CCIE Blog

A voice lab rack usually utilizes dedicated piece of hardware to simulate PSTN switch. Commonly, you can find a Cisco router in this role, with a number of E1/T1 cards set to emulate ISDN network side. It perfectly suits the function, switching ISDN connections between the endpoints. Additionally, it is often required to have an “independent” PSTN phone connected to the PSTN switch, in order to represent “outside” dialing patterns - such as 911, 999, 411 1-800/900 numbers. The most obvious way to do this is to enable a CallManager Express on the PSTN router, and register either hardware IP Phone or any of IP Soft-phones (such as IP Blue or CIPC) with the CME system.

However, there is another way to accomplish the same goal using IOS functionality solely. It relies on the IP-to-IP gateway feature, called “RTP loopback” session target. It is intended to be used for VoIP call testing, but could be easily utilized to loopback incoming PSTN calls to themselves. Let’s say we want PSTN router to respond to incoming calls to an emergency number 911. Here is how a configuration would look like:

PSTN:
voice service voip
 allow-connections h323 to h323
!
interface Loopback0
 ip address 177.254.254.254 255.255.255.255
!
dial-peer voice 911 voip
 destination-pattern 911
 session target ipv4:177.254.254.254
 incoming called-number 999
 tech-prefix 1#
!
dial-peer voice 1911 voip
 destination-pattern 1#911
 session target loopback:rtp
 incoming called-number 1#911

The trick is that only IP-to-IP calls could be looped back. Because of that, we need to redirect the incoming PSTN call to the router itself first, in order to establish an incoming VoIP call leg.

While this approach permit VoIP call testing, it lack one important feature, available with “real” PSTN phone: placing calls from the PSTN phone to in-rack phones. While this may seem a serious issue, you can always use “csim start” command on the PSTN router to overcome this obstacle. Have fun!

IPMA is yet another well-known CCM application that you may encounter on your CCIE Voice lab exam. While IPMA Proxy mode is clearly a legacy approach to configure this application its still a topic you could see in the lab. Before we discuss the configuration steps, let’s take a quick overview of a simplified model for IPMA Proxy mode operations. Refer to the diagram for IP Phone extension numbers.

The whole purpose of IPMA proxy is to intercept calls going directly to manager’s IP Phone primary line (“1001”), and proceed them using IPMA configurable call routing logic – usually divert calls to the assistant’s so-called Proxy line (“1112”). In other words, calls placed to manager phone, gets re-routed to assistant’s “proxy” line. Of course, manager has some control of the call-routing logic from it’s IP Phone, using special set of softkeys (plus a Web-interface for advanced configurations).

Now the whole idea of Proxy mode is to put the IPMA application in the call routing path between a caller and the manager’s primary extension. To accomplish this goal, manager’s primary line should be isolated into a separate partition – let’s call it PT_MANAGER. No other IP Phone in the system should have direct access to this partition – their respective CSSes should not contain this partition. Let’s name the CSS used by all IP Phones in the system as CSS_DEFAULT.

Now recall that IPMA is a Java application running in Cisco Tomcat server. IPMA uses CTI interface to control various call-routing components in the CallManager. Specifically, a CTI Route Point should be created in the CallManager system, and IPMA application should take control of it. Next, a “wildcard” extension “100X” should be associated with the CTI RP line and placed in partition PT_INTERNAL - the default partition used for all IP Phone lines within the system (Well, the DocCD recommends using a separate partition for the CTI RP – and indeed, this is a more flexible approach. However, for the sake of the configuration speed, it makes sense to use the minimum set of partitions). The “wildcard” extension number is actually used in configurations where many managers with the primary extension numbers in range “100X” should be covered with the IPMA application. If you are providing call coverage for just one manager’s phone, you can use “1001” here. Also, you may want to set the CFNA number to “100X” or “1001” – this will provide call routing backup in a case when IPMA application would happen to fail.

With the above configuration, when any phone in the system calls “1001” – the manager’s primary line, the call gets routed to “100X/PT_INTERNAL”, and eventually hits the IPMA application. At this point, the IPMA application may want to direct the call to the manager’s real line – “1001/PT_MANAGER” – and this is why the CTI RP should have a special CSS assigned, which has access to PT_MANAGER partition. Let’s name this CSS as CSS_IPMA. As a minimum, CSS_IPMA should contain PT_MANAGER and PT_INTERNAL – since the IPMA may need to redirect call to some other internal extension. (Note that “1001/PT_MANAGER” precedes “100X/PT_INTERNAL” when using CSS_IPMA. This order resolves the ambiguity, even in case when one assigns number “1001” to CTI RP).

To complete the picture, recall the proxy line on assistant’s phone. This is where IPMA application direct calls to by default. Since the assistant may need to direct the call back to manager’s phone, this proxy line should be configured to use CSS_IPMA as the Line CSS. With this setup, the proxy number “1112” is placed in PT_INTERNAL partition (so the CTI RP can reach it) and is allowed to call the manager’s primary line directly. Of course, the primary line of assistant’s phone (“1002”) has no special Line CSS configured, and will therefore hit the IPMA application when calling “1001”.

Per the recommended design, you should also create two intercom lines on both manager’s and assistant’s IP Phones. An intercom is simple a line, which has auto-answer with speakerphone turned on. On the opposite side, you just add a speed dial to reach the intercom number. Thus, you need to intercom lines plus two speed dials to accomplish the intercom configuration.

Now let’s move to the actual configuration. While CallManager has a special built-in IPMA Wizard, personally I’d prefer not to mess with it - unless you’re absolutely sure about what you are doing – the wizard will modify your partitions and CSSes, and may do that in the way you don’t expect. Configuring IPMA proxy mode manually takes a little more time, but once you understand it completely, it won’t take that much. Plus, you get full control of your configuration. So it’s a good idea to create your own IPMA configuration checklist, and use it during your practice. Here is how a checklist may look like.

[Call Intercept]

1) Create Partitions & CSSes: PT_MANAGER & CSS_IPMA
2) Create CTI RP, assign extension number “100X/PT_INTERNAL” to it, set CSS_IPMA as the device CSS. You may also use “1001” extension to cover just one manger
2.1) Set CFNA to “100X” or to “1001” if you provide call coverage for just one manager. This will provide call backup if the IPMA application fails.

[IP Phones]

1) Create a new Button Template, say “3+3 7960” to allow more than two lines on an IP Phone. You will need this template for assistant’s phone, to accommodate three lines: primary, proxy and intercom.

2) Configure the Manager’s Phone
2.1) Set Softkey template to “Standard IPMA Manager”
2.2) Configure the primary line in “PT_MANAGER”
2.3) Add an intercom line, “*1001” and a speed-dial to “*1002” to reach the assistant
2.4) Create IPMA IP Phone service & subscribe the IP Phone to it (URL could be found on DocCD)

3) Configure the Assistant’s Phone
3.1) Set Softkey template to “Standard IPMA Assisant”
3.2) Set Button Template to “3+3 7960” (assistant needs extra lines)
3.2) Add a proxy line “1112/PT_INTERNAL” and set the Line CSS to “CSS_IPMA” for this line
3.3) Add an intercom line, “*1002” and a speed-dial to “*1001” to reach the assistant

[Users]

1) Create a new user named “manager”
1.1) Allow it the use of CTI Application & CTI Super Provider
1.2) Associate this user with manager’s IP Phone

2) Create a new user named “assistant”
2.1) Allow it the use of CTI Application & CTI Super Provider
2.2) Associate this user with assistant’s IP Phone

3) Get back to “manager” user
3.1) Start the Cisco IPMA configuration dialog and disable automatic configuration
3.2) Configure the settings per your setup
3.3) Add a new assistant to the manager
3.4) Configure the assistant, matching proper primary manager’s line against the assistant’s proxy line

[IPMA Application]

1) Choose Service Parameters for Cisco IPMA Application
2) Configure CTI Manager IP Addresses (primary/backup). In simplest case just use your Publisher IP
3) Configure IPMA Application IP Addresses (primary/backup). In simplest case just use your Publisher IP
4) Set the CTI RP name for the IPMA application
5) Restart the Cisco Tomcat Windows Service or go to Tomcat manager interface at http://[IPMA server IP Address]/manager/list and restart the service there

[Verification]

1) Check that manager’s phone has IPMA softkey set on it’s screen
2) Install the Cisco IPMA Console Application and log in there as “assistant”
3) Place a call to manager’s primary line, ensure it get’s routed to the assistant phone, and pick it up from the IPMA console. Forward the call back to manager’s primary line
4) Configure from the manager’s phone to accept all calls and place a call to manager’s primary line once again

Making checklists for complex tasks is a must when preparing to CCIE Voice lab. The above list suggests a simplified manual approach to configure all IPMA application settings, in the order specifically optimized for speed. However, if you are pretty much comfortable with the IPMA Wizard, you can use it for your setup. Just make sure you performed a thorough verification after that.

The final note is about interaction of IPMA proxy mode with the voicemail system. Since we isolate the manager’s primary line in separate partition, we need to make sure MWI CSS is able to access it, in order to light the MWI lamp. Make sure you wont forget about it, since this may cost you some precious points.

Further reading:

Cisco IP Manager Assistant With Proxy Line Support

Catalyst QoS configuration for IP Telephony endpoints is one of the CCIE Voice labs topics. Many people have issues with that one, because of need to memorize a lot of SRND recommendations to do it right. The good news is that during the lab exam you have full access to the QoS SRND documents and UniverCD content. The bad news is that you won’t probably have enough time to navigate the UniverCD with comfort plus the reference configurations often have a lot of typos and mistakes in them.

Read the rest of this entry »

This is a good example of fragmentation and interleaving, applied in a complex context. To begin with, why whould anyone need to run Multilink PPP (MLPPP or MLP) with Interleaving over Frame-Relay? Well, back in days, when Frame-Relay and ATM were really popular, there was a need to interwork the two technologies: that is, transparently pass encapsulated packets between FR and ATM PVCs. (This is similar in concept with modern L2 VPN interworking, however it was specific to ATM and Frame-Relay). Let’s imagine a situation where we have slow ATM and Frame-Relay links, used to transport a mix of VoIP and data traffic. As we know, some sort of fragmentation and interleaving scheme should be implemented, in order to keep voice quality under control. Since there was no fragmentation scheme common to both ATM and Frame-Relay, people came with idea to run PPP (yet another L2 tech) over Frame-Relay and ATM PVCs and use PPP multilink and interleave feature to implement fragmentation. (Actually there was no good scheme for native fragmentation and interleaving with VoIP over ATM - the cell mode technology - how ironic!)

Before coming up with a configuration example, let’s discuss briefly how PPP Multilink and Interleave works. MLPPP is defined under RFC 1990, and it’s purpose is to group a number of physical links into one logical channel with larger “effective” bandwidth. As we discussed before, MLPPP uses a fragmentation algorithm, where one large frame is being split at Layer2 and replaced with a bunch of sequenced (by the use of additional MLPPP header) smaller frames which are then being sent over multiple physical links in parallel. The receiving side will then accept fragments, reorder some of them if needed, and assemble the pieces into complete frame using the sequence numbers.

So here comes the interleave feature: small voice packets are not fragmented by MLPPP (no MLPPP header and sequence number added) and are simply inserted (intermixed) among the fragments of large data packet. Of course, a special interleaving priority queue is used for this purpose, as we have discussed before.

To summarize:

1) MLPPP uses fragmentation scheme where large packets are sliced in pieces and sequence numbers are added using special MLPPP headers
2) Small voice packets are interleaved with fragments of large packets using a special priority queue

We see that MLPPP was originally designed to work with multiple physical links at the same time. However, PPP Multilink Interleave only works with one physical link. The reason is that voice (small) packets are being sent without sequence numbers. If we were using multiple physical links, the receiving side may start accepting voice packets out of their original order (due to different physical link latencies). And since voice packets bear no fragmentation headers, there is no way to reorder them. In effect, packets may arrive to their final destination out of order, degrading voice quality.

To overcome this obstacle, Multiclass Multilink PPP (MCMLPPP or MCMLP) has been introduced in RFC 2886. Under this RFC, different “fragment streams” or classes are supported at sending and receiving sides, using independent sequence numbers. Therefore, with MCMLPPP voice packets may be sent using MLPPP header with separate sequence numbers space. In result, MCMPPP permits the use of fragmentation and interleaving over multiple physical links at time.

Now back to our MLPPPoFR example. Let’s imagine the situation where we have two routers (R1 and R2) connected via FR cloud, with physical ports clocked at 512Kpbs and PVC CIR values equal to 384Kbps (There is no ATM interworking in this example). We need to provide priority treatment to voice packets and enable PPP Multilink and Interleave to decrease serialization delays.

[R1]---[DLCI 112]---[Frame-Relay]---[DLCI 211]---[R2]

Start by defining MQC policy. We need to make sure that software queue gives voice packets priority treatmet, or else interleaving will be useless

R1 & R2:

!
! Voice bearer
!
class-map VOICE
 match ip dscp ef

!
! Voice signaling
!
class-map SIGNALING
 match ip dscp cs3

!
! CBWFQ: priority treatment for voice packets
!
policy-map CBWFQ
 class VOICE
  priority 48
 class SIGNALING
  bandwidth 8
 class class-default
  fair-queue

Next create a Virtual-Template interface for PPPoFR. We need to calculate the fragment size for MLPPP. Since physical port speed is 512Kpbs, and required serialization delay should not exceed 10ms (remember, fragment size is based on physical port speed!), the fragment size must be set to 512000/8*0,01=640 bytes. How is the fragment size configured with MLPPP? By using command ppp multilink fragment delay - however, IOS CLI takes this delay value (in milliseconds) and multiplies it by configured interface (virtual-template) bandwidth (in our case 384Kbps). We can actually change the virtual-template bandwidth to match the physical interface speed, but this would affect the CBWFQ weights! Therefore, we take the virtual-template bandwidth (384Kpbs) and adjust the delay to make sure the fragment size matches the physical interace rate is 512Kpbs. This way, the “effective” delay value would be set to “640*8/384 = 13ms” (Fragment_Size/CIR*8) to accomodate the physical and logical bandwidth discrepancy. (This may be unimportant if our physical port speed does not differ much from PVC CIR. However, if you have say PVC CIR=384Kbps and port speed 768Kbps you may want to pay attention to this issue)

R1:
interface Loopback0
 ip address 177.1.101.1 255.255.255.255
!
interface Virtual-Template 1
 encapsulation ppp
 ip unnumbered Loopback 0
 bandwidth 384
 ppp multilink
 ppp multilink interleave
 ppp multilink fragment delay 13
 service-policy output CBWFQ

R2:
interface Loopback0
 ip address 177.1.102.1 255.255.255.255
!
interface Virtual-Template 1
 encapsulation ppp
 ip unnumbered Loopback 0
 bandwidth 384
 ppp multilink
 ppp multilink interleave
 ppp multilink fragment delay 13
 service-policy output CBWFQ

Next we configure PVC shaping settings by using legacy FRTS configuration. Note that Bc is set to CIR*10ms.

R1 & R2:
map-class frame-relay SHAPE_384K
frame-relay cir 384000
frame-relay mincir 384000
frame-relay bc 3840
frame-relay be 0

Finally we apply all the settings to the Frame-Relay interfaces:

R1:
interface Serial 0/0/0:0
 encapsulation frame-relay
 frame-relay traffic-shaping
!
! Virtual Template bound to PVC
!
interface Serial 0/0/0:0.1 point-to-point
 no ip address
 frame-relay interface-dlci 112 ppp virtual-template 1
  class SHAPE_384K

R2:
interface Serial 0/0/1:0
 encapsulation frame-relay
 frame-relay traffic-shaping
!
! Virtual Template bound to PVC
!
interface Serial 0/0/1:0.1  point-to-point
 no ip address
 no frame-relay interface-dlci 221
 frame-relay interface-dlci 211 ppp virtual-Template 1
  class SHAPE_384K

Verification

Two virtual-access interfaces have been cloned. First for the member link:

R1#show interfaces virtual-access 2
Virtual-Access2 is up, line protocol is up
  Hardware is Virtual Access interface
  Interface is unnumbered. Using address of Loopback0 (177.1.101.1)
  MTU 1500 bytes, BW 384 Kbit, DLY 100000 usec,
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation PPP, LCP Open, multilink Open
  Link is a member of Multilink bundle Virtual-Access3   <---- MLP bundle member
  PPPoFR vaccess, cloned from Virtual-Template1
  Vaccess status 0x44
  Bound to Serial0/0/0:0.1 DLCI 112, Cloned from Virtual-Template1, loopback not set
  Keepalive set (10 sec)
  DTR is pulsed for 5 seconds on reset
  Last input 00:00:52, output never, output hang never
  Last clearing of "show interface" counters 00:04:17
  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
  Queueing strategy: fifo       <---------- FIFO is the member link queue
  Output queue: 0/40 (size/max)
  5 minute input rate 0 bits/sec, 0 packets/sec
  5 minute output rate 0 bits/sec, 0 packets/sec
     75 packets input, 16472 bytes, 0 no buffer
     Received 0 broadcasts, 0 runts, 0 giants, 0 throttles
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
     86 packets output, 16601 bytes, 0 underruns
     0 output errors, 0 collisions, 0 interface resets
     0 output buffer failures, 0 output buffers swapped out
     0 carrier transitions

Second for the MLPPP bundle itself:

R1#show interfaces virtual-access 3
Virtual-Access3 is up, line protocol is up
  Hardware is Virtual Access interface
  Interface is unnumbered. Using address of Loopback0 (177.1.101.1)
  MTU 1500 bytes, BW 384 Kbit, DLY 100000 usec,
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation PPP, LCP Open, multilink Open
  Open: IPCP
  MLP Bundle vaccess, cloned from Virtual-Template1   <---------- MLP Bundle
  Vaccess status 0x40, loopback not set
  Keepalive set (10 sec)
  DTR is pulsed for 5 seconds on reset
  Last input 00:01:29, output never, output hang never
  Last clearing of "show interface" counters 00:03:40
  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
  Queueing strategy: Class-based queueing    <--------- CBWFQ is the bundle queue
  Output queue: 0/1000/64/0 (size/max total/threshold/drops)
     Conversations  0/1/128 (active/max active/max total)
     Reserved Conversations 1/1 (allocated/max allocated)
     Available Bandwidth 232 kilobits/sec
  5 minute input rate 0 bits/sec, 0 packets/sec
  5 minute output rate 0 bits/sec, 0 packets/sec
     17 packets input, 15588 bytes, 0 no buffer
     Received 0 broadcasts, 0 runts, 0 giants, 0 throttles
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
     17 packets output, 15924 bytes, 0 underruns
     0 output errors, 0 collisions, 0 interface resets
     0 output buffer failures, 0 output buffers swapped out
     0 carrier transitions

Verify the CBWFQ policy-map:

R1#show policy-map interface
 Virtual-Template1 

  Service-policy output: CBWFQ

    Service policy content is displayed for cloned interfaces only such as vaccess and sessions
 Virtual-Access3 

  Service-policy output: CBWFQ

    Class-map: VOICE (match-all)
      0 packets, 0 bytes
      5 minute offered rate 0 bps, drop rate 0 bps
      Match: ip dscp ef (46)
      Queueing
        Strict Priority
        Output Queue: Conversation 136
        Bandwidth 48 (kbps) Burst 1200 (Bytes)
        (pkts matched/bytes matched) 0/0
        (total drops/bytes drops) 0/0

    Class-map: SIGNALING (match-all)
      0 packets, 0 bytes
      5 minute offered rate 0 bps, drop rate 0 bps
      Match: ip dscp cs3 (24)
      Queueing
        Output Queue: Conversation 137
        Bandwidth 8 (kbps) Max Threshold 64 (packets)
        (pkts matched/bytes matched) 0/0
        (depth/total drops/no-buffer drops) 0/0/0

    Class-map: class-default (match-any)
      17 packets, 15554 bytes
      5 minute offered rate 0 bps, drop rate 0 bps
      Match: any
      Queueing
        Flow Based Fair Queueing
        Maximum Number of Hashed Queues 128
        (total queued/total drops/no-buffer drops) 0/0/0

Check PPP multilink status:

R1#ping 177.1.102.1 source loopback 0 size 1500

Type escape sequence to abort.
Sending 5, 1500-byte ICMP Echos to 177.1.102.1, timeout is 2 seconds:
Packet sent with a source address of 177.1.101.1
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 64/64/64 ms

R1#show ppp multilink

Virtual-Access3, bundle name is R2
  Endpoint discriminator is R2
  Bundle up for 00:07:49, total bandwidth 384, load 1/255
  Receive buffer limit 12192 bytes, frag timeout 1000 ms
  Interleaving enabled            <------- Interleaving enabled
    0/0 fragments/bytes in reassembly list
    0 lost fragments, 0 reordered
    0/0 discarded fragments/bytes, 0 lost received
    0x34 received sequence, 0x34 sent sequence   <---- MLP sequence numbers for fragmented packets
  Member links: 1 (max not set, min not set)
    Vi2, since 00:07:49, 624 weight, 614 frag size <------- Fragment Size
No inactive multilink interfaces

Verify the interleaving queue:

R1#show interfaces serial 0/0/0:0
Serial0/0/0:0 is up, line protocol is up
  Hardware is GT96K Serial
  MTU 1500 bytes, BW 1536 Kbit, DLY 20000 usec,
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation FRAME-RELAY, loopback not set
  Keepalive set (10 sec)
  LMI enq sent  10, LMI stat recvd 11, LMI upd recvd 0, DTE LMI up
  LMI enq recvd 0, LMI stat sent  0, LMI upd sent  0
  LMI DLCI 1023  LMI type is CISCO  frame relay DTE
  FR SVC disabled, LAPF state down
  Broadcast queue 0/64, broadcasts sent/dropped 4/0, interface broadcasts 0
  Last input 00:00:05, output 00:00:02, output hang never
  Last clearing of "show interface" counters 00:01:53
  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
  Queueing strategy: dual fifo                        <--------- Dual FIFO
  Output queue: high size/max/dropped 0/256/0         <--------- High Queue
  Output queue: 0/128 (size/max)                      <--------- Low (fragments) queue
  5 minute input rate 0 bits/sec, 0 packets/sec
  5 minute output rate 0 bits/sec, 0 packets/sec
     47 packets input, 3914 bytes, 0 no buffer
     Received 0 broadcasts, 0 runts, 0 giants, 0 throttles
     1 input errors, 1 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
     47 packets output, 2149 bytes, 0 underruns
     0 output errors, 0 collisions, 4 interface resets
     0 output buffer failures, 0 output buffers swapped out
     1 carrier transitions
  Timeslot(s) Used:1-24, SCC: 0, Transmitter delay is 0 flags

Further Reading

Reducing Latency and Jitter for Real-Time Traffic Using Multilink PPP
Multiclass Multilink PPP
Using Multilink PPP over Frame Relay

Current as of Jan 2008.

IOS 12.4(5b)
CME 3.3
CUE 2.1.3
CRS (IPCC Express) 4.0.1
CallManager 4.1(3)sr3b
Unity 4.0(5)
Catalyst OS 7.6 (6500)
IOS 12.1 (3550)

Inbound
1. QoS Policy Propagation through Border Gateway Protocol (BGP) (QPPB)
2. Input common classification
3. Input ACLs
4. Input marking (class-based marking or Committed Access Rate (CAR))
5. Input policing (through a class-based policer or CAR)
6. IP Security (IPSec)
7. Cisco Express Forwarding (CEF) or Fast Switching

Outbound
1. CEF or Fast Switching
2. Output common classification
3. Output ACLs
4. Output marking
5. Output policing (through a class-based policer or CAR)
6. Queueing (Class-Based Weighted Fair Queueing (CBWFQ) and Low Latency Queueing (LLQ)), and Weighted Random Early Detection (WRED)