[AusNOG] Cisco GRE Tunnel weirdness

joe at apcs.com.au joe at apcs.com.au
Fri Jan 3 21:14:24 EST 2014

  Thanks for the info. Some of the network in the middle does appear to 
be Telstra related, but not directly in their xDSL/EU side of things. 
Could be a coincidence but helpful none-the-less.

  Traffic is roughly 100-130mbit during peak times. Both devices are SNMP 
monitored, CPU stayed steady before and during the problems (well, the 
CPU dropped in a similar ratio to the bandwidth throughput while the 
problem was occurring), and this was both ends. show proc cpu, I have to 
admit I only look at the summary at the top to get overall CPU usage, 
which matched our SNMP monitoring indicating only 10-15% load during the 

  Tony - if it is not too much trouble, can you shoot me a message when 
you see your problems disappear? Coming into the weekend we are OK with 
the backup path as it is not as mission critical (thank god for 
redundancies) so we will be researching and attempting a few things over 
the weekend. Would just be good to know when those problems subside, see 
if it coincides with our problems.


On 2014-01-03 18:19, Tony wrote:
> Seemingly unrelated, but today two separate customer issues that have
> come to me:
> 1. A 2M p2p Telstra service where usable MTU across the link has
> dropped overnight to approx 200 bytes in size (from 1500). This caused
> OSPF to drop on the link and forced traffic onto a 3G backup service
> (fault logged with carrier)
> 2. Another customer reported that GRE has stopped working to their
> firewall that we provide IP connectivitiy to. Nothing changed by us,
> nothing changed by them, they are scheduling a reboot of the firewall.
> To top it off I've been having wierd issues on my link at home for the
> last couple of days (resold Telstra DSL port) and have just checked
> and for some reason the MTU on the virtual-access interface on our LNS
> for my service is not 1460 instead of the 1500 that it was previously.
> Setting "adjust-mss" on the LAN of my router has resolved my inability
> to access web stuff that was timing out, but leaves me no closer to
> knowing WHY the MTU has suddenly changed.
> Coincidence or conspiracy, who knows....
> As someone else said, try monitoring CPU & interface utilisation
> (graph via SNMP) to see whether that is taking a hit during your times
> of slowness. Is this a GRE tunnel over Internet or something else ?
> What speed ? 3945's are grunty enough to handle a fair amount of GRE
> traffic, but not if they are sustaining/filtering a DDOS attack at the
> same time.
> regards,
> Tony.
> -------------------------
>  FROM: "joe at apcs.com.au" <joe at apcs.com.au>
>  TO: ausnog at lists.ausnog.net
>  SENT: Friday, 3 January 2014 4:49 PM
>  SUBJECT: [AusNOG] Cisco GRE Tunnel weirdness
> Hi List,
>  I have a GRE tunnel between 2 sites over a link limited to 1500 MTU.
>  As such we have mtu set to 1440 and mss-adjust to 1400 on both ends.
> This is overly cautious probably but it was working.
>  Anyway - it had been working quite fine for some time, but randomly
> we
> started seeing massive performance issues. Bandwidth throughput halved
> and ping times sky rocketed (~50ms to ~1000ms). We tried bringing down
> the tunnel and back up, no luck, and even power cycled each end (Cisco
> 3945's), no luck.
>  We have confirmed that the config's had not been changed for weeks.
> Neither end had crashed and rebooted. The tunnel itself did not go
> down
> between 'working' and 'not working'. Performance and ping times via
> the
> tunnel endpoint address' is fine, proving (to me) that the networks
> between the 2 sites are not the issue, but the tunnel itself. No links
> are saturated, and CPU performance is quite tame (both before and
> during
> the issue)
>  For now we have gone back to backup path but I haven't been able to
> find similar problems online, and my own Cisco tunnel experience
> leaves
> me empty so far.
>  Has anyone experienced a similar issue? A working tunnel suddenly
> having major performance issues?
> Thanks,
> Joe
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