Cisco CCNP or BSCI Exam Tutorial: Using The OSPF Command “Area Range”
Your BSCI and CCNP exam success depends on knowing the details, and one such fine detail is knowing the proper way to summarize routes in OSPF. path summarization is not just a test of your binary conversion abilities, yet knowing where and when to summarize routes. It will not surprise any kind of CCNA or CCNP accreditation candidate that OSPF provides us the most options for path summarization, and therefore more details to know!
OSPF offers us 2 options for route summarization designs. In a previous tutorial, all of us looked at the “summary-address” control, and today we’ll look at the appropriate use of the “area range” control.
The “area range” control should be used on an Area boundary Router (ABR) to summarize paths being advertised from one OSPF area to another. In this guide, R1 is acting as an ABR, with interfaces in both Area 0 and region 1. Four loopbacks have been placed into R1’s Area 1 .
R1(config)#router ospf 1
R1(config-router)#network 12. 0. 0. zero 0. 255. 255. 255 a 1
R1(config-router)#network 13. zero. 0. 0. 255. 255. 255 a 1
R1(config-router)#network 14. 0. 0. zero 0. 255. 255. 255 a 1
R1(config-router)#network 15. zero. 0. 0. 255. 255. 255 a 1
the particular routing table of an OSPF neighbor, R2, shows all four routes.
R2#show ip path ospf
12. 0. zero. 0/32 is subnetted, one subnets
O IA twelve. 12. 12. 12 [110/65] via 172. 12. 123. 1, 00: 18: 52, Serial0
thirteen. 0. 0. 0/32 will be subnetted, 1 subnets
u IA 13. 13. thirteen. 13 [110/65] via 172. 12. 123. 1, 00: 18: forty two, Serial0
14. 0. zero. 0/32 is subnetted, one subnets
O IA fourteen. 14. 14. 14 [110/65] via 172. 12. 123. 1, 00: 18: 32, Serial0
fifteen. 0. 0. 0/32 will be subnetted, 1 subnets
u IA 15. 15. fifteen. 15 [110/65] via 172. 12. 123. 1, 00: 18: thirty-two, Serial0
To keep the redirecting tables of downstream routers smaller but still have the preferred IP connectivity, we can use the area range command upon R1 to summarize these 4 routes. The key to keep in mind with the area range command is that the area number given in the particular command is the area that contains the destinations, NOT the area that will receive the summary path.
R1(config)#router ospf 1
R1(config-router)#area 1 range 12. zero. 0. 0 252. zero. 0. 0
R2 right now shows a single summary path that can be used to reach all four remote control networks.
R2#show ip path ospf
O IA twelve. 0. 0. 0/6 [110/65] via 172. 12. 123. 1, 00: 00: 21, Serial0
oddly enough, there’s now an additional route in R1’s redirecting table.
R1#show ip path ospf
O 12. zero. 0. 0/6 is an overview, 00: 07: 53, Null0
When you configure summary paths in OSPF, a route to null0 will be installed into the OSPF routing table of the router performing the summarization. This helps to prevent routing spiral. Any packets destined for the routes that have been summarized will have a longer match in the redirecting table, and packets that do not match one of the described routes but do match up the summary route will be dropped.
