[New post] Understanding what OSPF does from the data flows.

Colin Paice posted: " 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 My router sending configuration information to the " ColinPaice Understanding what OSPF does from the data flows. Colin Paice Jan 29 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 My router sending configuration information to the remote router The remote router sending acknowledgments back to my router The remote router sending configuration information to my router (the same as 1. above, but in the opposite direction) 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. 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-LSAI 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 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. Comment Like You can also reply to this email to leave a comment. Unsubscribe to no longer receive posts from ColinPaice. Change your email settings at manage subscriptions. Trouble clicking? Copy and paste this URL into your browser: http://colinpaice.blog/2023/01/29/understanding-what-ospf-does-from-the-data-flows/ Powered by WordPress.com