OSPF LSA types

The first time I read about LSA types, It was a little blur for me to understand.
Then I read the CCNP routing and switching (route 300-101) from Kevin Wallace, and all became clear.

LSA are defined by these terms in OSPF Database :

LSA Type 1: Router Link States
LSA Type 2: Net Link States
LSA Type 3: Summary Net Link States
LSA Type 4: ASBR Summary LSA
LSA Type 5: External Link States

LSA type 1 is originated by each router. Its ID is the router-ID of the router which created this LSA. This LSA stays within the area.

LSA type 2 represents the DR for a multi-access network. If network type is defined as point-to-point (ip ospf network-type …), no DR is elected, so no Type 2 is created.

Both of these types are not flooded outside of their area.

We will use this network :

R1#show ip ospf database
OSPF Router with ID (1.1.1.1) (Process ID 1)
Router Link States (Area 0)
Link ID ADV Router Age Seq# Checksum Link count
1.1.1.1 1.1.1.1 26 0x80000003 0x0084CD 1
2.2.2.2 2.2.2.2 27 0x80000010 0x002F0C 1
Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
192.168.1.1 1.1.1.1 26 0x80000001 0x00417D
Summary Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
192.168.2.0 2.2.2.2 1858 0x80000002 0x008D3A
192.168.7.0 1.1.1.1 33 0x80000001 0x007651
192.168.30.0 2.2.2.2 1858 0x80000002 0x005853
192.168.40.0 2.2.2.2 1858 0x80000002 0x00F3AC
192.168.77.0 1.1.1.1 33 0x80000001 0x007B05

On top, this is a troncated output from R1 we can see:

  • 2 type 1 : one for each router in Area 0, LS-ID is router-id
  • 1 type 2 : one for the pseudonode on link between R1 et R2, Link-ID is the IP address of the DR on the link
  • 5 type 3 : generated by ABR R2 to represent Intra area link in Area 1. ABR is converting intra area Type 2 to type 3 and flooding to other connected area
                Summary ASB Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
5.5.5.5 2.2.2.2 1588 0x80000002 0x006AB2

The output above is showing information related to redistribution from ASBR R3 to OSPF. We have the following LSA :

  • 1 type 4 : List the RID of the ASBR, created by ABR R2, and not the ASBR itself

R2 is creating the LSA T4 to represente R5 but note that R1 is also re-originate this LSA in others Areas.

On R2 :
Summary ASB Link States (Area 0)
Routing Bit Set on this LSA
[...]
LS Type: Summary Links(AS Boundary Router)
Link State ID: 5.5.5.5 (AS Boundary Router address)
Advertising Router: 2.2.2.2
[...]

On R7 :
R7#sho ip osp da asbr-summary
OSPF Router with ID (7.7.7.7) (Process ID 1)
Summary ASB Link States (Area 2)
Routing Bit Set on this LSA
[...]
LS Type: Summary Links(AS Boundary Router)
Link State ID: 5.5.5.5 (AS Boundary Router address)
Advertising Router: 1.1.1.1

Author: Kévin SAS

I am Kévin SAS and live in France. I am an engineer in network and security. I worked for a lot of customers in these domains : Wireless, LAN, Datacenter, VoIP with several brands like Cisco, HPE, Aruba, Palo Alto amongst others. Previously working for a service provider. I currently work for an integrator while giving me lots of opportunity to learn. This blog stores some technicals notes I wanted to share.

Leave a Reply

Your email address will not be published. Required fields are marked *