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Opened 3 years ago

#20266 new defect

when using ppp and configuring an 'ethernet' alias for a wan interface the mtu is incorrectly applied.

Reported by: anonymous Owned by: developers
Priority: high Milestone: Chaos Calmer 15.05
Component: packages Version: Trunk
Keywords: ppp ipv6 modem pppoe pppoa Cc: 3

Description

In short if you follow http://wiki.openwrt.org/doc/howto/access.modem.through.nat for an adsl connection where the 'modem' interface is configured to be PPP and mtu fix is specified then the result is a situation where PPP sets the MTU for the 'modem' interface to the MTU of the PPPX interface.

That is,
you will see something like:

```
pppoe-wan Link encap:Point-to-Point Protocol  
          inet addr:151.xx.xx.175  P-t-P:151.xx.xx.72  Mask:255.255.255.255
          UP POINTOPOINT RUNNING NOARP MULTICAST  MTU:1492  Metric:1
   ...

'modem' is eth1
eth1      Link encap:Ethernet  HWaddr E8:DE:xx:xx:xx:14  
          UP BROADCAST RUNNING MULTICAST  MTU:1492  Metric:1
  ...

The issue is that because the ethernet interface on eth1 should have an mtu of 1500 (at least in my case) and PPP sets the mtu for the PPP interface and eth1 to 1492. This results in fragmentation because of the overhead of PPP(oe - in my case) will result in packets larger than the MTU of eth1/modem - this also breaks ipv6.

I was able to address this issue by configuring eth1/modem like so:

config interface modem
	option ifname eth1
	option proto static
	option ipaddr 169.254.1.1
	option netmask 255.255.255.0
        option mtu 1500

This results in an mtu of 1500 for the modem ethernet interface and allows PPP to operate without un-needed fragmentation.

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