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Loose source record routing (LSRR) can be used to ensure measurements for a given site are directed down the appropriate site-links for that site. The underlying network must explicitly allow LSRR packets for this technique to work. Once a target has been identified, the active probes need t be directed down the different site-links for that site. Each active probe can be sent with a different IP address in the LSRR field. When the network attempts to route an active probe toward a given site, it can use the IP address specified in the LSRR field to determine which site-link should be used.
Active measurement uses the next-hop specified per link in a link-group to set the LSRR field. For example, suppose a VPN included two sites, Chicago and Seattle, each with tunnels over three providers, UUNET, AT&T, and Sprint. The basic configuration would look like this:
module engine
link-group chicago-site
source-route active-measurement
link uunet next-hop 10.0.1.1
link att next-hop 10.0.1.2
link sprint next-hop 10.0.1.3
end
link-group seattle-site
source-route active-measurement
link uunet next-hop 10.0.2.1
link att next-hop 10.0.2.2
link sprint next-hop 10.0.2.3
end
active-measurement group chicago-targets
use link-group chicago-site
target 10.0.1.100 icmp 1 per-second
end
active-measurement group seattle-targets
use link-group seattle-site
target 10.0.2.100 icmp 1 per second
end
end
Whenever a probe is sent toward 10.0.1.100, in the chicago-targets group, the probes will be sent with the LSRR values specified by the next-hop link options in the chicago-site link-group. For example, an active probe that is meant to be sent down the UUNET tunnel toward Chicago will have an LSRR value of 10.0.1.1. The underlying network routing needs to be configured to ensure routing toward 10.0.1.1 will result in choosing the UUNET tunnel toward Chicago. Consult your router vendor’s documentation for information on configuring LSRR properly.
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