Cisco IP SLA UDP-Echo

According to Cisco documentation, IP SLA Responder is optional for the Cisco IP SLA UDP Echo operation. However, I experienced a different situation in my EVE-NG home lab.

Topology

I am using the same topology as in Cisco IP SLA track with Delay topic.

Cisco IP SLA UDP-Echo configuration

The basic configuration of Cisco IP SLA UDP-Echo is:

ip sla {SLA-OP-ID}

udp-echo {DST-IP} {DST-PORT} [source-ip ]

and its scheduling.

R2#sh ip sla config 1
IP SLAs Infrastructure Engine-III
Entry number: 1
Owner:
Tag:
Operation timeout (milliseconds): 5000
Type of operation to perform: udp-echo
Target address/Source address: 192.168.100.5/192.168.100.6
Target port/Source port: 5000/0
Type Of Service parameter: 0x0
Request size (ARR data portion): 16
Verify data: No
Data pattern:
Vrf Name:
Control Packets: enabled
Schedule:
   Operation frequency (seconds): 60  (not considered if randomly scheduled)
   Next Scheduled Start Time: Start Time already passed
   Group Scheduled : FALSE
   Randomly Scheduled : FALSE
   Life (seconds): 3600
   Entry Ageout (seconds): 3600
   Recurring (Starting Everyday): FALSE
   Status of entry (SNMP RowStatus): Active
Threshold (milliseconds): 5000
Distribution Statistics:
   Number of statistic hours kept: 2
   Number of statistic distribution buckets kept: 1
   Statistic distribution interval (milliseconds): 20
Enhanced History:
History Statistics:
   Number of history Lives kept: 0
   Number of history Buckets kept: 15
   History Filter Type: None

The result does not seem good at first.

R2#sh ip sla stat 1
IPSLAs Latest Operation Statistics

IPSLA operation id: 1
        Latest RTT: NoConnection/Busy/Timeout
Latest operation start time: 21:02:56 UTC Wed Nov 15 2017
Latest operation return code: No connection
Number of successes: 0
Number of failures: 4
Operation time to live: 3382 sec


R2#sh ip sla stat 1
IPSLAs Latest Operation Statistics

IPSLA operation id: 1
        Latest RTT: NoConnection/Busy/Timeout
Latest operation start time: 21:02:56 UTC Wed Nov 15 2017
Latest operation return code: No connection
Number of successes: 0
Number of failures: 4
Operation time to live: 3363 sec

In the debug messages the timeout error is clear.

R2#
Nov 15 21:04:01.655: IPSLA-OPER_TRACE:OPER:1 Timeout

Nov 15 21:04:01.655: IPSLA-OPER_TRACE:OPER:1 Ctrl msg: id=41, type=1, len=52, dest_ip=192.168.100.5, enablePort=5000, duration=5000

Nov 15 21:04:01.659: IPSLA-OPER_TRACE:OPER:1 src_ip=192.168.100.6, src_port=0

Nov 15 21:04:01.659: IPSLA-OPER_TRACE:OPER:1 table_id=0, topo_id=0 pktinfo_tableid = 0

And then I activated IP SLA Responder on the target Cisco device and it worked

R4(config)#ip sla responder
R4(config)#
R2#sh ip sla stat 1
IPSLAs Latest Operation Statistics

IPSLA operation id: 1
        Latest RTT: NoConnection/Busy/Timeout
Latest operation start time: 21:03:56 UTC Wed Nov 15 2017
Latest operation return code: No connection
Number of successes: 0
Number of failures: 5
Operation time to live: 3295 sec

R2#sh ip sla stat 1
IPSLAs Latest Operation Statistics

IPSLA operation id: 1
        Latest RTT: 20 milliseconds
Latest operation start time: 21:04:56 UTC Wed Nov 15 2017
Latest operation return code: OK
Number of successes: 1
Number of failures: 5
Operation time to live: 3285 sec
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