If the ISC link alone goes down when the remote peer is alive, both the MLAG peers forward the south-bound traffic, resulting in duplication of traffic. However, this does not result in traffic loops. This is because the remote node load shares to both the MLAG peers and does not forward the traffic received on one of the load shared member ports to other member ports of the same load shared group.
Starting in ExtremeXOS 15.5, health check messages can also be exchanged on an alternate path by separate configuration – typically the “Mgmt” VLAN (Virtual LAN). If the peer is alive when the ISC link alone goes down, one of the MLAG peers disables its MLAG ports to prevent duplicate south-bound traffic to the remote node. To reduce the amount of traffic on the alternate path, health check messages are initiated on the alternate path only when the ISC link goes down. When the ISC link is up, no health check messages are exchanged on the alternate path.
When the MLAG switch misses 3 consecutive health check messages from the peer, it declares that the MLAG peer is not reachable on the ISC link. It then starts sending out health check messages on the alternate path to check if the peer is alive. When the first health check message is received from the MLAG peer on the alternate path, it means that the peer is alive. In this scenario, one of the MLAG peers disables its MLAG ports to prevent duplication of south-bound traffic to the remote node.
Note
The MLAG switch having the lower IP address for the alternate path VLAN disables its ports.When the ISC link comes up and the switch starts receiving health check messages on the ISC control VLAN, the ports that were disabled earlier have to be re-enabled. This action is not performed immediately on the receipt of the first health check message on the ISC control VLAN. Instead the switch waits for 2 seconds before enabling the previously disabled ports. This is done to prevent frequent enabling and disabling of MLAG ports due to a faulty ISC link up event.
Each switch sends its MLAG peer information about the configuration and status of MLAGs that are currently configured over the ISC link.
This information is checkpointed over a TCP connection that is established between the MLAG peers after the keep-alive protocol has been bootstrapped.
The checkpoint messages exchanged between the MLAG peers over the TCP connection are sent in plain text and can be subjected to spoofing. Starting from EXOS 15.5 a provision is provided as part of this feature to secure the checkpoint connection against spoofing.
A key for authentication must be configured on both the MLAG peer switches. This key will be used in calculating the authentication digest for the TCP messages. TCP_MD5SIG socket option will be used for authentication and so only MD5 (Message-Digest algorithm 5) authentication is supported. The configured key will be used in setting up TCP_MD5SIG option on the checkpoint socket. The same key must be configured on both the MLAG peers. The checkpoint connection will not be established if different keys are configured on the MLAG peer switches.
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