Content Overview Dynamic routing protocols can help simplify the life of a network administrator. Dynamic routing makes it possible to avoid the time-consuming and exacting process of configuring static routes. Dynamic routing also makes it possible for routers to react to changes in the network and to adjust their routing tables accordingly, without the intervention of the network administrator. However, dynamic routing can cause problems. Some of the problems associated with dynamic distance vector routing protocols are discussed in this module, along with some of the steps that designers of the protocols have taken to solve the problems. Routing Information Protocol (RIP) is a distance vector routing protocol that is used in thousands of networks throughout the world. The fact that RIP is based on open standards and is very simple to implement makes it attractive to some network administrators, although RIP lacks the power and features of more advanced routing protocols. Because of its simplicity, RIP is a good beginning protocol for the networking student. This module also introduce RIP configuration and troubleshooting. Like RIP, Interior Gateway Routing Protocol (IGRP) is a distance vector routing protocol. Unlike RIP, IGRP is a Cisco-proprietary protocol rather than a standards-based protocol. While remaining very simple to implement, IGRP is a more complex routing protocol than RIP and it is able to use a number of factors to determine the best route to a destination network. This module will introduce IGRP configuration and troubleshooting. Students completing this module should be able to:
Content 7.1 Distance Vector Routing 7.1.1 Distance vector routing updates Routing table updates occur periodically or when the topology in a distance vector protocol network changes. It is important that a routing protocol be efficient in updating the routing tables. As with the network discovery process, topology change updates proceed systematically from router to router. Distance vector algorithms call for each router to send its entire routing table to each of its adjacent neighbors. The routing tables include information about the total path cost as defined by the metrics and the logical address of the first router on the path to each network contained in the table. Web Links Dynamic Routing http://cisn.metu.edu.tr/2001-3/ dynamic.php
Content 7.1 Distance Vector Routing 7.1.2 Distance vector routing loop issues Routing loops can occur when inconsistent routing tables are not updated due to slow convergence in a changing network.
  1. Just before the failure of Network 1, all routers have consistent knowledge and correct routing tables. The network is said to have converged. Assume for the remainder of this example that Router C's preferred path to Network 1 is by way of Router B, and the distance from Router C to Network 1 is 3.
  2. When Network 1 fails, Router E sends an update to Router A. Router A stops routing packets to Network 1, but Routers B, C, and D continue to do so because they have not yet been informed of the failure. When Router A sends out its update, Routers B and D stop routing to Network 1. However, Router C has not received an update. To Router C, Network 1 is still reachable via Router B.
  3. Now Router C sends a periodic update to Router D, indicating a path to Network 1 by way of Router B. Router D changes its routing table to reflect this good, but incorrect, information, and propagates the information to Router A. Router A propagates the information to Routers B and E, and so on. Any packet destined for Network 1 will now loop from Router C to B to A to D and back to again to C.
Web Links Distance Vector Protocols http://www.mcpprep.com/ WebHelp/ccna/ ccna_obj_42.htm
Content 7.1 Distance Vector Routing 7.1.3 Defining a maximum count The invalid updates of Network 1 will continue to loop until some other process stops the looping. This condition, called count to infinity, loops packets continuously around the network in spite of the fundamental fact that the destination network, Network 1, is down. While the routers are counting to infinity, the invalid information allows a routing loop to exist. Without countermeasures to stop the count to infinity process, the distance vector metric of hop count increments each time the packet passes through another router. These packets loop through the network because of wrong information in the routing tables. Distance vector routing algorithms are self-correcting, but a routing loop problem can require a count to infinity. To avoid this prolonged problem, distance vector protocols define infinity as a specific maximum number. This number refers to a routing metric which may simply be the hop count. With this approach, the routing protocol permits the routing loop to continue until the metric exceeds its maximum allowed value. The graphic shows the metric value as 16 hops. This exceeds the distance vector default maximum of 15 hops so the packet is discarded by the router. In any case, when the metric value exceeds the maximum value, Network 1 is considered unreachable. Web Links Avoiding Counting To Infinity In Distance Vector Routing http://www.uni-koblenz.de/~steigner/ labor/papers/ ripmti.pdf
Content 7.1 Distance Vector Routing 7.1.4 Elimination routing loops through split horizon Another possible source for a routing loop occurs when incorrect information that has been sent back to a router contradicts the correct information that the router originally distributed. Here is how this problem occurs:
  1. Router A passes an update to Router B and Router D, indicating that Network 1 is down. Router C, however, transmits an update to Router B, indicating that Network 1 is available at a distance of 4, by way of Router D. This does not violate split-horizon rules.
  2. Router B concludes, incorrectly, that Router C still has a valid path to Network 1, although at a much less favorable metric. Router B sends an update to Router A advising Router A of the new route to Network 1.
  3. Router A now determines that it can send to Network 1 by way of Router B; Router B determines that it can send to Network 1 by way of Router C; and Router C determines that it can send to Network 1 by way of Router D. Any packet introduced into this environment will loop between routers.
  4. Split-horizon attempts to avoid this situation. If a routing update about Network 1 arrives from Router A, Router B or Router D cannot send information about Network 1 back to Router A. Split-horizon thus reduces incorrect routing information and reduces routing overhead.
Web Links Configuring RIP http://www.cisco.com/en/US/products/sw/ iosswrel/ps1831/products_configuration_guide_ chapter09186a00800d97f7.html#xtocid20
Content 7.1 Distance Vector Routing 7.1.5 Route poisoning Route poisoning is used by various distance