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 administrative distance for RIP is 120; it has the highest distance of interior routing protocols and is therefore the least likely to be selected.

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: Enhanced IGRP will automatically redistribute between itself and IGRP as long as both processes are running the same autonomous system number.

Enhanced IGRP also automatically redistributes between the LAN protocol and Enhanced IGRP. EIGRP for IPX automatically redistributes; IPX for RIP/SAP and Enhanced IGRP for AppleTalk similarly redistribute automatically into RTMP.

Answer: The command show ip route displays the administrative distance for each route; the number is in brackets, coupled with the metric for the route.

Answer: Redistribution is required when there is more than one routing protocol for IP running within the organization and when every part of the network needs connectivity to all the networks.

Answer: The reason not to overlap routing protocols is that it will increase network traffic, router CPU processing, and memory because of the additional protocol updates. This additional traffic and CPU and memory requirements complicate the routing process. Now, the decision is not simply between multiple paths, but also between the various routing protocols that are advertising them.

Answer: A WAN link that is a dial-on-demand link is raised and maintained by the presence of traffic attempting to flow across the interface. Thus, every time a RIP update was sent, the path would be raised—or, more likely, kept up all the time.

Answer: The metric is used to select the best path when multiple paths are available to a remote network.

Answer: The command is used to match criteria in establishing the policy-based routing.
Access lists are used to specify the addressing of the packets to be affected.

Answer: This command is set on an incoming interface and enables the fast switching of policy routing. Before version 11.2 of the Cisco IOS, policy routing was processswitched. This caused some applications to time out, but the problem has now been resolved. Fast switching of policy routing is disabled by default. Therefore, it is necessary to manually configure it.

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: There is one IP routing table, and all the routes from all the different routing protocol are held in the one table. If a route has multiple paths from multiple protocols, one path will be selected based on the administrative distance.

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: IGRP has an administrative distance of 100, while OSPF has an administrative distance of 110. The IGRP path will be entered into the routing table if there are paths offered to the same destination from both protocols.

Answer: The command for configuring a route filter is as follows:
distribute list {access-list-number | name} out
[ interface-name | routing process | autonomous-system number]

Answer: A passive interface is an interface that will listen to routing updates but that will not propagate any updates for the protocol configured. It is used to prevent unnecessary traffic from being sent out of an interface. Usually a passive interface is configured when there are no routers to hear the updates on that network.

Answer: When the routing table is populated with networks that are provided by multiple routing protocols, the administrative distance is used to choose the best path to the remote network.

Answer: The concern with redistributing into a redundant network is that it is possible to select suboptimal routes. That is, if two routing protocols both have a path to the same destination, the path for the routing table will be selected based on administrative distance. This is a blanket solution that does not always render the best decision.

Answer: A default network is a route used when there is no entry for the remote network in the routing table. It is used to connect to the Internet and other routing domains when it is not practical to know all the available networks. It is sufficient to have an exit point from your network identified.

Answer: The metric is used within a routing protocol to select the best path to a remote network when there are multiple paths. When redistributing, it is not always possible to port the metric across because the metric is protocol-specific. The default metric throws the original metric away and substitutes a new metric for the new routing protocol.

Answer: The command to configure the administrative distance of a route is as follows:
distance weight [address mask [ access-list-number | name]] [ip]
A different command is used for EIGRP and BGP-4. The EIGRP command to change the administrative distance is as follows:
distance eigrp internal-distance external-distance

Answer: If a filter is applied to an incoming interface, the routing table is not checked unless the route is permitted. However, the router has to examine every incoming packet, which is expensive in resources. The outbound filter must go through the routing decision process the first time, after which the result is cached. Inbound filters are wider-ranging because they prevent routes from entering the router instead of filtering on each outgoing interface that is affected.

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.

Answer: Using the set commands modifies matching routes. If the criteria is met in the match command and the action was to permit, then the set criteria is initiated to control the routing as specified.

Answer: This command provides a list of IP addresses if there is no explicit route in the routing table for a destination. These addresses are those of next-hop routers or of the interfaces of adjacent routers. If multiple next-hop addresses are listed, then the first address is tried and the others are tried in turn.

Answer: The command show ip policy displays the route maps used for policy routing on the router’s interfaces. The command show route-map [map-name] displays the route maps.