Monitor link between external two sites

[DeXTeR-ITA]

Hi All!
New achivement required from Zabbix, I try to explain my scenario.
I have to monitor a connection between two sites (and more), but my server is in another location.

Zabbix srv <--> VPN <--> SRV1 <--> VPN2 <--> SRV2
___________________________<--> VPN2 <--> SRV3
___________________________<--> VPN2 <--> SRV4 etc...

I can't install zabbix server in SRV1/2.. etc.., but zabbix agent is already installed. The router for vpn2 not my, so I can't configure for snmp or other.
I need to know if the link si up or down, but how I can do this?? Is possible??

Can someone point me in the right direction?

Sorry for my english,
Thanks in advance!

[Alexei]

You may use active checks on the agent. If you have no data received from the active check (agent.ping, for example) fire a trigger. The trigger will onviously use function nodata(N), where N is a number of seconds.

[DeXTeR-ITA]

I need some help again...

I don't understand how it work active check on the agent side, I know, I have to put one row in zabbix_agentd.conf. But what parameters??

UserParameter=agent.ping,IPADDRESS ???

I try to find in manual and forum, but I don't figure out.
Thank you again!