[c-nsp] GRE packets drop

[Alex Wa]

Thanks Paul,
   
  We're using a management tool that sends an icmp echo request every 10 seconds, through the tunnel and outside the tunnel. the source address of packet not going through the tunnel is the same as the tunnel interface. I've been trying to see any pattern in the packet loss but didn't find anything that should be considered definitive or conclusive, but noted though, that after midnight there is a slight decrease in the packet loss, which increase later at morning hours. CPU and memory usage is normal, as OSPF timers (default). We've instructed the IT staff at remote side to gather more information from their ISP to try to figure out what's going on. 
   
  alejandro


Paul Cosgrove <paul.cosgrove at heanet.ie> wrote:
  Hi Alex,

You mentioned changing the MTU on one interface, but to get a full 
adjacency this would of course need to on both routers (or set OSPF to 
ignore MTU values). I guess this is what you meant.
Intermittent loss of small packets does not sound like an MTU problem 
though.

When you ping across without using the tunnel, are you using the source 
and destination IP addresses which are configured on the tunnel? If 
parallel links and load sharing are used anywhere along the path the 
src/dst ip details may affect which link your pings traverse (and one 
could be dropping packets).

If a hundred or so two hundred byte test pings do not show any errors, 
and if there is nothing unusual about the tunnel routers (e.g. high cpu, 
very low ospf timers or uplinks exceeding their provisioned rate), then 
it would be worth checking if the problem occurs any more frequently 
during working hours. There might be a QoS misconfiguration somewhere 
along the transit path.

Are you using any form of encryption on this traffic? Something to 
consider once the packet loss is resolved perhaps.

Paul.

Alex Wa wrote:
> Hi,
> 
> We're having a weird problem in one of our remote offices, connected through a GRE tunnel (using internet). The remote ISP, aparently is dropping randomly (or selectively) some 
> GRE packets. when we ping the remote physical interface through the tunnel some packet drops are observed, but if icmp packets don't get into the tunnel they are never dropped. this is causing the ospf adjacencies to go down for no reason and is giving us a hard time. We've changed the tunel interface MTU to 1400, but it didn't work. besides OSPF's hello are less than a hundred octets, and i don't see a reason for the ISP dropping so small packets. The interface connected to the provider is a Gigabit Ethernet. Any suggestion to avoid this behaviour? any hint?
> 
> Alejandro Wainshtok
> Network Engineer
> CCNP
> 
>
> 
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