[c-nsp] Cisco IOS ping utility reports lower RTT than possible

[Martin T]

Hi,

I have a following very simple network topology:

CISCO1921[Gi0/0] <-> [eno3]svr

Gi0/0 in Cisco 1921 ISR has 10.66.66.2/24 configured and eno3 in Linux
server has 10.66.66.1/24 configured. RTT on this link is 10ms:

svr$ ping -qc 100 -I 10.66.66.1 10.66.66.2
PING 10.66.66.2 (10.66.66.2) from 10.66.66.1 : 56(84) bytes of data.

--- 10.66.66.2 ping statistics ---
100 packets transmitted, 100 received, 0% packet loss, time 99098ms
rtt min/avg/max/mdev = 10.317/10.411/11.089/0.199 ms
svr$

However, when I ping from the Cisco 1921 router, then some results are
less than 10ms:

CISCO1921#ping ip 10.66.66.1 source 10.66.66.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.66.66.1, timeout is 2 seconds:
Packet sent with a source address of 10.66.66.2
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 8/9/12 ms
CISCO1921#

The same behavior seems to be present when using IP SLA "icmp-echo"
probes: https://i.imgur.com/Uu5uHAh.png ..or CISCO-PING-MIB:
https://i.imgur.com/gG5L310.png On both graphs I have plotted 21600
results.

For comparison, I replaced Cisco 1921 router with Juniper MX series
router like this:

Juniper-MX[ge-3/3/9] <-> [eno3]svr

ge-3/3/9 in Juniper MX series router has 10.66.66.2/24 configured and
eno3 in Linux server has 10.66.66.1/24 configured. When I execute the
"ping 10.66.66.1 source 10.66.66.2 count 1000 rapid" command in
Juniper router, then the summary of the results is following:

--- 10.66.66.1 ping statistics ---
1000 packets transmitted, 1000 packets received, 0% packet loss
round-trip min/avg/max/stddev = 10.333/11.315/54.519/4.506 ms

As expected, all the results are >10ms.

Maybe someone (from Cisco) can explain how Cisco IOS handles the ICMP
"echo reply" messages?


thanks,
Martin