15 Surprising Stats About ccna training london

[Donald Aichele]

To pass the CCNA exam, you have to be able to write and ccna course london troubleshoot access lists. As you climb the ladder toward the CCNP and CCIE, you'll see more and more uses for ACLs. Therefore, you had better know the basics!

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The use of "host" and "any" confuses some newcomers to ACLs, so let's take a look at that first.

It is acceptable to configure a wildcard mask of all ones or all zeroes. A wildcard mask of 0.0.0.0 means the address specified in the ACL line must be matched exactly a wildcard mask of 255.255.255.255 means that all addresses will match the line.

Wildcard masks have the option of using the word host to represent a wildcard mask of 0.0.0.0. Consider a configuration where only packets from IP source 10.1.1.1 should be allowed and all other packets denied. The following ACLs both do that.

R3#conf t

R3(config)#access-list 6 permit 10.1.1.1 0.0.0.0

R3(config)#conf t

R3(config)#access-list 7 permit host 10.1.1.1

The keyword any can be used to represent a wildcard mask of 255.255.255.255.

R3(config)#access-list 15 permit any

Another often overlooked detail is the order of the lines in an ACL. Even in a two- or three-line ACL, the order of the lines in an ACL is vital.

Consider a situation where packets sourced from 172.18.18.0 /24 will be denied, but all others will be permitted. The following ACL would do that.

R3#conf t

R3(config)#access-list 15 deny 172.18.18.0 0.0.0.255

R3(config)#access-list 15 permit any

The previous example also illustrates the importance of configuring the ACL with the lines in the correct order to get the desired results. What would be the result if the lines were reversed?

R3#conf t

R3(config)#access-list 15 permit any

R3(config)#access-list 15 deny 172.18.18.0 0.0.0.255

If the lines were reversed, traffic from 172.18.18.0 /24 would be matched against the first line of the ACL. The first line is "permit any", meaning all traffic is permitted. The traffic from 172.18.18.0/24 matches that line, the traffic is permitted, and the ACL stops running. The statement denying the traffic from 172.18.18.0 is never run.

The key to writing and troubleshoot access lists is to take just an extra moment to read it over and make sure it's going to do what you intend it to do. It's better to realize your mistake on paper instead of once the ACL's been applied to an interface!

Earning your CCNA certification means you have to notice details about Cisco routers that others might miss, and that's true in the exam room and on the job. In today's tutorial, we'll take a look at multilink PPP - a topic that seems simple enough, but has details that trip up some CCNA certification candidates.

ISDN BRI (Basic Rate Interface) has two B-channels that both carry data, and they each have a capacity of 64 KBPS. Interestingly enough, they don't share the load by default - the first channel has to be at capacity before the second channel starts taking some of the load. It's more efficient to have the channels share the load before then, and we can do just that by configuring PPP multilink. (Note that it's called PPP multilink for a reason. We can't configure it if the BRI interface is running at the default of HDLC.)

Configuring PPP multilink is simple enough. PPP must be running on the interface to begin with, and then you simply configure the interface with the "PPP multilink" command.

R1(config)#int bri0

R1(config-if)#encapsulation ppp

R1(config-if)#ppp multilink

Here's where knowing the details comes in handy. An additional command, "dialer load-threshold", is needed to define the capacity level of the first b-channel that will bring up the second b-channel. For example, let's say we want the second channel to come up when the first channel hits 50% of its capacity. It's easy to think that we would put "50" at the end of the dialer load-threshold command, but that's wrong! Look at what IOS Help tells us about this command.

R1(config-if)#dialer load-threshold ?

Load threshold to place another call

This value is based on 255, not 100. To bring the second channel up as described earlier, we need to enter a value that is 50% of 255:

R1(config-if)#dialer load-threshold 127

The dialer load-threshold command has other options not shown here. The interface can be configured to consider only incoming or outgoing traffic for this command, or both.

This is just one of the many details you must master in order to earn your CCNA certification. Take it from me - it's worth it. Keep studying!