Multi Area OSPF

1© 2002, Svenska-CNAP Halmstad UniversitySession NumberVersion 2002-1

Multi Area OSPFCarl Bergenhem - 030404

[email protected]

222© 2002, Svenska-CNAP / Halmstad University.Version 2002-1

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

InternalInternal: Routers with all their interfaces within the same area

BackboneBackbone: Routers with at least one interface connected to area 0

ASBRASBR: (Autonomous System Boundary Router): Routers that have at least one interface connected to an external internetwork (another autonomous system)

ABRABR: (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 TypesLSA1 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......


Area Types

External (type 5) routesSummary (type 3,4) routes

Default route Default route

Default route Accepted


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

ASBR

RIP


Stub Example


Stub Example

ABR

ASBR


Stub Example

ABR

ASBR

No Type 5 LSAs


Stub Example

ABR

ASBR

No Type 5 LSAs

Route to 0.0.0.0/0 via RTB






(desc. routes within AS)


(desc. routes to ASBR)






Totally Stubby Example

ABR

ASBR

No Type 3, 4, or 5 LSAs

Route to 0.0.0.0/0 via RTB

(no more IA routes)

Totally Stubby Area

no summary






(desc. routes within AS)


(desc. routes to ASBR)






NSSA Example


NSSA Example

ABR

ASBR

No Type 5 LSAs

3,4 OK

Type 7 OK


NSSA Example

ABR

ASBR

No Type 3, 4 or 5 LSAs

Type 7 OK

no summary


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).


RouterELoop 1.10.202.206/24

RouterALoop 1.0.202.206/24 RouterB

Loop 2.0.202.206/24

RouterCLoop 1.2.202.206/24

RouterDLoop 2.2.202.206

Switch

Switch

RouterFLoop 2.10.202.206/24

ASBRLoop 1.5.202.206/24

AS-Rem ote

10.0.0.0/8

192.10.10.0

/24206.202.0.0/24

192.10.5.0/24

206.202.1.0/24206.202.2.0/24

OSPFArea 1

OSPFArea 51

OSPFArea 0

.2

.1

.2

.1

.2.1

.3

.4.1

.2

.1

Loop 162.10.5.1/16

RIPRIP

.1

.2

Multi Area OSPF

Normal Areas

LSA 1 Orig.

LSA 1 Originated

LSA 1’s are flooded out other interfaces within the same area.

LSA 1 flooded

LSA 1 flooded

LSA 1 Router LSA




Loop 2.0.202.206/24



Switch

Switch


ASBRLoop 1.5.202.206/24

AS-Rem ote

10.0.0.0/8

192.10.10.0

/24206.202.0.0/24

192.10.5.0/24

206.202.1.0/24206.202.2.0/24

OSPFArea 1

OSPFArea 51

OSPFArea 0

.2

.1

.2

.1

.2.1

.3

.4.1

.2

.1

Loop 162.10.5.1/16

RIPRIP

.1

.2

Multi Area OSPF

Normal Areas

LSA 2

LSA 2’s being sent

LSA 2No LSA 2’s for Area 51 – No DR elected because no other routers on the broadcast segment

LSA 2 flooded

LSA 2 Network LSA




Loop 2.0.202.206/24



Switch

Switch


ASBRLoop 1.5.202.206/24

AS-Rem ote

10.0.0.0/8

192.10.10.0

/24206.202.0.0/24

192.10.5.0/24

206.202.1.0/24206.202.2.0/24

OSPFArea 1

OSPFArea 51

OSPFArea 0

.2

.1

.2

.1

.2.1

.3

.4.1

.2

.1

Loop 162.10.5.1/16

RIPRIP

.1

.2

Multi Area OSPF

Normal Areas

LSA 1LSA 1

LSA 1

LSA 1’s are sent as LSA 3’s into other areas.

LSA 3

LSA 3

LSA 3 Summary LSA




Loop 2.0.202.206/24



Switch

Switch


ASBRLoop 1.5.202.206/24

AS-Rem ote

10.0.0.0/8

192.10.10.0

/24206.202.0.0/24

192.10.5.0/24

206.202.1.0/24206.202.2.0/24

OSPFArea 1

OSPFArea 51

OSPFArea 0

.2

.1

.2

.1

.2.1

.3

.4.1

.2

.1

Loop 162.10.5.1/16

RIPRIP

.1

.2

Multi Area OSPF

Normal Areas

LSA 1 LSA 1

LSA 1LSA 1

LSA 1

LSA 1’s are sent as LSA 3’s into other areas.

LSA 3

LSA 3

LSA 3

LSA 3

LSA 3 Summary LSA



RouterALoop 1.0.202.206/24

RouterBLoop 2.0.202.206/24



Switch

Switch


ASBRLoop 1.5.202.206/24

AS-Rem ote

10.0.0.0/8

192.10.10.0

/24206.202.0.0/24

192.10.5.0/24

206.202.1.0/24206.202.2.0/24

OSPFArea 1

OSPFArea 51

OSPFArea 0

.2

.1

.2

.1

.2.1

.3

.4.1

.2

.1

Loop 162.10.5.1/16

RIPRIP

.1

.2

Normal Areas

LSA 4

LSA 4

LSA 4

LSA 4LSA 4

LSA 5’s flooded

LSA 4 ASBR Summary LSA







Switch

Switch


ASBRLoop 1.5.202.206/24

AS-Rem ote

10.0.0.0/8

192.10.10.0

/24206.202.0.0/24

192.10.5.0/24

206.202.1.0/24206.202.2.0/24

OSPFArea 1

OSPFArea 51

OSPFArea 0

.2

.1

.2

.1

.2.1

.3

.4.1

.2

.1

Loop 162.10.5.1/16

RIPRIP

.1

.2

Normal Areas

LSA 5

LSA 5

LSA 5

LSA 5LSA 5

LSA 5’s flooded

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 AS External LSA







Switch

Switch


ASBRLoop 1.5.202.206/24

AS-Rem ote

10.0.0.0/8

192.10.10.0

/24206.202.0.0/24

192.10.5.0/24

206.202.1.0/24206.202.2.0/24

OSPFArea 1

OSPFArea 51

OSPFArea 0

.2

.1

.2

.1

.2.1

.3

.4.1

.2

.1

Loop 162.10.5.1/16

RIPRIP

.1

.2

Stub Area

LSA 4

LSA 4

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

LSA 4

LSA 5

Blocked XLSA 5LSA 5 LSA 5

LSA 3

LSA 3

Default route to ABR injected

Stub Areas







Switch

Switch


ASBRLoop 1.5.202.206/24

AS-Rem ote

10.0.0.0/8

192.10.10.0

/24206.202.0.0/24

192.10.5.0/24

206.202.1.0/24206.202.2.0/24

OSPFArea 1

OSPFArea 51

OSPFArea 0

.2

.1

.2

.1

.2.1

.3

.4.1

.2

.1

Loop 162.10.5.1/16

RIPRIP

.1

.2

Stub Area

LSA 4

LSA 4LSA 4

X BlockedLSA 5

Blocked XLSA 5LSA 5 LSA 5

LSA 3

LSA 3

Default route to ABR injected

X

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

Totally Stubby Areas


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

Documents

Multi Area OSPF