How neutron calculates new routing path

asked 2015-03-07 04:44:43 -0500

minherz gravatar image

Hello, I am looking to learn how neutron agent (probably L3) calculates a new routing path when VM on compute node wants to communicate with some destination. Does it use neutron API to learn about network topology or it uses its internal structures to simulate path resolving like in real network? If the latter is correct, then what happens when a network topology is changed in neutron DB (due user intervention for by other actions) and the "local" data is invalid?

Thank you

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answered 2015-03-12 08:28:20 -0500

minherz gravatar image

I've got the following information. It shows a partial picture.

It seems that in many cases (most cases for L3) agents use ARP to discover the route. By this, it simulates behavior of the real L2/L3 network components. According to Assaf Muller the current version of neutron L3 agent does not support any sort of dynamic routing, and a Neutron virtual router may only have access to up to one external network. It can be (Directly) connected to many internal networks and performs static routing. If you connect a router to one external network ( and two internal networks (, then you can observe the router's routing table and see that it has three routes to said network, as well as a default gateway (The external network's gateway). L3 agent has a period task (1 minute hardcoded) that gets all of the routers hosted on the agent from the DB. If some error condition is met, then the periodic task doesn't do anything.

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answered 2015-03-10 08:19:54 -0500

smaffulli gravatar image

There is a thread on the Development mailing list How neutron calculate routing path. It would be great if you could post a summary of your findings here, too, for clarity and future readers.

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I thought that you posted an answer and not a reference to my question in another mailing list :) I'll update this post with the info I get. However, I'd appreciate anyone who has updated info to share it here as well

minherz gravatar imageminherz ( 2015-03-12 08:23:19 -0500 )edit

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Asked: 2015-03-07 04:44:43 -0500

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Last updated: Mar 12 '15