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

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s