopback interface.
```
set interfaces lo0.0 family inet address 1.1.1.1/32
set interfaces lo0.0 family inet6 address 2001:db8::1/128
```
\
Unlike on IOS, we cannot create multiple loopback interfaces.
```
[edit]
admin@R1# set interfaces lo1 uni
^
device value outside range 0..0 for '1' in 'lo1'.
```
\
Nor can we assign multiple units to the loopback. (However, we can have multiple units on Lo0 if they are in different routing-instances. You will see routing-instances in the *Junos Routing* section).
```
admin@R1# set interfaces lo0 unit 1 family inet address 1.2.3.4/24
[edit]
admin@R1# commit
[edit interfaces lo0]
'unit 1'
if_instance: Multiple loopback interfaces not permitted in master routing instance
error: configuration check-out failed
```
\
If you need more loopback IP addresses, you would just assign multiple IPs to lo0.0. (There is no such thing as a secondary address in Junos. See the previous lab, *Editing Configuration (Part 1)* for more details).
```
set interfaces lo0 unit 0 family inet address 1.1.1.1/32
set interfaces lo0 unit 0 family inet address 1.2.3.4/24
```
\
We should now be able to ping R2:
```
admin@R1> ping 10.1.2.2
PING 10.1.2.2 (10.1.2.2): 56 data bytes
64 bytes from 10.1.2.2: icmp_seq=0 ttl=64 time=28.333 ms
64 bytes from 10.1.2.2: icmp_seq=1 ttl=64 time=1.626 ms
admin@R1> ping 2001:db8:1:2::2
PING6(56=40+8+8 bytes) 2001:db8:1:2::1 --> 2001:db8:1:2::2
16 bytes from 2001:db8:1:2::2, icmp_seq=0 hlim=64 time=486.540 ms
16 bytes from 2001:db8:1:2::2, icmp_seq=1 hlim=64 time=1.557 ms
```
\
We can also check the MTU. Note that the L3 MTU (`protocol inet` and `inet6`) are automatically 14 bytes less than the L2 (Ethernet) MTU.
```
admin@R1> show interfaces ge-0/0/0 | match "ge-0/0/0|MTU"
Physical interface: ge-0/0/0, Enabled, Physical link is Up
Link-level type: Ethernet, MTU: 9000, MRU: 9008, LAN-PHY mode, Speed: 1000mbps, BPDU Error: None,
Logical interface ge-0/0/0.0 (Index 345) (SNMP ifIndex 552)
Protocol inet, MTU: 8986
Protocol inet6, MTU: 8986
Protocol multiservice, MTU: Unlimited
```
\
We can also see interface descriptions:
```
admin@R1> show interfaces descriptions
Interface Admin Link Description
ge-0/0/0 up up Connection to R2
```
### Further Reading