SR Linux interfaces

This chapter describes SR Linux interface types, subinterfaces, and support for Link Aggregation Groups (LAGs). See the ‟Interfaces” chapter in the SR Linux Configuration Basics Guide for configuration examples.

SR Linux interface types

On the SR Linux, an interface is any physical or logical port through which packets can be sent to or received from other devices.

The SR Linux supports the following interface types:

  • Loopback

    Loopback interfaces are virtual interfaces that are always up, providing a stable source or destination from which packets can always be originated or received. The SR Linux supports up to 256 loopback interfaces system-wide, across all network instances. Loopback interfaces are named loN, where N is 0 to 255.

  • System

    The system interface is a type of loopback interface that has characteristics that do not apply to regular loopback interfaces:

    • The system interface can be bound to the default network-instance only.

    • The system interface does not support multiple IPv4 addresses or multiple IPv6 addresses.

    • The system interface cannot be administratively disabled. When configured, it is always up.

    The SR Linux supports a single system interface named system0. When the system interface is bound to the default network-instance, and an IPv4 address is configured for it, the IPv4 address is the default local address for multi-hop BGP sessions to IPv4 neighbors established by the default network-instance, and it is the default IPv4 source address for IPv4 VXLAN tunnels established by the default network-instance. The same functionality applies with respect to IPv6 addresses / IPv6 BGP neighbors / IPv6 VXLAN tunnels.

  • Network

    Network interfaces carry transit traffic, as well as originate and terminate control plane traffic and in-band management traffic.

    The physical ports in line cards installed in the SR Linux are network interfaces. A typical line card has a number of front-panel cages, each accepting a pluggable transceiver. Each transceiver may support a single channel or multiple channels, supporting one Ethernet port or multiple Ethernet ports, depending on the transceiver type and its breakout options.

    In the SR Linux CLI, each network interface has a name that indicates its type and its location in the chassis. The location is specified with a combination of slot number and port number, using the following formats:


    For example, interface ethernet-2/1 refers to the line card in slot 2 of the SR Linux chassis, and port 1 on that line card.

    On 7220 IXR-D3 systems, the QSFP28 connector ports (ports 1/3-1/33) can operate in breakout mode. Each QSFP28 connector port operating in breakout mode can have four breakout ports configured, each operating at 25G. Breakout ports are named using the following format:


    For example, if interface ethernet 1/3 is enabled for breakout mode, its breakout ports are named as follows:

    • ethernet 1/3/1

    • ethernet 1/3/2

    • ethernet 1/3/3

    • ethernet 1/3/4

  • Management

    Management interfaces are used for out-of-band management traffic. The SR Linux supports a single management interface named mgmt0.

    The mgmt0 interface supports the same functionality and defaults as a network interface, except for the following:

    • Packets sent and received on the mgmt0 interface are processed completely in software.

    • The mgmt0 interface does not support multiple output queues, so there is no output traffic differentiation based on forwarding class.

    • The mgmt0 interface does not support pluggable optics. It is a fixed 10/100/1000-BaseT copper port.

  • Integrated Routing and Bridging (IRB)

    IRB interfaces enable inter-subnet forwarding. Network instances of type mac-vrf are associated with a network instance of type ip-vrf via an IRB interface.

LAG interfaces

A LAG, based on the IEEE 802.1ax standard (formerly 802.3ad), increases the bandwidth available between two network devices, depending on the number of links installed. A LAG also provides redundancy if one or more links participating in the LAG fail. All physical links in a LAG combine to form one logical interface.

LAGs can be either statically configured, or formed dynamically with Link Aggregation Control Protocol (LACP). Load sharing is executed in hardware, which provides line rate forwarding for all port types. A LAG consists of ports of the same speed.


On the SR Linux, each type of interface can be subdivided into one or more subinterfaces. A subinterface is a logical channel within its parent interface.

Traffic belonging to one subinterface can be distinguished from traffic belonging to other subinterfaces of the same port using encapsulation methods such as 802.1Q VLAN tags.

While each port can be considered a shared resource of the router that is usable by all network instances, a subinterface can only be associated with one network instance at a time. To move a subinterface from one network instance to another, you must disassociate it from the first network instance before associating it with the second network instance.

You can configure ACL policies to filter IPv4 and IPv6 packets entering or leaving a subinterface.

The SR Linux supports policies for assigning traffic on a subinterface to forwarding classes or remarking traffic at egress before it leaves the router. DSCP classifier policies map incoming packets to the appropriate forwarding classes, and DSCP rewrite-rule policies mark outgoing packets with an appropriate DSCP value based on the forwarding class.

DHCP relay

DHCP relay refers to the router's ability to act as an intermediary between DHCP clients requesting configuration parameters, such as a network address, and DHCP servers when the DHCP clients and DHCP servers are not attached to the same broadcast domain, or do not share the same IPv6 link (in the case of DHCPv6).

SR Linux supports DHCP relay for IRB subinterfaces and Layer 3 subinterfaces. The DHCP server network can be in the same IP-VRF network-instance of the Layer 3 subinterfaces that require DHCP relay, or it can be in a different IP-VRF network-instance or the default network instance.