Why you should change the ISIS Redistribution Metric in a Cisco ACI fabric
Today, during the upgrade preparation for a customer, I ran the Cisco ACI upgrade validation/precheck script and one of the things that alerted was the ISIS Redistribution Metric for MPod/MSite.
The Link leads to the Cisco ACI Best Practices Quick Summary Guide, which provides some details about the ISIS Redistribution Metric setting and why it’s important.
Details about the ISIS Redistribution Metric
The ISIS Redistribution Metric defines with which value Cisco ACI redistributes routes from other routing protocols (e.g. OSPF for Multipod).
In a Cisco ACI Multi-Pod deployment, the ISIS Redistribution Metric is the metric set for Cisco ACI infra TEP routes when spine nodes redistribute these routes from a routing protocol (such as OSPF) into ISIS. These redistributed ISIS routes are advertised to leaf nodes in the same pod so that those can reach to the other pod through the spine nodes.
During normal operation this is not an issue, but when upgrading the ACI fabric, it can lead to some longer convergence times.
When a spine node reboots or newly joins a fabric, until the spine node stabilizes and completes the policy download from the Cisco APIC, the node tries to advertise ISIS redistributed routes with the higher metric. This is known as “overload mode.” If the ISIS Redistribution Metric is kept at the default value of 63, which is the maximum, the overload functionality is ineffective, since the metric for overload and non-overload is the same. This results in potential longer convergence times after a spine node reboots in a Cisco ACI Multi-Pod setup. By lowering the value, leaf nodes can prefer other stable spine nodes to reach the other pods.
Recommendation for the ISIS Redistribution Metric
Cisco recommends as best practice to set a lower value than 63, which is the default.
The best practice is set this metric to 62 or lower as opposed to the maximum 63, which is the default.
How to change the ISIS Redistribution Metric?
Version lower than 5.0(1)
Go to Fabric -> Fabric Policies -> Policies -> Pod -> ISIS Policy Default -> ISIS metric for redistributed routes
According to Cisco this is non-disruptive.
Version higher than 5.0(1)
System -> System Settings -> ISIS Policy -> ISIS metric for redistributed routes
According to Cisco this is non-disruptive.