Link Aggregation¶

A link aggregate, or lag, allows multiple physical interfaces to be combined into a single logical interface, providing increased bandwidth (in some cases) and redundancy (primarily). Two modes of qualifying lag member ports are available:

static: Active members selected based on link status (carrier)
lacp: IEEE 802.3ad Link Aggregation Control Protocol

In LACP mode, LACPDUs are exchanged by the link partners to qualify each lag member, while in static mode only carrier is used. This additional exchange in LACP ensures traffic can be forwarded in both directions.

Traffic distribution, for both modes, across the active lag member ports is determined by the hash policy¹. It uses an XOR of the source, destination MAC addresses and the EtherType field. This, IEEE 802.3ad-compliant, algorithm will place all traffic to a particular network peer on the same link. Meaning there is no increased bandwidth for communication between two specific devices.

Tip

Similar to other interface types, naming your interface lagN, where N is a number, allows the CLI to automatically infer the interface type as LAG.

Basic Configuration¶

Creating a link aggregate interface and adding member ports:

admin@example:/> configure
admin@example:/config/> edit interface lag0
admin@example:/config/interface/lag0/> set lag mode static
admin@example:/config/interface/lag0/> end
admin@example:/config/> set interface eth7 lag-port lag lag0
admin@example:/config/> set interface eth8 lag-port lag lag0
admin@example:/config/> leave

A static lag responds only to link (carrier) changes of member ports. E.g., in this example egressing traffic is continuously distributed over the two links until link down on one link is detected, triggering all traffic to be steered to the sole remaining link.

LACP Configuration¶

LACP mode provides dynamic negotiation of the link aggregate. Key settings include:

admin@example:/> configure
admin@example:/config/> edit interface lag0
admin@example:/config/interface/lag0/> set lag mode lacp
admin@example:/config/interface/lag0/> set lag lacp mode passive
admin@example:/config/interface/lag0/> set lag lacp rate fast
admin@example:/config/interface/lag0/> set lag lacp system-priority 100

LACP mode supports two operational modes:

active: Initiates negotiation by sending LACPDUs (default)
passive: Waits for peer to initiate negotiation

Note

At least one end of the link must be in active mode for negotiation to occur.

The LACP rate setting controls protocol timing:

slow: LACPDUs sent every 30 seconds, with 90 second timeout (default)
fast: LACPDUs sent every second, with 3 second timeout

Link Flapping¶

To protect against link flapping, debounce timers can be configured to delay link qualification. Usually only the up delay is needed:

admin@example:/config/interface/lag0/lag/link-monitor/> edit debounce
admin@example:/config/interface/lag0/lag/link-monitor/debounce/> set up 500
admin@example:/config/interface/lag0/lag/link-monitor/debounce/> set down 200

Operational Status, Overview¶

Like other interfaces, link aggregates are also available in the general interfaces overview in the CLI admin-exec context. Here is the above static mode aggregate:

admin@example:/> show interfaces
INTERFACE       PROTOCOL   STATE       DATA                                    
lo              ethernet   UP          00:00:00:00:00:00
                ipv4                   127.0.0.1/8 (static)
                ipv6                   ::1/128 (static)
.
.
.
lag0            lag        UP          static: balance-xor, hash: layer2
│               ethernet   UP          00:a0:85:00:02:00
├ eth7          lag        ACTIVE
└ eth8          lag        ACTIVE

Same aggregate, but in LACP mode:

admin@example:/> show interfaces
INTERFACE       PROTOCOL   STATE       DATA                                    
lo              ethernet   UP          00:00:00:00:00:00
                ipv4                   127.0.0.1/8 (static)
                ipv6                   ::1/128 (static)
.
.
.
lag0            lag        UP          lacp: active, rate: fast (1s), hash: layer2
│               ethernet   UP          00:a0:85:00:02:00
├ eth7          lag        ACTIVE      active, short_timeout, aggregating, in_sync, collecting, distributing
└ eth8          lag        ACTIVE      active, short_timeout, aggregating, in_sync, collecting, distributing

Operational Status, Detail¶

In addition to basic status shown in the interface overview, detailed LAG status can be inspected:

admin@example:/> show interface lag0
name                : lag0
index               : 25
mtu                 : 1500
operational status  : up
physical address    : 00:a0:85:00:02:00
lag mode            : static
lag type            : balance-xor
lag hash            : layer2
link debounce up    : 0 msec
link debounce down  : 0 msec
ipv4 addresses      :
ipv6 addresses      :
in-octets           : 0
out-octets          : 2142

Same aggregate, but in LACP mode:

admin@example:/> show interface lag0
name                : lag0
index               : 24
mtu                 : 1500
operational status  : up
physical address    : 00:a0:85:00:02:00
lag mode            : lacp
lag hash            : layer2
lacp mode           : active
lacp rate           : fast (1s)
lacp aggregate id   : 1
lacp system priority: 65535
lacp actor key      : 9
lacp partner key    : 9
lacp partner mac    : 00:a0:85:00:03:00
link debounce up    : 0 msec
link debounce down  : 0 msec
ipv4 addresses      :
ipv6 addresses      :
in-octets           : 100892
out-octets          : 111776

Member ports provide additional status information:

Link failure counter: number of detected link failures
LACP state flags: various states of LACP negotiation:
active: port is actively sending LACPDUs
short_timeout: using fast rate (1s) vs. slow rate (30s)
aggregating: port is allowed to aggregate in this LAG
in_sync: port is synchronized with partner
collecting: port is allowed to receive traffic
distributing: port is allowed to send traffic
defaulted: using default partner info (partner not responding)
expired: partner info has expired (no LACPDUs received)
Aggregator ID: unique identifier for this LAG group
Actor state: LACP state flags for this port (local)
Partner state: LACP state flags from the remote port

Example member port status:

admin@example:/> show interface eth7
name                : eth7
index               : 8
mtu                 : 1500
operational status  : up
physical address    : 00:a0:85:00:02:00
lag member          : lag0
lag member state    : active
lacp aggregate id   : 1
lacp actor state    : active, short_timeout, aggregating, in_sync, collecting, distributing
lacp partner state  : active, short_timeout, aggregating, in_sync, collecting, distributing
link failure count  : 0
ipv4 addresses      :
ipv6 addresses      :
in-octets           : 473244
out-octets          : 499037

Example: Switch Uplink with LACP¶

LACP mode provides the most robust operation, automatically negotiating the link aggregate and detecting configuration mismatches.

A common use case is connecting a switch to an upstream device:

admin@example:/> configure
admin@example:/config/> edit interface lag0
admin@example:/config/interface/lag0/> set lag mode lacp

Enable fast LACP for quicker fail-over:

admin@example:/config/interface/lag0/> set lag lacp rate fast

Add uplink ports

admin@example:/config/interface/lag0/> end
admin@example:/config/> set interface eth7 lag-port lag lag0
admin@example:/config/> set interface eth8 lag-port lag lag0

Enable protection against "link flapping".

admin@example:/config/interface/lag0/> edit lag link-monitor
admin@example:/config/interface/lag0/lag/link-monitor/> edit debounce
admin@example:/config/interface/lag0/lag/link-monitor/debounce/> set up 500
admin@example:/config/interface/lag0/lag/link-monitor/debounce/> set down 200
admin@example:/config/interface/lag0/lag/link-monitor/debounce/> top

Add to bridge for switching

admin@example:/config/interface/lag0/lag/link-monitor/debounce/> end
admin@example:/config/> set interface lag0 bridge-port bridge br0
admin@example:/config/> leave

(source MAC XOR destination MAC XOR EtherType) MODULO num_links ↩