Understanding what OSPF does from the data flows.

I found that understanding the flows between to OSPF nodes helped me understand OSPF.

I used Wireshark to trace the data sent from my OSPF router with id 9.2.3.4.

There are four basic flows

1. My router sending configuration information to the remote router
2. The remote router sending acknowledgments back to my router
3. The remote router sending configuration information to my router (the same as 1. above, but in the opposite direction)
4. My router sending acknowledgements back to the remote router (the same as 2., but in the opposite direction).

It looks like a lot of data flowing – but I focused on my router sending information to the remote router.

Background information

Link state information helps others build a map of the configuration. This gives status information about the links.

Each router sends “new” information to the remote end of the connection; for example a Link State Update. The remote end acknowledges these with a Link State Acknowledgement.

While the local router is sending stuff to the remote router, the remote router is sending it’s configuration information to the local router.

Once the configuration information has been exchanged, and the configuration information stabilises, there is still a periodic “Hello Packet” between each router. This is a heartbeat to tell the remote end that the local end is still alive. The “Hello Packet” is sent out typically every 10 seconds. Updates are sent out around the “Hello Packet” time, so changes typically propagate through the network, 10 seconds a hop.

Information is exchanged via Link State Advertisement (LSA) which advertises the state of a link.

• LSA type 1 is for routers, it contains information about routers
• LSA type 2 is for networks, it contains information about IP addresses

Stub areas.

If you had all boxes in one big area – every box will know about other boxes. This may not scale well.

You can create areas, for example an area could be a country. Areas are connected together through the backbone area, area 0. An area, such as area1, can have information such as for addresses in area 17, go via the default routing to the backbone, and let the router where area 1 joins the backbone area sort out the routing.

Nodes in area 1need fewer definitions – as the definitions just say “go by the backbone”

Summary

I restarted my laptop, and it joined the network.
It’s configuration was

OSPF router id 1.2.3.4

Somewhere else in the network a node received two flows

• Flow 1
  • I am router, 9.2.3.4
  • Type 1 Router-LSA. I have 3 direct connections
    • Remote end’s IP address 10.1.1.2, my address 10.1.1.1
    • Remote end’s IP address 10.1.3.2, my address 10.1.3.1
    • Remove end’s IP address 10.1.0.3, my address 10.1.0.3
  • Type 2- Network LSA
    • Attached routers 1.2.3.4 and 9.2.3.4
• Flow 2
  • I am router 1.2.3.4
  • Type 1 Router LSA
    • I have IP address 10.1.0.0 type stub
    • I have IP address 12.1.0.1 type stub.

If the configuration changes, such as a new address is added to the node, the data broadcast is the current configuration.

Each system supporting OSPF gets the same information and can build up a database of the network, and can make informed routing decisions.

Changing the configuration

Adding an address to a link

I used the command

sudo ip -4 addr add 12.12.0.1 dev enp0s31f6

to add an additional IP address to the Ethernet connection on my laptop. The command

ip -4 addr gave

enp0s31f6: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
    inet 10.1.0.2/24 brd 10.1.0.255 scope global noprefixroute enp0s31f6
       valid_lft forever preferred_lft forever
    inet 12.13.0.1/32 scope global enp0s31f6
       valid_lft forever preferred_lft forever
    inet 12.14.0.1/32 scope global enp0s31f6
       valid_lft forever preferred_lft forever
    inet 12.12.0.1/32 scope global enp0s31f6
       valid_lft forever preferred_lft foreverd

This cause a flow to the z/OS system, saying “this is all the IP addresses I know about”.

LS Update Packet
 Number of LSAs: 1
 LSA-type 1 (Router-LSA), len 72
  LS Type: Router-LSA (1)
  Link State ID: 1.2.3.4
  Advertising Router: 1.2.3.4
   Number of Links: 4
    Type: Transit  ID: 10.1.0.3        Data: 10.1.0.2        Metric: 100
     Type: Stub     ID: 12.13.0.1      Data: 255.255.255.255 Metric: 100
     Type: Stub     ID: 12.14.0.1      Data: 255.255.255.255 Metric: 100
     Type: Stub     ID: 12.12.0.1      Data: 255.255.255.255 Metric: 100

The transit address was the original address.

The stub address(es) were added manually.

Deleting an address to a link

I used the command

sudo ip -4 addr del 12.12.0.1 dev enp0s31f6

to remove the link I had previously added.

This cause a flow to the z/OS system, saying “this is all the IP addresses I know about” – omitting the address I had just deleted.

LS Update Packet
 Number of LSAs: 1
 LSA-type 1 (Router-LSA), len 72
  LS Type: Router-LSA (1)
  Link State ID: 1.2.3.4
  Advertising Router: 1.2.3.4
   Number of Links: 4
     Type: Transit ID: 10.1.0.3  Data: 10.1.0.2        Metric: 100
     Type: Stub    ID: 12.13.0.1 Data: 255.255.255.255 Metric: 100
     Type: Stub    ID: 12.14.0.1 Data: 255.255.255.255 Metric: 100
     Type: Stub    ID: 12.12.0.1 Data: 255.255.255.255 Metric: 100

One way flows in more detail

The “Hello packet”

• I have Source OSPF router 9.2.3.4, area 0.0.0.0

DB Description

Source OSPF router 9.2.3.4, area 0.0.0.0

DB Description

“I know about…”

Source ospf router 9.2.3.4, area 0.0.0.0

• LSA-type 1 (Router-LSA) Link State ID 1.2.3.4 advertising 1.2.3.4
• LSA-type 1 (Router-LSA) Link State ID 9.2.3.4 advertising 9.2.3.4
• LSA-type 1 (Router-LSA) Link State ID 10.1.1.2 advertising 10.1.1.2
• LSA-type 2 (Network-LSA) Link State ID 10.1.0.2 advertising 1.2.3.4
• LSA-type 2 (Networ-kLSA) Link State ID 10.1.1.2 advertising 10.1.1.2

Link state update

“Here is information about the links and the IP addresses”.

Source router 9.2.3.4, Area 0.0.0.0

• LSA-type 1 (Router-LSA) Link State ID 1.2.3.4 advertising 1.2.3.4
  • Links: Type Transit ID 10.1.0.2 Data 10.1.0.2 Metric 100
• LSA-type 1 (Router-LSA) Link State ID 9.2.3.4 advertising 9.2.3.4
  • Links: Type Transit ID 10.1.0.2 Data 10.1.0.3 Metric 100
  • Links: Type Stub ID 10.1.1.0 Data 255.255.255.0 Metric 1000
• LSA-type 1 (Router-LSA) Link State ID 10.1.1.2 advertising 10.1.1.2
  • Links: Type Transit ID 10.1.1.1 Data 10.1.1.2 Metric 1
• LSA-type 2 (Network-LSA) Link State ID 10.1.0.2 advertising 1.2.3.4
  • Attached router: 1.2.3.4
  • Attached router 9.2.3.4
• LSA-type 2 (Network-LSA) Link State ID 10.1.1.2 Advertising 10.1.1.2
  • Attached router 10.1.1.2
  • Attached router 9.2.3.4

DB Description

I have OSPF router 9.2.3.4, Area 0.0.0.0

I support external routing

Link state update (2)

Source router 9.2.3.4, Area 0.0.0.0

Link State Type Router

• LSA-type 1 (Router-LSA) Link State ID 9.2.3.4 advertising 9.2.3.4
  • Links: Type Transit ID 10.1.0.2 Data 10.1.0.3 Metric 100
  • Links: Type Transit ID 10.1.1.1 Data 10.1.1.1 Metric 1000
• LSA-type 1 (Router-LSA) Link State ID 10.1.1.1 advertising 9.2.3.4
  • Attached router: 9.2.3.4
  • Attached router: 10.1.1.2

Hello Packet

Periodically (every 10 or so seconds) there is a Hello Packet flow, which acts as a heartbeat to let the remote end the know the local end is still alive.