Router for SRST function only

[inhamit]

Hi, One of the our client want to use Different make WAN router (not cisco) for WAN connectivity. At the same time, they wanted the backup call manager option for their cisco phones. Can we use SRST router somewhere middle of the network without any connection to WAN? router to be used for SRST function only.

[Nithin Eluvathingal]

SRST (Survivable Remote Site Telephony): SRST is a Cisco feature that allows a router at a remote site to provide basic call processing functionality for IP phones when the connection to the central Cisco Unified Communications Manager (CUCM) is lost. In other words, if the WAN link between the remote site and the central CUCM fails, the router switches to SRST mode, and the phones in that office or site can still make outgoing calls using the local resources.

Now, let’s understand the requirement of WAN In the context of SRST and IP telephony:

WAN (Wide Area Network): The WAN is the network that connects different sites or offices within an organization. It allows data and voice traffic to flow between locations. In the context of IPT , the WAN link connects the remote site (where the SRST router and IP phones are located) to the central CUCM.

Could you clarify what you’re asking in this question? "Can we use SRST router somewhere middle of the network without any connection to WAN? router to be used for SRST function only."

[inhamit]

Thanks. Cisco SRST router is connected to access switch in the network, not to WAN links. There is different WAN router in the network (HP router) which connects to WAN link and provide connection between two sites.

[Nithin Eluvathingal]

It’s not a requirement to connect the SRST router to the WAN link. It can be located anywhere within the SITE B network.

[JamesHawkins]

When SRST was first a thing I think the IP phones just tried to register to the IP address of their default gateway is connection to CallManager was lost. This effectively meant that the SRST router had to be the WAN router.

The CallManager SRST configuration then changed so that the IP address of the SRST router could be specified. This happened many releases ago so it would be very unusual to encounter a working CUCM cluster that still had the restriction.

[Roger Kallberg]

To add to the information shared by @Nithin Eluvathingal you need have a connection to the PSTN in the router that provides the SRST functionality.

[inhamit]

Is it a requirement to connect the SRST to PSTN? When WAN connection failed, all outside services will go down. This site is in offshore so i dont see possibility of PSTN there.

[Roger Kallberg · Accepted solution]

If you require the second site to be able to make and receive calls from PSTN in SRST mode it is a must. Without it the phones will be able to call other phones at the same site, but nothing else.