OSPF – NOTES

OSPF Neighbor Formation:

• INIT
  • ACL blocking traffic one way
• 2-WAY
• Exstart
• Exchange
  • MTU Mismatch
• Loading
• Full
  • Loading to Full – Desired for neighbor adjacency

Network Types:

• Point to Point
  • No DR/BDR.
  • Two Devices.
  • Multicast approach – 224.0.0.5 – All SPF Routers Address
  • Automatic network type under HDLC.
  • Hello 10, Dead 40, Wait 40, Retransmit 5
• Broadcast
  • DR/BDR
    • Eliminates mesh of neighbor adjacencies in multiaccess network.
    • Minimizes number of LSA updates when network changes.
    • 224.0.0.6 – AllDRouters – DR/pseudonode communication
• Non-Broadcast MultiAccess
  • Legacy Frame Relay
  • DR/BDR – Packets are unicast, not multicast.
• Point to Multipoint
  • Collection of Point-to-Point links. No DR/BDR
• OSPF Virtual Link
  • Bandaid for connecting all areas to backbone area 0.

Path Preference:

• Cost – Sum of all outbound interfaces.
• Cost can be adjusted by interface
  • router(config-if)#bandwidth <value>
  • router(config-if)#ip ospf cost <value>
    • default reference value is 100Mbps
• router#show ip ospf border-routers
  • Displays all ABRs and ASBRs.
  • Displays cost to reach prefixes.
• Path Selection List:
  • 1. Longest prefix/specific match
  • 2. Intra-area routes
  • 3. Inter-area routes
  • 4. E1
  • 5. E2
  • 6. NSSA1
  • 7. NSSA2

LSA Types:

1. Router – Flooded within Area – Intra
2. Network – Used by Designated Router. Advises on other adjacencies in multiaccess network.
3. Network Summary – How prefixes are advertised from one area to another. IA – Inter-Area.
4. ASBR Summary – Used to identify router that is ASBR.
5. AS External – Prefixes that come in from ASBR (Redistribution – E1, E2).
6. Group Membership – Was used for Multicast-OSPF – not really used.
7. NSSA External – Allows prefixes to be advertised into Not So Stubby Areas.
8. External Attributes – iBGP alternative – not really used.
9. – 11 – Opaque LSAs – Opened up OSPF for other applications to include information.

Network Types:

• Backbone/0:
  • LSA #s – 1, 2, 3, 4, 5
• Non Backbone, Non Stub:
  • LSA #s – 1, 2, 3, 4, 5
• Stub:
  • LSA #s – 1, 2, 3
• Totally Stub:
  • LSA #s – 1, 2
  • LSA # 3 default route only!
• Not So Stubby Area
  • LSA #s – 1, 2, 3, 4, 7

OSPF OPTIMIZATIONS:

• ISPF
  • Incremental Shortest Path First calculations
  • Stops entire tree within area from reconverging
  • R1(config-router)#ispf
• Hello & Dead Timers
  • R1(config-if)#ip ospf hello-interval <# in seconds>
  • R1(config-if)#ip ospf dead-interval <# in seconds>
  • Breaks neighborships until both neighbors match intervals
  • show ip ospf int <interface> – shows the timers on the interface
• R1(config-router)#max-lsa <#>
  • Number of non-self generated LSAs that this router can keep in database

Prefix Filtering/Suppression:

• Prefix filter with route map (ex. deny single prefix)
  • R1(config)#access-list 1 permit 1.1.1.0 0.0.0.255
  • R1(config)#route-map <NAME> deny 10
  • R1(config-route-map)#match ip address 1(ACL #)
  • R1(config-route-map)#route-map <NAME> permit 20
  • R1(config-route-map)#end
  • R1(config)#router ospf 1
  • R1(config-router)#distribute-list route-map <NAME> in
    • Feature works inbound on router
  • Only applies to local router. Downstream routers will still have route if they typically would with normal OSPF operations.
• Filter-List (ex. deny single prefix – LSA Type 3 Filtering)
  • R1(config)#ip prefix-list <NAME> deny 1.1.1.0/24
  • R1(config)#ip prefix-list <NAME> permit 0.0.0.0/0 le 32
  • R1(config)#router ospf 1
  • R1(config-router)#area 0 filter-list prefix <NAME> in
    • Feature works both inbound and outbound.
• Route Poisoning
  • R1(config)#access-list 1 permit 1.1.1.0 0.0.0.255
  • R1(config)#router ospf 1
  • R1(config-router)#distance 255 0.0.0.0 255.255.255.255 1(access-list number)
  • Makes route look unreachable – AD of 255
• Internal or Area Summarization
  • ON ABR:
    • R1(config-router)#area 10 range 1.0.0.0 255.0.0.0
    • Summarization for inter-area routes
• External Summarization
  • ON ASBR:
    • R1(config-config)#summary-address 1.0.0.0 255.0.0.0