Example: Configuring the RTR Next-Hop Verification Feature

This topic describes how to configure the RTR next-hop verification feature. Although this configuration example uses Fast Ethernet interfaces, E Series routers support next-hop verification on any type of lower-layer interface.

Requirements

This example uses the following software and hardware components:

Note:

  • Before you configure IP, you must create the lower-layer interfaces over which IP traffic flows.
  • All IP configurations will be removed from the interface when you issue the no ip interface command in Interface Configuration mode.

Overview

Figure 12 shows a sample configuration that illustrates the next-hop verification feature. In this example, two Fast Ethernet interfaces are configured between a remote system and an E Series router: Fast Ethernet interface 4/0 and Fast Ethernet interface 4/1. At any given time, only one of these interfaces forwards IP traffic, even though the associated layer 2 interfaces may be up concurrently.

On the E Series router, Fast Ethernet interfaces 4/0 and 4/1 are configured as unnumbered IP interfaces. In addition, each interface has an RTR probe configured as an echo type that sends requests over the interface to determine its availability. RTR 10 sends requests over Fast Ethernet interface 4/0, and RTR 11 sends requests over Fast Ethernet interface 4/1.

In this example, both RTR 10 and RTR 11 use the IP address of the remote system (10.1.1.2) as the target address. When you configure multiple RTR entries to use the same target address, you must set the receive-interface attribute to specify the interface on which the probe expects to receive responses. (See Step 4c.) This action enables the router to map incoming responses to the proper RTR entry, even when multiple RTR entries have the same target address.

Figure 12: Sample Configuration for Next-Hop Verification

Sample Configuration for Next-Hop Verification

The ip route command is issued for each interface with the verify rtr and last-resort keywords to establish the necessary static routes. (See Steps 6 and 7.) This command causes the results described in Table 6, based on the status of the associated RTR operations.

Table 6: Next-Hop Verification Results for Sample Configuration

RTR 10 Status

RTR 11 Status

Results

Up

Up

The router installs an equal-cost multipath (ECMP) route to 10.1.1.2 in the routing table, using Fast Ethernet interfaces 4/0 and 4/1 as the next hops.

Up

Down

The router installs a route to 10.1.1.2, using Fast Ethernet interface 4/0 as the next hop.

Down

Up

The router installs a route to 10.1.1.2, using Fast Ethernet interface 4/1 as the next hop.

Down

Down

Although both RTR operations are down, the last-resort keyword instructs the router to install an ECMP route to 10.1.1.2, using Fast Ethernet interfaces 4/0 and 4/1 as the next hops.

When all of the RTR operations associated with your static routes are down, you can control which route is installed in the routing table by including the last-resort keyword in the ip route verify rtr command only for the route that you want to install.

Configuring RTR Next-Hop Verification

To configure the next-hop verification example shown in Figure 12:

  1. Configure a loopback interface, and assign an IP address and mask to the interface.
    host1(config)#interface loopback 0 host1(config-if)#ip address 10.1.1.1 255.255.255.255 host1(config-if)#exit
  2. Configure Fast Ethernet port 4/0 with an unnumbered primary IP interface associated with the loopback interface configured in Step 1.
    host1(config)#interface fastEthernet 4/0 host1(config-if)#ip unnumbered loopback 0 host1(config-if)#exit
  3. Repeat Step 2 for Fast Ethernet port 4/1.
    host1(config)#interface fastEthernet 4/1 host1(config-if)#ip unnumbered loopback 0 host1(config-if)#exit
  4. Define probe RTR 10 for Fast Ethernet interface 4/0.
    1. Assign an operation number to the RTR probe, and access RTR Configuration mode. For information, see Configuring the Probe Type for RTR.
      host1(config)#rtr 10 host1(config-rtr)#
    2. Configure the RTR probe as an echo type, and set the IP destination address and source interface.

      You must configure the RTR probe as an echo type to use next-hop verification. For information, see Configuring the Probe Type for RTR.

      host1(config-rtr)#type echo protocol ipIcmpEcho 10.1.1.2
      source fastEthernet 4/0
    3. Specify the interface on which the RTR probe expects to receive responses.

      You must set the receive-interface attribute when multiple RTR operations use the same target address. For information, see Setting the Receiving Interface for the RTR Entry.

      host1(config-rtr)#receive-interface fastEthernet 4/0
    4. (Optional) Configure optional probe characteristics, such as the frequency and samples-of-history kept. For information, see Configuring the Probe Characteristics for RTR.
      host1(config-rtr)#frequency 1 host1(config-rtr)#samples-of-history-kept 0
    5. Exit RTR Configuration mode.
      host1(config-rtr)#exit
    6. Enable the probe to react to the test-failure event and the test-completion event.

      You must configure both the test-failure and test-completion reaction conditions to use next-hop verification. For information, see Setting the Reaction Conditions for the RTR Probe.

      host1(config)#rtr reaction-configuration 10 test-failure 3 host1(config)#rtr reaction-configuration 10 test-completion
    7. Schedule the probe operation. For information, see Scheduling the RTR Probe.
      host1(config)#rtr schedule 10 life 3 host1(config)#rtr schedule 10 restart-time 1 host1(config)#rtr schedule 10 start now
  5. Repeat Step 4 to define RTR 11 for Fast Ethernet interface 4/1.
    host1(config)#rtr 11 host1(config-rtr)#type echo protocol ipIcmpEcho 10.1.1.2
    source fastEthernet 4/1
    host1(config-rtr)#receive-interface fastEthernet 4/1 host1(config-rtr)#frequency 1 host1(config-rtr)#samples-of-history-kept 0 host1(config-rtr)#exit host1(config)#rtr reaction-configuration 11 test-failure 3 host1(config)#rtr reaction-configuration 11 test-completion host1(config)#rtr schedule 11 life 3 host1(config)#rtr schedule 11 restart-time 1 host1(config)#rtr schedule 11 start now
  6. Establish a static route associated with RTR 10.

    This command creates a static route and installs it in the routing table only if RTR 10 is currently reachable or if no other static route to IP destination address 10.1.1.2 is usable.

    host1(config)#ip route 10.1.1.2 255.255.255.255 10.1.1.2 fastEthernet 4/0 verify rtr 10 last-resort
  7. Establish a static route associated with RTR 11.

    This command creates a static route and installs it in the routing table only if RTR 11 is currently reachable or if no other static route to IP destination address 10.1.1.2 is usable.

    host1(config)#ip route 10.1.1.2 255.255.255.255 10.1.1.2 fastEthernet 4/1 verify rtr 11 last-resort

    When both RTR 10 and RTR 11 are unreachable, you can control which static route is installed in the routing table by including the last-resort keyword in the ip route verify rtr command only for the route that you want to install.

    Note: For detailed information about the commands for configuring RTR probes, see Response Time Reporter.

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