Subinterfaces

Updated: April 24, 2026

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Explains subinterface concepts, covering their architecture, benefits, and use cases in routing environments.

Subinterfaces in Cisco Catalyst SD-WAN are logical interfaces that are configured on a physical interface to enable flexible network segmentation.

Interface speed

Interface speed is the rate at which data is transmitted over a physical or logical interface. When a Cisco IOS XE Catalyst SD-WAN device starts, the Cisco Catalyst SD-WAN software autodetects the SFPs present in the router and sets the interface speed accordingly. The software then negotiates the interface speed with the device at the remote end of the connection to establish the actual operating speed. For non-physical interfaces, such as those used for the system IP address and loopback interfaces, the interface speed defaults to 10 Mbps. In Cisco SD-WAN Controller and Cisco SD-WAN Manager systems, the initial interface speed is 1000 Mbps, with the operating speed negotiated with the remote device. The controller interface speed may vary depending on the virtualization platform, the NIC used, and the drivers present in the software

Interface MTU

MTU (Maximum Transmission Unit) is the largest size, in bytes, of a packet that can be sent on an interface without fragmentation. In Cisco Catalyst SD-WAN, by default, all interfaces have an MTU of 1500 bytes.

Here is the lookup table summarizing the MTU range for Cisco IOS XE Catalyst SD-WAN devices by release and interface type:


Release	Interface Type	MTU Range (bytes)
Cisco IOS XE Catalyst SD-WAN Release 17.4.1a and earlier		576 through 2000
Cisco IOS XE Catalyst SD-WAN Release 17.4.1a and later	1 GE interfaces	576 through 9216
Cisco IOS XE Catalyst SD-WAN Release 17.5.1a and later	10 GE and 100 GE interfaces	576 through 9216

For Cisco SD-WAN Validator, Cisco SD-WAN Manager, and Cisco SD-WAN Controllers, you can configure interfaces to use ICMP to perform path MTU (PMTU) discovery. When PMTU discovery is enabled, the device automatically negotiates the largest MTU size that the interface supports in an attempt to minimize or eliminate packet fragmentation.

On Cisco IOS XE Catalyst SD-WAN device, the Cisco Catalyst SD-WAN BFD software automatically performs PMTU discovery on each transport connection (that is, for each TLOC, or color). BFD PMTU discovery is enabled by default, and it is recommended that you use it and not disable it.

Note

BFD is a data plane protocol and so does not run on Cisco SD-WAN Validator, Cisco SD-WAN Manager, and Cisco SD-WAN Controllers.

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