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 16.16.16.16 255.0.0.0interface Loopback17 Ip Address 17.17.17.17 255.0.0.0interface Loopback18 Ip Address 18.18.18.18 255.0.0.0interface Loopback19 Ip Address 19.19.19.19 255.0.0.0R1(config)router Ospf 1R1(config-router)redistribute Connected SubnetsR1(config-router)summary-address 16.0.0.0 252.0.0.0The Summary Address Appears On R2, A Downstream Router.R2show Ip Route OspfO E2 16.0.0.06 11020 Via 172.12.123.1, 00:00:05, Serial0Let's Go Back To R1 And Look At Its OSPF Table.R1show Ip Route Ospf O 16.0.0.06 Is A Summary, 00:01:51, Null0Where 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 17.0.0.08 Is Directly Connected, Loopback17C 16.0.0.08 Is Directly Connected, Loopback16C 19.0.0.08 Is Directly Connected, Loopback19C 18.0.0.08 Is Directly Connected, Loopback18O 16.0.0.06 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!
