# Message nd6_setmtu0: new link MTU on re0 (576) is too small for IPv6



## Kaminar (May 9, 2022)

These lines always appear in the log during boot

```
re0: link state changed to DOWN
re0: link state changed to UP
nd6_setmtu0: new link MTU on re0 (576) is too small for IPv6
re0: link state changed to DOWN
re0: link state changed to UP
```
These messages start to appear from FreeBSD 12.2-RELEASE.

I have an IPv4 network connection. It is set in the rc.conf file


> ifconfig_re0="SYNCDHCP"


Is there any problem with the network?


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## SirDice (May 9, 2022)

It's complaining the MTU you seem to get from DHCP is really small, too small for IPv6. But as you don't use IPv6 this shouldn't matter. That said, 576 is an awfully small MTU, even for IPv4. That can't be correct. 

When you have a connection what is the output of `ifconfig re0`? Mask out your IP if it's your actual internet address, I'm more interested in the other information it prints.


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## Kaminar (May 11, 2022)

```
$ ifconfig re0
re0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 576
    options=8209b<RXCSUM,TXCSUM,VLAN_MTU,VLAN_HWTAGGING,VLAN_HWCSUM,WOL_MAGIC,LINKSTATE>
    ether XX:XX:XX:XX:XX:XX
    inet XXX.XXX.XXX.XXX netmask 0xffffff00 broadcast 255.255.255.255
    media: Ethernet autoselect (100baseTX <full-duplex>)
    status: active
    nd6 options=29<PERFORMNUD,IFDISABLED,AUTO_LINKLOCAL>
```


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## SirDice (May 11, 2022)

Kaminar said:


> mtu 576


That seems like a really bad MTU. Normal ethernet has an MTU of 1500. Those low value MTUs were common during the dialup modem days. 576 is the smallest MTU you can set for IPv4. 

How is re0 connected? Is it going straight into your ISP's modem/router?

Edit: Found an old thread with the same issue: https://forums.freebsd.org/threads/my-isp-giving-a-mtu-of-576.67860/


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## Kaminar (May 11, 2022)

re0 is connected via a cable to my TP-LINK TL-R860 cable router. The router TL-R860 is connected to the ISP's router via a cable.

I have another PC with Ubuntu installed, which is connected to the same router via a cable. The command `ip addr` shows mtu 1500.


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