Type 1 LSA (also called Router LSA) contains information about directed connected links in the area to which the router belong. They are flooded to all routers in that area. If the router is an ABR (Area Border Router), it generates type 1 LSAs for all the areas to which it is connected and send those LSAs to all neighbors in corresponding areas.
For example in the picture exposed above you see that ABR router has three links, two of them connected in area 0 and another one in area 1. So, it will generate one LSA for area 0 which will include links to R1 and R2, this LSA will be sent to all neighbors in area 0, i.e. to R1 and R2. Another LSA will be build for area 1 that will include links in area 1 which is link to R3, this LSA will be sent to R3 because R3 is only neighbor in area 1. This process also happens from opposite side, R1 with R2 send to ABR their type 1 LSAs with all directly connected links in area 0, R3 does the same with its link connected in area 1. LSA received by ABR from R2 is also transmitted to R1 and vice versa, R1 LSA is also carried to R2. I this way, every router in the area knows about all the other routers in the area and their enabled OSPF links. In next picture I added more details to our OSPF domain, let’s consider it and have a closer look on how router LSAs work .
In this article I’m not going to show configurations for the routers (you can download GNS3 project from the attachment), all configurations are done according to the map above. We have four routers and two areas, where for each router I set router-id as follow: 126.96.36.199, 188.8.131.52, 184.108.40.206, 220.127.116.11. All ip addresses are configured on interfaces and OSPF is also configured. To get a quick picture of OSPF database (LSDB) use
show ip ospf database command as follow:
ABR#show ip ospf database OSPF Router with ID (18.104.22.168) (Process ID 1) Router Link States (Area 0) Link ID ADV Router Age Seq# Checksum Link count 22.214.171.124 126.96.36.199 1215 0x8000000B 0x0009F7 1 188.8.131.52 184.108.40.206 930 0x80000007 0x004722 2 220.127.116.11 18.104.22.168 1374 0x80000009 0x004CE3 3 Net Link States (Area 0) Link ID ADV Router Age Seq# Checksum 10.0.0.1 22.214.171.124 1374 0x80000002 0x0041CB Summary Net Link States (Area 0) Link ID ADV Router Age Seq# Checksum 10.0.2.0 126.96.36.199 1374 0x80000006 0x00DEFF Router Link States (Area 1) Link ID ADV Router Age Seq# Checksum Link count 188.8.131.52 184.108.40.206 336 0x80000006 0x001F41 2 220.127.116.11 18.104.22.168 1375 0x80000008 0x00A1B8 2 Summary Net Link States (Area 1) Link ID ADV Router Age Seq# Checksum 10.0.0.0 22.214.171.124 1375 0x80000004 0x00DA3E 10.0.1.0 126.96.36.199 1375 0x80000004 0x00EDF3 ABR#
As you can see, this output is from ABR, and colored in orange there are type 1 LSAs for area 0 and for area 1. As you might be expecting there are three router LSAs for area 0, one issued by R1, one by R2 and one by ABR. In area 1, there are two router LSAs, one issued by R3 and one by ABR. If you issue the same command on another router in area 0, you will not see any LSAs related to other areas than 0.
R1#show ip ospf database OSPF Router with ID (188.8.131.52) (Process ID 1) Router Link States (Area 0) Link ID ADV Router Age Seq# Checksum Link count 184.108.40.206 220.127.116.11 157 0x8000000C 0x0007F8 1 18.104.22.168 22.214.171.124 56 0x80000008 0x004523 2 126.96.36.199 188.8.131.52 297 0x8000000A 0x004AE4 3 Net Link States (Area 0) Link ID ADV Router Age Seq# Checksum 10.0.0.1 184.108.40.206 297 0x80000003 0x003FCC Summary Net Link States (Area 0) Link ID ADV Router Age Seq# Checksum 10.0.2.0 220.127.116.11 297 0x80000007 0x00DC01 R1#
This command gives us a brief look on router LSAs, it shows Link state ID, advertising router, age, sequence number, checksum, and number of links the LSA represent.
To have a detailed view on router LSA use show
ip ospf database router, or even more, you can specify Link ID or advertising router to filter LSAs like below:
R1#show ip ospf database router adv-router 18.104.22.168 OSPF Router with ID (22.214.171.124) (Process ID 1) Router Link States (Area 0) Routing Bit Set on this LSA LS age: 6 Options: (No TOS-capability, DC) LS Type: Router Links Link State ID: 126.96.36.199 Advertising Router: 188.8.131.52 LS Seq Number: 8000000B Checksum: 0x48E5 Length: 60 Area Border Router Number of Links: 3 Link connected to: another Router (point-to-point) (Link ID) Neighboring Router ID: 184.108.40.206 (Link Data) Router Interface address: 10.0.1.1 Number of TOS metrics: 0 TOS 0 Metrics: 64 Link connected to: a Stub Network (Link ID) Network/subnet number: 10.0.1.0 (Link Data) Network Mask: 255.255.255.252 Number of TOS metrics: 0 TOS 0 Metrics: 64 Link connected to: a Transit Network (Link ID) Designated Router address: 10.0.0.1 (Link Data) Router Interface address: 10.0.0.1 Number of TOS metrics: 0 TOS 0 Metrics: 10 R1#
The output is from ABR, where three links are described, but actually there are two physical links. The first two links describe connection between R2 and ABR, and the third one – connection between R1 and ABR. Here is also included cost for corresponding links.
In case there are any topology changes in area 1, only R3 and ABR will do spf calculations, R1 and R2 won’t. But if for example the link between ABR and R2 goes down, ABR will age out immediately LSA issued by R2 by setting age to 3600 and will send it to R1. ABR also will resend its own LSA with removed link to R2 within it, and with increased sequence number. This will lead to SPF calculations on all routers in area 0.