Multiarea OSPF CCNP 1 Advanced Routing Chapter 5

Multiarea OSPF CCNP 1: Advanced Routing Chapter 5

Overview 1. 2. 3. 4. 5. 6. OSPF Review Multiarea OSPF Operation and Configuration Area Types: Stub, Totally Stub, and NSSA Virtual Links Monitoring and Verifying Multiarea OSPF Pitfalls: “What could possibly go wrong? ”

Open Shortest Path First OSPF is a scalable link-state protocol that determines the best path between two networks by comparing the aggregate costs of all possible paths. • Extremely scalable • Sends incremental updates • Establishes a relationship between neighbor routers • Hierarchical • Supports VLSM

The OSPF Database • Each OSPF-enabled interface is considered a “link”. • All links are assimilated into the SPF database. • show ip ospf database • You too will be assimilated into the collective. Resistance is futile. http: //www. routergod. com/sevenofnine

OSPF Features • Scalability • OSPF is considered by most to be the 2 nd most scalable IP IGP • OSPF is used in both Service Provider and Enterprise networks • Incremental Updates • Fast convergence time • Light on bandwidth • Heavy on router CPU cycles

OSPF Neighbor Relationships • Point-to-point links: OSPF detects and connects to neighboring OSPF routers, and the routers exchange link state databases. • Multiaccess links: OSPF detects and connects to neighboring OSPF routers. A designated Router (DR) and Backup Designated Router (BDR) are elected. The routers exchange link state databases with the DR, and their tables are also sent to the BDR. • Verification/Troubleshooting: • Router#show ip ospf neighbors • If you don’t see a DR, guess what? • For added fun: • Router(config)#alias exec sion show ip ospf neighbors

Hierarchy • OSPF is based on a 2 -layer hierarchy • Backbone and Non-backbone • An OSPF domain can be divided in areas • Each area has its own link state database • Why?

5. 1. 1 What problem are you trying to solve?

5. 1. 1 What problem are you trying to solve?

OSPF Areas • An area can contain as many as “ 50 ish” routers • This number varies, based on the number and stability of links and the resources on the routers • Backbone area should be relatively small • Backbone area should consist of stable links • Backbone Area = Area 0 = Teh Princess

OSPF Areas Backbone Router: Router with at least one interface in Area 0 Internal Router: Router with all interfaces in the same area Area Border Router (ABR): Router with interfaces in two or more different areas Autonomous System Boundary Router (ASBR): Router with at least one interface connected to a non -OSPF domain

Configuration Area 10 Area 0 R 1(config)#router ospf 1 R 1(config-router)#network 10. 3. 0. 0. 255 area 0 R 1(config-router)#network 10. 1. 0. 0. 255 area 10 R 1(config-router)#network 10. 2. 0. 0. 255 area 1 What kind of router is R 1? Internal Backbone ABR ASBR

Configuration Area 10 Area 0 R 1(config)#router ospf 1 R 1(config-router)#network 10. 3. 0. 0. 255 area 0 R 1(config-router)#network 10. 1. 0. 0. 255 area 10 R 1(config-router)#network 10. 2. 0. 0. 255 area 1 What kind of router is R 1? Backbone ABR

Configuration R 2(config)#router ospf 1 R 2(config-router)#network 10. 3. 0. 0. 255 area 0 R 2(config-router)#network 10. 1. 0. 0. 255 area 0 R 2(config-router)#network 10. 2. 0. 0. 255 area 0 What kind of router is R 2? Internal Backbone ABR ASBR

Configuration R 2(config)#router ospf 1 R 2(config-router)#network 10. 3. 0. 0. 255 area 0 R 2(config-router)#network 10. 1. 0. 0. 255 area 0 R 2(config-router)#network 10. 2. 0. 0. 255 area 0 What kind of router is R 2? Internal Backbone Look Ma, no diagram!

Configuration R 3(config)#router ospf 1 R 3(config-router)#network 10. 3. 0. 0. 255 area 10 R 3(config-router)#network 10. 1. 0. 0. 255 area 10 R 3(config-router)#network 10. 2. 0. 0. 255 area 10 What kind of router is R 3? Internal Backbone ABR ASBR

Configuration R 3(config)#router ospf 1 R 3(config-router)#network 10. 3. 0. 0. 255 area 10 R 3(config-router)#network 10. 1. 0. 0. 255 area 10 R 3(config-router)#network 10. 2. 0. 0. 255 area 10 What kind of router is R 3? Internal

Configuration R 4(config)#ip route 10. 4. 0. 0 255. 0. 0 serial 0/0 R 4(config)#router ospf 1 R 4(config-router)#network 10. 3. 0. 0. 255 area 0 R 4(config-router)#network 10. 1. 0. 0. 255 area 10 R 4(config-router)#network 10. 2. 0. 0. 255 area 10 R 4(config-router)#redistribute static What kind of router is R 4? Internal Backbone ABR ASBR

Configuration R 4(config)#ip route 10. 4. 0. 0 255. 0. 0 serial 0/0 R 4(config)#router ospf 1 R 4(config-router)#network 10. 3. 0. 0. 255 area 0 R 4(config-router)#network 10. 1. 0. 0. 255 area 10 R 4(config-router)#network 10. 2. 0. 0. 255 area 10 R 4(config-router)#redistribute static What kind of router is R 4? Backbone ABR ASBR

Configuration R 5(config)#router ospf 1 R 5(config-router)#network 10. 3. 0. 0. 255 area 1 R 5(config-router)#network 10. 1. 0. 0. 255 area 10 R 5(config-router)#network 10. 2. 0. 0. 255 area 10 What kind of router is R 5? Internal Backbone ABR ASBR

Configuration R 5(config)#router ospf 1 R 5(config-router)#network 10. 3. 0. 0. 255 area 1 R 5(config-router)#network 10. 1. 0. 0. 255 area 10 R 5(config-router)#network 10. 2. 0. 0. 255 area 10 What kind of router is R 5? XXXXXXXXXXXXXXXXXXXXXXXX Internal Backbone ABR ASBR Every area MUST have at least one link in Area 0.

OSPF LSA Types

LSA Types and Routing tables R 1#show ip route ospf O 6. 0. 0. 0/8 [110/65] via 5. 0. 0. 2, 00: 04: 23, Serial 0/0 O IA 7. 0. 0. 0/8 [110/65] via 5. 0. 0. 2, 00: 04: 23, Serial 0/0 O E 2 8. 0. 0. 0/8 [110/65] via 5. 0. 0. 2, 00: 04: 23, Serial 0/0 Type 2: O Type 3: O IA (How to reach ABRs) Type 4: O IA (How to reach ASBRs) Type 5: O E 1 or O E 2

OSPF Stub Areas • A stub area has only one entry and exit point. • Topology changes in other areas do not affect stub areas. The ABR is still the only way out.

Stub Areas and LSAs 5. 2. 2

Stub vs. Totally Stub Similarities • • • ABR does not advertise Type 4 or 5 LSAs into the area There should be no OE 1 or OE 2 routes in the area ABR advertises a default route to the internal routers Neither area can have an ASBR in it Neither area can be the backbone Differences • Totally stub does not advertise Type 3 LSAs either • Stub routing table: O, O IA, and default route • Totally stub routing table: O and default route

5. 4. 3 Stub and Totally Stub Configuration Stub Area R 1(config-router)#area 1 stub R 2(config-router)#area 1 stub • Command must be entered on all routers in the area Totally Stubby Area R 1(config-router)#area 1 stub no-summary R 2(config-router)#area 1 stub • ABR is configured with stub no-summary • Internal routers are configured with stub

NSSA • Many service providers have OSPF areas that have only one exit point, but the areas contain an ASBR • Solution: Not-So-Stubby Areas (NSSAs) • The ASBR originates a Type 7 LSA • The ABR converts from Type 7 to Type 5

NSSA Configuration Not-So-Stubby Area R 1(config-router)#area 1 nssa R 2(config-router)#redistribute static • Command must be entered on all routers in the area • ASBR must be configured to inject non-OSPF routes Totally Not-So-Stubby Area R 1(config-router)#area 1 nssa no-summary R 2(config-router)#area 1 stub • ABR is configured with nssa no-summary • Internal routers are configured with nssa

5. 6. 3 Stub and NSSA • By default, stub ABRs advertise a default route. • By default, NSSA ABRs do not. • To force an NSSA to advertise a default route: R 1(config-router)#area 1 nssa defaultinformation-originate

5. 3. 2 Summarizing with OSPF • Only ABRs and ASBRs can summarize • Routes should be summarized as/before they are advertised to Area 0 • ABR: R 1(config-router)#area 1 range 10. 0 255. 0. 0. 0 • ASBR: R 1(config-router)#summary-address 10. 0 255. 0. 0. 0

5. 5. 1 Breaking the Area 0 Rule • Every area must have at least one interface on at least one router in Area 0. • Area 0 must be contiguous. • Problem: Mergers • Problem: Extra. Nets

OSPF Virtual Links R 1(config-router)#area 1 virtual-link 3. 3 R 3(config-router)#area 1 virtual-link 1. 1 • Virtual links are established by router IDs • Can be used to join a discontiguous Area 0 • Can be used to “tunnel” a remote area to Area 0 Note: YOU SHOULD NOT FIND YOURSELF IN THIS SITUATION.

OSPF Pitfalls • OSPF Path Selection • OSPF over NBMA • OSPF over DDR

Multiarea OSPF CCNP 1: Advanced Routing Chapter 5