finding right mtu size for sdwan

[hmc250000]

We are in the process of migrating our MPLS and DMVPN network to SDWAN. Our MPLS and DMVPN routers all had an mtu size of 1400 configured for the VPN tunnel interfaces.

Now we use the same transport networks for SDWAN, so the same MPLS and Internet circuits are being used for the SDWAN devices. We are just replacing the MPLS/DMVPN routers with SDWAN devices.

Can I assume we are supposed to configure similar mtu settings? Or better yet how do we go about finding the right mtu size for the SDWAN overlay tunnels?

[svemulap@cisco.com]

hi hmc250000 -

In SDWAN environment, BFD PMTU is used to determine PMTU between two SDWAN TLOCs (aka between two end nodes)
You can verify this via:

vEdge Platform: show interface (interface MTU and TCP MSS Adjust)
cEdge Platform: show sdwan tunnel statistics (tunnel-mtu and tcp-mss-adjust)

BFD PMTU discovery happens dynamically and current default timer is 20 min.
TCP-MSS is adjusted based on the PMTU discovery.

HTH.

[hmc250000]

Sorry I'm having a hard time understanding this. I still don't know how to troubleshoot mtu issues from the dashboard and find the right mtu size.

[svemulap@cisco.com · Accepted solution]

Not sure, what issues are you running into ?
Check the SD-WAN Design Guide on the basics of the solution and how it works.
https://www.cisco.com/c/en/us/td/docs/solutions/CVD/SDWAN/cisco-sdwan-design-guide.html?dtid=osscdc000283#SDWANRouting

Some other useful links:
https://www.cisco.com/c/en/us/support/docs/routers/sd-wan/214510-troubleshoot-bidirectional-forwarding-de.html
https://www.cisco.com/c/en/us/support/docs/routers/vedge-router/215359-bidirectional-forwarding-detection-flap.html

HTH