[Olsr-users] Routing problem with bad ETX

Simon Ebnicher (spam-protected)
Tue Apr 30 17:06:11 CEST 2013

we have set up a small OLSR mesh network with three routers (TP-Link
TL-WDR3600 running OpenWrt 12.09rc1, olsrd 0.6.3). We placed two routers
near to each other, and the third router at quite some distance.
R1 <----ETX:~1----> R2 <---------------ETX: 1.x to 2.x-----------> R3

The ETX on the direct link between R1 and R3 is usually below 2 (about
1.3 to 1.7), so the direct route is preferred. However, the direct route
is completely unusable for data transfer (e.g., ping reports >95% packet
loss). As soon as two hop route via R2 is used, ping and other traffic
works fine.
We are wondering why the unreliable link is rated too optimistically. We
suspect that the Hello and TC multicast packets (which are used to
calculate the ETX) may have a better reception probability than unicast
packets (e.g., because they are sent at lower rates, no RTS/CTS). This
would lead to ETX values that do not reflect the link accurately.

Can anyone confirm or explain this behaviour and maybe offer a solution?

Best regards,
Simon Ebnicher

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