[cisco-voip] GK w/ MGCP/SRST Gateways - Toll Bypass Routing

Mike Brooks 2xccie2b at gmail.com
Wed Feb 25 14:37:30 EST 2009

Hi everyone,

I have a tough question for you guys.

Say you have a network in which you have multiple SRST offices that are
centrally managed by a a single cluster running CallManager 5.x.  Calls
within each office are obviously routed by the callmanager but between the
SRST offices the calls are routed by a centrally located gatekeeper.  Calls
to and from "other" clusters, "non-srst", are routed by the gatekeeper to
and from these SRST offices as well.

The issue is that the SRST offices are actually MGCP gateways not H323.
Therefore, instead of having connections between the SRST gateways and the
GK you have trunks between the GK and the SRST office.  Each trunk
represents an SRST office. The gatekeeper contains a seperate zone for each
SRST office.  Calls destined to other SRST offices are routed to the
gatekeeper.  The gatekeeper performs call routing based on the zone prefixes
and checks if enough bandwidth exists between SRST offices (trunks) to make
the call and routes it back to the SRST cluster identifying the appropriate
trunk.  If not enough bandwidth is not available then the GK rejects the
call and the call is routed out to the PSTN.

The problem is when an MGCP gateway goes in SRST mode the gatekeeper does
not know the office is not reachable.  The gatekeeper just knows the trunk
is still accessible which does not represent the status of the actually
gateway/office.  Therefore calls would be routed directly to Voicemail.
CFUR is not available on CM 5.x.  Therefore all offices (including
operations ;-) ) would not be able to call phones in that office at all when
that site is in SRST mode.

It would require manual intervention of someone going into the gatekeeper
and lowering the bandwidth statement for the zone that represents the SRST
office.  Then the gatekeeper would reject the call and the call would be
routed across the PSTN and into the office that is in SRST mode.

So my questions is ....  does anyone know of a dynamic way to have the
gatekeeper poll the MGCP gateway and if that MGCP gateway is not reachable
then it .... rejects the call  ... rather than forwarding to the gateways
associated trunk ... and directly to voicemail.   For example, if the MGCP
gateway is not reachable it lowers the bandwidth statement for the
associated trunk to 1.

Perhaps something to do with IPSLA....

I know its a complex situation, just wondering if anyone had "any"

Your input would be greatly appreciated.


Mike Brooks
CCIE# 16027 (R&S)
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