#20266: when using ppp and configuring an 'ethernet' alias for a wan interface
the
mtu is incorrectly applied.
----------------------------------------+----------------------------------
Reporter: anonymous | Owner: developers
Type: defect | Status: new
Priority: high | Milestone: Chaos Calmer (trunk)
Component: packages | Version: Trunk
Keywords: ppp ipv6 modem pppoe pppoa |
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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.
--
Ticket URL: <https://dev.openwrt.org/ticket/20266>
OpenWrt <http://openwrt.org>
Opensource Wireless Router Technology
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