WORKSHOPS

Split Horizon The Split Horizon rule states that it is never useful to send information about a route back in the direction from which it came, as the router sending you the information is nearer the route advertised.

Poison Reverse Updates RIP version 1 implemented "Split Horizon with Poison Reverse Updates ." This differs from standard Split Horizon by sending updates to neighbors with a hop count of 16 for routes learned about from those neighbors.

Hold-Downs The larger a network gets, the more difficult rapid convergence becomes. Take a look at the "Sample Internetwork With Complex Connectivity"chart, below left, to examine what happens if the link between B and C becomes unavailable. When the invalid timer expires, B and C will notify their neighbors of the link failure via a triggered update. All router devices receiving this news will issue triggered updates to their neighbors, indicating the change in topology. Problems occur if, for example, Router J is made aware of the change before Router F. Router J adjusts its routing table to reflect that the B-to-C link is no longer available; however, if Router F has not heard of this change and sends a regular update message (assuming that the B-to-C link is still available) to J, J will reinstate the B-to-C link in its routing table. Without hold-downs, t his behavior can perpetuate for up to 20 minutes in a large network using standard RIP timers. The result is that packets get routed to a black hole and need to be retransmitted for this 20-to-30-minute period.

To resolve this, RIP uses hold-down timers. The hold-down rule states that when a route is removed, no update to that route will be accepted for a given period of time. In our example, this means Router J will ignore the update from Router F attempting to reinstate the B-to-C route. This gives the triggered updates time to get to all other routers, ensuring that any new routes are not just reinstating an old link. The downside of this scheme is delayed adoption of a new route when an old route fails.

Problems With RIP In large heterogeneous networks connecting Ethernet segments to serial-line wide area links and dial-up links, a few problems make the use of RIP far from ideal.

First, as the network grows, destinations that require a metric of more than 15 become unreachable. This is parti cularly bad if a network administrator applies a metric of more than 1 to a link, to indicate slow transmission time.

The overly simplistic metric generates a suboptimal routing table, resulting in packets being sent over slow (or otherwise costly) links when better paths are available. Other routing protocols use a more complex metric to include measurement of delay, bandwidth, reliability and load.

RIP-enabled devices will accept RIP updates from any device. This enables a misconfigured device to easily disrupt an entire network. Other prot ocols allow neighbors to be defined as the only ones to accept routing updates from.

RIP does not carry subnet-mask information in updates. It assumes all interfaces on the network have the same mask. Because of this, the network can run out of usable Internet-compatible addresses earlier than it should.

On larger internetworks, convergence time is unacceptably slow (more than 5 minutes) for most commercial applications such as database access or financia l transactions.

The Split Horizon with Poison Reverse Update algorithm in RIP counters routing loops only between adjacent routers. Other routing protocols employ more sophisticated mechanisms to counter larger routing loops, allowing the safe use of a zero hold-down value.

RIP updates use more bandwidth than other protocols, primarily because the entire routing table is sent in updates.

Chris Lewis is a product support manager at ILX Systems in New York.
He can be reached at chrisl@ilx.com.