Creating Multiple OSPF Areas. 2001, Cisco Systems, Inc. 6-1

Transcription

1 Creating Multiple OSPF Areas 6-1

2 Issues with Maintaining a Large OSPF Network I am only receiving LSAs, no data. OSPF The SPF is running too often for me to route. OSPF OSPF OSPF OSPF My routing table is too big, and I am running low on memory. BSCI v

3 The Solution: OSPF Hierarchical Routing Area 0 Area 1 Area 2 Autonomous System Consists of areas and autonomous systems Minimizes routing update traffic BSCI v

4 Types of OSPF Routers (cont.) Area 1 Backbone Area 0 Area 2 Routers Routers External AS BSCI v

5 Types of OSPF Routers (cont.) Area 1 Backbone Area 0 Area 2 Routers Backbone/ Routers Routers External AS BSCI v

6 Types of OSPF Routers (cont.) Area 1 Backbone Area 0 Area 2 Routers ABR and Backbone Router Backbone/ Routers Routers External AS ABR and Backbone Router BSCI v

7 Types of OSPF Routers (cont.) Area 1 Backbone Area 0 Area 2 Routers ABR and Backbone Router Backbone/ Routers Routers ASBR and Backbone Router External AS ABR and Backbone Router BSCI v

8 LSAs in OSPF Database Area 1 Area 0 DR External Network ABR ASBR Router External AS Summary p1r3#show ip ospf database OSPF Router with ID ( ) (Process ID 1) Router Link States (Area 1) Link ID ADV Router Age Seq# Checksum Link count x xD482 4 Net Link States (Area 1) Link ID ADV Router Age Seq# Checksum x xAD9A Summary Net Link States (Area 1) Link ID ADV Router Age Seq# Checksum x xE6F8 BSCI v

9 Calculating Costs for Summary and AS External Routes Area 0 R4 10 R3 E1 E1 10 R R 3 s Cost to: AS1 (E1) via R1 = 1795 AS1 (E1) via R3 = 1785 AS1 BSCI v

10 Calculating Costs for Summary and AS External Routes (cont.) Area 1 Area 0 R5 E1 10 R4 R3 E E1 R R 5 s Cost to: AS1 (E1) via R1 = 1815 AS1 (E1) via R3 = R 3 s Cost to: AS1 (E1) via R1 = 1795 AS1 (E1) via R3 = 1785 AS1 BSCI v

11 Types of Areas BSCI v

12 Types of Areas (cont.) Backbone Area 0 Interconnects areas; accepts all LSAs. BSCI v

13 Types of Areas (cont.) Stub Area Backbone Area 0 Does not accept external LSAs. Interconnects areas; accepts all LSAs. BSCI v

14 Types of Areas (cont.) Stub Area Backbone Area 0 Totally Stubby Area Does not accept external LSAs. Interconnects areas; accepts all LSAs. Does not accept external or summary LSAs. BSCI v

15 OSPF Operation Across Multiple Areas 6-15

16 Forwarding Packets in a Multiarea Network Area 1 Area 0 Area 50 ABR1 ABR2 BBone Data To Destination Network To ABR1 To Backbone To ABR2 BSCI v

17 Flooding LSUs to Multiple Areas (cont.) Area 1 Area 0 Area 50 Stub ABR1 ABR2 BBone Type 1 BSCI v

18 Flooding LSUs to Multiple Areas (cont.) Area 1 Area 0 Area 50 Stub ABR1 ABR2 BBone Type 1 Type 3 Type 3 BSCI v

19 Flooding LSUs to Multiple Areas (cont.) RIP Area 1 Area 0 Area 50 Stub ABR1 ABR2 BBone Type 1 Type 3 Type 3 Type 5 Type 5 Default BSCI v

20 Flooding LSUs to Multiple Areas (cont.) Routing Table Intra-Area Routes Interarea Routes Area 1 Area 1 Area 0 External (Non-OSPF Routes) Area 1 RIP BSCI v

21 Using and Configuring OSPF Multiarea Components 6-21

22 Configuring OSPF ABRs Area 0 ABR Area 1 E S A E0 B C S1 <Output Omitted> interface Ethernet0 ip address ! <Output Omitted> router ospf 77 network area 0 <Output Omitted> interface Ethernet0 ip address ! interface Serial0 ip address <Output Omitted> router ospf 50 network area 1 network area 0 BSCI v

23 Using Stub and Totally Stubby Areas Area 50 Stub RIP Area 0 Area 1 Totally Stubby Non-Cisco Router ABR1 ASBR BBone ABR2 Summary Summary Summary Default Default External External Default BSCI v

24 Stub and Totally Stubby Area Restrictions R3 Area R4 Single Exit Point X Typically a single exit point into area; if multiple exit points, suboptimal paths may be selected An ASBR cannot be internal to stub Area is not the backbone Area 0 Virtual links are not allowed External AS BSCI v

25 Configuring Stub and Totally Stubby Areas Router(config-router)# area area-id stub [no-summary] Creates a stub area Router(config-router)# area area-id default-cost cost Specifies cost for default route sent into stub area BSCI v

26 OSPF Stub Area Configuration Example External AS E0 R3 S S0 Area 0 Stub Area 2 R4 R3# interface Ethernet 0 ip address interface Serial 0 ip address router ospf 100 network area 0 network area 2 area 2 stub R4# interface Serial 0 ip address router ospf 15 network area 2 area 2 stub BSCI v

27 OSPF Totally Stubby Configuration Example External AS E0 R3 S S0 Area 0 Totally Stubby Area 2 R4 R3# router ospf 100 network area 0 network area 2 area 2 stub no-summary R4# router ospf 15 network area 2 area 2 stub BSCI v

28 Multiple-Area NBMA Environment Area 0 R1 Frame Relay Area 1 (Stub Area) BSCI v

29 Multiple-Area NBMA Environment (cont.) Area 1 Area 0 R1 Frame Relay Area 2 Area 3 Area 4 BSCI v

30 Supporting Route Summarization Area 0 Backbone Summarization ABRs Area 1 x Minimizes number of routing table entries Localizes impact of a topology change Reduces LSAs and saves CPU BSCI v

31 Supporting VLSM OSPF carries subnet mask information Use hierarchical addressing scheme BSCI v

32 Using Route Summarization Area 1 ABR Area 0 A B C Routing Table for B O O O O O O Summarization LSAs Sent to Router C IA IA Interarea (IA) summary link carries mask One entry can represent several subnets BSCI v

33 Configuring Route Summarization Router(config-router)# area area-id range address mask Consolidates interarea (IA) routes on an ABR Router(config-router)# summary-address address mask [not-advertise] [tag tag] Consolidates external routes, usually on an ASBR BSCI v

34 Route Summarization Configuration Example Area 0 Interface Addresses ( Mask) Interface Addresses ( Mask) Area 1 R1 R2 R Area 2 R1# router ospf 100 network area 1 network area 0 area 0 range area 1 range R2# router ospf 100 network area 2 network area 0 area 0 range area 2 range BSCI v

35 Verifying OSPF Operation 6-35

36 show ip ospf Command Router# show ip ospf border-routers Lists the ABRs and ASBRs in the autonomous system Router# show ip ospf virtual-links Displays the status of the virtual link Router# show ip ospf process-id Displays statistics about each area to which the router is connected Router# show ip ospf database Displays the contents of the OSPF link-state database BSCI v