Confusions in ATM

Unanswered Question
Mar 19th, 2010

Hi folks,

I am a new member here and this is my first post.

Right now I am studying ATM and have lots of confusions. ATM communication happens through VPI/VCI. What are these and what is there significance.

What is a PVC ?

Also, how can we configure multiple pvcs on single atm interface, if you need number, it is 2000 PVCs. How can we do that.

Please reply with the config also (just gimme an example, i will be able to understand).

What is IMA ? Please provide details.

I will really appreciate your help.

Thanks in advance

Regards

Ratnesh

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John Patrick Lopez Fri, 03/19/2010 - 15:22

I'll try to answer your questions base on what we are doing.


Usually, these VPI/VCI pairs are the layer2 identifier when trasmitted over the ATM switches (i.e. MGX 8850s or Lucent PSAX). A PVC as the name implies is a Permanent Virtual Circuit so it means permanent. VPI/VCI is like the DLCI of a Frame-relay circuit. Just like frame-relay, ATM also has point-to-point subinterfaces and multipoint subinterfaces. You can create multiple point-to-point subinterfaces if you want to have multiple point-to-point virtual circuits. Same as a multipoint, a multipoint sub-if can have multiple neighbors or peers on a single subinterface.


I don't know if you are sure to make 2000 VCs on a single ATM interface as it will be pain in the butt. When the interface goes down, all 2000VCs will also go down. And you will defin need a very powerful router to do that and I do not know if there is one. (I'm guessing on this part as I haven't read the white paper or data sheet of the routers)


HTH,

John

ratnesh.nagori Sat, 03/20/2010 - 00:05

Hi John,

Yes the router I am using is a powerfull one .

Can you please reply with config, how to create the PVC on ATM interface ?

Regards

Ratnesh    

Giuseppe Larosa Sat, 03/20/2010 - 05:36

Hello Ratnesh,

there are features that can be of help in your scenario like ILMI PVC discovery.

ILMI is the protocol spoken between a router ATM DTE interface and the connected ATM switch port.

see

Configuring PVC Discovery Example

The following example shows how to enable PVC Discovery on an ATM main  interface 2/0. The keyword subinterface is  used so that all discovered PVCs with a VPI value of 1 will be assigned  to the subinterface 2/0.1. For further information, refer to the section  "Configuring  PVC Discovery" earlier in this chapter.

interface atm 2/0
 pvc RouterA 0/16 ilmi
 exit
 atm ilmi-pvc-discovery subinterface
 exit
!
interface atm 2/0.1 multipoint
 ip address 172.21.51.5 255.255.255.0

see
http://www.cisco.com/en/US/docs/ios/atm/configuration/guide/atm_cfg_atm_ps6441_TSD_Products_Configuration_Guide_Chapter.html#wp1005115


Other possible tools for scalability can be the VC classes

If you look at the document from the beginning you will see that configuration ranges from a totally manual
configuration where the network admin specifies the PVC to be used and what remote device can be
reached on that specific VPI/VCI value.
At the other end there are the VC classes and the ILMI PVC autodiscovery features that takes advantage
of ATM signalling protocols for forms of auto-provisioning.

Another important note is the following:
each Cisco router platform has a limit on the maximum number of interfaces (including physical
and logical interfaces like loopbacks or tunnels) that can be configured on device.

Devices like C10000, C6400, or C7200 with specific license have an increased IDB count to deal
with high  number of subinterfaces

see
http://www.cisco.com/en/US/products/sw/iosswrel/ps1835/products_tech_note09186a0080094322.shtml

so not all platforms could support 2000 ATM subinterfaces.
To be noted that using multipoint  ATM subinterfaces one subif is associated to multiple
and potentially many ATM PVCs as shown in ATM ILMI PVC discovery example.

Hope to help
Giuseppe



John Patrick Lopez Wed, 03/24/2010 - 05:13

Hi Ratnesh,


Here's an example of point-to-point PVC.


RouterA:

interface ATM1/0.108 point-to-point

bandwidth 12500

ip address 10.200.20.137 255.255.255.252

ip accounting output-packets

delay 101

pvc 80/108

  protocol ip 10.200.20.138 broadcast

  vbr-nrt 2470 2470

  oam-pvc manage 3

  oam retry 3 3 1

  encapsulation aal5snap

!


RouterB:

interface ATM1/0.108 point-to-point

bandwidth 12500

ip address 10.200.20.138 255.255.255.252

delay 101

pvc 20/108

  protocol ip 10.200.20.137 broadcast

  vbr-nrt 2470 2470

  oam-pvc manage 3

  oam retry 3 3 1

  encapsulation aal5snap


HTH,

John

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