[outages] Level3 Chicago
tozz at tozz.net
Wed Aug 19 14:47:25 EDT 2009
Yes, and when pure prioritization is not available, ISPs
will use rate limiting too which is one of the ways
Level 3 restricts ICMP to the cpu on the core devices.
Also, the car device is an edge router so there could be
congestion on a customer port too when higher response times
are seen on the other side of a hop. Response times that settle
could be the control plane/data plane issue or could be once
traffic gets to a far end there's an asymetric path that goes
a different return path rather than back across the link seen on
the forward traceroute. All these are why simple pings and
traceroutes don't always tell the story.
* Devon True was thought to have said:
> 'Jeremy Chadwick' wrote:
> > That's an excellent question -- and one I've always wondered myself.
> > This is purely speculative, but I believe outbound ICMP (e.g. sent from
> > the router to whatever src solicited it) is what's de-prioritised.
> > Someone more familiar with Cisco and Juniper equipment might know for
> > certain.
> Usually packets destined to the control-plane of the system are
> prioritized based on criteria. It is better to let routing control
> protocols (e.g. ospf, bgp, isis) through first than someone pinging or
> running a traceroute. Packets *through* the router take the normal
> forwarding path and are not affected by this system.
> There may be system defaults based on hardware/software, but Cisco has
> CoPP and I believe Juniper uses a firewall on the lo0 interface (been a
> while since I touched one) for user-defined rules.
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