I have a L2 port-channel, up and running, between two Cisco switches 3560 and 3750E. Traffic is load-balanced fine by src-dest-ip, but I have one problem: I can't ping the radio links connected to each one of the ports of the bundle from my NMS server. I am sure that this is a problem related to arp-mac address tables and load-balanced traffic. The MAC table entries are correct to the portchannel interface.

Any idea to resolve the connectivity issue? I can't separate the two VLANs passing through the portchannel, and static mac assignation to the portchannel interface doesn't work.

The devices sometimes respond to ping and sometimes not, and it depends on the device from which I am pinging.

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1 Answer 1


You are running into a problem with the port-channel hash algorithm. To the switch, the port-channel is now the interface, not the physical interfaces, and which physical interface is used is up to the hash algorithm. The algorithm you are using is apparently giving you a different interface at different times. You probably use one that takes into account the source address, too.

Also, the algorithm may cause you to need to change the link where a radio connects if the hash is going to send the traffic for a MAC address out the opposite link, or you may need to change the MAC addresses on your radios.

You can try to change the algorithm to see if that fixes your problem.

Understanding EtherChannel Load Balancing and Redundancy on Catalyst Switches:

Catalyst 3750/3560

The Catalyst 3750/3560 series switch can support up to eight compatibly configured Ethernet interfaces in an EtherChannel. The EtherChannel provides full-duplex bandwidth up to 800 Mbps (Fast EtherChannel) or 8 Gbps (Gigabit EtherChannel) between your switch and another switch or host. With Cisco IOS Software Release 12.2(20)SE and earlier, the number of EtherChannels has a limit of 12. With Cisco IOS Software Release 12.2(25)SE and later, the number of EtherChannels has a limit of 48.

EtherChannel balances the traffic load across the links in a channel through the reduction of part of the binary pattern that the addresses in the frame form to a numerical value that selects one of the links in the channel. EtherChannel load balancing can use MAC addresses or IP addresses, source or destination addresses, or both source and destination addresses. The mode applies to all EtherChannels that are configured on the switch. You configure the load balancing and forwarding method with use of the port-channel load-balance {dst-ip | dst-mac | src-dst-ip | src-dst-mac | src-ip | src-mac} global configuration command.

You can find out which interface is used in the EtherChannel to forward traffic based on the load balancing method. The command for this determination is test etherchannel load-balance interface port-channel number {ip | mac} [source_ip_add | source_mac_add] [dest_ip_add | dest_mac_add].

While what you are doing may work for the traffic, it is a very bad idea. Trying to run a port channel across links that changes speed on the link can cause problems.

  • Thanks for your answer. I have changed hash algorithm and dst-mac I resolved 3/4 devices connectivity without passing all the traffic for one link as i need due to bw limitations. I figure that if i would change the mac address of this remaining device i would solve it. I know it is not a good idea...at least the radios are similar and very stable...
    – RS1983
    Jan 27, 2017 at 10:51

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