Split horizon route advertisement
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In computer networking, split-horizon route advertisement is a method of preventing routing loops in distance-vector routing protocols by prohibiting a router from advertising a route back onto the interface from which it was learned.
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[edit] Example
In this example, network node A routes packets to node B in order to reach node C. The links between the nodes are distinct point-to-point links.
According to the split-horizon rule, node A does not advertise its route for C (namely A to B to C) back to B. On the surface, this seems redundant since B will never route via node A because the route costs more than the direct route from B to C. However, if the link between B and C goes down, and B had received a route from A, B could end up using that route via A. A would send the packet right back to B, creating a loop. With the split-horizon rule in place, this particular loop scenario cannot happen, improving convergence time in complex, highly-redundant environments.
There are scenarios where the split-horizon rule may fail. Suppose 6 nodes(A,B,C,D,E,F) are each connected to every other node. The lowest cost routes will establish over time. Now if one of the nodes fails. Split-horizon only protects each node from where their current path is received from. But the same information received from the other nodes will be transmitted as well.
If A connects to E via B and C. And if D connects to E via A->B->C. Suppose B-C fail. A will not advertise its route to E to B directly due to the split-horizon rule. However, as each node is connected to each other. D will advertise the route to E to B (D->A->B->C-E). The split horizon rule stops D from advertising the route to A. Not to B.
Such scenarios and multiple links exist in practical networks. And it is the network administrator who needs to work out such intricacies.
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[edit] Poison reverse
Split-horizon routing with poison reverse[1] is a variant of split-horizon route advertising in which a router actively advertises routes as unreachable over the interface over which they were learned by setting the route metric to infinite (16). The effect of such an announcement is to immediately remove most looping routes before they can propagate through the network.
The main disadvantage of poison reverse is that it can significantly increase the size of routing announcements in certain fairly common network topologies, but it allows to improve the overall efficiency of the network in case of faults.
[edit] Implementations
The split-horizon method is effective and simple to implement, and is therefore used by most distance-vector protocols. It is notably used by:
- RIP
- IGRP
- EIGRP
- VPLS uses the split horizon technique to avoid loops in the forwarding plane.
- Babel applies split-horizon to wired links only
[edit] See also
- Split-horizon DNS, a similar concept in DNS performing selective publishing of information.
- Route poisoning
[edit] References
[edit] External links
- "Split Horizon" and "Split Horizon with Poison Reverse" in Convergence in RIP Internetworks, from Microsoft TechNet.
