Problems with FreeBSD (amd64 stable/11) router

[Chris Ross]

> On Dec 6, 2016, at 09:34, Ryan Stone <rysto32 at gmail.com> wrote:
> 
> Let me confirm I understand what's happening:
> 
> 1) You want to use your router to vlan-tag traffic from your network, and then send it out of a lagg over bce interfaces.  The bxe interfaces have their MTU set to 1500 and the vlan interface to 1496

  I believe this is correct.  All traffic is using vlan interfaces, including the external network connection.  But they are all over a lagg on two bce’s.

> 2) The TiVo is sending packets with a payload size of 1500 and the DF bit set.
> 
> If this is the case, then the problem is simply that when the packets are passed through the vlan interface, the payload of the packets exceeds the MTU, but as the DF bit is set, the packets cannot be fragmented.  Your choices are either to use a 1500 byte MTU on the vlan interface (assuming that the network that you are routing to can accept 1518 byte packets), or only advertise a 1496 byte MTU in your internal network.

  Perhaps I misunderstood, but I thought that the router should send an ICMP in this case (that it cannot fragment the packets due to the DF bit), which would then cause the TiVo to send smaller packets.  But passing that detail for now;

  You mention “only advertise a 1496 byte MTU in [my] internal network.”  I tried doing this by setting an “interface-mtu” option in the DHCP response to the device, but it didn’t obey that option.  Do you know of another way to “advertise” MTU’s on the internal network?

  You also mention using a higher MTU on the network.  I hadn’t thought of this, but presume it would work.  I would only need support for that MTU on the bce’s, and in the ethernet switches, correct?  The ethernet switches I have are Dell PowerConnect 2724 and 2824 switches, which claim to support jumbo frames.  I’ll have to find out if I have to _do_ anything to support that, but it should work.  Thanks for the suggestion, I’ll look into that…

                                - Chris

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