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CCNP ROUTE 642-902 Exam Foundation Learning: Implementing Path Control

  • Sample Chapter is provided courtesy of Cisco Press.
  • Date: Aug 5, 2010.


In this chapter, you learned about implementing path control. The chapter focused on the following topics:

  • Redundant network considerations including resiliency, availability, adaptability, performance, support for network and application services, predictability, and asymmetric traffic.
  • Path control tools including a good addressing design, redistribution and other routing protocol characteristics, passive interfaces, distribute lists, prefix lists, administrative distance, route maps, route tagging, offset lists, Cisco IOS IP SLAs, and PBR. (Advanced tools covered briefly include Cisco IOS OER, virtualization, and Cisco WAAS.)
  • Offset lists, a mechanism for increasing incoming and outgoing metrics to routes learned via EIGRP or RIP. Configuration of offset lists is performed with the offset-list {access-list-number | access-list-name} {in | out} offset [interface-type interface-number] router configuration command. Verification of offset lists can be performed with the traceroute command, the show ip route command, and the show ip eigrp topology command.
  • Cisco IOS IP SLAs, which use active traffic monitoring, generating traffic in a continuous, reliable, and predictable manner, to measure network performance. IOS IP SLAs can be used in conjunction with other tools, including the following:
    • Object tracking, to track the reachability of specified objects
    • Cisco IOS IP SLAs probes, to send different types of probes toward the desired objects
    • Route maps with PBR, to associate the results of the tracking to the routing process
    • Static routes with tracking options, as a simpler alternative to PBR
  • Cisco IOS IP SLAs terminology, including the following:
    • All the Cisco IOS IP SLAs measurement probe operations are configured on the IP SLAs source, either by the CLI or through an SNMP tool that supports IP SLAs operation. The source sends probe packets to the target.
    • There are two types of IP SLAs operations: those in which the target device is running the IP SLAs responder component, and those in which the target device is not running the IP SLAs responder component (such as a web server or IP host).
    • An IP SLAs operation is a measurement that includes protocol, frequency, traps, and thresholds.
  • Configuring IOS IP SLAs, including the use of the following commands:
    • The ip sla operation-number global configuration command (or the ip sla monitor operation-number global configuration command) to begin configuring a Cisco IOS IP SLAs operation and enter IP SLA configuration mode (or rtr configuration mode).
    • The icmp-echo {destination-ip-address | destination-hostname} [source-ip {ip-address | hostname} | source-interface interface-name] IP SLA configuration mode command (or the type echo protocol ipIcmpEcho {destination-ip-address | destination-hostname} [source-ipaddr {ip-address | hostname} | source-interface interface-name] rtr configuration mode command) to configure an IP SLAs ICMP echo operation.
    • The frequency seconds IP SLA configuration submode command (or rtr configuration submode command) to set the rate at which a specified IP SLAs operation repeats.
    • The timeout milliseconds IP SLA configuration submode command (or rtr configuration submode command) to set the amount of time a Cisco IOS IP SLAs operation waits for a response from its request packet.
    • The ip sla schedule operation-number [life {forever | seconds}] [start-time {hh:mm[:ss] [month day | day month] | pending | now | after hh:mm:ss}] [ageout seconds] [recurring] global configuration mode command (or the ip sla monitor schedule operation-number [life {forever | seconds}] [start-time {hh:mm[:ss] [month day | day month] | pending | now | after hh:mm:ss}] [ageout seconds] [recurring] global configuration mode command) to configure the scheduling parameters for a single Cisco IOS IP SLAs operation.
    • The track object-number ip sla operation-number {state | reachability} global configuration command (or the track object-number rtr operation-number {state | reachability} global configuration command) to track the state of an IOS IP SLAs operation, and enter track configuration mode.
    • The delay {up seconds [down seconds] | [up seconds] down seconds} track configuration command to specify a period of time to delay communicating state changes of a tracked object.
    • The ip route prefix mask {ip-address | interface-type interface-number [ip-address]} [dhcp] [distance] [name next-hop-name] [permanent | track number] [tag tag] global configuration command to establish a static route that tracks an object.
  • Verifying Cisco IOS IP SLAs, including the use of the show ip sla configuration [operation] command (or the show ip sla monitor configuration [operation] command), and the show ip sla statistics [operation-number] [details] command (or the show ip sla monitor statistics [operation-number] [details] command).
  • Using PBR to control path selection, providing benefits including source-based transit provider selection, QoS, cost savings, and load sharing. PBR is applied to incoming packets; enabling PBR causes the router to evaluate all packets incoming on the interface using a route map configured for that purpose.
  • Configuring and verifying PBR, including the following steps:
    • Choose the path control tool to use; for PBR, route-map commands are used.
    • Implement the traffic-matching configuration, specifying which traffic will be manipulated; match commands are used within route maps.
    • Define the action for the matched traffic, using set commands within route maps.
    • Optionally, fast-switched PBR or CEF-switched PBR can be enabled. Fast-switched PBR must be enabled manually. CEF-switched PBR is automatically enabled when CEF switching is enabled and PBR is enabled.
    • Apply the route map to incoming traffic or to traffic locally generated on the router.
    • Verify path control results, using show commands.
  • PBR match commands, including the following:
    • The match ip address {access-list-number | name} [...access-list-number | name] route map configuration command
    • The match length min max route map configuration command
  • PBR set commands, including the following four which are evaluated in this order (as soon as a destination address or interface has been chosen, other set commands for changing the destination address or interface are ignored):
    • The set ip next-hop ip-address [...ip-address] route map configuration command, which affects all packet types and is always used if configured.
    • The set interface type number [...type number] route map configuration command. If there is no explicit route for the destination address of the packet in the routing table (for example, if the packet is a broadcast or is destined for an unknown address), the set interface command has no effect and is ignored. A default route in the routing table is not considered an explicit route for an unknown destination address.
    • The set ip default next-hop ip-address [...ip-address] route map configuration command. A packet is routed to the next hop specified by the set ip default next-hop command only if there is no explicit route for the packet's destination address in the routing table. A default route in the routing table is not considered an explicit route for an unknown destination address.
    • The set default interface type number [...type number] route map configuration command. A packet is routed to the next hop specified by the set default interface command only if there is no explicit route for the packet's destination address in the routing table. A default route in the routing table is not considered an explicit route for an unknown destination address.
  • Other PBR set commands, including the following:
    • The set ip tos [number | name] route map configuration command, used to set the 5 CoS bits. Values 0 through 15 are used; one of the bits is reserved.
    • The set ip precedence [number | name] route map configuration command, used to set the 3 IP precedence bits in the IP packet header.
    • The set ip next-hop verify-availability [next-hop-address sequence trackobject] route-map configuration command to configure policy routing to verify the reachability of the next hop of a route map before the router performs policy routing to that next hop.
  • Commands to configure PBR on an interface, including the following:
    • The ip policy route-map map-tag interface configuration command, configured on the interface that receives the packets, not on the interface from which the packets are sent
    • The ip local policy route-map map-tag global configuration command, to apply a route map to packets originating on the router
  • Commands to verify PBR, including the show ip policy command, the show route-map [map-name] command, the debug ip policy command, the traceroute command, and ping command.
  • Advanced path control tools, including the following:
    • Cisco IOS OER, which uses tools such as Cisco IOS IP SLAs to automatically detect network service degradation and to make dynamic routing decisions and adjustments based on criteria such as response time, packet loss, jitter, path availability, traffic load distribution, and so forth
    • Virtualization, such as the use of VRF tables, VLANs, VPNs, and GRE tunnels
    • Cisco WAAS, including the use of WCCP to redirect normal traffic flows into Cisco WAAS devices
7. References | Next Section Previous Section

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