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OSPF Router-ID (RID)

The RID is A 32-bit dotted decimal value which is used to uniquely identify a specific OSPF-enabled device in an autonomous system.   Uniquely means that different devices shouldn’t have the same router-id configured.

RID-Single-Area

It is a best practice to hard code the Router-ID but is optional.

OSPF has its own automatic RID election process:

  • The IP Address of the first loopback interface.
  • If there is no Loopback interface, then the Router ID will be the highest IP Address a physical interface.

We have to understand that the Router-ID looks like an IP address in format, but is not.   Is just and ID and is used to represent a Router in the OSPF topology graph.   We can assign something like 0.0.0.1, 0.0.10.10, for example.

With RID election, we are subject to undesired changes or outages when the process is cleared.   Another reason to manually assign the RID is because other technologies such OSPF virtual links.

Configuring RID

Configuring the RID in OSPF is very straight-forward.  It’s done using the command router-id x.x.x.x under the process.

!
router ospf 1
 router-id 1.1.1.1
!

If the OSPF process was previously started or pre-configured, you will receive a notification informing that it will be necessary to clear the process for the change to take effect.

WARNING RGE

Yes, Clear the OSPF process is a Resume Generating Event if done in production, especially during business hours.

Clearing the OSPF process is a DISRUPTIVE command which forces OSPF to reconverge.

To clear the OSPF process use the clear ip ospf process command in exec mode.

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OSPF LSA Types

This post will cover only LSA Types for OSPFv2.  The OSPFv3 LSAs will be covered in further posts.

In order to fully understand how OSPF works is necessary understand of all of the OSPF LSA types.

Type 1 – Router LSA

Type 1 LSA is generated by every router and contains information about directly connected links in the area.

  • Describe the state of all router interfaces and their cost of the link to the neighbor and the IP prefix.
  • The ADV Router will always be the Router ID of the device injecting the LSA into the area.

Type 2 – Network LSA

Type 2 LSAs are present only on multi-access links for Broadcast and Nonbroadcast network types.

  • Generated by the DR and is never flooded outside of the area.
  • Lists all attached routers including the DR.
  • Gives information about the subnet mask on that segment.

Type 3 – Network Summary LSA

Type 3 LSAs describes the routes to neighbors outside the area (inter-area routes) but within the OSPF domain.

  • Generated by ABRs and is flooded between areas.
  • Include cost from ABR to the network.
  • The Link-State ID is the IP address of the subnet that is being advertised.

Type 4 – ASBR Summary LSA

Type 4 LSAs describes the routes to ASBRs.

  • Generated by the ABRs and is flooded between areas except stub areas.
  • The Link-State ID is the Router ID of the ASBR described.

 Type 5 – External LSA

Type 5 LSAs describes the routes to networks that are external to the AS.

  • Generated by ASBR and is flooded between areas.
  • The Link-State ID is network number advertised in the LSA.

Type 6 – Multicast OSPF LSA

This type of LSA is used for multicast applications and is not supported by Cisco.  However, the presence of Type 6 LSAs generates Syslog messages when the router receives the link-state advertisements.   To suppress the Syslog messages use the “ignore lsa mospf” process command.

Type 7 – NSSA External LSA

Type 7 LSAs describes the routes that are external to the AS in the NSSA area.

  • Generated by NSSA ASBR
  • The ABR translate LSA type 7 to LSA type 5 when sends from NSSA into Area Zero.

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