Hi, since you also brought in the network layer into the discussion, I think it would be good to discuss some more generic questions than what you asked originally (specifically about serial/frame relay):
a.) Why do I need to assign an encapsulation on the physical interface? b.) Why can I have only one encapsulation on a physical interface? c.) Why can't I specify an encapsulation on the subinterface level? My answers: a.) "Encapsulation" in this context really means "data format". The purpose of specifying an encapsulation on the physical interface is to let the router know what is the ultimate format that should enter and especially *leave* the router on that particular *physical* interface. You can't receive/send data on a subinterface per se, you can only *logically* assign the already received data to a subinterface based on the information you extracted from the data you already received. So what would you do without having an encapsulation assigned to the physical interface? Would you check for every possible L2 format, say ATM cells on Ethernet interfaces? :) b.) If the encapsulation/decapsulation is done in hardware, and the interface hardware supports only one encapsulation per physical interface at a time, as Larry alluded to earlier, then you obviously can have only one encapsulation per physical interface. More importantly though: how would you decide in which format the information needs to be sent out if you had more than one encapsulation? c.) In general, you can. In the case of Ethernet interfaces for instance, you can specify encapsulation on a subinterface level. In fact I seem to remember that you *had* to specify it on a subinterface level for a while, and it was in fact the encapsulation type that selected the subinterface. The reason for this behavior is that "Ethernet" has two sub-layers within Layer 2, so even there you have an implied encapsulation assigned to the physical interface (the lower of the two sub-layers, the IEEE 802.3/Ethernet II format), and only the higher layer (layer 802.2) encapsulation is assigned to a subinterface. There was a lengthy discussion about Ethernet encapsulations on this list a few days ago, btw. At this point, the more specific question is in order: d.) Why can I not specify any encapsulation on a Serial sub-interface in IOS? Well, perhaps because the encapsulation you specified at the physical level (see above why you have to have that) took care of everything that you would characterize as "encapsulation". This of course doesn't mean that all the packets assigned to a subinterface are the same, but for some reason we don't speak about "IP encapsulation" vs. "IPX encapsulation" and the like. >If it's possible to assign a network address ( IP / IPX viz., ) to the >FR sub-interface why not be able to specify encap as well..?? Network addresses don't specify the data format, they are the data themselves. If you wanted to ask "how come I can run IP and IPX on the same interface", then the answer is "because something at a lower layer will usually indicate what kind of packet you are receiving". Thanks, Zsombor At 08:55 AM 6/26/2003 +0000, Srivathsan Ananthachari wrote: >If it's possible to assign a network address ( IP / IPX viz., ) to the >FR sub-interface why not be able to specify encap as well..?? > >I hope I'm not draggin it.../ > >Srivathsan > >-----Original Message----- >From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of >Mwalie W >Sent: Thursday, June 26, 2003 12:42 PM >To: [EMAIL PROTECTED] >Subject: RE: Subinterfaces [7:71421] > > >Hi, > >Well, encapsulation is done prior to placing the packets(Frames) to the >physical medium - I guess it should be done on the physical interface. > >I would be interested in what other members have to say, but I think it >makes sense that it should be on the physical interface. > >Mwalie Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=71440&t=71421 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
