Troubleshooting OSPF Route Advertisement

This topic is to discuss the following lesson:

Thank you for explaining OSPF troubleshooting in a simple language. This helps in understanding OSPF better.

The first scenario doesn’t match with the troubleshooting. For example, “We can see that R1 has learned about network 10.2.2.0 /24 from R2 but there’s nothing in the routing table of R2. What’s going on? Let’s check if OSPF is enabled:”. There is nowhere on the scenario that states, configure network address 10.2.2.0 /24, so I’m not expecting to see network 10.2.2.0 /24 in my routing table.

Hi Alex,

You are right, this should be the 2.2.2.0/24 network. I just fixed it in the article. Thanks for letting me know!

Rene

Super Explanation :slight_smile:

Completely agree , Super Explanation !
Nice concept to include such a topic and very well explained !

After going through this article , I instantly checked if there is anything like this for BGP :slight_smile:
Would really appreciate if you can add similar article for BGP i.e. the basic checkpoints of the protocol and most likely the only you might encounter ever.

thanks,Amit

Hi Amit,

We do have something similar for BGP:

Troubleshooting BGP Neighbor Adjacency

Troubleshooting BGP Route Advertisement

Rene

Hi Rene ,

It helped me revise very quickly for an interview.
There’s a little printing mistake under the section ‘OSPF summarization
commands’ :
“The network engineer that configured R1 and R2 expects to see network 172.16.0.0 /26 in the routing table of R1”, It is to be printed as 172.16.0.0/16.
Many Thanks .

Thank you Sutandra, I just fixed this.

Hi team,

I suppose there is a little typo under a section “OSPF Network Command”. In the configuration below only host 10.1.1.1/32 has been advertised into OSPF.

image

Thanks

1 Like

Thank you @bvesel, just fixed this one.

Rene

Hi Team,

i was doing a LAB today about OSPF topology is given below

Here i have 3 routers on multi-access network so i created 3 loopback on R1 and advertised them under OSPF. so now the thing is i am able to see all loopbacks under routing table of each router but not seeing them in LSDB can you explain why,

R1

R1#show ip ospf interface 
Loopback2 is up, line protocol is up 
  Internet Address 1.1.1.1/32, Area 0 
  Process ID 1, Router ID 192.168.20.1, Network Type LOOPBACK, Cost: 1
  Loopback interface is treated as a stub Host
Loopback3 is up, line protocol is up 
  Internet Address 2.2.2.1/24, Area 0 
  Process ID 1, Router ID 192.168.20.1, Network Type LOOPBACK, Cost: 1
  Loopback interface is treated as a stub Host
Loopback100 is up, line protocol is up 
  Internet Address 100.100.100.1/24, Area 0 
  Process ID 1, Router ID 192.168.20.1, Network Type LOOPBACK, Cost: 1
  Loopback interface is treated as a stub Host
FastEthernet0/0 is up, line protocol is up 
  Internet Address 192.168.20.1/24, Area 0 
  Process ID 1, Router ID 192.168.20.1, Network Type BROADCAST, Cost: 1
  Transmit Delay is 1 sec, State BDR, Priority 1 
  Designated Router (ID) 192.168.20.2, Interface address 192.168.20.2
  Backup Designated router (ID) 192.168.20.1, Interface address 192.168.20.1
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
    oob-resync timeout 40
    Hello due in 00:00:01
  Supports Link-local Signaling (LLS)
  Index 1/1, flood queue length 0
  Next 0x0(0)/0x0(0)
  Last flood scan length is 1, maximum is 1
  Last flood scan time is 0 msec, maximum is 0 msec
  Neighbor Count is 1, Adjacent neighbor count is 1 
    Adjacent with neighbor 192.168.20.2  (Designated Router)
  Suppress hello for 0 neighbor(s)


R1#sh ip ospf database 

            OSPF Router with ID (192.168.20.1) (Process ID 1)

		Router Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum Link count
192.168.10.1    192.168.10.1    932         0x80000002 0x00213E 1
192.168.20.1    192.168.20.1    1450        0x80000011 0x00704B 4
192.168.20.2    192.168.20.2    931         0x8000000B 0x0063CC 2

		Net Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum
192.168.10.2    192.168.20.2    931         0x80000001 0x00DA76
192.168.20.2    192.168.20.2    1251        0x80000002 0x00EC4F

not seeing any loopback in LSDB

but on R3 i am seeing them under routing table.

R3

R3#show ip ospf database 

            OSPF Router with ID (192.168.10.1) (Process ID 1)

		Router Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum Link count
192.168.10.1    192.168.10.1    211         0x80000002 0x00213E 1
192.168.20.1    192.168.20.1    733         0x80000011 0x00704B 4
192.168.20.2    192.168.20.2    212         0x8000000B 0x0063CC 2

		Net Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum
192.168.10.2    192.168.20.2    212         0x80000001 0x00DA76
192.168.20.2    192.168.20.2    532         0x80000002 0x00EC4F

route

 1.0.0.0/32 is subnetted, 1 subnets
O       1.1.1.1 [110/3] via 192.168.10.2, 00:16:23, FastEthernet0/1
 2.0.0.0/32 is subnetted, 1 subnets
O       2.2.2.1 [110/3] via 192.168.10.2, 00:16:23, FastEthernet0/1
 100.0.0.0/32 is subnetted, 1 subnets
O       100.100.100.1 [110/3] via 192.168.10.2, 00:16:23, FastEthernet0/1
C    192.168.10.0/24 is directly connected, FastEthernet0/1
O    192.168.20.0/24 [110/2] via 192.168.10.2, 00:16:23, FastEthernet0/1

Hello Ankush

This issue has to do with the way we read the OSPF database. Specifically, each LSA has a Link ID and an Adv Router that are shown in the table. These fields do not show the actual destinations being advertised but identify the LSAs exchanged between the OSPF routers.

The ADV router is the router ID of the router that advertised that particular LSA. In your case, looking at the OSPF database of R3, your loopbacks are contained within the LSAs that have the ADV Router of 192.168.20.1. This IP address is the router ID of R1. This can be confirmed by the fact that you have a link count of 4 (three loopbacks and one physical link) for that router.
As an aside, in your case, the router used the highest IP address on an interface to define the router ID. You have loopbacks as well, but those must have been added after the router ID was established. (if you reset the OSPF process, your router ID will change on R1 based on the OSPF router ID selection process).

For LSA type 1, the Link ID is the same as the ADV Router column. However, for type 2 LSAs, the Link ID refers to the router ID of the DR on the segment.

The point here is that you won’t actually see the destination networks within the OSPF database on the router with this command. In order to see the actual contents of the Type 1 LSA, you must use the show ip ospf database router command, which will show you details of the Type 1 LSAs that the router has received.

For more info no how to read the OSPF database, take a look at the following lesson:

I hope this has been helpful!

Laz