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Authentication, Authorization, and Accounting (AAA) are independent security services that, when implemented, will ensure that only appropriate administrators access the network hardware and execute appropriate commands Cisco has grouped these three security functions together to help you configure AAA consistently and better protect your network architecture AAA gives you the access controls you need for your network devices The benefits of using AAA include Increased flexibility and control Scalability Standardized authentication methods Multiple backup systems AAA uses protocols such as RADIUS and TACACS+ to administer its security functions These protocols can be applied to a server that functions as a RADIUS security server, TACACS+ security server, or simply "a security server" The AAA security functions use a router or access server acting as a network access server (NAS) to establish communication with the security server The NAS does not have to communicate to a separate security server it can use internal securityserver functionality Figure 16-1 is an example of a relationship between an NAS and a separate security server, configured to use AAA

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[ v ( x 0 + x , y0 ) v ( x 0 , y0 )] u( x 0 + x , y0 ) u( x 0 , y0 ) w + i lim = lim x 0 x 0 x z x v u +i = x x

Before you can use any of the services provided by AAA network security, you need to enable AAA on the NAS by executing the following command:

Because only a single device can use any network at a time, network nodes must have a way to access the network media without stepping on each other s data packets Wired Ethernet networks use carrier sense multiple access/collision detection (CSMA/CD), as you ll recall from previous chapters, but Wi-Fi networks use carrier sense multiple access/collision avoidance (CSMA/CA) Let s compare both methods EXAM TIP Wired Ethernet networks use carrier sense multiple access/ collision detection (CSMA/CD) Wi-Fi networks use carrier sense multiple access/collision avoidance (CSMA/CA) How do multiple devices share the network media, such as a cable This is fairly simple: each device listens in on the network media by measuring the level of voltage currently on the wire If the level is below the threshold, the device knows that it s clear to send data If the voltage level rises above a preset threshold, the device knows that the line is busy and it must wait before sending data Typically, the waiting period is the length of the current frame plus a short, predefined silence period called an interframe space (IFS) So far, so good but what happens when two devices both detect that the wire is free and try to send data simultaneously As you probably guessed, packets transmitted on the network from two different devices at the same time will corrupt each other, thereby canceling each other out This is called a collision Collisions are a fact of networking life So, how do network nodes deal with collisions They either react to collisions after they happen, or they take steps to avoid collisions in the first place CSMA/CD is the reactive method With CSMA/CD, each sending node detects the collision and responds by generating a random timeout period for itself, during which it doesn t try to send any more data on the network this is called a backoff Once the.

If the derivative f ( z ) exists, we must obtain the same answer even if we choose another way for ( x , y) to go to zero Now we use the other option, approaching the origin along the y axis Hence we set x = 0 This time we have w u( x 0 , y0 + y) u( x 0 , y0 ) [ v ( x 0 , y0 + y) v ( x 0 , y0 )] = + z i y y