CUCM 8.5 and SRST with non-Cisco branch routers

[news2010a]

Imagine data center runs Cisco CUCM 8.5.

Branch offices (on MPLS) have Cisco IP Phones, Cisco Cat 3560 switches PoE.

However routers on branch offices are Juniper.

Imagine Juniper supports a SRST Avaya card that can connect to POTS line.

Any chance to make this Juniper/Avaya SRST card interoperate in this Cisco CUCM environment for SRST purposes? Which role does CUCM 8.5 plays during a SRST scenario in the branch office?

[Aaron Harrison · Accepted solution]

Hi

Chances will be v. slim. CUCM plays no role at all in SRST failures - the router performs the functions provided by CUCM, and the phones decide for themselves when to connect to SRST and when to connect back to CUCM.

As such the router you have would need to support SCCP and maybe SIP registrations, from Cisco handsets, and provision the lines etc automatically based on info supplied by the phones... It really needs to be a Cisco device.

You can use a seperate ISR/ISR G2 to provide the SRST function and serve as a local (or just for SRST) point of connection to the PSTN. It doesn't need to be the default gateway  - in fact from a resilience point of view it's better not to be. If your WAN router also hosts SRST, and the WAN router fails - you have no telephony at all. As a seperate device the resilience is greatly improved.

Aaron

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[news2010a]

It makes sense.
It seems if I pick a 1941 for the remote branches I could support up to 35 VoIP users right?

What would be the best way for the Cisco IP Phones to find the 1941 SRST router if the connectivity to the CUCM in the data center is lost via the MPLS link? Is that the IP address for the 1941 SRST router configured in the VoIP phones directly, so that they can fall back to the 1941 SRST and register at that point?

[Aaron Harrison]

Hi

You would configure the SRST reference on a device pool, and then assign phones to that device pool. If the router doesn't have capacity for the full number of phones on this site you can have two device pools for the site, one with SRST, one without..

I don't see 1941 on the SRST compatibility guides..http://www.cisco.com/en/US/docs/voice_ip_comm/cusrst/requirements/guide/srs88spc.html

Aaron

[news2010a]

Very good table you sent. Thanks.

Hey, do you know when it lists that the Cisco 880 can handle "max total IP phones=4", does it mean 4 people calling at the same time (4 concurrent calls)?

If that is not the case, I would go with1861 (just need to make sure it is not approaching EOL/EOS).

Funny I got info on 1941 SRST from a blog post here after a google search.

Supported Platforms and Memory Specifications: SCCP/SIP IP Phones*
Cisco Platform	Maximum Total IP Phones	Maximum SCCP/SIP IP Phones	Maximum Total Ephone-dns or Virtual Ports	Recommended DRAM	Recommended Flash Memory
Cisco 880 SRST ****	4	4	50	512	128
Cisco 1861	15	15	48	256	128

[Aaron Harrison]

Hi

It's concurrent registered phones. So make sure the specified number is more than the number of phones you have - or you'll end up having to pick out key phones for survivability.

Good luck

Aaron

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