Cisco 3560G MTU options

ciscocisco-catalystmtu

Is there any way to limit the MTU per L2 interface on this switch if you configure the system MTU for Jumbo frames?

Next question assuming no, are there any Cisco stackable switches with this capability?

Best Answer

Catalyst 3750/3560 Series switches support an MTU of 1998 bytes for all 10/100 interfaces. All Gigabit Ethernet interfaces support jumbo frames up to 9000 bytes. The default MTU and jumbo frame size is 1500 bytes. You cannot change the MTU on an individual interface. You must set the MTU globally. Reset the switch afterwards for the MTU change to take effect. Gigabit Ethernet ports are not affected by the system mtu command; 10/100 ports are not affected by the system mtu jumbo command. If you do not configure the system mtu jumbo command, the setting of the system mtu command applies to all Gigabit Ethernet interfaces. If Gigabit Ethernet interfaces are configured to accept frames greater than the 10/100 interfaces, jumbo frames that ingress on a Gigabit Ethernet interface and egress on a 10/100 interface are dropped.

I'm not aware of any Cisco switches that support a per L2 interface MTU size. The Nexus class switches can use class of service, So in order to do increased MTU for your iSCSI traffic, you should create a policy-map identifying your iSCSI traffic, place the traffic in a separate queue with increased MTU.

Related Solutions

Cisco – catalyst 3750x – jumbo frames on svi

From the configuration guide:

Frames sizes that can be received by the switch CPU are limited to 1998 bytes, no matter what value was entered with the system mtu or system mtu jumbo commands. Although frames that are forwarded or routed are typically not received by the CPU, in some cases packets are sent to the CPU, such as traffic sent to control traffic, SNMP, Telnet, or routing protocols.

But if it's received over a trunk it shouldn't be routed in the first place and even if it was the jumbo MTU should still apply.

Routed packets are subjected to MTU checks on the output ports. The MTU value used for routed ports is derived from the applied system mtu value (not the system mtu jumbo value). That is, the routed MTU is never greater than the system MTU for any VLAN. The routing protocols use the system MTU value when negotiating adjacencies and the MTU of the link. For example, the Open Shortest Path First (OSPF) protocol uses this MTU value before setting up an adjacency with a peer router. To view the MTU value for routed packets for a specific VLAN, use the show platform port-asic mvid privileged EXEC command.

Can you try this command:

show platform port-asic mvid

So it seems 1998 is max on SVI.

Ethernet – Internet IP MTU must not exceed 1500 bytes

There is a very important distinction between the Internet, and your own LAN, (or even WAN circuits.)

On the Internet, 1500 is the max MTU. (End of discussion.)

On your own LAN, (or in some cases WAN too) you can do whatever you want. As technologies expanded, 1500 MTU was no longer needed. So things like jumbo frames came into use.

Best Answer

Related Solutions

Cisco – catalyst 3750x – jumbo frames on svi

Ethernet – Internet IP MTU must not exceed 1500 bytes

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