Get Cisco CCNA / CCNP Certification|OSPF E2 vs. E1 Routes

When preparing for CCNA and CCNP success, you’ve got to know the many details of OSPF.   Learn about these two vital OSPF route types and how they differ from each other in this free tutorial from Chris Bryant, CCIE #12933.

Get Cisco CCNA / CCNP Certification|OSPF E2 vs. E1 Routes

OSPF is a major topic on both the CCNA and CCNP exams, and it's also the topic that requires the most attention to detail.  Where dynamic routing protocols such as RIP and IGRP have only one router type, a look at a Cisco routing table shows several different OSPF route types.
R1#show ip route

CCNP Certification Exam |Floating Static Routes on cisco

Cisco CCNA  / CCNP Certification Exam Tutorial:  Floating Static Routes

Knowing when and how to configure floating static routes is an important skill on your Cisco certification exams, as well as working in real-world networks.  Learn all about floating static routes from Chris Bryant, CCIE #12933.

CCNP Certification Exam |Floating Static Routes on cisco

 

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!