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Most of the references say "RIP is not scalable" hence can be used only in smaller networks. But none say "WHY?" What is that in RIP that actually preventing it from scaling to larger networks? And HOW OSPF overcomes the disadvantage of RIP?

21

Most of the references say "RIP is not scalable" hence can be used only in smaller networks. But none say "WHY?" What is that in RIP that actually preventing it from scaling to larger networks? And HOW OSPF overcomes the disadvantage of RIP?

Summary

  • RIPv1 floods routes frequently (every 30 seconds), which introduces large CPU loads as the size of the routing table increases. This is compounded by the reality that RIP recalculates metrics for every route, every time it floods the route out a new interface (regardless of whether there was a topology change or not). As the number of routes increases, this is prevents RIP from scaling as well as other protocols.
  • RIPv1 is classful
  • OSPF floods routes infrequently. If there is a topology change in the network, only the LSAs changed are flooded; metrics are calculated on these changes. As such, on-demand route calculations, on LSAs which are flooded infrequently, make OSPF scale well.
  • OSPF is a classless protocol, which supports CIDR, which also makes it a more scalable protocol than RIPv1

RIPv1 Details:

RIP is a Distance Vector protocol; all Distance Vector protocols run the Bellman-Ford algorithm. At a high level, this means:

  • All routes in the routing table are periodically announced through all interfaces.
  • RIP floods routes out every RIP interface every 30 seconds. Since RIP routes by rumor, this means every router in the topology must work in direct proportion to the size of the routing table every 30 seconds. The CPU load and traffic jitter implications of this become scary as you approach thousands of routes (particularly on CPU-based routers with no hardware forwarding).
  • The RIP protocol itself has a fixed maximum hop-count at 15 hops (which is small if you need to do any form of path-weighting).
  • Protocols based on Bellman-Ford algorithms are prone to routing loops, and count-to-infinity issues.

OSPF Details:

By way of contrast, OSPF is a link-state protocol running Dijkstra's algorithm. As such:

  • Each router only announces its directly connected and redistributed routes in routing updates (called LSAs).
  • Each router floods their own LSA every 30 minutes by default (because the route refresh timer is 3600 seconds, or 1 hour)
  • LSAs are also flooded when triggered by changes in the routing table
  • Routers use Dijkstra's algorithm to perform distributed LSA path calculations only when necessary.
  • 2
    Is there a reason behind comparing RIPv1 to OSPF? Why not compare against v2? – Ryan Foley Jan 17 '14 at 17:12
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    @fizzle, by convention when someone says RIP, they mean RIPv1; that said, RIPv2 doesn't substantially change RIP's scalability: it introduced classless routing, but it still blasts the full routing table every 30 seconds, and recalculates the table at every hop. If you have ever operated a large RIP network with flapping links, you would understand how bad this is... random routing black holes open up and eat good traffic because it takes so long to propagate updates. RIP and RIPv2 have their place, but it's not in a large network. – Mike Pennington Jan 17 '14 at 18:19
  • Good insight, I've just never seen a network still utilizing RIP. It might be worth mentioning OSPF's ability to authenticate neighbors, although, that doesn't directly answer the OP. – Ryan Foley Jan 17 '14 at 21:22
  • Thanks @mike-pennington for an answer that is satisfying ;-) – gulam Jan 18 '14 at 9:58
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Just to add to what Mike has already explained, RIP recalculates its routes and announces all of them every 30 secs. In a network with thousands of routers and tens of thousands of routes, that's a LOT of routes being calculated -- the routers would be too busy to actually forward any traffic.

As you probably already learned, RIP's maximum metric is 15 hops. That limits the size of the network.

RIP has no hierarchy. Imagine a world-wide network, and every time a link goes up and down in Singapore, the router in Iceland has to recalculate all its routes. There's no way to isolate one region from another.

  • Doesn't the last part (all routers have to recalculate routes) apply to OSPF as well? – grawity Jan 19 '14 at 0:26
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    @grawity OSPF has hierarchy, namely areas, which allow you to hide topology changes in one part of the network from affecting other parts. So if Singapore and Iceland are in different areas, then they don't have to calculate each others routes. – Ron Trunk Jan 19 '14 at 12:44

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