System information

10-26

Port Status and Basic Configuration

Jumbo Frames

Figure 10-13. Forwarding Jumbo frames Through Non-Jumbo Ports

Jumbo frames can also be forwarded out non-jumbo ports when the jumbo

frames received inbound on a jumbo-enabled VLAN are routed to another,

non-jumbo VLAN for outbound transmission on ports that have no mem-

berships in other, jumbo-capable VLANs. Where either of the above

scenarios is a possibility, the downstream device must be configured to

accept the jumbo traffic. Otherwise, this traffic will be dropped by the

downstream device.

Troubleshooting

A VLAN is configured to allow jumbo frames, but one or more ports

drops all inbound jumbo frames. The port may not be operating at 1 giga-

bit or higher. Regardless of a port’s configuration, if it is actually operating at

a speed lower than 1 gigabit, it drops inbound jumbo frames. For example, if

a port is configured for Auto mode (speed-duplex auto), but has negotiated a

100 Mbps speed with the device at the other end of the link, then the port

cannot receive inbound jumbo frames. To determine the actual operating

speed of one or more ports, view the Mode field in the output for the following

command:

show interfaces brief < port-list >

A non-jumbo port is generating “Excessive undersize/giant frames”

messages in the Event Log. The switch can transmit outbound jumbo traf-

fic on any port, regardless of whether the port belongs to a jumbo VLAN. In

this case, another port in the same VLAN may be jumbo-enabled through

membership in a different, jumbo-enabled VLAN, and may be forwarding

jumbo frames received on the jumbo VLAN to non-jumbo ports. Refer to

“Outbound Jumbo Traffic” on page 10-25.

Jumbo-Enabled VLAN

VLAN 10

Non-Jumbo VLAN

VLAN 20

Port 3 belongs to both VLAN 10 and VLAN 20.

Jumbo packets received inbound on port 3 can be

forwarded out the Non-Jumbo ports 4, 5, and 6.