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Jumbo frames
The maximum transmission unit(MTU) is the maximum size IP frame the switch can receive for Layer 2 frames
inbound on a port. The switch drops any inbound frames larger than the MTU allowed on the port. Ports operating
at a minimum of 1 Gbps can accept forward frames of up to 9220 bytes (including four bytes for a VLAN tag)
when configured for jumbo traffic. You can enable inbound jumbo frames on a per-VLAN basis. That is, on a
VLAN configured for jumbo traffic, all ports belonging to that VLAN and operating at a minimum of 1 Gbps allow
inbound jumbo frames of up to 9220 bytes.
Operating rules
•
Required port speed: This feature allows inbound and outbound jumbo frames on ports operating at a
minimum of 1 Gbps.
•
GVRP operation: A VLAN enabled for jumbo traffic cannot be used to create a dynamic VLAN. A port
belonging to a statically configured, jumbo-enabled VLAN cannot join a dynamic VLAN.
•
Port adds and moves: If you add a port to a VLAN that is already configured for jumbo traffic, the switch
enables that port to receive jumbo traffic. If you remove a port from a jumbo-enabled VLAN, the switch
disables jumbo traffic capability on the port only if the port is not currently a member of another jumbo-enabled
VLAN. This same operation applies to port trunks.
•
Jumbo traffic sources: A port belonging to a jumbo-enabled VLAN can receive inbound jumbo frames
through any VLAN to which it belongs, including non-jumbo VLANs. For example, if VLAN 10 (without jumbos
enabled) and VLAN 20 (with jumbos enabled) are both configured on a switch, and port 1 belongs to both
VLANs, port 1 can receive jumbo traffic from devices on either VLAN. For a method to allow only some ports in
a VLAN to receive jumbo traffic, see Configuring a maximum frame size on page 177.
Jumbo traffic-handling
•
Configuring a voice VLAN to accept jumbo frames is not recommended. Voice VLAN frames are typically
small, and allowing a voice VLAN to accept jumbo frame traffic can degrade the voice transmission
performance.
•
You can configure the default, primary, and/or (if configured) the management VLAN to accept jumbo frames
on all ports belonging to the VLAN.
•
When the switch applies the default MTU (1522-bytes including 4 bytes for the VLAN tag) to a VLAN, all ports
in the VLAN can receive incoming frames of up to 1522 bytes. When the switch applies the jumbo MTU (9220
bytes including 4 bytes for the VLAN tag) to a VLAN, all ports in that VLAN can receive incoming frames of up
Chapter 6 Port Traffic Controls
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Troubleshooting
