Conference noted::decnis

Hi Jos�,

>    
>    We are working in a project that requires the connection of 80 remote
>    offices to a central site using Frame Relay.
>    
>    Although I was intended that the DECnis was designed as a central
>    router to connect remote offices I have found that this router has very
>    important limitations:
>    
>    - why only 32 PVCs are supported by physical interface (in X.25 the
>    most basic processor supports 128 active SVCs)?. Is it planned to
>    increase this number?
> 

	The number of 32 PVCs per interface is pretty well fixed into the
	basic design on the DECnis, and although it would not be impossible 
	to come up with a method that might allow a higher number of PVC
	we would need a good business case before starting on this work.


	A few questions:

	If you had 80 PVCs on a 2Mbps link this only gives each circuit
	only ~19Kbps bandwidth per PVC. 

	Many of our customer in the past have been worried about a single 
	wire/interface failure taking down their whole network.


   
>    - why DECnis only routes TCP/IP over PPP in Frame Relay links?. Is
>    there any technical reason to show our customers that it is better to
>    encapsulate TCP/IP over PPP instead of using TCP/IP over native Frame
>    Relay (RFC 1490 ?). Encapsulating over TCP/IP probably envolves
>    aditional overhead. Is it planned to support RFC 1490?
 

	We currently support encapsulation in either PPP or 
	CHDLC (cisco own HDLC, and default encapsulation) format.

	As I write this note I have just completed coding the linecard
	forwarding code to add RFC1490 encapsulation to the DECnis
	code base. This code will ship in a later version on the 
	DECnis V4.0 baselevel.

	However from a overhead point of view the differences between
	the 3 protocols, RFC1490 is sometimes the least efficient..!



			PPP		CHDLC		RFC1490
			===		=====		=======

	IP		4		2		2
	OSI		4		3		1
	DECnet		4		2		8
	Bridge (Enet)	6		2		8
	Bridge (FDDI)	7		NA		9



>   
>    The RouteAbout router that normaly is used as a back end router
>    supports 64 PVCs by serial interface and also routes TCP/IP over 
>    Frame Relay using PPP or native mode.  
>    
 
	I beleive that only the RouteAbout central offers 64 PVCs, and
	this has only fairly recently been increased from 32.

	I will check this with Neil Turner next time I see him.


Cheers Tony

    Dear Tony,
    
    
    Fist of all thanks a lot for your quick answer.
    
    Regarding your comments:
    
    In my opinion I think only 32 PVC for a 2 Mbytes line is a very
    important limitation. Consider that 128-256 SVCs are supported under
    X.25 links which normally are not intended to work over more than 64 Kbps.
    At least in Spain, it is very common to contact Frame Realy PVCs of 16
    Kbps. 
    
    In our particular case, that is the project we are working in Spain, it
    should be enough to split the 80 remote connections between two serial
    lines (due to performance and also for redundancy). 
    
    But using the DECnis we need three lines and what it is worst, 
    we cannot setup a line as a backup for the other lines, because 
    every physical line only supports 32 PVCs.
    
    Therefore, if it is not planned to increase the number of PVCs
    supported by physical line, in our project we will have to replace 
    two DECnis, which are already installed, by CISCOs.
    
    Sincerely,
    
    
    Jose Luis Platas.
    
    pd.: By the way, according to your reply I guess that overhead of 1 means 
    the most efficient ?.
    
    
               

We are not going to increase the number of PVCs supported on each line to more
than 32 so for this network you will have to use some other router. Good luck
getting the network to be usable with that many PVCs on a link and even more
luck will be required to get the fail-over situation to be viable. It could even
be better to have no service at all for half the users than a completely
useless service for all the users.

I would strongly recommend using prioritisation and per-PVC flow control in this
sort of situation. You'll need to use the very latest cisco release as cisco
admit that fair-queueing over FR doesn't work in IOS V11.2

John

Had you  considered  RouteAbout  Central  as an alternative to DECNIS?.  Not
quite sure of your requirements, but since you seem to be using RA's for the
"back end router", why not look at RA Central for the "front end"?.

Mark

    Thanks a lot for your answers.
    
    
    Now assuming that DECnis will not be able to support such configuration
    we will evaluate the RouteAbout Central alternative before installing
    Ciscos.
    
    
    Sincerely,
    
    
    Jos� Luis Platas.