MGCP Switchover

• MGCP GW sends keepalives to primary CUCM every 15 sec using empty NTFY message
• CUCM acknowledges the message using 200OK
• If no-response is received within 30 sec, MGCP will consider CUCM dead and will try to switchover to next CUCM.
• In case secondary isn’t available, the gateway can fall back to using the default H.323 session application, if it is configured.

Note: The difference between switchover and fallback. Switchover between CUCM servers while fallback will take place when all CUCM servers aren't responding.

• If the Primary CUCM fails:
• D-channel will remain up since Q921 messages terminate on the gateway
• Backhaul will go down since Q931 messages terminate on CUCM
• During CUCM Switchover, calls will be treated as follow:
• Active calls will be preserved because D-Channel is up
• New calls will be rejected because backhaul is to CUCM is down
• Calls in Transition state will be cleared
• The messaging will be as follow:

• After NTFY messages being sent for 30 seconds without response, MGCP Gateway will send RSIP Graceful message to Primary Failed CUCM (without a response from CUCM)

Jan 27 21:49:11 MST: MGCP Packet sent to 10.170.4.11:2427--->

RSIP 329754022 *@kl2951-router.shelfdrilling.com MGCP 0.1

RM: graceful

<--- p="">

• MGCP Gateway will send RSIP Restart message to Secondary CUCM

Jan 27 21:49:11 MST: MGCP Packet sent to 10.170.4.10:2427--->

RSIP 329754024 *@kl2951-router.shelfdrilling.com MGCP 0.1

RM: restart

<--- p="">

• Secondary CUCM will respond with 200OK Acknowledgement confirming that it is alive

Jan 27 21:49:11 MST: MGCP Packet received from 10.170.4.10:2427--->

200 329754024

<--- p="">

• Secondary CUCM will send AUEP messages for each Voice Port or PRI Channel requesting X (RequestIdentifer), I (ConnectionId), A (Capabilities)

Jan 27 21:49:11 MST: MGCP Packet received from 10.170.4.10:2427--->

AUEP 760 S0/SU0/DS1-0/1@kl2951-router.shelfdrilling.com MGCP 0.1

F: X, A, I

<--- p="">

• MGCP Gateway will respond with 200OK acknowledgement providing the requested details.

Jan 27 21:49:11 MST: MGCP Packet sent to 10.170.4.10:2427--->

200 760

I: 6

X: 1

L: p:10-20, a:PCMU;PCMA;G.nX64, b:64, e:on, gc:1, s:on, t:10, r:g, nt:IN;LOCAL, v:T;G;D;L;H;R;ATM;SST;PRE

L: p:10-220, a:G.729;G.729a;G.729b, b:8, e:on, gc:1, s:on, t:10, r:g, nt:IN;LOCAL, v:T;G;D;L;H;R;ATM;SST;PRE

L: p:10-110, a:G.726-16;G.728, b:16, e:on, gc:1, s:on, t:10, r:g, nt:IN;LOCAL, v:T;G;D;L;H;R;ATM;SST;PRE

L: p:10-70, a:G.726-24, b:24, e:on, gc:1, s:on, t:10, r:g, nt:IN;LOCAL, v:T;G;D;L;H;R;ATM;SST;PRE

L: p:10-50, a:G.726-32, b:32, e:on, gc:1, s:on, t:10, r:g, nt:IN;LOCAL, v:T;G;D;L;H;R;ATM;SST;PRE

M: sendonly, recvonly, sendrecv, inactive, loopback, conttest, data, netwloop, netwtest

<--- p="">

• If there is no active call, I will be empty and X will be '0'.
• If there is an active call, I will point to ConnectionId of the call and X will point to RequestIdentifier of the call

• Once Secondary CUCM completes the query of AUEP, it will send AUCX message using I (ConnectionId) requesting for C (CallId) and M (ConnectionMode)

Jan 27 21:49:11 MST: MGCP Packet received from 10.170.4.10:2427--->

AUCX 790 S0/SU0/DS1-0/1@kl2951-router.shelfdrilling.com MGCP 0.1

I: 6

F: C, M

<--- p="">

• MGCP GW will acknowledge the message with 200OK and provide the details

Jan 27 21:49:11 MST: MGCP Packet sent to 10.170.4.10:2427--->

200 790

C: D000000002924996000000F500000001

M: sendrecv

<--- p="">

• Secondary CUCM will request MGCP Gateway to monitor DTMF Events using RQNT message. MGCP will acknowledge the request

Jan 27 21:49:12 MST: MGCP Packet received from 10.170.4.10:2427--->

RQNT 791 S0/SU0/DS1-0/1@kl2951-router.shelfdrilling.com MGCP 0.1

X: 1

R: R/iu

Q: process,loop

<--- p="">

Jan 27 21:49:12 MST: MGCP Packet sent to 10.170.4.10:2427--->

200 791 OK

<--- p="">

• Secondary CUCM will send Q.931 Status Enquiry message to the PRI device attached to the gateway to confirm the status of any calls CUCM believes are preserved.

Jan 27 21:49:12 MST: ISDN Se0/0/0:15 Q931: TX -> STATUS_ENQ pd = 8  callref = 0x0001

Jan 27 21:49:12 MST: ISDN Se0/0/0:15 Q931: RX <- pd="8<span" status="" style="mso-spacerun: yes;"> 

callref = 0x8001

        Cause i = 0x829E - Response to STATUS ENQUIRY or number unassigned

        Call State i = 0x0A

Note: You can't enable or disable MGCP call preservation at GW or CUCM level unlike H323.

Note the difference between SIP/H323 & MGCP. Here call preservation matters if its PRI or non-PRI endpoint because of backhaul concept. However, in SIP/H323, it won't matter since those protocols are independent.

• During fallback, MGCP gateway will keep probing CUCM servers and once any of them is restored, MGCP GW (in fallback mode) will Rehome to register with that CUCM.
• The message will be RSIP Graceful > RSIP Restart > 200OK > AUEP > 200OK
• There are multiple modes of switchback:
• Graceful (after last call completed) - Default
• Immediate (even with active calls)
• Never (disable switchback)
• Schedule (schedule switchback)
• Uptime (wait for x time of CUCM restoration before switchback. This is guard time to avoid flapping)