Making RSVP work over DVMPN
When using RSVP Call Admission Control (CAC) for VoIP, DMVPN and RSVP have limitations that prevent RSVP from working over DMVPN. If you have VoIP and you can’t use location based CAC, RSVP is the only answers. So what’s the problem with RSVP over DMVPN? The root of the problem is RSVP’s loop prevention mechanism. In this post I’ll describe an original solutions to make RSVP CAC work over DMVPN.
RSVP Split-Horizon on Tunnels
RSVP has a little known behavior used for loop prevention. It is similar to the split-horizon rule of many Distance Vector routing protocols and is described in RFC2205:
“[S]tate that is received through a particular interface must never be forwarded out the same interface.” [1]
When RSVP is set to be mandatory for call setup between two locations, RSVP has to successfully establish a reservation for each one way RTP audio stream. That reservation is done by voice gateways acting as the RSVP agents for IP phones. IP phones do not have RSVP running, but rely on voice gateways for that functionality.
Normally, when using DMVPN Phase 3, the initial packets sent between two spoke sites, match a route with the next-hop of the Continue reading