Link redundancy in a cluster with LACP
A Citrix ADC cluster provides link redundancy for LACP to ensure that all nodes have the same partner key.
To understand the need for link redundancy, let us consider the example of the following cluster setup along with the accompanying cases (with attention to case 3):
In this setup, interfaces I1, I2, I3 and I4 are bound to LACP channel with KEY 5. On the partner side, I1 and I2 are connected to Switch 1 to form a single LA channel with KEY 1. Similarly, I3 and I4 are connected to Switch 2 to form a single LA channel with KEY 2.
Now let us consider the following cases to understand the need for link redundancy:
Case 1: Switch 1 is up and Switch 2 is down
In this case, cluster LA on both the nodes would stop receiving LACPDUs from Key2 and would start receiving LACPDUs from Key1. On both the nodes, cluster LA is connected to KEY 1 and I1 and I2 will be UP and channel on both the nodes would be UP.
Case 2: Switch1 goes down and Switch2 becomes UP
In this case, cluster LA on both the nodes would stop receiving LACPDUs from Key1 and would start receiving LACPDUs from Key2. On both the nodes, cluster LA is connected to Key2 and I3 and I4 will be UP and channel on both the nodes would be UP.
Case 3: Both Switch1 and Switch2 are UP
In this case, it is possible that cluster LA on node1 chooses Key1 as its partner and cluster LA on node2 chooses Key2 as its partner. This means that I1 on node1 and I4 on node2 are receiving traffic which is undesirable. This can happen because the LACP state machine is node-level and chooses its partners on first-come first-serve basis. To solve these concerns, link redundancy of dynamic cluster LA is supported. To configure link redundancy on a channel or interface, you must enable it and optionally specify the threshold throughput as follows: set channel CLA/1 -linkRedundancy ON -lrMinThroughput <positive_integer> The throughput of the partner channels is checked against the configured threshold throughput. The partner channel that satisfies the threshold throughput is selected in first-in-first-out (FIFO) manner. If none of the partner channel meets the threshold, or if threshold throughput is not configured, the partner channel with the maximum number of links is selected.
The threshold throughput can be configured from NetScaler 11 onwards.