OSPF Areas Why OSPF is Complicated Part 2 2005/03/11 (C) Herbert - PowerPoint PPT Presentation

OSPF – Areas Why OSPF is Complicated Part 2 2005/03/11 (C) Herbert Haas

“An algorithm must be seen to be believed” Donald .E. Knuth

OSPF Areas  To improve performance divide the whole OSPF domain in multiple Areas  Restrict Router LSA and Network LSA within these Areas  All areas must be connected to the so-called "Backbone Area"  "Area 0" 2005/03/11 (C) Herbert Haas 3

ABR Area Border Router (ABR): Note: Terminates Router LSAs Network Summary LSAs and Network LSAs are Distance Vector Forwards Network Summary LSAs Area 0 updates !!! LSA 3 3 A LSA 3 S L ABR LSA 3 ABR ABR LSA 3 2 LSA 3 A LSA 1 LSA 3 S L LSA 3 LSA 2 L Area 1 S A 1 LSA 1 LSA 2 Area 2 Area 5 Router LSA LSA 1 Network LSA LSA 2 Network Summary LSA LSA 3 2005/03/11 (C) Herbert Haas 4

ASBR When an ABR receives an Autonomous System AS External LSA it emits ASBR Summary LSAs Border Router (ASBR) to all routers Imports foreign routes via AS External LSA Area 0 LSA 5 LSA 5 LSA 4 LSA 5 LSA 4 4 ABR A S L ABR ABR LSA 5 LSA 5 4 A LSA 4 S L 4 ASBR LSA 5 A LSA 5 LSA 5 S L LSA 5 Area 1 LSA 5 5 A S L Area 2 Area 5 Router LSA LSA 1 Network LSA ASBR Summary LSA LSA 2 LSA 4 Network Summary LSA AS External LSA LSA 3 LSA 5 2005/03/11 (C) Herbert Haas 5

Stub Area Area 0 LSA 5 LSA 3 LSA 4 4 ABR A S L ABR LSA 2 LSA 5 ABR LSA 5 LSA 3 3 A LSA 3 ASBR LSA 1 S LSA 5 L LSA 5 Area 1 AS External LSA and ASBR Summary LSA are not sent into a Stub Area Stub Area 5 Area 2 Router LSA LSA 1 Network LSA ASBR Summary LSA LSA 2 LSA 4 Network Summary LSA AS External LSA LSA 3 LSA 5 2005/03/11 (C) Herbert Haas 6

Totally Stubby Area Cisco Specific Area 0 LSA 5 LSA 3 LSA 4 4 ABR A S L ABR LSA 2 LSA 5 ABR LSA 5 LSA 3 ASBR LSA 1 LSA 5 LSA 5 Area 1 No external or summary LSA are sent into a Totally Totally Stubby Area Area 5 Stubby Area 2 Router LSA LSA 1 Network LSA ASBR Summary LSA LSA 2 LSA 4 Network Summary LSA AS External LSA LSA 3 LSA 5 2005/03/11 (C) Herbert Haas 7

Not So Stubby Area (NSSA) ABR will translate the Type 7 LSA into a Type 5 LSA only if the Type 7 LSA has the P-bit set to 1 Area 0 LSA 5 LSA 5 LSA 3 LSA 4 4 ABR A S L ABR LSA 2 LSA 5 ABR LSA 5 LSA 3 3 LSA 3 A S L ASBR LSA 1 LSA 5 LSA 7 LSA 5 Area 1 LSA 7 ASBR ASBR advertizes routes NSSA of another routing Area 5 Area 2 domain via NSSA External LSA Router LSA LSA 1 Network LSA ASBR Summary LSA LSA 2 LSA 4 Network Summary LSA AS External LSA LSA 3 LSA 5 NSSA External LSA LSA 7 2005/03/11 (C) Herbert Haas 8

Summarization  Efficient OSPF address design requires hierarchical addressing  Address plan should support summarization at ABRs Area 0 2 2 20/8 / 8 21/8 22.1.0.0/16 20.1.0.0/16 ... ... 22.254.0.0/16 20.254.0.0/16 21.1.0.0/16 ... Area 30 21.254.0.0/16 Area 10 Area 20 2005/03/11 (C) Herbert Haas 9

Virtual Links  Another way to Area 0 connect to area 0 using a point-to-point ABR unicast tunnel Virtual Link  Transit area must Area 1 have full routing information ABR  Must not be stub area  Bad Design! Area 2 2005/03/11 (C) Herbert Haas 10

Virtual Link Example  Now router 3.3.3.3 has Area 0 an interface in area 0 4.0.0.1  Thus router 3.3.3.3 Router 1.1.1.1 5.0.0.1 becomes an ABR  Generates summary 5.0.0.2 LSA for network Router Area 1 2.2.2.2 7.0.0.0/8 into area 1 and 6.0.0.2 area 0  Also summary LSAs in 6.0.0.3 Router area 2 for all the 3.3.3.3 information it learned 7.0.0.3 from areas 0 and 1 Area 2 2005/03/11 (C) Herbert Haas 11

Virtual Link Configuration Example router ospf 5 Area 0 network 4.0.0.0 0.255.255.255 area 0 network 5.0.0.0 0.255.255.255 area 1 4.0.0.1 area 1 virtual-link 3.3.3.3 Router 1.1.1.1 5.0.0.1 5.0.0.2 Router Area 1 2.2.2.2 6.0.0.2 router ospf 5 6.0.0.3 network 7.0.0.0 0.255.255.255 area 2 Router network 6.0.0.0 0.255.255.255 area 1 3.3.3.3 area 1 virtual-link 1.1.1.1 7.0.0.3 Area 2 2005/03/11 (C) Herbert Haas 12

GRE instead of Virtual Link  Alternative solution  Good: Transit area can be a also a stub area  Bad: All traffic is encapsulated  Not only routing traffic  Increased overhead 2005/03/11 (C) Herbert Haas 13

Summary  Area concept supports large networks  Keeps topology table small  Reduces routing traffic  But additional LSA types necessary  Inter-Area Routing is Distance Vector  Originally OSPF designed for ToS routing – too resource greedy! 2005/03/11 (C) Herbert Haas 14

Quiz  When should we split the OSPF domain into areas?  What about Areas and addressing plans?  Why must all areas be connected to the backbone area? 2005/03/11 (C) Herbert Haas 15