When you're preparing to pass the CCNA exam and earn this coveted Cisco certification, you've got to be totally prepared for the many kinds of binary and subnetting questions Cisco may throw at you. You also have to be familiar with the different manners in which a subnet mask can be expressed, as in the following:
255.255.255.0
/24
Believe it or not, those two values are exactly the same. The first mask is written out in the more familiar dotted decimal format, and you know by looking at those first three octets that every bit is set to "1", since the maximum value of such an octet is 255.
The second value represents the exact same mask, only this value is expressed in prefix notation. This particular value would be pronounced "slash twenty-four", and the 24 represents the number of consecutive ones that are set in the subnet mask.
Those of us who hate to type numbers are particularly appreciative of this, since it means you'll have to type a lot less numbers to represent a subnet mask. In addition, it's a lot easier to discuss masks in prefix notation than dotted decimal. ("I thought about using a two-fifty-five two-fifty-five two-fifty-five zero mask ,but then decided to use a two-fifty-five two-fifty-five two-fifty-five one-twenty-eight mask...")
Be sure you're comfortable with prefix notation before taking your CCNA exam. As with Cisco documentation, you'll most likely see masks expressed in both dotted decimal and prefix notation, and you've got to be ready to use the both as well!
When you're studying to pass the Cisco CCNA and CCNP certification exams, you quickly learn that there's always something else to learn. (You'll really pick up on this in your CCIE studies, trust me!) Today we'll take a look at an often-overlooked topic in Frame Relay, the encapsulation type. You don't exactly change this on a daily basis in production networks (not if you want to stay employed, anyway!), but it's an important exam topic that you must be familiar with.
The DCE and DTE must agree on the LMI type, but there's another value that must be agreed upon by the two DTEs serving as the endpoints of the VC. The Frame encapsulation can be left at the default of Cisco (which is Cisco-proprietary), or it can be changed to the industry-standard IETF, as shown below. If a non-Cisco router is the remote endpoint, IETF encapsulation must be used. Note that the default of Cisco isn't listed as an option by IOS Help, so you better know that one by heart!
R1(config)#int s0
R1(config-if)#encap frame ?
ietf Use RFC1490/RFC2427 encapsulation
R1(config-if)#encap frame ietf
What if a physical interface is in use and some remote hosts require Cisco encapsulation and others require IETF? The encapsulation type can be configured on a per-PVC basis as well. One encap type can be used on the interface, and any map statements that require a different encap type can have that specified in the appropriate map statement. In the following example, all PVCs will use the default Cisco encapsulation type except for PVC 115. The frame map statement using that PVC has ietf specified.
R1(config)#int s0/0
R1(config-if)#encap frame
R1(config-if)#frame map ip 172.12.123.3 123 broadcast
R1(config-if)#frame map ip 172.12.123.2 122 ietf broadcast
show frame map shows us that the mapping to DLCI 123 is using Cisco encapsulation, and DLCI 122 is using IETF.
R1#show frame map
Serial0 (up): ip 172.12.123.3 dlci 123(0x7B,0x1CB0), static
broadcast, CISCO, status defined, active
Serial0 (up): ip 172.12.123.2 dlci 122(0x7B,0x1CB0), static
broadcast, ietf, status defined, active
Just remember that Cisco is the default, and all PVCs will use Cisco unless you specify IETF in the frame map statement itself. You could also change the entire interface to use IETF for all mappings with the frame-relay encapsulation IETF command. For Cisco exams, as well as work on production networks, it's always a good idea to know more than one way to do something!
Chris Bryant has sinced written about articles on various topics from CISCO CCNA, Personal Desktop and Cisco CCNP. Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage , home of free CCNA and CCNP tutorials! Pass the. Chris Bryant's top article generates over 27100 views. Bookmark Chris Bryant to your Favourites.