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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  TCP/IP Routing Protocols (Gateway Protocols)                 9  TCP/IP Interior Routing Protocols (RIP, OSPF, GGP, HELLO, IGRP, EIGRP)                      9  TCP/IP Routing Information Protocol (RIP, RIP-2 and RIPng)                           9  RIP Fundamentals and General Operation

RIP Protocol Limitations and Problems 1 2 3 4 RIP Version-Specific Message Formats and Features

Split Horizon With Poisoned Reverse

This is an enhancement of the basic split horizon feature. Instead of omitting routes learned from a particular interface when sending RIP Response messages on that interface, we include those routes but set their metric to “RIP infinity”, 16. So in the example above, Router B would include the route to Network 1 in its transmissions on Network 2, but it would say the cost to reach N1 was 16, instead of its real cost (which is 2).

The “poisoned reverse” refers to the fact that we are poisoning the routes that we want to make sure routers on that interface don't use. Router A will see Router B advertise Network 1 but with a cost of 16, which serves as an explicit message to Router A: “there is absolutely no way for you to get to Network 1 through Router B”. This provides more insurance than the regular split horizon feature, because if the link from Router A to Network 1 dies as we described in the previous topic, Router A will know for certain that it can't try to get a new route through Router B. Figure 175 shows how split horizon with poisoned reverse works.

This technique also works in normal circumstances (meaning, when there is no issue such as a broken link to a network). In that case, Router A will receive updates from RB with a cost of 16 on a periodic basis, but RA will never try to reach Network 1 through Router B anyway, since it is directly connected to Network 1 (cost of 1).

Figure 175: RIP Problem Solving Using Split Horizon With Poisoned Reverse

The top panel in this diagram (#1) shows the same example as in Figure 174. In #2, as before, the link between RA and N1 is broken, just as RB is ready to send out its routine update. However, the split horizon with poisoned reverse feature means it sends different messages on its two links; on the network that connects it to RA, it sends a route advertisement with a cost of 16. In #3, Router A receives this, which it will discard, ensuring no counting to infinity problem occurs. On RA’s next cycle it will update RB to tell it that N1 is no longer reachable.

RIP Protocol Limitations and Problems 1 2 3 4 RIP Version-Specific Message Formats and Features

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