Hi,
GK
RAS
Cluster1 H225/H245 Cluster2
GK will use RAS messages before proceed with the call.
1. Phone1 in CCM1 dials a Phone2 in CCM2
2. CCM1 sends an ARQ to GK
3. GK sends an ACF with CCM2 IPaddr to CCM1
4. CCM1 sends a H225 message to CCM2
5. CCM2 sends a ARQ to GK
6. GK sends an ACF to CCM2 (GK check available resources, if succesful sends the ACF, if not the ARJ)
7. CCM2 should proceed H225
8. CCM1|CCM2 interchange H245
9. Phone1 and Phone2 are connected now.
You may use Route Lists (Including PSTN GWs and ICT) in CCM to use alternate routes in case of WAN failures.
http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_implementation_design_guide_chapter09186a0080447501.html#wp1043988
HTH
//Gonzalo