Junos Basics – Aggregated Ethernet Interfaces (LACP)

In my previous Junos Basics post I covered configuring an 802.1Q Trunk between a Juniper EX2200C and a Cisco 2960S. This post will expand upon the previous one by bundling two interfaces together on each switch to form an aggregated link for the trunk.

There are a few proprietary standards for aggregating ethernet links, but Juniper uses the IEEE 802.3ad standard and Cisco can also be configured to use this. The 802.3ad standard is known as Link Aggregation Control Protocol (LACP). LACP can be configured in either Active or Passive mode – in Active mode a switch will always try and form an LACP link with the other side, and in Passive mode a switch will form an LACP link if the other side is in Active mode.


On the Cisco side, the config steps are very simple:

  • specify the interfaces to be aggregated
  • set the protocol to LACP
  • create a Channel Group and specify the LACP mode
  • set the Port Channel interface as a trunk
  • specify which VLAN’s are allowed over the trunk
Cisco2960S(config)#int range gi1/0/47-48
Cisco2960S(config-if-range)#channel-protocol lacp
Cisco2960S(config-if-range)#channel-group 1 mode passive
Cisco2960S(config)#interface po1
Cisco2960S(config-if)#switchport mode trunk
Cisco2960S(config-if)#switchport trunk allowed vlan 100,200

Onto the Juniper side, the first step is to specify the number of aggregated links on the switch:

rich@EX2200C# set chassis aggregated-devices ethernet device-count 1

Next, we have to remove the logical unit configuration from the interfaces that are to be bundled, as logical units are not allowed on aggregated links:

delete interfaces ge-0/1/1 unit 0
delete interfaces ge-0/1/0 unit 0

Next, set the interfaces to use LACP (802.3ad) and to be members of a logical aggregated ethernet port (ports begin with ae):

set interfaces ge-0/1/0 ether-options 802.3ad ae0
set interfaces ge-0/1/1 ether-options 802.3ad ae0

Then we need to set the LACP mode for our new aggregated interface. We’ll make the Juniper side Active, so that it initiates the transmissison of LACP packets:

set interfaces ae0 aggregated-ether-options lacp active

Finally, we need to set the aggregated link to be a trunk, and tell it which VLAN’s to trunk:

set interfaces ae0 unit 0 family ethernet-switching port-mode trunk
set interfaces ae0 unit 0 family ethernet-switching vlan members [SALES IT]

To verify our config, we’ll start on the Cisco side and check the Etherchannel summary:

Cisco2960S#show etherchannel summary
Flags:  D - down        P - bundled in port-channel
        I - stand-alone s - suspended
        H - Hot-standby (LACP only)
        R - Layer3      S - Layer2
        U - in use      f - failed to allocate aggregator

        M - not in use, minimum links not met
        u - unsuitable for bundling
        w - waiting to be aggregated
        d - default port

Number of channel-groups in use: 1
Number of aggregators:           1

Group  Port-channel  Protocol    Ports
1      Po1(SU)         LACP      Gi1/0/47(P) Gi1/0/48(P)

Then we can confirm the trunk config:

Cisco2960S#show interfaces trunk

Port        Mode             Encapsulation  Status        Native vlan
Po1         on               802.1q         trunking      1

Port        Vlans allowed on trunk
Po1         100,200

Port        Vlans allowed and active in management domain
Po1         100,200

Port        Vlans in spanning tree forwarding state and not pruned
Po1         100,200

And on the Juniper side:

rich@EX2200C> show lacp interfaces
Aggregated interface: ae0
    LACP state:       Role   Exp   Def  Dist  Col  Syn  Aggr  Timeout  Activity
      ge-0/1/0       Actor    No    No   Yes  Yes  Yes   Yes     Fast    Active
      ge-0/1/0     Partner    No    No   Yes  Yes  Yes   Yes     Slow   Passive
      ge-0/1/1       Actor    No    No   Yes  Yes  Yes   Yes     Fast    Active
      ge-0/1/1     Partner    No    No   Yes  Yes  Yes   Yes     Slow   Passive
    LACP protocol:        Receive State  Transmit State          Mux State
      ge-0/1/0                  Current   Slow periodic Collecting distributing
      ge-0/1/1                  Current   Slow periodic Collecting distributing

From the above output we can see that our individual interfaces are both Active, with the partner end Passive. For a detailed explanation of the output see this article from Juniper, but suffice to say the Mux State of Collecting and Distributing means the LACP protocol is working correctly.

We can also confirm the trunk is up and trunking for VLAN’s 100 and 200:

rich@EX2200C> show ethernet-switching interfaces
Interface    State  VLAN members        Tag   Tagging  Blocking
ae0.0        up     IT                  200   tagged   unblocked
                    SALES               100   tagged   unblocked

I hope this has been a useful explanation.  In my next Junos Basics post I’ll cover first hop redundancy using VRRP.

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