Ping latency difference ?

[tedauction]

Hello, I notice that ping latency to my router and switch is slower (4-5ms) compared to ping latency to server behind the router and switch (1-2ms).

Can anyone explain why this might be the case ? Do servers respond faster to pings than routers/switches ?

Thank you.

[Julio E. Moisa]

Hi

First we can define the latency as the time used for a packet to traverse from a Point A to a Point B. Now we must remember that the ping is reciprocal, we sent an icmp packet and we wait for a response to be successful.

Usually the communication from a PC to its gateway will be really fast, with latency around 1ms. If you PC want to reach a server it will pass for a router or L3 devices that will make a routing process to select the best path to the server and once the path is selected, the packet will be sent to the server, this routing process will take a time. 

The latency could be affected by bad cabling, network cards, network congestion, old devices (router, switches, etc) over the path to the destination, etc.

4 to 5ms is acceptable. 

Hope it is useful

:-)

[lpassmore · Accepted solution]

Also remember that routers and switches are specifically tuned to pass traffic, not to respond quickly.  When the traffic is destined for the server, the router and the switch pass this in hardware.  The ping packets destined for the router or switch need to be passed up to that device's software layers which can often add a few ms especially if the device is busy.

[Joseph W. Doherty · Accepted solution]

Additionally, routers and switches generally treat ping responses as low priority task.  (Likely true of most hosts too.)

Some Cisco routers and switches support IP SLA responders, which will timestamp a ping response, in such a way, most of the internal processing latency can be discounted.  I.e. this feature allows for a much more precise measure of just network latency.