Migrating a configuration between vendors

Replacing a device from one vendor with another invites a line-by-line translation. That approach mostly works, and the places it does not are the places that hurt, because they are silent.

Translate intent, not syntax

Start by writing down what the device does in plain language: it terminates these VLANs, routes between them, filters this traffic, connects to these neighbours, and provides these services. Then configure the new device to do that.

A line-by-line translation carries across every accumulated workaround, including the ones that existed only because of the old platform’s quirks.

Defaults differ, silently

This is the main hazard. A behaviour that was implicit on the old platform may need to be explicit on the new one, and a configuration that does not mention it will simply behave differently.

Spanning tree mode and default priorities. Whether an interface is routed or switched by default. Default administrative distances. Whether an access list ends in an implicit deny. Timers. Whether the default is to forward or to drop.

The dangerous ones are permissive: a device that forwards something the old one blocked, because the rule that blocked it was implicit.

Check the feature exists, not just the keyword

Similar names do not guarantee identical behaviour. Verify the implementation, particularly for anything involving timers, hashing or load distribution, where two vendors can both be standards-compliant and produce different results.

Test what the users do

Build a test list from the traffic that actually matters — these applications, from these subnets, to these destinations, including the ones that only run at month end. Run it before the change on the old device and after on the new. Differences are the migration’s real output.

Keep the old device

Do not wipe it. Rack it, powered off, cabled and ready, for at least a few weeks. The fastest rollback available is powering the old one back on, and the problems that justify a rollback tend to surface days later, not during the window.