OSPF quick-ref

Purpose of this list is to be a quick ref; get it in the brain and dump it on the empty notepad you get at the written so you can use it during the exam.

Internal Metrics

Cost = 10^8 / bandwidth in bps

Administrative Distance

intra-area = 110
inter-area = 110
external = 110

Internals:

OSPF uses sub-protocols

  • Hello
  • Exchange
  • Flooding

Packet types

  1. Hello – neighborship
  2. Database description – summarise DB content
  3. Link State Request – Request LSAs (only during Exchange, loading or Full)
  4. Link State Update – list of LSAs to be updated (used in flooding)
  5. Link State Acknowledgement – Ack flooded packets

Topology change: Flood type 4 (LS Update) and 5 (LS Ack)

LSA types

  1. Router (local or intra-area)
  2. Network (local or intra-area)
  3. ABR summary (inter-area / summary-link)
  4. ASBR summary (inter-area / summary-link)
  5. AS external route (external)
  6. Multicast group LSA (non Cisco)
  7. NSSA external (only seen in an NSSA)
  8. n/a
  9. Opaque LSA: Link-local scope
  10. Opaque LSA: Area-local scope
  11. Opaque LSA: AS scope

A Type 4 LSA is used by ABRs to inform ‘areas’ where to find an ASBR, which advertises itself using a type 1 LSA within it’s own area.

Type 7 to 5 conversion: The router with the highest RID (i.e. ABR) within an NSSA will be responsible for translating Type 7 LSAs into Type 5 LSAs.

Type 5 LSA is flooded throughout the domain, except stub and NSSAs, the rest is local to the area.

Opaque LSAs: Used for MPLS (TE), their implementation is vendor specific.

Allowed LSAs

  • Stub: local: 1, 2; inter-area: 3
  • NSSA: local: 1, 2; inter-area: 3; external: 7
  • A default (3) exist if the ABR has a default route, otherwise use the ‘always’ keyword

LSA timers

  • LSA RefreshTime: every 30 minutes
  • After group pacing (IOS 12.0) refresh is per paced group and not at a fixed interval
  • LSA MaxAge: 60 minutes

Neighbor status

  1. Down – initial status
  2. Attempt – try to contact neighbor
  3. Init – hello received
  4. 2-Way – bi-direction (received hello with our ROUTERID in it)
  5. ExStart – 1st step towards adjacency
  6. Exchange – sending data description packets
  7. Loading – sending LS request packets
  8. Full – adjancency is up

SPF: Runs twice: once for routers, once for networks

Area # Range: Used for summarising routes (LSA 1 & 2) on Area borders (creates LSA type 3)

Routing order: Intra area routing first (by cost) then consider inter area, external comes last

M’cast

  • 224.0.0.5 – all OSPF routers (LAN & NMBA)
  • 224.0.0.6 – all DR routers (LAN)

Default timers

  • LAN: Hold: 10, Dead 40
  • NMBA: Hold: 30, Dead: 120

Router types

  • IR – Internal router
  • ABR – Area Border Router (always have a link to area 0)
  • ASBR – Autonomous System Border Router
  • Backbone Routers

An ABR can be an ASBR

Network types

  • Broadcast (LAN)
  • NMBA (F/R, ATM, X.25)
  • Point-to-Multipoint
  • Point-to-Point

Area types

  • Backbone = Area 0 (0.0.0.0 if you relate the 32 bit area code to an IPv4 address)
  • Normal = Allowed LSA types: 1, 3, 4 & 5 (no LSA type 7)
  • Stub = No LSA type 5 allowed
  • Totally Stub = No LSA type 3, 4 & 5 allowed, except default summary route(s)
  • NSSA = No LSA type 5 allowed, use LSA type 7 which are converted to type 5 at the NSSA ABR (if the P-bit is set)
  • NSSA Totally Stub = No LSA type 3, 4 & 5 allowed, except default summary route(s). LSA type 7 which are converted to type 5 at the NSSA ABR are allowed

Various bits (flags)

  • P-bit = Propagate : If ’0′ then the NSSA ABR shouldn’t translate the LSA type 7 to a type 5
  • E-bit = External : An NSSA ASBR must set the External bit in its router (type-1) LSAs (both in the backbone and in the NSSA area)
  • B-bit = Border : An NSSA ASBR must set the Border bit in its router (type-1) LSAs (both in the backbone and in the NSSA area)
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