The MPLS-OPS Archive

[Ravindra Botkar <ravibotkar@yahoo.com>]

Problem with cell interleaving :

A diagram might help you to understand the case :-

Router A ---- Router B -------Router C ------ FEC A
                                       |
                                        ------FEC B

Router C will assign different lables for FEC A and B

Router B will assign same label for both the FECs
since they are reachable from the same interface.
Router A will also assign the same label.

Now if Router A forward cells (belonging to different
FEC) to Router B. Then Router B will not be able to
differentiate between the two as the labels get mapped
to VPI/VCI. This problem is due to Cell Interleaving.

There are 2 approaches to the problem :-

1) First to send all the cells belonging to only FEC A
by clubbing them together at Router A and then the
second clubbing for FEC B. -- Memory Intensive in case
there are lots of FECs

2) Second approach is where router A will ask for
different labels for the FECs from downstream router
B..which is Downstream-On-Demand Label Distribution.

Hence ATM LSRs use Downstream-On-Demand Label
Distribution.

Regards,

Ravindra





 

--- Ian <ipotts@msbx.net> wrote:

> Hello Ravindra,
> 
> Thank you for your reply.  My understanding is that
> a Non-ATM LSR uses
> independent LSP control, not ordered.  This is where
> my question comes from.
> 
> If Non-ATM LSRs can use independent LSP control, why
> can't ATM LSRs do the
> same?
> 
> 
> Thank you,
> Ian
> 
> 
> 
> Quoting Ravindra Botkar <ravibotkar@yahoo.com>:
> 
> > Hello Ian, 
> > 
> > Maybe this could help you to understand :-
> > 
> > Non-ATM LSR --> Frame Mode MPLS 
> > 1) Downstream Unsolicited Label Distribution (as
> they
> > are  not subject to Cell interleaving problem ) 
> > 2) Ordered LSP Control (Prevents  data  
> forwarding
> > across partially established paths) 
> > 3) Liberal Retention Mode (faster adaptation to
> state
> > and routing changes in the network)
> > 
> > ATM-LSR --> Cell Mode MPLS (MPLS enabled ATM) 
> > 1) Downstream-On-Demand Label Distrbution (Avoids
> Cell
> > Interleaving Problem but eats up the VCI
> resources) 
> > 2) Ordered LSP Control (Prevents  data  
> forwarding
> > across partially established paths)  
> > 3) Conservative Retention Mode (minimize no of
> labels
> > VPI/VCI thereby optimizing resources of each ATM
> LSR )
> > 
> > Regards,
> > Ravi
> > 
> > --- Ian <ipotts@msbx.net> wrote:
> > 
> > > Hello Ravindra,
> > > 
> > > Thank you for your reply.  Couldn't the same be
> > > argued for frame LSRs?  Couldn'
> > > t independent label distribution also lead to
> > > partially established tunnels for
> > > a period of time leading to drops?  
> > > 
> > > Many Thanks,
> > > Ian
> > > 
> > > 
> > > 
> > > Quoting Ravindra Botkar <ravibotkar@yahoo.com>:
> > > 
> > > > 
> > > > Independent label distribution could lead to
> data
> > > > forwarding across partially established paths,
> > > thus
> > > > creating black holes....
> > > > 
> > > > Regds,
> > > > Ravindra 
> > > > 
> > > > --- Ian <ipotts@msbx.net> wrote:
> > > > 
> > > > > Hello,
> > > > > 
> > > > > Would you please tell me why ATM switches do
> > > ordered
> > > > > label not independent label
> > > > > distribution.  In other words when the ATM
> > > switch
> > > > > gets a label request, why
> > > > > doesn't it immediately allocate a label and
> send
> > > it
> > > > > upstream, why does it need
> > > > > to forward a label request downstream and
> not
> > > reply
> > > > > upstream until it gets a
> > > > > response?
> > > > > 
> > > > > I have heard that it is something to do with
> the
> > > ATM
> > > > > switch not being able to do
> > > > > a layer 3 lookup?
> > > > > 
> > > > > Thanks in advance.
> > > > > 
> > > > > 
> > > > > 
> > > > > 
> > > > > 
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