Multiarea OSPF
CCNP 1: Advanced Routing
Chapter 5
Overview
1.
2.
3.
4.
5.
6.
OSPF Review
Multiarea OSPF Operation and Configuration
Area Types: Stub, Totally Stub, and NSSA
Virtual Links
Monitoring and Verifying Multiarea OSPF
OSPF Pitfalls: “What could possibly go wrong?”
Open Shortest Path First
OSPF is a scalable link-state protocol that determines
the best path between two networks by comparing the
aggregate costs of all possible paths.
•Extremely scalable
•Sends incremental updates
•Establishes a relationship between neighbor routers
•Hierarchical
•Supports VLSM
The OSPF Database
• Each OSPF-enabled
interface is considered a
“link”.
• All links are assimilated
into the SPF database.
• show ip ospf database
• You too will be assimilated
into the collective.
Resistance is futile.
http://www.routergod.com/sevenofnine
OSPF Features
•Scalability
•OSPF is considered by most to be the 2nd most scalable IP IGP
•OSPF is used in both Service Provider and Enterprise networks
•Incremental Updates
•Fast convergence time
•Light on bandwidth
•Heavy on router CPU cycles
OSPF Neighbor Relationships
• Point-to-point links: OSPF detects and connects to neighboring
OSPF routers, and the routers exchange link state databases.
• Multiaccess links: OSPF detects and connects to neighboring OSPF
routers. A designated Router (DR) and Backup Designated Router
(BDR) are elected. The routers exchange link state databases with
the DR, and their tables are also sent to the BDR.
• Verification/Troubleshooting:
• Router#show ip ospf neighbors
• If you don’t see a DR, guess what?
• For added fun:
• Router(config)#alias exec sion show ip ospf neighbors
Hierarchy
•OSPF is based on a 2-layer hierarchy
•Backbone and Non-backbone
•An OSPF domain can be divided in areas
•Each area has its own link state database
•Why?
5.1.1
What problem are you trying to solve?
5.1.1
What problem are you trying to solve?
OSPF Areas
• An area can contain as many as “50ish” routers
• This number varies, based on the number and
stability of links and the resources on the routers
• Backbone area should be relatively small
• Backbone area should consist of stable links
• Backbone Area = Area 0 = Teh Princess
OSPF Areas
Backbone Router: Router with
at least one interface in
Area 0
Internal Router: Router with
all interfaces in the same
area
Area Border Router (ABR):
Router with interfaces in two
or more different areas
Autonomous System
Boundary Router (ASBR):
Router with at least one
interface connected to a
non-OSPF domain
Configuration
Area 1
Area 10
Area 0
R1(config)#router ospf 1
R1(config-router)#network 10.3.0.0 0.0.255.255 area 0
R1(config-router)#network 10.1.0.0 0.0.255.255 area 10
R1(config-router)#network 10.2.0.0 0.0.255.255 area 1
What kind of router is R1?
Internal
Backbone
ABR
ASBR
Configuration
Area 1
Area 10
Area 0
R1(config)#router ospf 1
R1(config-router)#network 10.3.0.0 0.0.255.255 area 0
R1(config-router)#network 10.1.0.0 0.0.255.255 area 10
R1(config-router)#network 10.2.0.0 0.0.255.255 area 1
What kind of router is R1?
Backbone
ABR
Configuration
R2(config)#router ospf 1
R2(config-router)#network 10.3.0.0 0.0.255.255 area 0
R2(config-router)#network 10.1.0.0 0.0.255.255 area 0
R2(config-router)#network 10.2.0.0 0.0.255.255 area 0
What kind of router is R2?
Internal
Backbone
ABR
ASBR
Configuration
R2(config)#router ospf 1
R2(config-router)#network 10.3.0.0 0.0.255.255 area 0
R2(config-router)#network 10.1.0.0 0.0.255.255 area 0
R2(config-router)#network 10.2.0.0 0.0.255.255 area 0
What kind of router is R2?
Internal
Backbone
Look Ma, no diagram!
Configuration
R3(config)#router ospf 1
R3(config-router)#network 10.3.0.0 0.0.255.255 area 10
R3(config-router)#network 10.1.0.0 0.0.255.255 area 10
R3(config-router)#network 10.2.0.0 0.0.255.255 area 10
What kind of router is R3?
Internal
Backbone
ABR
ASBR
Configuration
R3(config)#router ospf 1
R3(config-router)#network 10.3.0.0 0.0.255.255 area 10
R3(config-router)#network 10.1.0.0 0.0.255.255 area 10
R3(config-router)#network 10.2.0.0 0.0.255.255 area 10
What kind of router is R3?
Internal
Configuration
R4(config)#ip route 10.4.0.0 255.255.0.0 serial 0/0
R4(config)#router ospf 1
R4(config-router)#network 10.3.0.0 0.0.255.255 area 0
R4(config-router)#network 10.1.0.0 0.0.255.255 area 10
R4(config-router)#network 10.2.0.0 0.0.255.255 area 10
R4(config-router)#redistribute static
What kind of router is R4?
Internal
Backbone
ABR
ASBR
Configuration
R4(config)#ip route 10.4.0.0 255.255.0.0 serial 0/0
R4(config)#router ospf 1
R4(config-router)#network 10.3.0.0 0.0.255.255 area 0
R4(config-router)#network 10.1.0.0 0.0.255.255 area 10
R4(config-router)#network 10.2.0.0 0.0.255.255 area 10
R4(config-router)#redistribute static
What kind of router is R4?
Backbone
ABR
ASBR
Configuration
R5(config)#router ospf 1
R5(config-router)#network 10.3.0.0 0.0.255.255 area 1
R5(config-router)#network 10.1.0.0 0.0.255.255 area 10
R5(config-router)#network 10.2.0.0 0.0.255.255 area 10
What kind of router is R5?
Internal
Backbone
ABR
ASBR
Configuration
R5(config)#router ospf 1
R5(config-router)#network 10.3.0.0 0.0.255.255 area 1
R5(config-router)#network 10.1.0.0 0.0.255.255 area 10
R5(config-router)#network 10.2.0.0 0.0.255.255 area 10
What kind of router is R5?
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
Internal
Backbone
ABR
ASBR
Every area MUST have at least one link in Area 0.
OSPF
LSA
Types
LSA Types and Routing tables
R1#show ip route ospf
O
6.0.0.0/8 [110/65] via 5.0.0.2, 00:04:23, Serial 0/0
O IA 7.0.0.0/8 [110/65] via 5.0.0.2, 00:04:23, Serial 0/0
O E2 8.0.0.0/8 [110/65] via 5.0.0.2, 00:04:23, Serial 0/0
Type 2: O
Type 3: O IA (How to reach ABRs)
Type 4: O IA (How to reach ASBRs)
Type 5: O E1 or O E2
OSPF Stub Areas
• A stub area has only one entry and exit point.
• Topology changes in other areas do not affect
stub areas. The ABR is still the only way out.
Stub Areas and LSAs
5.2.2
Stub vs. Totally Stub
Similarities
•
•
•
•
•
ABR does not advertise Type 4 or 5 LSAs into the area
There should be no OE1 or OE2 routes in the area
ABR advertises a default route to the internal routers
Neither area can have an ASBR in it
Neither area can be the backbone
Differences
• Totally stub does not advertise Type 3 LSAs either
• Stub routing table: O, O IA, and default route
• Totally stub routing table: O and default route
5.4.3
Stub and Totally Stub Configuration
Stub Area
R1(config-router)#area 1 stub
R2(config-router)#area 1 stub
• Command must be entered on all routers in the area
Totally Stubby Area
R1(config-router)#area 1 stub no-summary
R2(config-router)#area 1 stub
• ABR is configured with stub no-summary
• Internal routers are configured with stub
NSSA
• Many service providers have OSPF areas
that have only one exit point, but the areas
contain an ASBR
• Solution: Not-So-Stubby Areas (NSSAs)
• The ASBR originates a Type 7 LSA
• The ABR converts from Type 7 to Type 5
NSSA Configuration
Not-So-Stubby Area
R1(config-router)#area 1 nssa
R2(config-router)#area 1 nssa
R2(config-router)#redistribute static
• Command must be entered on all routers in the area
• ASBR must be configured to inject non-OSPF routes
Totally Not-So-Stubby Area
R1(config-router)#area 1 nssa no-summary
R2(config-router)#area 1 stub
• ABR is configured with nssa no-summary
• Internal routers are configured with nssa
5.6.3
Stub and NSSA
• By default, stub ABRs advertise a default route.
• By default, NSSA ABRs do not.
• To force an NSSA to advertise a default route:
R1(config-router)#area 1 nssa defaultinformation-originate
5.3.2
Summarizing with OSPF
• Only ABRs and ASBRs can summarize
• Routes should be summarized as/before they
are advertised to Area 0
• ABR:
R1(config-router)#area 1 range 10.0.0.0
255.0.0.0
• ASBR:
R1(config-router)#summary-address
10.0.0.0 255.0.0.0
5.5.1
Breaking the Area 0 Rule
• Every area must have at least one interface on
at least one router in Area 0.
• Area 0 must be contiguous.
• Problem: Mergers
• Problem: ExtraNets
OSPF Virtual Links
R1(config-router)#area 1 virtual-link 3.3.3.3
R3(config-router)#area 1 virtual-link 1.1.1.1
• Virtual links are established by router IDs
• Can be used to join a discontiguous Area 0
• Can be used to “tunnel” a remote area to Area 0
Note: YOU SHOULD NOT FIND YOURSELF IN
THIS SITUATION.
OSPF Pitfalls
• OSPF Path Selection
• OSPF over NBMA
• OSPF over DDR
Multiarea OSPF
CCNP 1: Advanced Routing
Chapter 5