Management and Configuration Guide K/KA/KB.15.15

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.)
Table 23 Maximum frame sizes for mirroring
Frame mirrored to remote portFrame
mirrored to
local port
VLAN tagMaximum
frame size
Frame type configuration
IPv4 headerDataData
541464151801518Non-jumbo (default config.)Untagged
549162921609216Jumbo
1
on all VLANs
541464n/a
2
01518Jumbo
1
On all but source
VLAN
541468152241522Non-jumboTagged
549164921849220Jumbo
1
on all VLANs
541468n/a
2
41522Jumbo
1
On all but source
VLAN
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.
364 Monitoring and Analyzing Switch Operation