Overview

Gatekeeper

A Gatekeeper is a H.323 element that is used to group gateways into logical zones and perform call routing between them. Gateways are responsible for edge routing decisions between the Public Switched Telephone Network (PSTN) and the H.323 network. Cisco gatekeepers handle the core call routing among devices in the H.323 network and provide centralized dial plan administration.

Zone and Technology Prefixes

Zone Prefixes

A zone prefix is the part of the called number that identifies the zone to which a call hops off. Zone prefixes are usually used to associate an area code to a configured zone.

The Cisco gatekeeper determines if a call is routed to a remote zone or handled locally. For example, according to this configuration excerpt, gatekeeper (GK) A forwards 214……. calls to GK-B. Calls to area code (512) are handled locally.

gatekeeper

zone local GK-A abc.com

zone remote GK-B abc.com 172.22.2.3 1719

zone prefix GK-B 214…….

zone prefix GK-A 512…….

Technology Prefixes

Cisco gatekeepers use technology prefixes to route calls when there is no E.164 addresses registered (by a gateway) that matches the called number. In fact, this is a common scenario because most Cisco IOS gateways only register their H.323 ID (unless they have Foreign Exchange Station (FXS) ports configured). Without E.164 addresses registered, the Cisco gatekeeper relies on two options to make the call routing decision:

• With the Technology Prefix Matches option, the Cisco gatekeeper uses the technology prefix appended in the called number to select the destination gateway or zone.
• With the Default Technology Prefixes option, the Cisco gatekeeper assigns default gateway(s) for routing unresolved call addresses. This assignment is based on the gateways’ registered technology prefix.

Some notes on call routing decision process

1. For local zones, if there is no tech-prefix or default tech-prefix configured, the target must be registered or the call will fail.
2. For remote zones, if there is a tech-prefix but no default tech-prefix the call will fail unless hopoff is configured.  Even if the zone prefix matches, the GK will see the tech-prefix as a part of the called number.  The two GKs are not aware of the tech-prefixes configured on the opposing GK.  The format of this command is:

Gw-type-prefix 1# hopoff <gatekeeper hostname>.

There are four ways that call can be routed locally:

Hop-off prefix match

Tech prefix match and local Gateway found

Target registered

Default tech-prefix and local Gateway found

Address Resolution

CUBE

Cube is used to terminate and re-originate calls.  The following connection types are supported:

• H.323 to H.323
• H.323 to SIP
• SIP to H.323
• SIP to SIP

A concept called VIA-ZONES is used for Gatekeepers.  A via-zone-enabled gatekeeper is capable of recognizing via-zones and sending traffic to via-zone gateways.

In the lab, it is highly likely that the GK and the CUBE will be on the same router.

Zone local …

Invia – Use if the calls come from/to a Gatekeeper (LRQ)

Outvia – Use if the calls come from/to a Gateway (ARQ)

enable-intrazone—All intra zone calls must use the via-zone gatekeeper

In essence, CUBE is just a Gateway that registers to the Gatekeeper.  Calls are terminated on the CUBE and then re-originated.  Both incoming and outgoing dial-peers must be configured.

CUBE Example
CUBE and Gatekeeper on same Gateway
interface FastEthernet0/0.20
encapsulation dot1Q 20
ip address 10.10.200.3 255.255.255.0
ip helper-address 10.10.210.11
h323-gateway voip interface
h323-gateway voip id VIA ipaddr 10.10.110.1 1719
h323-gateway voip h323-id HQ-RTR
h323-gateway voip tech-prefix 1#

dial-peer voice 3000 voip
destination-pattern 3…
session target ras
dtmf-relay h245-alphanumeric
no vad

URL available during lab:

http://www.cisco.com/en/US/partner/docs/ios/voice/cubegk/configuration/guide/12_4t/ve_12_4t_book.html

Faststart and MTP

Faststart bypasses H.245.  Similar to SIP early offer.

Inbound example:

SIP phone (CME) –> GK –>UCM – since SIP phone is doing early offer, then H225 trunk needs to be configured for inbound faststart.

Outbound faststart –

UCM –> GK(CUBE) –> ITSP – If ITSP needs SIP early offer, faststart must be configured on UCM. Must use MTP since UCM can’t do outbound faststart.

CUBE doesn’t support empty H245 capabilities set.  Uncheck H245 capabilities set in UCM.

h323
emptycapability
h225 id-passthru
h225 connect-passthru
h245 passthru tcsnonstd-passthru

*** DONT FORGET THAT A VOIP DIAL-PEER MUST BE CONFIGURED ON THE GATEWAY FOR IT TO REGISTER TO THE GATEKEEPER ***

Some sample configurations

Gatekeeper

! config a local zone named test – it will use the highest loopback if not assigned manually

zone local TEST voiplab.com

! assign a prefix to a zone and set the primary gateway use to route the calls for the prefix

zone prefix 1… gw-priority 10 10.10.210.11

! assign a secondary Gateway

zone prefix 1… gw-priority 9 10.10.210.10