It may help to think of it this way. When you have a single physical and logical interface, it is easy for the router to determine how to process the incoming/outgoing traffic, it just uses the attributes assigned to the interface, that is its only option.
When you add a subinterface, while you are not adding another physical interface, from the router's perspective and for routing/policy functions you are. The router now has to be able to differentiate incoming/outgoing traffic on that interface using some parameter, and determine which sub-interface the traffic belongs to so that it can set policies, perform filtering etc. So, what methods do you have to give the router some way of differentiating traffic when you have a single physical, but multiple logical interfaces? You have frame relay dlcis (WAN), atm pvcs (WAN) and vlan assignments for Ethernet (LAN). What you have done now is enabled the router to differentiate incoming/outgoing traffic and determine by some parameter which sub-interface the traffic is assigned to. If you have point to point serial interfaces, you can run frame relay back to back with sub-interfaces to test this, I can't find the link right now but I am sure it is in the archives somewhere. -----Original Message----- From: Rich [mailto:[EMAIL PROTECTED] Sent: Thursday, September 04, 2003 11:05 PM To: [EMAIL PROTECTED] Subject: Stuck on Subinterfaces - HELP! [7:74854] Could someone help a CCNP student who is really confused? I am currently studying for the BSCI Routing Exam using the Sybex Books on a home Lab of 4 2500's and 1 2600. It has been working great and I have always been very pleased with the Sybex Series. Unfortunately they have never covered Subinterfaces well enough and many of the Labs In the CCNP BSCI book are using them without much explanation. They have various Labs that use OSPF, IS-IS, BGP etc. to route IP over Serial Subinterfaces on what I see as just a "Plain old LAN". All they do is show the IP Addresses and Networks already arranged, some on Serial Subinterfaces, and go right into the Routing Protocol configurations. They don't say anything at this point about using a Frame Relay, ATM, IPX, or ISL for VLAN's on them in this book. Those topics are covered in the Remote Access and Switching Books. My problem is: when I set up Subinterfaces on the Serial Ports with IP Addresses, set the clocking, and then bring up the interfaces, they all show as Interface Up and Line Protocol up - But I just can't seem to Ping any of the IP's on the Serial ports if they, or the other end they are attached to, are Subinterfaces. If I can't Ping I sure can't route right? When I stick to regular physical interfaces, everything works great. Am I missing something important? If any of you Cisco Experts out there could offer any suggestions, I would hugely appreciate it. I'm kind of stuck on Stall right now and can't move on to any of the other Labs until I resolve this. Thanks. Rich. **Please support GroupStudy by purchasing from the GroupStudy Store: http://shop.groupstudy.com FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=74896&t=74854 -------------------------------------------------- **Please support GroupStudy by purchasing from the GroupStudy Store: http://shop.groupstudy.com FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html
