Re: selection of CPR/CBSR in pim

[Ahmed A-G Helmy <ahelmy@catarina.usc.edu>]

> Hi, Ahmed
> 
> I have a question on the selection of CRP/CBSR in the pim protocol.
> 
> Since the selction of RP will affect te performance of the routing 
> protocol, can 
> I use a random # generator to generate the 0 or 1 in the list [...]. 

you can, and you probably will get different performances, depending on 
the elected RP, the distribution of senders and members, membership 
dynamics and traffic patterns !

PIM-SM protocol spec does not provide a mechanism to optimize the RP 
placement (it is not part of the protocol, and it is believed by lots to 
be a complicated and impratical task due to membership dynamics).

> My question is,
> 
> (1) Is every router in the topology has to be configured? looks likes 
> it is the > case, right?

no, the routers come up with a default of non-RP and non-BSR (check 
pim-init.tcl, proc initialize, flags CRP and CBSR).
it suffices only to have one router configured as CRP or CBSR to things 
to work.

> 
> (2) What does "1" in the list[...] mean? the router has been elected as a 
> candidate RP/BSR?? and what does "0" mean ?

I admit that the list API is somewhat esoteric and better APIs may be 
developed... you can read pim-init.tcl, proc configure to know exactly 
how these 1s and 0s are being processed.
 Order is important in the list; the 1st element is the CRP setting, if 1 
means the router is a CRP, the 2nd element is the CBSR setting, if 1 
means the router is a CBSR, and if 1 then the 3rd element (if exists) is 
the setting of the priority of the CBSR (default is 0).

> 
> (3) Why in some pim node configurations, the list only contains one argument, 
> the first argument? so is that right to say every router is being 
> configured as 
> being CRP(in the case of "1") and not being a CRP (in the case of "0"), what 
> about CBSR &priority field configuration, ignored or there is some sort of 
> default? 

default is non-CBSR (in which case you don't care about the priority),... 
see above.

> 
> (4) If the CBSR has been configured as "1" in 2nd argument of list[...], what 
> should be the priority field-the 3rd argument in the list[...]? should 
> that be > "0" or "1", and what "1" or "0" in that field stands for?

if left out then priority is 0 by default, otherwise the 3rd argument is 
the priority.


note that the current pim implementation doesn't implement detailed CBSR 
or CRP elections,... the RP is selected in a centralized fashion (the 
highest ID CRP is the RP)... do if you are interested in dynamics and the 
effect of the locations of CBSR, the convergence time and transient 
behavior, then you'd have to add the detailed distributed algorithm to 
the pim code.

hope this helps,

Regards,
-Ahmed

> 
> pls help to clarify.
> 
> Tks,
> 
> shuqian Yan
> 
>