Situation: two ethernet connections, between a switch and a server.
At the switch end, the two interfaces are configured for LACP.
At the server end, they're configured as normal ethernet interfaces.
How do the switch interfaces behave?
I believe that they'll behave as two normal ethernet interfaces and forward the traffic in parallel to the server as if they had no LACP config applied to them. The server will pick one interface and use it for outbound traffic.
The switch interfaces might be configured for LACP but until they form an LACP trunk with the server end interfaces, nothing happens to the normal traffic. In reality it's halving the bandwidth available between them because they're both forwarding the same traffic.
Is this correct please?

Edit: The interfaces are configured as outbound mirrored ports.

  • 3
    If one of the sides doesn't negotiate LACP, the port channel will not form and they will be just like normal interfaces
    – Alex
    Commented Feb 19, 2015 at 13:27

2 Answers 2


You didn't specify a manufacturer, I will assume Cisco switches though most other vendors should behave similarly.

If the channel group's mode is active, the interfaces will not forward traffic, since the switch will actively be trying to form a channel and if the channel negotiation fails, the port channel will be "down".

If the channel group's mode is passive or analog to passive, the interfaces will forward traffic normally and will listen for LACP/PaGP negotiations. This will bring the interfaces up and listen for traffic on the interfaces. If the switch sees these LACP packets from the host and a negotiation commences, the channel will be negotiated and packets will be forwarded over the port channel interface and not the individual interfaces.

From the server's perspective, at the IP layer, if the destination host is in the subnet defined by the network configuration on the interface, the server will attempt to ARP this address. If somehow both interfaces were connected to the same subnet (most OS will show a warning or disallow this behavior), they may ARP out both interfaces. So once the ARP is received on a given interface, the server will know to send the packets out this interface, and the switch will also identify which IP and MAC are tied to each individual interface (but the forwarding behavior will be controlled by the below).

If you are purely talking about Ethernet packets, and not IP, the server will forward the frames using whatever interface is specified. If I am not mistaken, in Linux the interface MUST be specified; in Windows it will probably use the interface with the highest (top) priority in the network interface bindings. This behavior varies OS to OS.

From the switch's perspective, the switch will flood frames with a MAC address out all interfaces until it learns which port a given MAC address is on. It will learn this port by listening for a frame with the source MAC address. So if 0111.2222.3333 is sending a frame to 0111.2222.3334, the switch will flood the frame out all ports in that VLAN

0111.2222.3333 (Fa0/1) -> 0111.2222.3334 will flood to all ports

Until it sees a reply

0111.2222.3334 (Fa0/2) -> 0111.2222.3333 (Fa0/1)

Then it will commence forwarding all traffic to the specific port that issued these frames.

There are a number of edge cases here that might bring more confusion such as the potential spanning tree interaction, but this covers the basics.

  • The network vendor is Juniper and the server is an application server, OS unknown at present, but thank for the extensive reply. I also forgot to mention a huge point, added above.
    – fixit9660
    Commented Feb 21, 2015 at 9:10
  • @fixit9660: In the mirrored case, the ports won't accept any traffic from the server at all. If the ports are promiscuous on the server, the server will listen to all traffic they hear on the interface including traffic for MAC addresses not including the server. Juniper doesn't support LAG on outbound mirrored ports in EX-series, if instead what you meant was that the inbound interface is a LAG, then I think the same information still applies. Probably best to withdraw this and ask an entirely new question.
    – Arima
    Commented Feb 22, 2015 at 0:31

It all depends on the mode of LACP you have configured on the switch. For eg, under the interfaces, you type 'channel-group 1 mode on/active/passive'

If you have selected 'active', then the switch will look for the other end of the connection to be either in the active or passive mode. If it detects either, then an LACP port-channel gets formed and traffic gets load-balanced among the ports. If not, they will be treated as regular ports and spanning-tree will kill one of the 2 ports.

However, if you have selected the mode 'on'(which you never should), then it will force a port-channel on the switch. If the other side does not form a port-channel, then there is possibility of a loop, which can cause a network meltdown.

  • 1
    On is not LACP.
    – cpt_fink
    Commented Feb 20, 2015 at 8:03

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