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ovn: Add a case of policy based routing.
OVN currently supports multiple gateway routers (residing on different chassis) connected to the same logical topology. When external traffic enters the logical topology, they can enter from any gateway routers and reach its eventual destination. This is achieved with proper static routes configured on the gateway routers. But when traffic is initiated in the logical space by a logical port, we do not have a good way to distribute that traffic across multiple gateway routers. This commit introduces one particular way to do it. Based on the source IP address or source IP network of the packet, we can now jump to a specific gateway router. This is very useful for a specific use case of Kubernetes. When traffic is initiated inside a container heading to outside world, we want to be able to send such traffic outside the gateway router residing in the same host as that of the container. Since each host gets a specific subnet, we can use source IP address based policy routing to decide on the gateway router. Rationale for using the same routing table for both source and destination IP address based routing: Some hardware network vendors support policy routing in a different table on arbitrary "match". And when a packet enters, if there is a match in policy based routing table, the default routing table is not consulted at all. In case of OVN, we mainly want policy based routing for north-south traffic. We want east-west traffic to flow as-is. Creating a separate table for policy based routing complicates the configuration quite a bit. For e.g., if we have a source IP network based rule added, to decide a particular gateway router as a next hop, we should add rules at a higher priority for all the connected routes to make sure that east-west traffic is not effected in the policy based routing table itself. Signed-off-by: Gurucharan Shetty <email@example.com> Acked-by: Ben Pfaff <firstname.lastname@example.org>
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