Interface

Network interfaces are the foundation of any network device, serving as the connection points to other devices and networks. On Juniper devices, interfaces can be configured with various options including link aggregation (LACP), logical units, VLANs, MTU and MSS settings to accommodate different network requirements.

This guide covers the essentials of Juniper interface configuration with practical examples.

Physical Interfaces

Physical interfaces on Juniper devices are named according to their type, FPC slot, and port number. For example:

• ge-0/0/0: Gigabit Ethernet interface in slot 0, port 0

• xe-0/0/12: 10 Gigabit Ethernet interface in slot 0, port 12

Link Aggregation (LACP)

Link Aggregation Control Protocol (LACP) allows you to bundle multiple physical interfaces into a single logical interface, providing increased bandwidth and redundancy.

Configuring Link Aggregation

1. First, assign physical interfaces to an aggregated Ethernet interface (ae):

interfaces {
    xe-0/0/12 {
        gigether-options {
            802.3ad ae0;                # Assign to ae0 bundle
        }
    }
    xe-0/0/13 {
        gigether-options {
            802.3ad ae0;                # Assign to ae0 bundle
        }
    }
}

1. Then, configure the aggregated interface with LACP:

interfaces {
    ae0 {
        vlan-tagging;                   # Enable VLAN tagging
        mtu 9000;                       # Set MTU to 9000 bytes
        aggregated-ether-options {
            lacp {
                active;                 # Active LACP mode (initiates LACP negotiations)
                periodic fast;          # Fast periodic rate (sends LACP packets every second)
            }
        }
    }
}

LACP Modes:

• active: Actively initiates LACP negotiations

• passive: Responds to LACP negotiations but doesn't initiate them

LACP Periodic Options:

• fast: Sends LACP packets every 1 second

• slow: Sends LACP packets every 30 seconds (default)

Logical Units (Subinterfaces)

Logical units, also called subinterfaces, are used to create multiple logical interfaces on a single physical interface. Each unit can have its own VLAN ID, IP address, and protocols.

Configuring Logical Units

interfaces {
    ae0 {
        unit 10 {                       # Logical unit 10
            description "Peering: IX Connection";  # Description for SNMP / Manage
            vlan-id 10;                 # Associate with VLAN 10
            family inet {               # IPv4 configuration
                address 192.0.2.127/24;
            }
            family inet6 {              # IPv6 configuration
                address 2001:db8::127/64;
            }
        }
    }
}

You can have multiple units on a single interface, each with different VLAN IDs:

interfaces {
    ae0 {
        unit 10 {
            vlan-id 10;
            family inet {
                address 192.0.2.127/24;
            }
        }
        unit 20 {
            vlan-id 20;
            family inet {
                address 198.51.100.127/24;
            }
        }
    }
}

VLAN Configuration

VLANs (Virtual LANs) are used to segment network traffic. There are several ways to configure VLANs on Juniper devices:

1. VLAN Tagging

interfaces {
    ae0 {
        vlan-tagging;                   # Enable 802.1Q VLAN tagging
        unit 10 {
            vlan-id 10;                 # Specify VLAN ID for this unit
        }
    }
}

2. VLAN Ranges (For trunk ports)

interfaces {
    ge-0/0/0 {
        vlan-tagging;
        unit 0 {
            vlan-range 10-20;           # Allow VLANs 10 through 20
            family ethernet-switching {
                interface-mode trunk;
            }
        }
    }
}

3. Native VLAN

interfaces {
    ge-0/0/0 {
        vlan-tagging;
        native-vlan-id 1;               # Untagged traffic belongs to VLAN 1
        unit 0 {
            family ethernet-switching {
                interface-mode trunk;
                vlan {
                    members [ 1-10 ];
                }
            }
        }
    }
}

MTU Configuration

MTU (Maximum Transmission Unit) defines the largest packet size that can be transmitted on an interface. The default MTU on most interfaces is 1500 bytes, but you can increase it for applications like jumbo frames.

Interface-Level MTU

interfaces {
    ae0 {
        mtu 9000;                       # Set MTU to 9000 bytes for jumbo frames
    }
}

MSS Configuration

MSS (Maximum Segment Size) defines the largest amount of data that a device can receive in a single TCP segment, For tunneled traffic, This is very useful and necessary.

MSS for an Interface

interfaces {
    ae0 {
        unit 0 {
            family inet {
                mtu 1500;
                tcp-mss 1500;
            }
            family inet6 {
                mtu 1500;
                tcp-mss 1500;
            }
        }
    }
}

Applying Groups to Interfaces

Groups allow you to apply a common configuration to multiple interfaces or units:

interfaces {
    ae0 {
        unit 10 {
            apply-groups sampling;      # Apply the "sampling" group configuration
        }
    }
}

This is particularly useful for consistent configurations across multiple interfaces, such as applying traffic sampling, security settings, or QoS parameters.

Complete Example

Here's a complete example combining the concepts discussed:

interfaces {
    xe-0/0/12 {
        gigether-options {
            802.3ad ae0;
        }
    }
    xe-0/0/13 {
        gigether-options {
            802.3ad ae0;
        }
    }
    ae0 {
        vlan-tagging;
        mtu 9000;
        aggregated-ether-options {
            lacp {
                active;
                periodic fast;
            }
        }
        unit 10 {
            apply-groups sampling;
            description "Peering: Internet Exchange";
            vlan-id 10;
            family inet {
                address 192.0.2.127/24;
            }
            family inet6 {
                address 2001:db8::127/64;
            }
        }
    }
}

Verification Commands

To verify your interface configuration, use these commands:

show interfaces ae0 extensive
show interfaces ae0.10
show lacp interfaces ae0
show interfaces terse | match ae0

Best Practices

1. Documentation: Always use descriptive names and include descriptions

2. MTU Consistency: Ensure MTU is consistent across physical interfaces in an aggregated bundle

3. LACP Configuration: Use active mode on both ends for fastest failover detection

4. Interface Ranges: For similar configurations on multiple interfaces, use interface ranges

5. Commit Confirmed: When making interface changes remotely, use "commit confirmed" to prevent lockouts