Chapter 04: Manipulating Routing Updates (Part03)
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Controlling Routing Update Traffic

Add a note here Routing updates compete with user data for bandwidth and router resources, yet routing updates are critical because they carry the information that routers need to make sound routing decisions. To ensure that the network operates efficiently, you must control and tune routing updates. Information about networks must be sent where it is needed and filtered from where it is not needed. No one type of route filter is appropriate for every situation. Therefore, the more techniques you have at your disposal, the better your chance of having a smooth, well-run network.

Add a note hereThis section discusses controlling the updates sent and received by dynamic routing protocols and controlling the routes redistributed into routing protocols. In many cases, you do not want to prevent all routing information from being advertised; you might want to block the advertisement of only certain routes. For example, you could use such a solution to prevent routing loops when implementing two-way route redistribution with dual redistribution points. The following are some ways to control or prevent dynamic routing updates from being generated:

  • Add a note here Passive interface— A passive interface prevents routing updates for the specified protocol from being sent through an interface.

  • Add a note here Default routes— A default route instructs the router that if it does not have a route for a given destination, it should send the packet to the default route. Therefore, no dynamic routing updates about the remote destinations are necessary.

  • Add a note here Static routes— A static route allows routes to remote destinations to be manually configured on the router. Therefore, no dynamic routing updates about the remote destinations are necessary.

  • Add a note here Route maps— Route maps are complex access lists that allow conditions to be tested against a packet or route, and then actions taken to modify attributes of the packet or route.

  • Add a note here Distribute lists— A distribute list allows an access list to be applied to routing updates.

  • Add a note here Prefix lists— A prefix list is a specialized access list designed to filter routes.

Add a note herePassive interfaces were discussed earlier in the “The passive-interface Command” section. Static and default routes were discussed in Chapter 1; specifics related to controlling routing updates are explored in the next section, which is followed by a discussion of route maps, distribute lists, and prefix lists.

Add a note here Static and Default Routes

Add a note here Static routes are routes that you manually configure on a router. Static routes are used most often to do the following:

  • Add a note hereDefine specific routes to use when two autonomous systems must exchange routing information, rather than having entire routing tables exchanged.

  • Add a note hereDefine routes to destinations over a WAN link to eliminate the need for a dynamic routing protocol—that is, when you do not want routing updates to enable or cross the link.

Add a note hereWhen configuring static routes, keep in mind the following considerations:

  • Add a note hereWhen using static routes instead of dynamic routing updates, all participating routers must have static routes defined so that they can reach remote networks. Static route entries must be defined for all routes for which a router is responsible. To reduce the number of static route entries, you can define a default static route—for example, ip route 0.0.0.0 0.0.0.0 S0/0/1.

  • Add a note hereIf you want a router to advertise a static route in a routing protocol, you might need to redistribute it.

Add a note hereYou can configure default routes for routing protocols on Cisco routers. For example, when you create a default route on a router running RIP, the router advertises an address of 0.0.0.0. When a router receives this default route, it forwards any packets destined for a destination that does not appear in its routing table to the default route you configured.


Summary

Add a note hereIn this chapter, you learned about manipulating routing updates. The chapter focused on the following topics:

  • Add a note hereNetwork performance issues, including excessive routing updates, the presence of any route maps or filters, and the number of routing protocols running in the same autonomous system. Solutions to these issues include design changes, using passive interfaces, and route filtering (access lists, route maps, distribute lists, and prefix lists).

  • Add a note hereReasons for using more than one routing protocol (migration, host system needs, mixed-vendor environment, political and geographical borders, MPLS VPNs), how routing information can be exchanged between them (referred to as redistribution), and how Cisco routers operate in a multiple routing protocol environment.

  • Add a note hereHow route redistribution is always performed outbound. The router doing redistribution does not change its routing table.

  • Add a note hereIssues arising when redistributing routes, including routing loops, incompatible routing information, and inconsistent convergence times.

  • Add a note hereThe roles that the administrative distance and the routing metric play in route selection. Lower administrative distances are considered more believable (better). Lower metrics are considered better, within the same routing protocol. When redistributing, a router assigns a seed metric to redistributed routes, using the default-metric router configuration command, or specified as part of the redistribute command either with the metric option or by using a route map.

  • Add a note hereThe redistribution techniques, one-point and multipoint:

    • Add a note here The two methods of one-point route redistribution: one way and two way. Suboptimal routing is a possible issue with these techniques.

    • Add a note hereThe two methods of multipoint route redistribution: one way and two way. Multipoint redistribution is likely to introduce potential routing loops.

    • Add a note hereTo prevent routing issues use one of the following options: Redistribute a default route from the core autonomous system into the edge autonomous system, and redistribute routes from the edge routing protocols into the core routing protocol; redistribute multiple static routes about the core autonomous system networks into the edge autonomous system, and redistribute routes from the edge routing protocols into the core routing protocol; redistribute routes from the core autonomous system into the edge autonomous system with filtering to block out inappropriate routes; redistribute all routes from the core autonomous system into the edge autonomous system, and from the edge autonomous system into the core autonomous system, and then modify the administrative distance associated with redistributed routes so that they are not the selected routes when multiple routes exist for the same destination.

  • Add a note hereConfiguration of redistribution between various IP routing protocols:

    • Add a note hereTo redistribute into RIP, use the redistribute protocol [process-id] [match route-type] [metric metric-value] [route-map map-tag] router configuration command.

    • Add a note hereTo redistribute into OSPF, use the redistribute protocol [process-id] [metric metric-value] [metric-type type-value] [route-map map-tag] [subnets] [tag tag-value] router configuration command.

    • Add a note hereTo redistribute into EIGRP, use the redistribute protocol [process-id] [match route-type] [metric metric-value] [route-map map-tag] router configuration command.

  • Add a note hereUsing the passive-interface type number [default] router configuration command to prevent a routing protocol’s routing updates from being sent through the specified router interface.

  • Add a note hereHow to manipulate the administrative distance of routes to influence the route-selection process:

    • Add a note hereUse the distance administrative-distance [address wildcard-mask [ip-standard- list] [ip-extended-list]] router configuration command for all protocols.

    • Add a note hereAlternatively, for EIGRP, use the distance eigrp internal-distance external-distance router configuration command.

    • Add a note hereAlternatively for OSPF, use the distance ospf {[intra-area dist1] [inter-area dist2] [external dist3]} router configuration command.

    • Add a note hereAlternatively, for BGP, use the distance bgp external-distance internal-distance local-distance router configuration command.

  • Add a note hereUsing the show ip route [ip-address] and traceroute [ip-address] commands to verify route redistribution.

  • Add a note here Using the ip route 0.0.0.0 0.0.0.0 interface or ip default-network network-number global configuration command to configure default routes.

  • Add a note hereUsing route maps for route filtering during redistribution, PBR, NAT, and BGP.

  • Add a note hereThe characteristics of route maps, configured using the route-map map-tag [permit | deny] [sequence-number] global configuration command:

    • Add a note hereRoute maps allow some conditions to be tested against the packet or route in question using match commands. If the conditions match, some actions can be taken to modify attributes of the packet or route; these actions are specified by set commands.

    • Add a note hereA collection of route-map statements that have the same route map name is considered one route map.

    • Add a note hereWithin a route map, each route-map statement is numbered and therefore can be edited individually.

    • Add a note hereThe default for the route-map command is permit, with a sequence-number of 10.

    • Add a note hereOnly one condition listed on the same match statement must match for the entire statement to be considered a match. However, all match statements within a route-map statement must match for the route map to be considered matched.

    • Add a note hereWhen used with a redistribute command, a route-map statement with permit indicates that the matched route is to be redistributed, while a route-map statement with deny indicates that the matched route is not to be redistributed.

  • Add a note hereConfiguring route maps for PBR, using the ip policy route-map map-tag interface configuration command.

  • Add a note here Table 4-15 lists some of the variety of match criteria that can be defined. Table 4-16 lists some of the variety of set commands that are available.

  • Add a note hereDistribute lists, allowing an access list to be applied to routing updates:

    • Add a note hereThe distribute-list {access-list-number | name} out [interface-name | routing-process [routing-process parameter]] router configuration command assigns the access list to filter outgoing routing updates. This command filters updates going out of the interface or routing protocol specified in the command, into the routing process under which it is configured.

    • Add a note hereThe distribute-list {access-list-number | name} [route-map map-tag] in [interface-type interface-number] router configuration command assigns the access list to filter routing updates coming in through an interface. This command filters updates going into the interface specified in the command, into the routing process under which it is configured.

  • Add a note herePrefix lists, an alternative to distribute lists, with improvements in performance, support for incremental modifications, a more user-friendly command-line interface, and greater flexibility. Prefix lists are configured with the ip prefix-list {list-name | list-number} [seq seq-value] {deny | permit} network/length [ge ge-value] [le le-value] global configuration command. The ge-value and le-value range is length < ge-value < le-value <= 32. An exact match is assumed when neither ge nor le is specified.

  • Add a note here Whether a prefix in a prefix list is permitted or denied is based on the following rules:

    • Add a note hereAn empty prefix list permits all prefixes.

    • Add a note hereIf a prefix is permitted, the route is used. If a prefix is denied, the route is not used.

    • Add a note herePrefix lists consist of statements with sequence numbers. The router begins the search for a match at the top of the prefix list, which is the statement with the lowest sequence number.

    • Add a note hereWhen a match occurs, the router does not need to go through the rest of the prefix list. For efficiency, you might want to put the most common matches (permits or denies) near the top of the list by specifying a lower sequence number.

    • Add a note hereAn implicit deny is assumed if a given prefix does not match any entries in a prefix list.

  • Add a note herePrefix list sequence numbers:

    • Add a note hereSequence numbers are generated automatically, unless you disable this automatic generation.

    • Add a note hereA prefix list is an ordered list. The sequence number is significant when a given prefix is matched by multiple entries of a prefix list, in which case the one with the smallest sequence number is considered the real match.

    • Add a note hereThe evaluation of a prefix list starts with the lowest sequence number and continues down the list until a match is found, in which case the permit or deny statement is applied to that network and the remainder of the list is not evaluated.

  • Add a note hereVerifying prefix lists can involve commands listed in Table 4-20.

  • Add a note hereHow multiple methods can be combined to effectively control routing updates.


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