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Community Member

Ping/ICMP and Priority

If I run pings across the WAN from device to device, is it true that when routers or switches get busy, they drop pings since they are low priority?

Our network does not run QOS or any form of it. I'm running pings from workstations, switches, routers, and/or applications such as PingPlotter.


Re: Ping/ICMP and Priority

Yes, that is correct. In routers and switches there is a slow path (process switching), and a fast path (CEF or MLS). The fast path is taken when packets entering the router need to be switched out another interface. These packets are interrupt-switched (with CEF on routers), or hardware-switched (MLS on switches).

When you send a ping to an IP address belonging to the router, it must be process switched (slow path), and it needs to wait for the process scheduler to allocate CPU resources to send a reply. If the CPU is busy doing other things, this reply can be delayed, or even lost.

Best practice for testing WAN links (or any links) is to pass the traffic between workstations, so that the traffic takes the fast-path.

Community Member

Re: Ping/ICMP and Priority

If I understand you correctly, ping tests from workstations should be treated like all other traffic, right?

Also, if I send multiple pings from my workstation to a router interface (ex: s/0, fe1/0, etc.), will this be handled as a slow path process?

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