Hi, As far as I know, split horizon is usually done by routing protocols in order to prevent the most common cases of routing loops.
Babel also implements split horizon, but the goal doesn't seem to be loop prevention — probably because it has more sophisticated ways of preventing routing loops. Instead, split horizon is an optimisation performed when an interface is known to be transitive. Quoting RFC 6126: 3.7.4. Split Horizon When running over a transitive, symmetric link technology, e.g., a point-to-point link or a wired LAN technology such as Ethernet, a Babel node SHOULD use an optimisation known as split horizon. When split horizon is used on a given interface, a routing update is not sent on this particular interface when the advertised route was learnt from a neighbour over the same interface. Split horizon SHOULD NOT be applied to an interface unless the interface is known to be symmetric and transitive; in particular, split horizon is not applicable to decentralised wireless link technologies (e.g., IEEE 802.11 in ad hoc mode). Thus my question: how well does Babel perform when having many neighbours on the same interface, with and without split horizon? More specifically, I'm looking for data (either operational or theoretical) regarding: - control traffic as a function of the number of neighbouring nodes on the same interface (with and without split horizon). I would expect it to be linear with split horizon and quadratic without split horizon, does that sound right? - hard limit on the number of neighbours, with and without split horizon (what would be the bottleneck? CPU usage? network throughput? possible implementation limitations?) Testing and measuring would not be so hard to do, but time-consuming. If somebody already has experience with this, any input is highly welcome :) The VPN topology I'm currently exploring (based on Tinc) has unusual properties: it is "mostly" transitive, meaning that it will usually look like a full mesh, but a number of edges will be missing. I'm wondering whether this topology would scale if disabling split horizon. On the other hand, what could go wrong if split horizon is enabled? Will some routes fail to propagate? Thanks, Baptiste
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