Carl Bergenhem - 030404 carl@bergenhem.com 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 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

OSPF Areas

OSPF Router Types

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.

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)

LSA Types See online material: chpt: 5.1.3

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

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.

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).

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......

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

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

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

Multi-area Example ABR RIP ASBR

Stub Example

Stub Example ABR ASBR

Stub Example ABR No Type 5 LSAs ASBR

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

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

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

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

NSSA Example

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

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

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).

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

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

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

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

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

LSA 5 AS External LSA Normal Areas LSA 5 LSA 5’s flooded 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

Stub Areas Stub Area LSA 3 LSA 4 LSA 4 LSA 3 LSA 5 LSA 5 LSA 4 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

X Blocked X Totally Stubby Areas Stub Area LSA 3 LSA 4 LSA 4 LSA 3 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

Virtual Links

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.

Virtual Links

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

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

Reserve slides

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. 

LSA Types