Network Bulls
www.networkbulls.com
Best Institute for CCNA CCNP CCSP CCIP CCIE Training in India
M-44, Old Dlf, Sector-14 Gurgaon, Haryana, India
Call: +91-9654672192
Two different technologies are used to provide remote-site redundancy for small and
medium remote sites in a CUCM environment. SRST and MGCP gateway fallback are the
key components of delivering fail-safe communication services, as shown in Figure 5-1.
Figure 5-1 SRST and MGCP Fallback
Main Site
CUCM supports Cisco Unified IP Phones at remote sites that are attached to Cisco multiservice
routers across the WAN. Before Cisco Unified SRST, when the WAN connection
between a router and the CUCM failed, or when connectivity with the CUCM was lost for
any reason, Cisco Unified IP Phones on the network became unusable for the duration of
the failure. The reason is that Cisco IP Phones demand Skinny Client Control Protocol
(SCCP) or session initiation protocol (SIP) connectivity to a call-processing agent such as
CUCM, and in the absence of signaling connectivity, the phones become fully unusable.
Cisco Unified SRST overcomes this problem and ensures that Cisco Unified IP Phones
offer continuous, although reduced, service by providing call-handling support for Cisco
Unified IP Phones directly from the Cisco Unified SRST router. The system automatically
detects a failure and uses Simple Network-Enabled Auto-Provision (SNAP) technology
to autoconfigure the branch office router to provide call processing for Cisco Unified IP
Phones that are registered with the router. When the WAN link or connection to the primary
CUCM subscriber is restored, call handling reverts to the primary CUCM.
MGCP gateway fallback is a mechanism that allows a Cisco IOS router to continue providing
voice gateway functions even when the MGCP call agent is not in control of the media
gateway. These voice gateway functions are implemented through a fallback mechanism
that activates the so-called default technology application. The gateway then works in the
same way as a standalone H.323 or SIP gateway by using its configured dial peers.
Remote-Site Redundancy Technologies
Remote-Site Redundancy Technologies
Table 5-1 lists the capabilities of different remote-site redundancy technologies.
Table 5-1 Remote-Site Redundancy Technologies
SRST
MGCP Fallback
Cisco Unified
SRST
Cisco Unified
SIP SRST
Cisco Unified
Communications
Manager Express
in SRST Mode
Provides
Redundancy For
MGCP-controlled
gateways
SCCP phones SIP phones SCCP phones
Delivered Service Fall back to
Cisco IOS default
technology
Basic telephony
service
Basic SIP
proxy service
Cisco Unified
Communications
Manager Express
Maximum
Number of Phones
720 (C3845) 480 (C3845) 240 (C3845)
ISDN Call
Preservation
No Yes(no
MGCP)
Yes (no
MGCP)
Yes (no MGCP)
Analog/CAS Call
Preservation
Yes Yes Yes Yes
To use SRST as your fallback mode on an MGCP gateway, SRST and MGCP fallback have
to be configured on the same gateway.
NOTE MGCP and SRST have had the capability to be configured on the same gateway
since Cisco IOS Software Release 12.2(11)T.
Cisco Unified SIP SRST provides a basic set of features to SIP-based IP Phones. It has to
be enabled and configured separately on Cisco IOS routers. In Cisco Unified SRST versions
before 3.4, it provides a SIP Redirect Server function; in subsequent versions, it acts as a
back-to-back user agent (B2BUA).
SIP is an Internet standard discussed in many Request for Comments (RFC). If you want
to supplement your knowledge of SIP, here is a good RFC to start with:
http://www.ietf.org/rfc/rfc3261 .txt
100 Chapter 5: Examining Remote-Site Redundancy Options
NOTE Before CUCM version 6, CUCME and SRST contained similar functionality
but existed as separate technologies. SRST always requires Cisco CallManager (CCM)
or CUCM, whereas CUCME can exist as a separate entity. However, with CUCM
version 6, CUCME in SRST mode can add functionality to SRST when configured to
provide survivability with CUCM. However, only SRST or CUCME can be configured
at any one time on an IOS router.
VoIP call preservation, as shown in Tables 5-2 and 5-3, sustains connectivity for topologies
in which signaling is handled by an entity (such as CUCM) that is different from the other
endpoint and brokers signaling between the two connected parties.
Call preservation is useful when a gateway and the other endpoint, which typically is a
Cisco Unified IP Phone, are co-located at the same site, and the call agent is remote and
therefore more likely to experience connectivity failures.
Table 5-2 Call Preservation Capabilities of Different Interfaces
Interface Type I Cisco Analog Interface Cards
Card Type WS-svccmm-24fxs Ws-x6624-fxs
1
VG224
"
VG248 ATA 186 and 188
H.323 Gateway No No No I Yes No
SIP Gateway No No No Yes No
MGCP
Gateway
No No No Yes No
Table 5-3 Call Preservation Capabilities of Different Interfaces, Continued
Interface Type Cisco Time-Division Multiplexing Interface Cards
Card Type T1 CAS | E1 CAS | T1 PRI E1 PRI | BRI
H.323 Gateway Yes Yes Yes Yes Yes
SIP Gateway Yes Yes Yes Yes Yes
MGCP Gateway Yes Yes No No No
No comments:
Post a Comment