Hi, can someone please help me in trying to understand the OSPF Database. Per my understanding the Database is structured per the Areas. Each Area will contain LSA's under it. I am confused as to what LSA's will appear under what area. For eg:
- The ASBR Summary LSA in the output below appears only under Area 51 but in other topologies and output I have seen it appear under Area 0 as well.
- Under the summary LSA I see 172.16.10.4 under the Area 0 section as well as the Area 51 section? Why is that.
OSPF Router with ID (192.168.2.1) (Process ID 1)
Router Link States (Area 0)
Link ID ADV Router Age Seq# Checksum Link count
172.16.10.5 172.16.10.5 751 0x8000000F 0x6F9C 1
192.168.1.1 192.168.1.1 540 0x80000012 0x8D3D 1
192.168.2.1 192.168.2.1 543 0x80000016 0x7E46 1
192.168.3.1 192.168.3.1 543 0x80000005 0x9C36 1
Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
172.16.1.2 192.168.2.1 539 0x8000000D 0xCFE8
Summary Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
172.16.10.4 192.168.3.1 617 0x80000001 0xD126
172.16.20.0 192.168.3.1 617 0x80000001 0xA746
172.16.51.1 192.168.2.1 544 0x80000005 0xA832
Router Link States (Area 51) Link ID ADV Router Age Seq# Checksum Link count
192.168.2.1 192.168.2.1 585 0x8000000B 0x855 1
Summary Net Link States (Area 51)
Link ID ADV Router Age Seq# Checksum
172.16.1.0 192.168.2.1 531 0x80000001 0xE22F
172.16.10.4 192.168.2.1 531 0x80000001 0xE215
172.16.20.0 192.168.2.1 531 0x80000001 0xB835
Summary ASB Link States (Area 51)
Link ID ADV Router Age Seq# Checksum
192.168.1.1 192.168.2.1 531 0x80000001 0x9EC4
Type-5 AS External Link States
Link ID ADV Router Age Seq# Checksum Tag
11.0.0.0 192.168.1.1 988
0x80000001 0x3FEA 0
12.0.0.0 192.168.1.1 988
0x80000001 0x32F6 0
13.0.0.0 192.168.1.1 988
0x80000001 0x2503 0
In addition say if i look at the sho ip ospf data asbr-summary output and see that for LSA's appearing under Area 51 it gives the message the
advertising router is not reachable. However I see the route to the advertising router in my routing table and am able to ping it as well. Could it be that the area 51 is upstream to the router I am looking at and hence because its upstream it doesn't make sense to put this routers LSA in the routing table since that would cause a routing loop and that is the reason why it says "Adv Router is not-reachable"