Connection Admission Control and Oversubscription

Connection Admission Control (CAC) happens at the VSI slave in the resource manager module. There are several CAC algorithms, including CAC based on bandwidth, logical connection numbers (LCNs), and enhanced CAC algorithms. Basic CAC is the default CAC algorithm for all service types except the ATM Forum UBR service category and MPLS service types. For these, the LCN CAC algorithm is used by default. Table 11-3 describes the effective cell rate (ECR) calculation used in basic CAC.

Table 11-3. ECR Values for Basic CAC

Service Category	ECR Value
CBR	PCR
VBR-RT, VBR-NRT	SCR
ABR	MCR
UBR	N/A

Enhanced CAC takes into the ECR calculation various QoS requirements.

It is important not to mistake CAC performed at the VSI slave with generic CAC or GCAC performed by PNNI at the controller.

Multiservice switching PNNI implementations have a set of CAC parameters that can be modified on a per-class basis. You can use the command dsppnportcac executed at the PNNI controller CLI to display those parameters and their default values. See Example 11-39.

Example 11-39. Displaying the Default CAC Parameters

 m8850-7a.7.PXM.a > dsppnportcac 1:1.2:2                 cbr:     rt-vbr:    nrt-vbr:        ubr:       abr:       sig: bookFactor:     100%        100%        100%        100%       100%       100% maxBw:     100.0000%   100.0000%   100.0000%   100.0000%  100.0000%  100.0000% minBw:       0.0000%     0.0000%     0.0000%     0.0000%    0.0000%    0.9928% maxVc:          100%        100%        100%        100%       100%       100% minVc:            0%          0%          0%          0%         0%         1% maxVcBw:          0           0           0           0          0          0 m8850-7a.7.PXM.a > 

The minimum values are guaranteed quantities, and the maximum values describe the upper limit based on shared resources, for both bandwidth and LCNs.

On the VSI slave side, you can display the load model showing available bandwidth and LCN values on a port's partition, as well as the per-service category, using the command dspload, as Example 11-40 shows.

Example 11-40. Using dspload

 m8850-7a.1.AXSM.a > dspload 2 1         +--------------------------------------------+         |   I N T E R F A C E    L O A D   I N F O   |         +--------------------------------------------+         | Maximum Channels         : 0004000         |         | Guaranteed Channels      : 0001000         |         | Igr Maximum Bandwidth    : 0096000         |         | Igr Guaranteed Bandwidth : 0096000         |         | Egr Maximum Bandwidth    : 0096000         |         | Egr Guaranteed Bandwidth : 0096000         |         | Available Igr Channels   : 0003995         |         | Available Egr Channels   : 0003995         |         | Available Igr Bandwidth  : 0058519         |         | Available Egr Bandwidth  : 0058519         |         +--------------------------------------------+         |          E X C E P T -- V A L U E S        |         +--------------------------------------------+         | SERV-CATEG | VAR-TYPE | INGRESS  | EGRESS  |         | VSI-SIG    | Avl Chnl | 0003995  | 0003995 |         | CBR        | Avl Chnl | 0003995  | 0003995 |         | VBR-RT     | Avl Chnl | 0003995  | 0003995 |         | VBR-nRT    | Avl Chnl | 0003995  | 0003995 |         | UBR        | Avl Chnl | 0003995  | 0003995 |         | ABR        | Avl Chnl | 0003995  | 0003995 |         +--------------------------------------------+         | VSI-SIG    | Avl Bw   | 0058519  | 0058519 |         | CBR        | Avl Bw   | 0058519  | 0058519 |         | VBR-RT     | Avl Bw   | 0058519  | 0058519 |         | VBR-nRT    | Avl Bw   | 0058519  | 0058519 |         | UBR        | Avl Bw   | 0058519  | 0058519 |         | ABR        | Avl Bw   | 0058519  | 0058519 |         +--------------------------------------------+ m8850-7a.1.AXSM.a > 

The interface load information shows per-partition quantities, and the except values show per-service category values within the VSI partition. The dspload command output shows that, by default, the values for all the service categories are the same and equal the per-partition values for both bandwidth and LCNs. All available channels are equal to 3995 LCNs, and the available bandwidth is 58,519 CPS. This is because the command dsppnportcac shows all minimum values equal to 0, except for the signaling service category, the resources of which are used.

The following examples demonstrate the relationship between the PNNI VSI master pnportcac values and the VSI slave load values.

CAC Example 1

In the first example, the maximum bandwidth for CAC for VBR-NRT service types is made null. The command cnfpnportcac in the PNNI controller is used while the PnPort is administratively down. See Example 11-41.

Example 11-41. Changing the Maximum CAC Bandwidth for nrt-VBR

 m8850-7a.7.PXM.a > dnpnport 1:1.2:2 m8850-7a.7.PXM.a > cnfpnportcac 1:1.2:2 nrtvbr -maxbw 0 WARNING: New CAC parameters apply to existing connections also m8850-7a.7.PXM.a > uppnport 1:1.2:2 

All the VBR-NRT connections that were traversing the PnPort 1:1.2:2 in the MGX-8850 were derouted (as you can see using the command dsppncons), and the load model looks as Example 11-42 shows.

Example 11-42. Checking the Load Model

 m8850-7a.1.AXSM.a > dspload 2 1         +--------------------------------------------+         |   I N T E R F A C E    L O A D   I N F O   |         +--------------------------------------------+         | Maximum Channels         : 0004000         |         | Guaranteed Channels      : 0001000         |         | Igr Maximum Bandwidth    : 0096000         |         | Igr Guaranteed Bandwidth : 0096000         |         | Egr Maximum Bandwidth    : 0096000         |         | Egr Guaranteed Bandwidth : 0096000         |         | Available Igr Channels   : 0003997         |         | Available Egr Channels   : 0003997         |         | Available Igr Bandwidth  : 0094897         |         | Available Egr Bandwidth  : 0094897         |         +--------------------------------------------+         |          E X C E P T -- V A L U E S        |         +--------------------------------------------+         | SERV-CATEG | VAR-TYPE | INGRESS  | EGRESS  |         | VSI-SIG    | Avl Chnl | 0003997  | 0003997 |         | CBR        | Avl Chnl | 0003997  | 0003997 |         | VBR-RT     | Avl Chnl | 0003997  | 0003997 |         | VBR-nRT    | Avl Chnl | 0003997  | 0003997 |         | UBR        | Avl Chnl | 0003997  | 0003997 |         | ABR        | Avl Chnl | 0003997  | 0003997 |         +--------------------------------------------+         | VSI-SIG    | Avl Bw   | 0094897  | 0094897 |         | CBR        | Avl Bw   | 0094897  | 0094897 |         | VBR-RT     | Avl Bw   | 0094897  | 0094897 |         | VBR-nRT    | Avl Bw   | 0000000  | 0000000 |         | UBR        | Avl Bw   | 0094897  | 0094897 |         | ABR        | Avl Bw   | 0094897  | 0094897 |         +--------------------------------------------+ m8850-7a.1.AXSM.a > 

From the dspload command output, you can see that you now have more available bandwidth and LCNs from the connections that took an alternative path. In addition, the except values for VBR-NRT available bandwidth are null. The subpartitioned bandwidth is shown in Figure 11-10.

Figure 11-10. Bandwidth Subpartitioning in CAC Example 1

NOTE

In a PNNI application, it is critical to match the per-class PNNI advertised link values used by GCAC (modified with the command cnfpnni-intf) with the VSI slave CAC values (modified with the command cnfpnportcac).

Performing only a modification in the CAC parameters can lead to unnecessary crankbacks or underutilized bandwidth. In particular, considering CAC Example 1, PNNI is unaware of the change in CAC. You configured no bandwidth for the VBR-NRT service types in the interface, but PNNI tries to route VBR-NRT connections over it, and GCAC passes. This scenario leads to crankbacks.

CAC Example 2

The second example involves changing the minimum bandwidth for CBR service types. The CAC values were set back to the defaults before beginning.

Using the command cnfpnportcac, the minimum bandwidth for CBR is changed to 40 percent (see Example 11-43).

Example 11-43. Using cnfpnportcac to Change the Minimum Bandwidth for CBR

 m8850-7a.7.PXM.a > cnfpnportcac 1:1.2:2 cbr -minbw 40 WARNING: New CAC parameters apply to existing connections also m8850-7a.7.PXM.a > m8850-7a.7.PXM.a > dsppnportcac 1:1.2:2                 cbr:     rt-vbr:    nrt-vbr:        ubr:       abr:       sig: bookFactor:     100%        100%        100%        100%       100%       100% maxBw:     100.0000%   100.0000%   100.0000%   100.0000%  100.0000%  100.0000% minBw:      40.0000%     0.0000%     0.0000%     0.0000%    0.0000%    0.9928% maxVc:          100%        100%        100%        100%       100%       100% minVc:            0%          0%          0%          0%         0%         1% maxVcBw:          0           0           0           0          0          0 m8850-7a.7.PXM.a > 

NOTE

The command dsppnportcac shows a minimum guaranteed bandwidth for the signaling service category of approximately 1 percent. This is done to reserve some bandwidth for the signaling VCs, because the signaling service types are critical. A minimum LCN of 1 percent is also configured by default.

In real life, the signaling VCs are more active when the interface doesn't have much traffic. For example, after an interface flap, the signaling VCs set up all the SVCs and SPVCs that were previously torn down, but there is no user traffic. In contrast, the more VCs that are created (and, therefore, the more user traffic), the less traffic there is on the signaling VCs.

You can display the per-class subpartition and except CAC values from the VSI slave using the command dspload, as shown in Example 11-44.

Example 11-44. Displaying the Subpartition with dspload

 m8850-7a.1.AXSM.a > dspload 2 1         +--------------------------------------------+         |   I N T E R F A C E    L O A D   I N F O   |         +--------------------------------------------+         | Maximum Channels         : 0004000         |         | Guaranteed Channels      : 0001000         |         | Igr Maximum Bandwidth    : 0096000         |         | Igr Guaranteed Bandwidth : 0096000         |         | Egr Maximum Bandwidth    : 0096000         |         | Egr Guaranteed Bandwidth : 0096000         |         | Available Igr Channels   : 0003995         |         | Available Egr Channels   : 0003995         |         | Available Igr Bandwidth  : 0093293         |         | Available Egr Bandwidth  : 0093293         |         +--------------------------------------------+         |          E X C E P T -- V A L U E S        |         +--------------------------------------------+         | SERV-CATEG | VAR-TYPE | INGRESS  | EGRESS  |         | VSI-SIG    | Avl Chnl | 0003995  | 0003995 |         | CBR        | Avl Chnl | 0003995  | 0003995 |         | VBR-RT     | Avl Chnl | 0003995  | 0003995 |         | VBR-nRT    | Avl Chnl | 0003995  | 0003995 |         | UBR        | Avl Chnl | 0003995  | 0003995 |         | ABR        | Avl Chnl | 0003995  | 0003995 |         +--------------------------------------------+         | VSI-SIG    | Avl Bw   | 0055893  | 0055893 |         | CBR        | Avl Bw   | 0093293  | 0093293 |         | VBR-RT     | Avl Bw   | 0055893  | 0055893 |         | VBR-nRT    | Avl Bw   | 0055893  | 0055893 |         | UBR        | Avl Bw   | 0055893  | 0055893 |         | ABR        | Avl Bw   | 0055893  | 0055893 |         +--------------------------------------------+ m8850-7a.1.AXSM.a > 

This example is summarized in Figure 11-11.

Figure 11-11. Bandwidth Subpartitioning in CAC Example 2

CAC Example 3

In the third example, four CBR connections use 2100 CPS in the PnPort. The initial PnPort CAC configuration is as Example 11-45 shows.

Example 11-45. Checking the PnPort CAC Initial Values

 m8850-7a.7.PXM.a > dsppnportcac 2:1.1:1                 cbr:     rt-vbr:    nrt-vbr:        ubr:       abr:       sig: bookFactor:     100%        100%        100%        100%       100%       100% maxBw:     100.0000%   100.0000%   100.0000%   100.0000%  100.0000%  100.0000% minBw:       0.0000%     0.0000%     0.0000%     0.0000%    0.0000%    0.6681% maxVc:          100%        100%        100%        100%       100%       100% minVc:            0%          0%          0%          0%         0%         1% maxVcBw:          0           0           0           0          0          0 m8850-7a.7.PXM.a > 

There is 0.6681 percent of guaranteed bandwidth and 1 percent of guaranteed LCN for the signaling service category. As Example 11-46 shows, using the command dsppnportrsrc, allows you to see the breakdown of actual values expressed in CPS and the number of channels.

Example 11-46. Using the Command dsppnportrsrc to Check Per-Class Resource Allocation

 m8850-7a.7.PXM.a > dsppnportrsrc 2:1.1:1                       cbr:    rt-vbr:   nrt-vbr:       ubr:       abr:      sig: Max TxCR CPS:        50000      50000      50000      50000      50000     50000 Max RxCR CPS:        50000      50000      50000      50000      50000     50000 MinGuar TxCR CPS:        0          0          0          0          0       334 MinGuar RxCR CPS:        0          0          0          0          0       334 Min Tx CLR:             10          8          6          6          6         8 Min Rx CLR:             10          8          6          6          6         8 Avl TxCR CPS:        47566      47566      47566      47566      47566     47566 Avl RxCR CPS:        47566      47566      47566      47566      47566     47566 OvSub AvTx CPS:      47566      47566      47566      47566      47566     47566 OvSub AvRx CPS:      47566      47566      47566      47566      47566     47566 # Avl Tx Chans:       1986       1986       1986       1986       1986      1995 # Avl Rx Chans:       1986       1986       1986       1986       1986      1995 m8850-7a.7.PXM.a > 

Regarding bandwidth subpartitioning per class of service in the partition, the guaranteed bandwidth for the signaling of 0.6681 percent out of 50,000 CPS equals 334 CPS. That is why the signaling service category has a guaranteed bandwidth of 334 CPS. You can see that the signaling service category uses 334 CPS because its available bandwidth equals the available bandwidth in other service categories.

Subtracting 334 CPS plus the used bandwidth (2100 CPS) from 50,000 CPS of total bandwidth gives you 47,566 CPS shared by all service categories.

Now the book factor is configured in this PnPort for the CBR service category. The book factor can be seen as the percentage utilization for the service category in the partition. A book factor of 50 percent means that the port is oversubscribed on a 2:1 ratio for the CBR service category. A book factor of 200 percent indicates an undersubscription factor of the same ratio. See Example 11-47.

Example 11-47. Configuring the bookfactor to Oversubscribe

 m8850-7a.7.PXM.a > cnfpnportcac 2:1.1:1 cbr -bookfactor 50 WARNING: New CAC parameters apply to existing connections also m8850-7a.7.PXM.a > 

In this configuration, the CBR service types are oversubscribed by 2:1. You can see this in the output of the command dsppnportrsrc, as Example 11-48 shows.

Example 11-48. Checking Oversubscription Using the Command dsppnportrsrc

 m8850-7a.7.PXM.a > dsppnportrsrc 2:1.1:1                       cbr:    rt-vbr:   nrt-vbr:       ubr:       abr:      sig: Max TxCR CPS:        50000      50000      50000      50000      50000     50000 Max RxCR CPS:        50000      50000      50000      50000      50000     50000 MinGuar TxCR CPS:        0          0          0          0          0       334 MinGuar RxCR CPS:        0          0          0          0          0       334 Min Tx CLR:             10          8          6          6          6         8 Min Rx CLR:             10          8          6          6          6         8 Avl TxCR CPS:        48616      48616      48616      48616      48616     48616 Avl RxCR CPS:        48616      48616      48616      48616      48616     48616 OvSub AvTx CPS:      97232      48616      48616      48616      48616     48616 OvSub AvRx CPS:      97232      48616      48616      48616      48616     48616 # Avl Tx Chans:       1986       1986       1986       1986       1986      1995 # Avl Rx Chans:       1986       1986       1986       1986       1986      1995 m8850-7a.7.PXM.a > 

NOTE

This example shows per-partition and per-class overbooking. Overbooking can also be performed on a per-connection basis.

You can see that the available bandwidth for all service types increased by 1050 CPS. This is because the four CBR connections with a total of 2100 CPS now have an effective rate of 2100 CPS divided by 2 (1050 CPS). The CBR service category is oversubscribed by a factor of 2. See Figure 11-12.

Figure 11-12. Oversubscription in CAC Example 3

In Figure 11-12, the switch does not create bandwidth for CBR type connections, but it applies the book factor to CBR call reservations. So if a CBR connection is configured with PCR = 1000 CPS, only 500 CPS is reserved in the load model, making the effective bandwidth twice as much.

NOTE

For SVC and SPVC calls, the actual connection bandwidth is used for the GCAC algorithm. However, PNNI advertises a link bandwidth equal to the actual link bandwidth divided by the book factor. This is done because a connection can span multiple PNNI links with a different book factor.

For example, on a 100 Mbps link with a configured book factor of 10 percent, PNNI advertises a link of 1000 Mbps. For a call of 20 Mbps, GCAC uses 20 Mbps, but the actual CAC performed by the VSI slave uses 2 Mbps.

In summary, the connection bandwidth is adjusted based on the book factor at the CAC level in the VSI slave, but not at the GCAC level in the PNNI process. At the PNNI level, the book factor is considered when links are advertised.

You now configure the minimum guaranteed bandwidth for the CBR service category equal to 10 percent of the total partition bandwidth. See Example 11-49.

Example 11-49. Changing the CAC Minimum Bandwidth for CBR

 m8850-7a.7.PXM.a > cnfpnportcac 2:1.1:1 cbr -minbw 10 WARNING: New CAC parameters apply to existing connections also m8850-7a.7.PXM.a > 

You have the available bandwidth and channels for all service classes as Example 11-50 shows.

Example 11-50. Checking Available Resources Using the Command dsppnportrsrc

 m8850-7a.7.PXM.a > dsppnportrsrc 2:1.1:1                       cbr:    rt-vbr:   nrt-vbr:       ubr:       abr:      sig: Max TxCR CPS:        50000      50000      50000      50000      50000     50000 Max RxCR CPS:        50000      50000      50000      50000      50000     50000 MinGuar TxCR CPS:     5000          0          0          0          0       334 MinGuar RxCR CPS:     5000          0          0          0          0       334 Min Tx CLR:             10          8          6          6          6         8 Min Rx CLR:             10          8          6          6          6         8 Avl TxCR CPS:        48616      44666      44666      44666      44666     44666 Avl RxCR CPS:        48616      44666      44666      44666      44666     44666 OvSub AvTx CPS:      97232      44666      44666      44666      44666     44666 OvSub AvRx CPS:      97232      44666      44666      44666      44666     44666 # Avl Tx Chans:       1986       1986       1986       1986       1986      1995 # Avl Rx Chans:       1986       1986       1986       1986       1986      1995 m8850-7a.7.PXM.a > 

You can see that the CBR service category has a guaranteed bandwidth of 5000 CPS. This 5000 CPS was subtracted from the available bandwidth of all the service types. All service types (including CBR) share 44,666 CPS (which equals 50,000 CPS total partition bandwidth minus 334 CPS reserved for signaling service category minus 5000 CPS) set aside for CBR service category.

The CBR service category has a total of 50,000 CPS minus 334 CPS, or 49,666 CPS (from which 5000 CPS is guaranteed and the rest is shared). The four CBR connections effectively use 1050 CPS, so the available bandwidth equals 48,616 CPS. The bandwidth set aside for the four CBR connections is taken from the CBR reserved guaranteed bandwidth.