Chapter 3 - Multi Area OSPF - Part 1

CCNA Advance

Chapter 3OSPF Multi Area part1

Multi-Area OSPF Operation

3Issues with Maintaining a Large OSPF Network

4The Solution: OSPF Hierarchical Routing

Consists of areas and autonomous systems Minimizes routing update traffic

5Types of OSPF Routers

InternaI: Routers with all their interfaces within the same areaBackbone: Routers with at least one interface connected to area 0ASBR: (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.

6Defining Virtual Links

Virtual links are used to connect a discontiguous area to area 0. A logical connection is built between router A and router B. Virtual links are recommended for backup or temporary connections.

7Configuring Virtual Links

8OSPF Virtual Link Configuration Example

9The show ip ospf virtual-links Command

10

LSA Types

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LSA Types

LSA Types 1 through 5 We will look at these in detail as we discuss areas in this chapter.LSA Type 6 MOSPF (Multicast OSPF) Not supported by Cisco. MOSPF enhances OSPF by letting routers use their link-state databases to buildmulticast distribution trees for the forwarding of multicast traffic.LSA 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).Type 8 Type 8 is a specialized LSA that is used in internetworking OSPF and Border Gateway Protocol (BGP).Types 9, 10, and 11 The opaque LSAs, types 9, 10, and 11, are designated for future upgrades to OSPF for application-specific purposes. For example, Cisco Systems uses opaque LSAs for Multiprotocol Label Switching (MPLS) with OSPF. Opaque LSAs are distributed using standard LSDB flooding mechanisms. Each type has a different flooding scope.

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LSA Type 1: Router LSA

One router LSA (type 1) for every router in an area Includes list of directly attached links Each link identified by IP prefix assigned to link and link type

Identified by the router ID of the originating router

Floods within its area only; does not cross ABR. On multi-access networks, sent to the DR. Denoted by just an O in the routing table or C if the network is directlyconnected. ABR will include a set of LSA 1s for each area it belongs to.

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LSA 1 - Router Link States

LSA 1: Sent within an area

ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID192.168.3.1/32

Pri 100Pri 200

Lo - RouterID192.168.4.1/32

LSA 1

LSA 1

LSA 1

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ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID192.168.3.1/32

Pri 100Pri 200

Lo - RouterID192.168.4.1/32

LSA 1s are flooded outother interfaces within thesame area.

LSA 1Originated

LSA 1flooded

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Denoted by just an O in the routing table, or a C Why is there only just anO for this network and not the other networks? Directly connected or via another area.

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LSA Type 2: Network LSA

One network (type 2) LSA for each transit broadcast or NBMA network in an area Includes list of attached routers on the transit link Includes subnet mask of link Advertised by the DR of the broadcast network Floods within its area only; does not cross ABR Denoted by just an O in the routing table or C if the network is directlyconnected. LSA 2s are in link state database for all routers within area, even those routers onnot on multi-access networks or DRs on other multi-access networks in the samearea. ABR may include a set of LSA 2s for each area it belongs to.

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LSA 2 - Network Link States

ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID192.168.3.1/32

Pri 100Pri 200

Lo - RouterID192.168.4.1/32

LSA 2DR

DR

LSA 2

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LSA 2 - Network Link States

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LSA Type 3: Summary LSA

Type 3 LSAs are used to flood network information to areas outside the originating area (interarea) Describes network number and mask of link. Advertised by the ABR of originating area. LSA 3s are flooded throughout the backbone (Area 0) and to other ABRs. Routes learned via LSA type 3s are denoted by an IA (Inter-area) in the routing table. Regenerated by subsequent ABRs to flood throughout the autonomous system. By default, routes are not summarized, and type 3 LSA is advertised for every subnet.

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LSA 3 Summary Net Link States

ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID192.168.3.1/32

Pri 100Pri 200

Lo - RouterID192.168.4.1/32

LSA 1

LSA 1

LSA 1LSA 3LSA 3LSA 3

LSA 3

LSA 3LSA 3

LSA 3

LSA 3

LSA 3

LSA 1s are sent as LSA 3s intoother areas by the ABRs.

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Routers only see the topology of the area they belong to. When a link in one area changes, the adjacent routers originate in LSA 1s and flood them within the area, causing intra-area (internal) routers to re-run the SPF and recalculating the routing table. ABRs do not announce topological information between areas. ABRs only inject routing information into other areas, which is basically a distance-vector technique.

22


ABRs calculate intra-area routes for directly attached areas and announce them to all other areas as inter-area routes, using LSA 3s. OSPF ABRs will only announce inter-area routes that were learned from the backbone area, area 0. The backbone area serves as a repository for inter area routes. This keeps OSPF safe from routing loops.

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Topology Change: Down Link

When a router detects a topology change it immediately sends out LSA 1s (Router LSAs) with the change. Trigger Update In the case of a down link, the age of the LSA is set to MaxAge(3,600 seconds) Routers that receive LSAs with the age equal to MaxAge remove this entry from their LSDB (Link State Data Base). Routers that receive the LSA 1s, within the area of the change, re-run their SPF algorithm, to build a new SPF tree and then make the changes to their IP routing tables.

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25


26


Summary (type 4) LSAs are used to advertise an ASBR to all other areas in theautonomous system. They are generated by the ABR of the originating area. They are regenerated by all subsequent ABRs to flood throughout the autonomous system. Type 4 LSAs contain the router ID of the ASBR not a network -> Describes thereachability to the ASBRs Included in routing table as an IA route. Same format as a LSA 3 - Summary LSA, except LSA 4 ASBR Summary LSA theNetwork Mask field is always 0 Exceptions: Not flooded to Stub and Totally Stubby networks. More on this later

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ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID192.168.3.1/32

Pri 100Pri 200

Lo - RouterID192.168.4.1/32

LSA 4

LSA 4LSA 4

Flooded throughout the backbone area to the Describes the reachability to the ASBRs (not a network)

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29

LSA Type 5: External LSA

External (type 5) LSAs are used to advertise networks from other autonomous systems. Type 5 LSAs are advertised and owned by the originating ASBR. Type 5 LSAs flood throughout the entire autonomous system -> Describes destination networks external to the Autonomous System (This OSPF Routing Domain) The advertising router ID (ASBR) is unchanged throughout the autonomous system. Type 4 LSA is needed to find the ASBR. By default, routes are not summarized. Denoted in routing table as E1 or E2 (default) route (soon) ASBR Router which redistributes routes into the OSPF domain. Exceptions: Not flooded to Stub and Totally Stubby networks. More on this later

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ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID192.168.3.1/32

Pri 100Pri 200

Lo - RouterID192.168.4.1/32

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ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID192.168.3.1/32

Pri 100Pri 200

Lo - RouterID192.168.4.1/32


LSA 5

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33

Interpreting the OSPF Database

34

Interpreting the Routing Table: Types ofRoutes

35

LSA 5 - AS External Link States

E1 vs. E2 External Routes External routes fall under two categories: (LSA 5) external type 1 external type 2 (default) The difference between the two is in the way the cost (metric) of the route is being calculated. The cost of a type 2 route is always the external cost, irrespective of the interior cost to reach that route. A type 1 cost is the addition of the external cost and the internal cost used to reach that route. A type 1 route is always preferred over a type 2 route for the same destination. More later

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Calculating Costs for E1 and E2 Routes

37

The show ip route Command

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Changing the Cost Metric

The cost, or metric, is an indication of the overhead to sendpackets over an interface. Default = (100 Mbps) / (bandwidth inMbps).

RouterA(config-if)#ip ospf cost interface-cost

Overrides the default cost calculation. Values from 1 to 65535can be defined.

RouterA(config-router)#auto-cost reference-bandwidth ref-bw

Sets the reference bandwidth to values other than 100 Mbps (legal values range from 1 to 4,294,967 in megabits per second).

Question ?

Thank you !

CCNA AdvanceMulti-Area OSPF OperationIssues with Maintaining a Large OSPF NetworkThe Solution: OSPF Hierarchical RoutingTypes of OSPF RoutersDefining Virtual LinksConfiguring Virtual LinksOSPF Virtual Link Configuration ExampleThe show ip ospf virtual-links CommandLSA TypesLSA TypesLSA Type 1: Router LSALSA 1 - Router Link StatesLSA 1 - Router Link StatesLSA 1 - Router Link StatesLSA Type 2: Network LSALSA 2 - Network Link StatesLSA 2 - Network Link StatesLSA Type 3: Summary LSALSA 3 Summary Net Link StatesLSA 3 Summary Net Link StatesLSA 3 Summary Net Link StatesTopology Change: Down LinkLSA 3 Summary Net Link StatesLSA 3 Summary Net Link StatesLSA Type 4: Summary LSALSA Type 4: Summary LSALSA Type 4: Summary LSALSA Type 5: External LSALSA Type 5: External LSALSA Type 5: External LSALSA Type 5: External LSAInterpreting the OSPF DatabaseInterpreting the Routing Table: Types ofRoutesLSA 5 - AS External Link StatesCalculating Costs for E1 and E2 RoutesThe show ip route CommandChanging the Cost MetricQuestion ?

Chapter 3 - Multi Area OSPF - Part 1

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