[c-nsp] C3750E routing jumbo frames

[Artyom Viklenko]

08.07.2010 01:09, David DeSimone пишет:
> I am curious if others have experienced this problem.
>
> We are setting up new solutions that require routing (not just
> switching) of jumbo frames between racks.  We'll set up a simple vlan
> extended across the core with 3750E's at the edges:
>
>      system mtu jumbo 9000
>      system mtu routing 9000
>
>      vlan 2242
>       mtu 9000
>
>      interface GigabitEthernet1/0/9
>       switchport trunk encapsulation dot1q
>       switchport trunk native vlan 2242
>       switchport trunk allowed vlan 1,2242
>       switchport mode trunk

Can you show output of 'sh int gi1/0/9'?
What MTU you see in it?
IMHO, you need mtu command in interface config.


>
>      (routing config not shown, simple OSPF setup)
>
> What I've seen a couple times now is that server admins report they
> cannot ping across to the other rack using jumbo packets.  Standard packet
> sizes are fine, just large packets fail to route.
>
> We waste some time verifying the router configuration and determine that
> it looks correct, then in desperation, we simply try re-issuing this
> command:
>
>      system mtu routing 9000
>
> Then suddenly the router "wakes up" and starts routing the jumbo frames
> correctly.  No other changes needed.  Before and after issuing the
> command, the MTU settings are the same:
>
>      # show system mtu
>
>      System MTU size is 1500 bytes
>      System Jumbo MTU size is 9000 bytes
>      Routing MTU size is 9000 bytes
>
> My only guess is that the jumbo MTU routing attribute somehow isn't
> applied to the VLAN when the router is initially configured, because
> there are currently no ports showing link in the VLAN, so the VLAN is
> invisibly "admin down" until a server is eventually plugged in.  It
> seems that I have to reissue the "mtu routing 9000" setting in order to
> get that VLAN to inherit the correct routing MTU setting.
>
> But I'm just guessing.  Anyone else seen this?
>


-- 
            Sincerely yours,
                             Artyom Viklenko.
-------------------------------------------------------
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