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A customer just upgraded aggregation layer switches and noticed traffic dropping for 120 seconds after the aggregation layer came online again. What is the best way to avoid having this traffic dropped given the topology below?
The reason is that the linkup delay timer is a feature that delays bringing downstream VSX links up, following a VSX device reboot or an ISL flap. The linkup delay timer has two phases: initial synchronization phase and link-up delay phase.
The initial synchronization phase is the download phase where the rebooted node learns all the LACP+MAC+ARP+STP database entries from its VSX peer through ISLP. The initial synchronization timer, which is not configurable, is the required time to download the database information from the peer.
The link-up delay phase is the duration for installing the downloaded entries to the ASIC, establishing router adjacencies with core nodes and learning upstream routes. The link-up delay timer default value is 180 seconds. Depending on the network size, ARP/routing tables size, you might be required to set the timer to a higher value (maximum 600 seconds).
When both VSX devices reboot, the link-up delay timer is not used.
Therefore, by configuring the linkup delay timer to include LAGs 101 and 102, which are part of the same VSX device as LAG 201, you can ensure that both devices have enough time to synchronize their databases and form routing adjacencies before bringing down their downstream links.