About ExpressRoute MTU PacketSize

Question

In my project, we plan to implement ExpressRoute.

• The configuration is a typical Private Peering configuration Connecting between Azure and on-premises via ExpressRoute

[Questions]

In the following Microsoft Express QA, the following articles related to ExpressRoute were mentioned.

I think both are about MTU packet size.

Which one should I refer to for the MTU packet size?

１．

https://learn.microsoft.com/en-us/azure/expressroute/expressroute-faqs#maximum-supported-limits

ExpressRoute supports a maximum TCP and UDP packet size of 1,400 bytes. Packet sizes larger than 1,400 bytes will get fragmented.

２．

https://learn.microsoft.com/en-us/azure/expressroute/expressroute-faqs#whats-the-maximum-mtu-supported

ExpressRoute supports the standard internet MTU of 1,500 bytes. See TCP/IP performance tuning for Azure VMs for tuning the MTU of your VMs.

Answer 1

Hello house

• MTU (Maximum Transmission Unit) refers to the maximum size of a packet that can be transmitted without fragmentation. The physical interface MTU on ExpressRoute is 1,500 bytes.
• However, effective payload size for TCP/UDP packets is 1,400 bytes due to overhead from headers (IP, TCP/UDP, etc.). This is why the first article mentions 1,400 bytes—it’s the maximum unfragmented payload size for TCP/UDP traffic.
• The second article is correct in stating that the MTU is 1,500 bytes, but it doesn't account for protocol overhead when discussing actual usable payload size.

The maximum transmission unit (MTU) is the largest size frame (packet plus network access headers) specified in bytes that can be sent over a network interface. The MTU is a configurable setting. The default MTU used on Azure VMs, and the default setting on most network devices globally, is 1,500 bytes.

fragmentation occurs when a packet is sent that exceeds the MTU of a network interface. The TCP/IP stack breaks the packet into smaller pieces (fragments) that conform to the interface's MTU. Fragmentation occurs at the IP layer and is independent of the underlying protocol (such as TCP). When a 2,000-byte packet is sent over a network interface with an MTU of 1,500, the packet is broken down into one 1,500-byte packet and one 500-byte packet.

Network devices in the path between a source and destination can either drop packets that exceed the MTU or fragment the packet into smaller pieces.

but use them for different purposes:

• Use the 1,500-byte MTU as the interface configuration baseline.
• Use the 1,400-byte TCP/UDP payload limit for application-level tuning to avoid fragmentation.

I hope this has been helpful!

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Answer 2

Hello Ganesh Patapati

It was very helpful. Thank you very much.