Maximum supported frame size

The IPv4 encapsulation of mirrored traffic adds a 54-byte header to each mirrored frame. If a resulting frame exceeds the MTU allowed in the network, the frame is dropped or truncated.

NOTE: Oversized mirroring frames are dropped or truncated, according to the setting of the [truncation] parameter in the mirror command. Also, remote mirroring does not allow downstream devices in a mirroring path to fragment mirrored frames.

If jumbo frames are enabled on the mirroring source switch, the mirroring destination switch and all downstream devices connecting the source switch to the mirroring destination must be configured to support jumbo frames.

Enabling jumbo frames to increase the mirroring path MTU

On 1-Gbps and 10-Gbps ports in the mirroring path, you can reduce the number of dropped frames by enabling jumbo frames on all intermediate switches and routers. (The MTU on the switches covered by this manual is 9220 bytes for frames having an 802.1Q VLAN tag, and 9216 bytes for untagged frames.)

Maximum frame sizes for mirroring

	Frame type configuration	Maximum frame size	VLAN tag	Frame mirrored to local port	Frame mirrored to remote port
	Frame type configuration	Maximum frame size	VLAN tag	Data	Data	IPv4 header
Untagged	Non-jumbo (default config.)	1518	0	1518	1464	54
	Jumbo^[1] on all VLANs	9216	0	9216	9162	54
	Jumbo^[1] On all but source VLAN	1518	0	n/a^[2]	1464	54
Tagged	Non-jumbo	1522	4	1522	1468	54
	Jumbo^[1] on all VLANs	9220	4	9218	9164	54
	Jumbo^[1] On all but source VLAN	1522	4	n/a^[2]	1468	54
^[1]Jumbo frames are allowed on ports operating at or above 1 Gbps ^[2]For local mirroring, a non-jumbo configuration on the source VLAN dictates an MTU of 1518 bytes for untagged frames, and an MTU of 1522 for tagged frames, regardless of the jumbo configuration on any other VLANs on the switch.

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