7.4 ATM Interfaces


ATM is a very popular high-speed WAN protocol. Many logical units can be configured on each port, with each logical unit configured for different QoS options. This can be a powerful tool for connectivity options. The disadvantages of ATM are extremely high overhead and an upward speed limitation of OC-12.

In addition to the large cell header overhead, the segmentation and reassembly (SAR) of packets to ATM cells and back also use many resources on an interface, further limiting the overall throughput. Juniper Networks has two ports per PIC for OC-3 ATM and one port per PIC for OC-12 ATM. In addition to the optical ATM interfaces, Juniper Networks implements ATM on electrical DS-3 and E-3 ports using coaxial cable. Juniper Networks supports standard PVCs on ATM logical units.

Juniper Networks enables either PVC encapsulation or ATM CCC encapsulation for cell relay on a physical interface. A variety of ATM encapsulations can be configured on the PVC logical units. The most common of the PVC encapsulations (and Juniper Networks default) is ATM-SNAP. ATM-NLPID and ATM-CISCO-NLPID are also available for routing packets. There are two options for CCC: cell relay and VC-Mux.

Juniper Networks supports three QoS classes on PVCs, defaulting to UBR as the lowest class. VBR and CBR are the other two classes supported, with CBR being the highest-priority class followed by VBR.

In addition to those features mentioned above, Juniper Networks supports integrated local management interface (ILMI) in its ATM implementation. ILMI allows management information to flow between directly connected ATM devices, such as the router and an ATM switch. ILMI is set at VPI 0 VCI 16 when configured on a physical interface.

7.4.1 Physical Interface Configuration

Since the optical transport for ATM is SONET/SDH, the same physical features can be configured under the options for SONET on an ATM physical interface, including APS. Aggregating links is one feature that cannot be configured on an ATM link. In addition to all of the options for SONET that are available, there are options for ATM that can be configured, such as enabling ILMI and configuring the virtual paths that can be used on the interface.

Juniper Networks ATM interfaces support more than 4,000 virtual circuits (VCs) per physical interface. To enable VC assignment per logical unit, you must first assign the virtual path to the physical interface and define the maximum VCs for that virtual path under the ATM options. The following example first shows the assignment of VPI 1 to ATM interface 0/1/1 on router New York, then the assignment of the maximum VC number that can be used:

 [edit interfaces at-0/1/0]  lab@newyork# set atm-options ? Possible completions: + apply-groups         Groups from which to inherit configuration data > ilmi                 Enable Interim Local Management Interface >  vpi                  Define a virtual path  [edit interfaces at-0/1/1] lab@newyork# set atm-options vpi 1 ? Possible completions:  maximum-vcs          Maximum number of virtual circuits on this VP  [edit interfaces at-0/1/1] lab@newyork# set atm-options vpi 1 maximum-vcs 250 [edit interfaces at-0/1/1] lab@newyork# 

If more virtual paths need to be configured, they can be assigned in a similar manner.

7.4.2 Logical Interface Configuration

The PVC in ATM is configured on the logical unit. Some of the unit logical unit parameters that can be configured include the following:

  • ATM PVC-type encapsulation

  • VPI (optional)/VCI assignment

  • Family addressing

  • Type of PVC

  • Any traffic-shaping QoS parameters

As mentioned earlier, the default encapsulation for ATM logical units is ATM-SNAP for standard ATM encapsulation of IP packets. The other possible encapsulation alternatives are shown below:

 [edit interfaces at-0/1/0 unit 200]  lab@newyork# set encapsulation ? Possible completions:   atm-ccc-cell-relay   ATM Cell Relay for CCC   atm-ccc-vc-mux       ATM VC for CCC   atm-cisco-nlpid      Cisco-compatible ATM NLPID encapsulation   atm-nlpid            ATM NLPID encapsulation   atm-snap             ATM LLC/SNAP encapsulation   atm-vc-mux           ATM VC multiplexing [edit interfaces at-0/1/1 unit 200] lab@newyork# 

The Network Layer Protocol ID (NLPID) is a field used in Frame Relay headers. Frame Relay “type service to ATM networks requires a Frame Relay “type encapsulation of the packets before they are segmented into the payloads of ATM cells. The IETF standardized one Frame Relay encapsulation and Cisco created another. The difference between the IETF-based NLPID and Cisco-based NLPID standards is as follows : VC-Mux is the VC-based-multiplexing encapsulation type used when each protocol gets its own VC. These encapsulation types for ATM are defined in IETF RFC 1483; although RFC 2684 renders 1483 obsolete, Juniper Networks documentation references 1483 explicitly.

When configuring the VPI/VCI for a PVC, if only one VP were assigned to the entire physical interface with the vpi # maximum-vcs # command, then only one would need to be configured with the VCI assignment. If more than one VP were assigned to the physical interface, the vpi.vci format would have to be used.

An ATM logical unit can be point-to-point or -multipoint. Juniper Networks ATM logical units default to point-to-point. If that is the type you want, then you do not have to specify as much during configuration. If you want multipoint configured, then you will have to configure a destination statement for each destination on the link. This is configured in the family inet hierarchy with the multipoint-destination (address/xx) command, where the address is the IP address and xx is the address prefix.

In Figure 7-13, router Chicago has an ATM PVC to router New York using VPI 2 VCI 200.

Figure 7-13. ATM Basic Configuration

graphics/07fig13.gif

Since the PVC is assigned values of VPI 2 VCI 200, VPI 2 must be assigned to the physical interface with the maximum number of VCs that can be assigned for that virtual path. After that, the specific unit is configured. Since router Chicago has more than one VPI assigned to the physical interface, the VPI must be designated in the unit configuration. For this example, the point-to-point logical unit type is illustrated , although it does not have to be, since it is the default. If it is manually typed in, it will be output in the configuration.

 [edit interfaces at-0/1/0]  lab@chicago# set atm-options vpi 2 maximum-vcs 250 [edit interfaces at-0/1/0] lab@chicago# edit unit 201 [edit interfaces at-0/1/0 unit 201] lab@chicago# set vci 2.200 [edit interfaces at-0/1/0 unit 201] lab@chicago# set family inet address 192.168.20.1/30 [edit interfaces at-0/1/0 unit 201] lab@chicago# set point-to-point [edit interfaces at-0/1/0 unit 201] lab@chicago# show point-to-point; vci 2.200; family inet {     address 192.168.20.1/30; } [edit interfaces at-0/1/0 unit 201] lab@chicago# 

Router New York is already configured appropriately to be connected to Chicago. The VPI and VCI are configured, along with the IP address.

 [edit interfaces at-0/1/0]  lab@newyork# show atm-options {     vpi 2 maximum-vcs 250; } unit 202 {     vci 2.200;     family inet {         address 192.168.20.2/30;     } } [edit interfaces at-0/1/0] lab@newyork# 

Notice in the output above from New York that even though the configuration does not show the point-to-point logical unit (because it was not manually entered into the configuration), it is shown in the logical unit status and statistics below:

 lab@newyork> show interfaces at-0/1/0  Physical interface: at-0/1/0, Enabled, Physical link is Up   Interface index: 13, SNMP ifIndex: 17   Link-level type: ATM-PVC, MTU: 4482, Clocking: Internal, SONET mode,   Speed: OC3, Loopback: None, Payload scrambler: Enabled   Device flags   : Present Running   Link flags     : None   Input rate     : 0 bps (0 pps)   Output rate    : 0 bps (0 pps)   SONET alarms   : None   SONET defects  : None   Logical interface at-0/1/0.202 (Index 13) (SNMP ifIndex 39)  Flags: Point-To-Point  SNMP-Traps Encapsulation: ATM-SNAP   Input packets : 4   Output packets: 4     Protocol inet, MTU: 4470, Flags: None       Addresses, Flags: Is-Preferred Is-Primary         Destination: 192.168.20.0/30, Local: 192.168.20.2     VCI 2.200       Flags: Active       Total down time: 0 sec, Last down: Never       Traffic statistics:        Input  packets:                    4        Output packets:                    4 lab@newyork> 

7.4.3 ATM QoS Configuration

One of the advantages of ATM is the maturity of the QoS. These features allow priority and guaranteed service to the PVCs that require it. Data that is time-sensitive or highest-priority can be sent along in the CBR class. Data that is more important than best-effort, but is not of the highest priority, can be forwarded on a logical unit assigned the VBR class. Any other logical units can be assigned UBR, whatever bandwidth is left when the other two classes have used what they need.

QoS is configured in the logical unit hierarchy with the set shaping command. UBR is not a choice if CBR or VBR is configured. If you want to change the setting to UBR, the CBR or VBR setting must be deleted. The following example shows possible completions for this command.

 [edit interfaces at-0/1/0 unit 201]  lab@chicago# set shaping ? Possible completions: + apply-groups         Groups from which to inherit configuration data  cbr                  Continuous bandwidth utilization (33000..271000000)  queue-length         Queue length (1..16383) >  vbr                  Variable bandwidth utilization  [edit interfaces at-0/1/0 unit 201] lab@chicago# 

Configuring for CBR only requires one value: the highest constant rate at which this logical unit is allowed to transmit.

 [edit interfaces at-0/1/0 unit 201]  lab@chicago# set shaping cbr ? Possible completions:   <cbr>                Continuous bandwidth utilization (33000..271000000) [edit interfaces at-0/1/0 unit 201] lab@chicago# 

CBR is defined in bits per second. Instead of typing out all of the numbers , letters may substitute for a number of zeros. The letter k can be substituted for 1,000, m for 1,000,000, and g for 1,000,000,000. If you set a CBR of 12m , this is 12 million bits per second, or bps. In addition to bps, cell per second, or cps, can be used as the CBR value to be configured. Each ATM cell has 384 payload bits after the header (48 bytes x 8 bits/byte = 384). To calculate the equivalent bits per second, multiply the number of cells by 384 with a small c behind the value. In the example below, 2,000 cps is configured as the CBR rate. When shown, the output will be converted into the bit rate as is shown below with a CBR of 768k (768k/2k = 384).

 [edit interfaces at-0/1/0 unit 201]  lab@chicago#  set shaping cbr 2000c  [edit interfaces at-0/1/0 unit 201] lab@chicago# show point-to-point; vci 2.200;  shaping {   cbr 768k;  } family inet {     address 192.168.20.1/30; } [edit interfaces at-0/1/0 unit 201] lab@chicago# 

VBR has several different parameters. Since the bandwidth allowed is variable, a normal rate limit and a quick temporary high limit are defined. In addition, the length of the temporary high limit is also defined. The normal limit for a VBR value is known as the sustained rate, whereas the temporary high limit for a VBR logical unit is known as the peak rate. The maximum number of cells allowed at the peak rate before the transmission must drop to the sustained rate is known as the burst length. The configuration options are shown below:

 [edit interfaces at-0/1/0 unit 201]  lab@chicago# set shaping vbr ? Possible completions:   burst                Burst size (1..255)   peak                 Peak rate (33000..271000000)   sustained            Sustained rate (33000..271000000) [edit interfaces at-0/1/0 unit 201] lab@chicago# 

Note

Even though the rates in the parentheses above seem to allow 271Mbps, the ATM interfaces on router New York are OC-3, with an actual bit rate limit of approximately 135.5Mbps. Juniper Networks implements a limit of 50 percent of line rate on OC-12 ATM interfaces for CBR and VBR values of 271Mbps. The following is an example of setting a VC to 384Kbps sustained rate (normal) and 512Kbps peak rate (high limit) and limiting the peak rate to a 32 cell burst.


 [edit interfaces at-0/1/0 unit 201]  lab@chicago# set shaping vbr sustained 384k peak 512k burst 32 [edit interfaces at-0/1/0 unit 201] lab@chicago# show point-to-point; vci 2.200; shaping {     vbr peak 512k sustained 384k burst 32; } family inet {     address 192.168.20.1/30; } [edit interfaces at-0/1/0 unit 201] lab@chicago# 

Building on the configuration for router Chicago, two more units, one each of UBR and CBR are added. In Figure 7-14, the ATM interface at-0/1/0 is shown with three logical units, each with a different priority assigned.

Figure 7-14. ATM QoS Configuration

graphics/07fig14.gif

You configure the unit 58 and unit 162 in the same manner as unit 201 . Do not forget that vpi 1 will have to be added in the atm-options on the physical interface because unit 58 has a VPI of 1. When it is complete, the configuration will look as follows:

 [edit interfaces at-0/1/0]  lab@chicago# show atm-options {     vpi 0 maximum-vcs 250;     vpi 2 maximum-vcs 250;     vpi 1 maximum-vcs 250; } unit 58 {     vci 1.90;     shaping {         cbr 1500000;     }     family inet {         address 192.168.60.201/30;     } } unit 162 {     vci 0.67;     family inet {         address 192.168.130.13/30;     } } unit 201 {     point-to-point;     vci 2.200;     shaping {         vbr peak 512k sustained 384k burst 32;     }     family inet {         address 192.168.20.1/30;     } } [edit interfaces at-0/1/0] lab@chicago# 

This has covered basic ATM interface configuration. VPIs are assigned to interfaces and then the VPI/VCI area is assigned to the logical units to connect to PVCs. QoS traffic priorities can be assigned to those logical units to give priority to certain logical units over others.



Juniper Networks Reference Guide. JUNOS Routing, Configuration, and Architecture
Juniper Networks Reference Guide: JUNOS Routing, Configuration, and Architecture: JUNOS Routing, Configuration, and Architecture
ISBN: 0201775921
EAN: 2147483647
Year: 2002
Pages: 176

flylib.com © 2008-2017.
If you may any questions please contact us: flylib@qtcs.net