CCNP Route FAQ: Controlling Routing Updates Across the Network

Answer: The methods that Cisco discusses as useful methods of controlling routing updates are as follows:

• Passive interfaces: Prevent network communication and thus an adjacency from being formed with neighboring routers.
• Changing the administrative distance on the route: Changes the natural order as laid down by Cisco on which routing protocol is more plausible than the others. A scale of weighting is applied to the protocols.
• Default routes: Instruct the interface on where to send the destination traffic if the routing table had no entry for that destination.
• Static routes: Offers the ability to manually configure the path to a destination network.
• Route update filtering: Offers the use of access lists to configure.

Answer: The two occasions that you do not want routing information to be propagated are as follows:

• If there is a WAN link where the cost of the link is based on network traffic. This may also have the added disincentive of being a WAN link that is a dial-on-demand link, which is raised and maintained by the presence of traffic attempting to flow across the interface.
• The other situation in which you should prevent routing updates from being propagated is when you are trying to prevent routing loops. If the routing domain has redundant paths that will be learned by different routing protocol, it is advisable to filter the propagation of one of the paths.

Answer: The main reasons for multiple protocols existing within an organization are as follows:

• The organization is transitioning from one routing protocol to another because the network has grown and there is a need for a more sophisticated protocol that will scale.
• Although a vendor solution is preferred, there is a mix of different vendors within the network, so the vendor solution is used in the areas available. This is particularly true in client/server networks.
• Historically, the organization was a series of small network domains that have recently been tied together to form one large enterprise network. The company may well have plans to transition to a single routing protocol in the future.
• Often after a merger or a takeover, when several companies become one, it takes planning, strategy, and careful analysis to determine the best overall design for the network.
• Politically, there are ideological differences among the different network administrators, which until now have not been resolved.
• In a very large environment, the various domains may have different requirements, making a single solution inefficient. A clear example is in the case of a large multinational corporation, where EIGRP is the protocol used at the access and distribution layer, but BGP is the protocol connecting the core.

Answer: The problems experienced as a result of multiple routing processes and their redistribution include the following:

• The wrong or a less efficient routing decision being made, referred to as the suboptimal path.
• A routing loop occurring, in which the data traffic is sent in a circle without ever arriving at the destination.
• The convergence time of the network increasing because of the different technologies involved. If the routing protocols converge at different rates, this may also cause problems. In some cases, this may result in timeouts and the temporary loss of networks.

Answer: The benefits of policy-based routing include the following:

• Organizations can determine traffic flow based on the origin of the traffic. They can send traffic owned by different groups across different paths.
• Quality of service (QoS) can be set in the IP header using the precedence or TOS bits. This allows certain traffic to be prioritized through the network.
• High-cost links can be raised on more specific criteria, which allows an efficient use of the resources available.
• Traffic can be send across multiple paths based on traffic characteristics.