11

I've encountered a problem on a cisco ws-c3750x-48ts-s where jumbo-frames won't cross a trunk-connection between a vwmare esx-host and the switch.

The access ports in the same vlan don't experience any problems.

I suspect that the problem lies with the mtu for the SVI for that vlan since it's derived from the system mtu value and not the system-mtu jumbo command :

*<hostname>*#show vlan mtu

VLAN    SVI_MTU    MinMTU(port)      MaxMTU(port)     MTU_Mismatch
---- ------------- ----------------  ---------------  ------------
1    1500          9198              9198              No
10   1500          9198              9198              No
20   1500          9198              9198              No
30   1500          9198              9198              No
31   1500          9198              9198              No
150  1500          9198              9198              No
151  1500          9198              9198              No
200  1500          9198              9198              No
250  1500          9198              9198              No
255  1500          9198              9198              No
444  1500          9198              9198              No

*<hostname>*#show interfaces vlan 150
Vlan150 is up, line protocol is up
  Hardware is EtherSVI, address is c89c.1d49.b245 (bia c89c.1d49.b245)
  Description: iSCSI
  Internet address is 10.150.7.254/21
  MTU 1500 bytes, BW 1000000 Kbit, DLY 10 usec,
     reliability 255/255, txload 1/255, rxload 1/255

Is there a way to increase the SVI MTU to 9198, since system mtu has a max-value of 1998?

  • Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer. – Ron Maupin Aug 8 '17 at 9:20
15

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.

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0

Try system mtu routing. See the link for caveats that may apply to your situation.

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