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
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