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Multi Area OSPF

Multi Area OSPF. Carl Bergenhem - 030404 carl@bergenhem.com. Issues with large OSPF nets. Frequent SPF calculations Large routing table Large link-state table (topological database). Issues with large OSPF nets. OSPF uses “Areas”. Hierarchical

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Multi Area OSPF

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  1. Multi Area OSPF Carl Bergenhem - 030404 carl@bergenhem.com

  2. Issues with large OSPF nets • Frequent SPF calculations • Large routing table • Large link-state table(topological database)

  3. Issues with large OSPF nets

  4. OSPF uses “Areas” • Hierarchical • separate large internetworks (autonomous system) into smaller internetworks, called areas. • Routing between the areas (called inter-area routing), • Routing within an area: intra-area • But many of the smaller internal routing operations are restricted within an area. • Hide unnecessary information, e.g. summarise

  5. OSPF Areas

  6. OSPF Router Types

  7. OSPF Router Types Internal: Routers with all their interfaces within the same area Backbone: Routers with at least one interface connected to area 0 ASBR: (Autonomous System Boundary Router): Routers that have at least one interface connected to an external internetwork (another autonomous system) ABR: (Area Border Router): Routers with interfaces attached to multiple areas.

  8. LSA • Link State Advertisement • Sent in a LSU (Link State Update) • Flooding = information distribution within an area • Different types of information • External route (outside the OSPF AS) • Summary routes • Inter-area routes (from different area but same OSPF AS) • Intra-area routes (from same area and same OSPF AS)

  9. LSA Types • See online material: chpt: 5.1.3

  10. LSA Types LSA1 contain the Router Link States, from all routers LSA2 contain the Net Link States, from the DR Intra-area routes (O) LSA3 contain the Summary Net Link States, from ABR (describes routes within AS) LSA4 contain the AS Summary Net Link States, from ASBR (describes routes to ASBR) Inter-area routes (O IA) LSA5 contain external route information (E1 or E2),from ASBR LSA7 contain NSSA External Link Entry, from ASBR translated into LSA5 at the ABR

  11. LSA Types (con’t) • Type 6 MOSPF (Multicast OSPF) Not supported by Cisco. • MOSPF enhances OSPF by letting routers use their link-state databases to build multicast distribution trees for the forwarding of multicast traffic.

  12. LSA Types (con’t) • Type 7 NSSA External Link Entry • Originated by an ASBR connected to an NSSA. • Type 7 messages can be flooded throughout NSSAs and translated into LSA Type 5 messages by ABRs. Routes learned via Type-7 LSAs are denoted by either a “N1” or and “N2” in the routing table. (Compare to E1 and E2).

  13. Area Types • Standard (accept all LSAs) • Backbone (accept all LSAs) • Stub area (not accept all LSAs) • Stub • Totally Stubby Area (TSA) • Not-so-stubby-area (NSSA) • Difference in what LSAs they accept......

  14. Area Types Default route Accepted  External (type 5) routesSummary (type 3,4) routes  Default route Default route

  15. Area Types - differences • What LSAs that may be flooded=> • What info an area can know (hide info!!) • External routes are injected into OSPF by ASBR: LSA5 • Type 1 (E1): cost metric increments as route is passed through OSPF domain • Type 2 (E2): cost metric remains the same as route is passed through OSPF domain

  16. Configuring Areas • stub = Stub area • LSA3,4 OK, LSA5 not OK • stub + no summary = Totally stubby area • LSA3,4 and 5 not OK • NSSA = Not so stubby area • LSA3,4 OK, LSA5 not OK, LSA7 OK • NSSA + no summary = NSSA totally stubby area.. • LSA3,4 and 5 not OK LSA7 OK

  17. Multi-area Example ABR RIP ASBR

  18. Stub Example

  19. Stub Example ABR ASBR

  20. Stub Example ABR No Type 5 LSAs ASBR

  21. Stub Example ABR No Type 5 LSAs Route to 0.0.0.0/0 via RTB ASBR

  22. LSA Types LSA1 contain the Router Link States, from all routers LSA2 contain the Net Link States, from the DR Intra-area routes (O) LSA3 contain the Summary Net Link States, from ABR (desc. routes within AS) LSA4 contain the AS Summary Net Link States, from ASBR (desc. routes to ASBR) Inter-area routes (O IA) LSA5 contain external route information (E1 or E2),from ASBR LSA7 contain NSSA External Link Entry, from ASBR translated into LSA5 at the ABR

  23. Totally Stubby Example no summary ABR Totally Stubby Area No Type 3, 4, or 5 LSAs Route to 0.0.0.0/0 via RTB (no more IA routes) ASBR

  24. LSA Types LSA1 contain the Router Link States, from all routers LSA2 contain the Net Link States, from the DR Intra-area routes (O) LSA3 contain the Summary Net Link States, from ABR (desc. routes within AS) LSA4 contain the AS Summary Net Link States, from ASBR (desc. routes to ASBR) Inter-area routes (O IA) LSA5 contain external route information (E1 or E2),from ASBR LSA7 contain NSSA External Link Entry, from ASBR translated into LSA5 at the ABR

  25. NSSA Example

  26. NSSA Example ABR No Type 5 LSAs 3,4 OK Type 7 OK ASBR

  27. NSSA Example no summary ABR No Type 3, 4 or 5 LSAs Type 7 OK ASBR

  28. LSA Types (con’t) • Type 7 NSSA External Link Entry • Originated by an ASBR connected to an NSSA. • Type 7 messages can be flooded throughout NSSAs and translated into LSA Type 5 messages by ABRs. Routes learned via Type-7 LSAs are denoted by either a “N1” or and “N2” in the routing table. (Compare to E1 and E2).

  29. LSA 1 Router LSA Multi Area OSPF Normal Areas LSA 1 Originated LSA 1 Orig. LSA 1 flooded LSA 1’s are flooded out other interfaces within the same area. LSA 1 flooded

  30. Multi Area OSPF Normal Areas LSA 2 Network LSA LSA 2’s being sent LSA 2 No LSA 2’s for Area 51 – No DR elected because no other routers on the broadcast segment LSA 2 flooded LSA 2

  31. LSA 3 Summary LSA Multi Area OSPF Normal Areas LSA 3 LSA 1 LSA 1 LSA 1 LSA 3 LSA 1’s are sent as LSA 3’s into other areas.

  32. LSA 3 Summary LSA Multi Area OSPF Normal Areas LSA 3 LSA 3 LSA 3 LSA 1 LSA 1 LSA 1 LSA 3 LSA 1 LSA 1 LSA 1’s are sent as LSA 3’s into other areas.

  33. LSA 4 ASBR Summary LSA Normal Areas LSA 5’s flooded LSA 4 LSA 4 LSA 4 LSA 4 LSA 4

  34. LSA 5 AS External LSA Normal Areas LSA 5’s flooded LSA 5 LSA 5 LSA 5 ASBR router ospf 1 redistribute rip metric 500 network 206.202.0.0 0.0.0.255 area 0 ip route 0.0.0.0 0.0.0.0 10.0.0.2 LSA 5 LSA 5

  35. Stub Areas LSA 3 LSA 4 LSA 4 LSA 3 LSA 5 LSA 5 LSA 4 LSA 5 LSA 5 Blocked X Default route to ABR injected Stub Area • All routers in the area must be configured as “stub” including the ABR: router ospf 1 network 206.202.0.0 0.0.0.255 area 0 network 206.202.1.0 0.0.0.255 area 1 area 1 stub

  36. Totally Stubby Areas LSA 3 LSA 4 LSA 4 LSA 3 LSA 5 LSA 5 LSA 4 LSA 5 LSA 5 X Blocked X Blocked X Default route to ABR injected Stub Area • All routers in the area must be configured as “stub” except the ABR “stub no summary”: router ospf 1 network 206.202.0.0 0.0.0.255 area 0 network 206.202.1.0 0.0.0.255 area 1 area 1 stub no-summary

  37. Virtual Links

  38. Virtual Links • A virtual link has the following two requirements: • It must be established between two routers that share a common area. • One of these two routers must be connected to the backbone.

  39. Virtual Links

  40. Virtual Links RTA(config)#router ospf 1 RTA(config-router)#network 192.168.0.0 0.0.0.255 area 51 RTA(config-router)#network 192.168.1.0 0.0.0.255 area 3 RTA(config-router)#area 3 virtual-link 10.0.0.2 ... RTB(config)#router ospf 1 RTB(config-router)#network 192.168.1.0 0.0.0.255 area 3 RTB(config-router)#network 192.168.2.0 0.0.0.255 area 0 RTB(config-router)#area 3 virtual-link 10.0.0.1

  41. OSPF verification commands • show ip ospf ? • list of possible commands • border-routers • database • neighbours • etc. • Be curious!!!!

  42. Reserve slides

  43. OSPF Area Types • Backbone area (Area 0) – A transit area, the central entity to which all other areas connect, all other areas must connect to this area to exchange route information. Accepts all LSAs. • Stub area - Area that does not accept information about routes external to the autonomous system (AS), such as routes from non-OSPF sources. If routers need to reach networks outside the AS, they use a default route. Does not accept LSAs. • Totally stubby area - Area that does not accept external AS routes and summary routes from other areas internal to the AS. Does not accept external or summary LSAs. A totally proprietary Cisco feature that uses a default route to reach networks outside the AS. • Not-so-stubby areas - Area that is similar to a stub area but allows for importing external routes as Type 7 LSAs and translation of specific Type 7 LSA routes into Type 5 LSAs. 

  44. LSA Types

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