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.
-- Devon
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