CCNA / CCNP Cisco Certification Exam: Caller ID Screening And Callback

To pass Cisco certification exams, you have to know more than one way to do things.  Learn another method of configuring ISDN callback from Chris Bryant, CCIE #12933.

CCNA / CCNP Cisco Certification Exam:  Caller ID Screening And Callback

CCNP Certification: How And Why To Build An Etherchannel

Etherchannels are in use all over the world, as well as on your CCNA and CCNP exams.  Learn how to build and support an EC in this tutorial from Chris Bryant, CCIE #12933.

 CCNP Certification: How And Why To Build An Etherchannel

CCNA and CCNP candidates are well-versed in Spanning-Tree Protocol, and one of the great things about STP is that it works well with little or no additional configuration. There is one situation where STP works against us just a bit while it prevents switching loops, and that is the situation where two switches have multiple physical connections.

Introduction To BGP Attributes|Cisco CCNA / CCNP Certification

BGP is one of the most complex topics you’ll tackle in your CCNP studies, but if you break it down to the fundamentals, you’ll pick it up quickly.  Chris Bryant, CCIE #12933, has written this free BGP tutorial on BGP attributes to help you do master BGP and pass the exams!

 Introduction To BGP Attributes|Cisco CCNA / CCNP Certification

 

BGP is one of the most complex topics you'll study when pursuing your CCNP, if not the most complex. I know from personal experience that when I was earning my CCNP, BGP is the topic that gave me the most trouble at first. One thing I keep reminding today's CCNP candidates about, though, is that no Cisco technology is impossible to understand if you just break it down and understand the basics before you start trying to understand the more complex configurations.

CCNP / BSCI Exam Tutorial: Route Summarization And The OSPF Null Interface

CCNP / BSCI Exam Tutorial:  Route Summarization And The OSPF Null Interface

Route summarization requires a knowledge of binary conversion, as well as knowing how each protocol responds to summarization.  Learn how OSPF does so from Chris Bryant, CCIE #12933.

CCNP exam success, particularly on the BSCI exam, demands you understand the details of route summarization.  This skill not only requires that you have a comfort level with binary conversions, but you have to know how and where to apply route summarization with each individual protocol.

You also have to know the "side effects" of route summarization.  With OSPF, there will actually be an extra interface created at the point of summarization, and this catches a lot of CCNP candidates by surprise.  Let's take a look at the null0 interface and how it relates to OSPF summarization.

On R1, the following networks are redistributed into OSPF, and then summarized.

interface Loopback16

 ip address 16.16.16.16 255.0.0.0


interface Loopback17

 ip address 17.17.17.17 255.0.0.0


interface Loopback18

 ip address 18.18.18.18 255.0.0.0

interface Loopback19

 ip address 19.19.19.19 255.0.0.0

R1(config)#router ospf 1

R1(config-router)#redistribute connected subnets

R1(config-router)#summary-address 16.0.0.0 252.0.0.0

The summary address appears on R2, a downstream router.

R2#show ip route ospf


O E2 16.0.0.0/6 [110/20] via 172.12.123.1, 00:00:05, Serial0


Let's go back to R1 and look at its OSPF table.

R1#show ip route ospf


O    16.0.0.0/6 is a summary, 00:01:51, Null0

Where did the null0 interface come from, and why is it there?  Packets sent to the null interface are dropped, and in this case, that's a good thing.

When you configure summary routes in OSPF, a route to null0 will be installed into the OSPF routing table.  This helps to prevent routing loops.  Any packets destined for the routes that have been summarized will have a longer match in the routing table, as shown below...

C    17.0.0.0/8 is directly connected, Loopback17

C    16.0.0.0/8 is directly connected, Loopback16

C    19.0.0.0/8 is directly connected, Loopback19

C    18.0.0.0/8 is directly connected, Loopback18

O  16.0.0.0/6 is a summary, 00:01:51, Null0


.. and packets that do not match one of the summarized routes but do match the summary route will be dropped.

Preventing routing loops when performing route redistribution and summarization is vital.  OSPF gives us a little help in that regard in this situation, and as you study more complex redistribution scenarios on your way to the CCNP and CCIE, you'll realize that we'll take all the help we can get!