I am currently about to implement a Frame-Relay network, starting off with one Central Site and two remote sites.
The remote sites don't need interconnectivty between themselves, therefore I will implement a 'Hub and Spoke' network design.
The questions I have are, 1. What are the advanteges/disadvantages to using sub-interfaces? 2. What are the practical limits (the central router is a 3640 w/48M of memory with the number of sub-interfaces/DLCI's as there will be additional sites added at a later stage (max. approx. 6)
For implementing a network of the size you mention there is no problems. There are no problems with using sub-interfaces and in a lot ways the advantages by far outweigh the disadvantages. There is something called IDB's (Interface Descriptor Blocks) which used to be a limitation but nowadays it doesn't really matter because you chances are you will oversubsribe the interface/router before you run of them. If you have problem with a shortage of IP addresses I suggest using IP unnumbered and ip route statements pointing staight to the interface. Alternatively you can also use a dynamic routing protocol on this config.
Subinterfaces were designed for this type of frame relay setup. Specifically, they benefit you if you run a dynamic protocol. Think of split horizon. Without subinterfaces, the hub learns of remote site a. It can't tell remote site b about it back out on the same interface (and vice versa). So a and b know nothing about one another. Of course, there are ways around that (I think map statements would work). But that was the general idea as I understand it.
1.Sub-interface is using to configure multiple logical PVC in one phycial serial interface ( T1 ).
2. The pritical limit is base on your routing table and traffic. If you are not running BGP on the router. If it is only using for Frame Relay with EIGRP or OSPF between couple site It is not problem. I am using 3640 with digital modem dialup ( PRI ) and have connect 9 offices on it and doing multicasting also. It is working fine. Do not need to worry about it. Just do "show process cpu and show memory " to check the CPU utilizaton and memory usage.
Using subinterfaces for frame relay is also a great advantage when using a backup line such as ISDN. When using the "backup delay" command in the ISDN interface config, you are specifying that when the Frame interface goes down, ISDN should come up. Without a subinterface, the PVC may break but the frame interface will still show up and up and the backup delay timer will not start. With sub interfaces, as soon as the subinterface's PVC breaks, the subinterface goes down and kicks in the backup delay cauing the ISDN link to come active. In this situation you are specifying that the ISDN line is backing up the subinterface instead of the physical interface.
1)With sub-interfaces, management is a breaze; easy to view stats, and easy to apply map-class statements.
2) Make sure you get traffic shaping working. It is paramount for F/R as your sites all have different access rates (CIR/EIR).
3) We use compression and tends to load the CPU of the router (we have 7 sites connected, most using compression) We use MRTG to monitor traffic flow, and you can also use MRTG to monitor CPU/memory usage. (Great tool!)
4) The 3640 is very adequate for what you want to do. We are using a 26XX to connect 7 sites, but compression is going to cause CPU/mem problems. MRTG shows that with full data transfers (to ALL sites), the router can hit 80% utilisation, which is probably the highest limit you should push the router.
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