OSPF Notes


Introduction

In this doc, we go over the OSPF neighborship types workflow and the troubleshooting tips, LSA type, Area types, summarization and more 

Pre-requisite

  • A good understanding of routing
  • Knowledge in BGP

Neighbour status & neighbourship troubleshooting tips

  1. Nothing: This means the router hasn't received anything
    Reason: This is the initial state, meaning we didn't get any packets. we need to check ACL, multicast, ping 224.0.0.5, debug IP packet, traceroute neighbour, and capture on the interface
  2. INIT: The router has received a hello but didn't see its own router-ID in the hello packet
    Reason: this could be that the neighbour didn't receive my hello, or the local router didn't send it. You need to check ACL, check multicast, ping 224.0.0.5, and debug IP packet
  3. 2WAY: router sees its IP in the neighbour hello packet the router only performs full adjacency to the DR and BDR, so this could be normal
  4. Exstast: the neighbours see each other and exchange link state info Reason: for a neighbour to flap between ecstasy or exchange to 2WAY could be a configuration error like the same router ID, corrupt packets, MTU, AREA type, AREA number, BUG...
  5. Exchange: the neighbour's exchange database info and descriptions Reason: for a neighbour to flap between ecstasy or exchange to 2WAY could be a configuration error like the same router ID, corrupt packets, MTU, AREA type, AREA number, BUG...
  6. FULL: the neighbours are up

LSA Type

LSA 1 - Router:
One per router per area lists the router ID and all interface IP addresses in that area. LSA1 Flooded only within its area of origin
LSA 2 - Network:
One per transit network created by the DR and or BDR on the subnet and represents the subnet and the router interface connected to the subnet flooded only within its area of origin
LSA 3 - Net Summary:
Generated by ABRs to represent networks in one area when advertising to another. Defines the subnets cost but not topology in that area
LSA 4 - ASBR summary:
Advertises a host route used to reach the ASBR. Floods only within their area of origin re-originated on ABRs
LSA 5 - AS external :
Created by ASBR for external routes injected to OSPF flooded to all regular areas
LSA 7 - NSSA external:
Created by ASBR inside an NSSA instead of LSA 5. Flooded only within its area of origin converted to LSA5 on ABR toward other Areas

Area types

Standard area
In the Standard area, type 1 and 2 LSAs are being flooded between routers within the area, and type 3 and 5 LSAs are flooded throughout all standard areas.
Backbone area (area 0)
The backbone area is somewhat the same as a standard area but is designed as the centre to which all other areas are connected.
Stub area
The ABR, instead of propagating LSA 5 and 4 into the area the ABR injects a type 3 LSA containing a default route into the stub area.
Totally stubby area
Like stub areas, totally stubby areas do not receive LSA 4 and 5 but also do not receive type 3 LSAs. They will receive a single default route injected by the ABR.
Not-so-stubby Areas
-In an NSSA area, the ASBR will generate LSA 7 to advertise an external route to an ABR, and ABR will convert LSA 7 to 5 and flood them to the rest of the domain.
-The ABR will not inject a default route into an NSSA unless explicitly configured.
- Type 3 LSAs will pass into and out of the area to eliminate type 3 LSAs, meaning configure a not-so-totally-stubby area, all ABRs must be configured with the no-summary command.

Summarization

Summarization of internal routes
Summarization of internal routes is performed on the ABRs all the prefixes from an area are passed into the backbone as type 3 interarea routes. When summarization is enabled, the ABR intercepts this process and instead injects a single type 3 LSA
Summarization of external routes
Summarization of external routes is performed on the ASBRs each route that is redistributed into OSPF from other protocols is advertised individually with an external LSA. To reduce the size of the OSPF LSDB, you can configure a summary for external routes. Summarization of external routes can be done on the ASBR for type 5 LSAs (redistributed routes) before injecting them into the OSPF domain.

OSPFv3

The Old Ways OSPF v2
in OSPF v2, the SPF is run more often than actually necessary because every time a subnet is changed, we need to rerun the SPF because of all the information that is present in LSA 1 and 2.

The New Way In OSPFv3
in OSPFv3, a lot of info moved from LAS 1 and 2 and to LSA 8 and 9. LSA 1 and 2 no longer carry any addressing information. They only carry a description of topology adjacencies ex what another object or a multiaccess network is connected to, using router RIDs as an address-independent way of referring to a neighbouring object. With this separation of topology and addressing information, OSPFv3 is significantly more efficient when it comes to scheduling an SPF run. If an interface address changes, only an updated Link LSA and Intra-Area-Prefix LSA will be originated and flooded.
OSPFv3 uses the link-local addresses as next-hop addresses

type 9 LSA (Intra-Area-Prefix LSA)
LSA9 carries IPv6 prefixes on individual interfaces of a router with the area flooding scope.

Type 8 LSAs
Each router advertises its link-local address in a type 8 LSA (Link LSA) sent out to the particular interface. Type 8 LSAs have the link flooding scope and are never flooded beyond the receiving neighbour on the link.

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