H.323 Gatekeeper Fundamentals (Configuring H.323 Gatekeepers) (Cisco Voice over IP (CVOICE)) Part 3

Gatekeeper Signaling: LRQ Sequential

For gatekeeper redundancy and load-sharing features, you can configure multiple gatekeepers to service the same zone or technology prefix by sending LRQs to two or more gatekeepers. Either the LRQs are sent sequentially to the gatekeepers or to all gatekeepers at the same time (blast).

Sequential forwarding of LRQs is the default forwarding mode. With sequential LRQ forwarding, the originating gatekeeper forwards an LRQ to the first gatekeeper in the matching list. The originating gatekeeper then waits for a response before sending an LRQ to the next gatekeeper on the list. If the originating gatekeeper receives an LCF while waiting, it will terminate the LRQ forwarding process.

If you have multiple matching prefix zones, you might want to consider using sequential LRQ forwarding instead of blast LRQ forwarding. With sequential forwarding, you can configure which routes are primary, secondary, and tertiary.

Figure 8-9 shows three gatekeepers to which Gatekeeper A can point. Gatekeeper A, whose configuration is provided in Example 8-1, will send an LRQ first to Gatekeeper B. Gatekeeper B will send a reply as either an LCF or an LRJ to Gatekeeper A. If Gatekeeper B returns an LCF to Gatekeeper A, the LRQ forwarding process will be terminated. If Gatekeeper B returns an LRJ to Gatekeeper A, then Gatekeeper A will send an LRQ to Gatekeeper C. Gatekeeper C will return either an LCF or LRJ to Gatekeeper A. Then Gatekeeper A will either terminate the LRQ forwarding process or start the LRQ process again with Gatekeeper D.

Figure 8-9 shows three gatekeepers to which Gatekeeper A can point. Gatekeeper A, whose configuration is provided in Example 8-1, will send an LRQ first to Gatekeeper B. Gatekeeper B will send a reply as either an LCF or an LRJ to Gatekeeper A. If Gatekeeper B returns an LCF to Gatekeeper A, the LRQ forwarding process will be terminated. If Gatekeeper B returns an LRJ to Gatekeeper A, then Gatekeeper A will send an LRQ to Gatekeeper C. Gatekeeper C will return either an LCF or LRJ to Gatekeeper A. Then Gatekeeper A will either terminate the LRQ forwarding process or start the LRQ process again with Gatekeeper D.

Figure 8-9 Sequential LRQ

Example 8-1 Sequential LRQ Configuration

Notice the zone prefix commands at the bottom of the router output. Because sequence is the default method for LRQ forwarding, the option seq can be included, and sequential LRQ forwarding will take place.

Note With sequential LRQs, there is a fixed timer when LRQs are sent. Even if Gatekeeper A gets an LRJ back immediately from Gatekeeper B, it will wait a fixed amount of time before sending the next LRQ to Gatekeeper C and Gatekeeper D. You can speed up this process by using the lrq lrj immediate-advance command.

Gatekeeper Signaling: LRQ Blast

In Figure 8-10 and Example 8-2, when blast LRQ is used, Gatekeeper A will simultaneously send LRQs to all three gatekeepers that match the zone prefix.

Figure 8-10 Blast LRQ

Example 8-2 Blast LRQ Configuration

If all three reply with a positive confirmation (that is, an LCF), Gatekeeper A chooses which one to use. Gatekeeper A can tailor the choice by using the cost and priority keywords at the end of the zone remote statement as follows:

The cost and priority command options need to be examined carefully for correct operation. The default cost is 50, in the range 1-100. In the example, you see the three gatekeepers have costs of 50, 51, and 52. This means Gatekeeper B has a lower cost than Gatekeeper C, and Gatekeeper C has a lower cost than Gatekeeper D. Therefore, Gatekeeper B will be selected first, and then Gatekeeper C, and finally Gatekeeper D.

The priority can also be set. The default for this option is also 50 in the range 1-100. In the example, the gatekeepers with a higher cost also have a lower priority. When each of the gatekeepers returns an LCF to Gatekeeper A, a decision as to which gatekeeper the call should be forwarded to can be made based on either cost or priority.

You can assign cost and priority values independently of each other. You might choose to assign only a cost or a priority to a specific gatekeeper. If the values you assign to a specific gatekeeper are higher or lower than the default values, and there are other gatekeepers that are using default values for cost and priority, call routing might take these unexpected paths. In the following syntax,

the blast option has been added to the zone prefix commands. This option is an important part of the configuration that is often overlooked. The blast option allows Gatekeeper A to simultaneously send LRQs to Gatekeeper B, Gatekeeper C, and Gatekeeper D. If the blast command option is omitted, the gatekeeper will use the default method, which is to choose the gatekeeper based on sequence.

To summarize, Gatekeeper A receives an ARQ from a gateway for 1408555xxxx. Gatekeeper A then blasts LRQs to all gatekeepers, which in this case are Gatekeeper B, Gatekeeper C, and Gatekeeper D. Gatekeeper A will use the cost and priority values to evaluate the received LCFs to determine where the call should be forwarded. In this case, if all the downstream gatekeepers respond with LCFs, Gatekeeper A will use the priority and cost values and choose Gatekeeper B as the gatekeeper to which to forward the call.

Intrazone Call Setup

Figure 8-11 shows the sequence of the signaling events and the basic signaling that takes place between a gateway and gatekeeper.

Figure 8-11 Intrazone Call Setup

The steps of this intrazone call setup are as follows:

1. Phone A dials the phone number 408 5555-2001 for Phone B.

2. Gateway A sends an ARQ to the gatekeeper, asking permission to call Phone B.

3. The gatekeeper does a lookup and finds Phone B registered to Gateway B and returns an ACF with the IP address of Gateway B.

4. Gateway A sends an H.225 call setup message to Gateway B with the phone number of Phone B.

5. Gateway B sends an H.255 call proceeding message to Gateway A.

6. Gateway B sends an ARQ to the gateway, asking permission to answer Gateway A’s call.

7. The gateway returns an ACF with the IP address of Gateway A.

8. Gateway B and Gateway A initiate an H.245 capability exchange and open logical channels.

9. Gateway B sets up a plain old telephone service (POTS) call to Phone B at 408 5552001.

10. When Phone B answers, Gateway B sends an H.245 call connect message to Gateway A.

11. Dual RTP streams flow between gateways.

Interzone Call Setup

Figure 8-12 shows how gatekeepers signal each other in a multizone gatekeeper network. It shows the sequence of RAS signaling events between gatekeepers, the LRQ RAS messages, and how an LRQ RAS message is used.

Figure 8-12 Interzone Call Setup

The basic gateway-to-gatekeeper signaling that occurs between zones is as follows:

1. Phone A dials the phone number 408 555-2001 for Phone B.

2. Gateway A sends Gatekeeper 1 an ARQ, asking permission to call Phone B.

3. Gatekeeper 1 does a lookup and does not find Phone B registered. Gatekeeper 1 does a prefix lookup and finds a match with Gatekeeper 2. Gatekeeper 1 sends an LRQ to Gatekeeper 2 and a RIP to Gateway A.

4. Gatekeeper 2 does a lookup, finds Phone B registered, and returns an LCF to Gatekeeper 1 with the IP address of Gateway B.

5. Gatekeeper 1 returns an ACF with the IP address of Gateway B.

6. Gateway A sends an H.225 call setup message to Gateway B with the phone number of Phone B.

7. Gateway B sends an H.225 call proceeding message to Gateway A.

8. Gateway B sends Gatekeeper 2 an ARQ, asking permission to answer the call from Gateway A.

9. Gatekeeper 2 returns an ACF with the IP address of Gateway A.

10. Gateway B and Gateway A initiate an H.245 capability exchange and open logical channels.

11. Gateway B sets up a POTS call to Phone B at 408 555-2001.

12. Gateway B sends a call connect to Gateway A.

13. Dual RTP streams flow between the gateways.

Call Disconnect

Figure 8-13 shows basic call-disconnect signaling between a gateway and a gatekeeper.

Figure 8-13 Call Disconnect

The RAS signaling messages used in this figure are DRQ and DCF. Phones A and B are in a conversation, and RAS signaling occurs as follows:

1. Phone B hangs up.

2. Gateway B sends a DRQ to Gatekeeper 2, disconnecting the call between Phones A and B. A DCF is received some time later.

3. Gateway B sends a Q.931 release complete message to Gateway A.

4. Gateway A sends a DRQ to Gatekeeper 1, disconnecting the call between Phones A and B. A DCF is received some time later.

5. Gateway A signals a call disconnect to the voice network.