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Knowledge Base

Dynamic Branch to Branch tunnels fail to establish after a link flap. (75287)

Last Updated: 6/10/2020Categories: InformationalTotal Views: 8451Language:
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Symptoms

VMware SD-WAN Edge has two active ISP links connected. In this example, the links are connected on the GE4 and GE5 interfaces.

Dynamic Branch to Branch tunneling is enabled in the configuration.

Dynamic Branch to Branch tunnels are established from both links to another Velocloud Edge. Meaning that a tunnel is formed from both GE4 and GE5.

One link flaps (Due to ISP issues, problem in the configuration, physical issues, etc) while the remaining link stays online, and when the link comes back the dynamic tunnels do not rebuild across it.

Purpose

Understand the behavior for Edges with multiple links and Dynamic Branch to Branch tunnels.

Cause

The expected behavior when the circuit using the GE5 interface goes down and comes back up is for the Edge to rebuild all the tunnels towards the VMware SD-WAN Gateways. Meaning that the Edge already had a tunnel established with the Gateway's using it's GE4 link and after GE5 comes back up the Edge should establish another tunnel towards the Gateways using the GE5 link.

It is not mandatory for an Edge to establish a dynamic branch to branch over GE5 immediately if another path already exists between the two Edges. Dynamic Branch to Branch tunnels would only be established between branches when there is live traffic between those two Edges.

Resolution

When an Edge needs to establish a dynamic tunnel with another Edge for Branch-to-Branch VPN traffic, it tries to establish the paths through all the available WAN links.

But when GE5 goes down and comes back up, because a path already exists between the two Edges on GE4, Dynamic Branch to Branch path or tunnel establishment does not go into effect for GE5 and hence the Edge does not initiate another path from the GE5 link after it flapped.

Should the GE4 link also go down and come back up, then the Edge would attempt to build paths via both WAN interfaces. This is the expected behavior.

When Dynamic Branch to Branch tunneling needs to be established, it is attempted over all the available WAN interfaces. But when one or more paths already exist, the existing path(s) are used. Keep in mind that these are "dynamic" tunnels and not meant to stay up indefinitely. Dynamic tunnels remain up as long as they are needed and are torn down when there is no traffic flow between the Edges.

To check which tunnels are established from the Edge to the Gateways, please go to Remote Diagnostics > 'Edge Name' > List Paths:

Detectable by VMware SkylineTM

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