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Example: Configuring an IS-IS Default Route Policy on Logical Systems

Understanding Default Routes

If a packet is received on a routing device, the device first checks to see if the IP destination address is on one of the device’s local subnets. If the destination address is not local, the device checks its routing table. If the remote destination subnet is not listed in the routing table, the packet is forwarded to the next hop toward the destination using the default route. The default route generally has a next-hop address of another routing device, which performs the same process. The process repeats until a packet is delivered to the destination.

The route evaluation process in each router uses the longest prefix match method to obtain the most specific route. The network with the longest subnet mask that matches the destination IP address is the next-hop network gateway.

The default route in IPv4 is designated as 0.0.0.0/0 or simply 0/0. Similarly, in IPv6, the default route is specified as ::/0. The subnet mask /0 specifies all networks, and is the shortest match possible. A route lookup that does not match any other route uses this route if it is configured and active in the routing table. To be active, the configured next-hop address must be reachable.

Administrators generally point the default route toward the routing device that has a connection to a network service provider. Therefore, packets with destinations outside the organization's local area network, typically destinations on the Internet or a wide area network, are forwarded to the routing device with the connection to that provider. The device to which the default route points is often called the default gateway.

Example: Configuring an IS-IS Default Route Policy on Logical Systems

This example shows logical systems configured on a single physical router and explains how to configure a default route on one logical system.

• Requirements
• Overview
• Configuration
• Verification

Requirements

No special configuration beyond device initialization is required before configuring this example.

Overview

This example shows a logical system redistributing a default route to other logical systems. All logical systems are running IS-IS. A common reason for a default route is to provide a path for sending traffic destined outside the IS-IS domain.

In this example, the default route is not used for forwarding traffic. The no-install statement prevents the route from being installed in the forwarding table of Logical System LS3. If you configure a route so it is not installed in the forwarding table, the route is still eligible to be exported from the routing table to other protocols. The discard statement silently drops packets without notice.

Figure 1 shows the sample network.

Figure 1: IS-IS Logical Systems with a Default Route to an ISP

Configuration

CLI Quick Configuration

To quickly configure this example, copy the following commands, paste them into a text file, remove any line breaks, change any details necessary to match your network configuration, and then copy and paste the commands into the CLI at the [edit] hierarchy level.

Step-by-Step Procedure

1. Configure the logical tunnel interfaces.
   [edit logical-systems LS3 interfaces lt-1/2/0]user@R1# set unit 3 description LS3->LS2user@R1# set unit 3 encapsulation ethernetuser@R1# set unit 3 peer-unit 4user@R1# set unit 3 family inet address 10.0.2.1/30user@R1# set unit 3 family isouser@R1# set unit 5 description LS3->LS1user@R1# set unit 5 encapsulation ethernetuser@R1# set unit 5 peer-unit 0user@R1# set unit 5 family inet address 10.0.1.1/30user@R1# set unit 5 family iso[edit logical-systems LS3 interfaces lo0 unit 3]user@R1# set family iso address 49.0001.1234.1600.2231.00
2. Enable IS-IS on the interfaces.
   [edit logical-systems LS3 protocols isis]user@R1# set interface lt-1/2/0.3user@R1# set interface lt-1/2/0.5user@R1# set interface lo0.3 passive

3. Configure the default route on Logical System LS3.

   [edit logical-systems LS3 routing-options]user@R1# set static route 0.0.0.0/0 discard user@R1# set static route 0.0.0.0/0 no-install

4. Configure the default route policy on Logical System LS3.

   [edit logical-systems LS3 policy-options]user@R1# set policy-statement isis-default from protocol static user@R1# set policy-statement isis-default from route-filter 0.0.0.0/0 exact user@R1# set policy-statement isis-default then accept

5. Apply the export policy to IS-IS on Logical System LS3.

   [edit logical-systems LS3 protocols isis]user@R1# set export isis-default

6. If you are done configuring the device, commit the configuration.

   [edit]user@R1# commit

Results

From configuration mode, confirm your configuration by issuing the show logical-systems LS3 command. If the output does not display the intended configuration, repeat the instructions in this example to correct the configuration.

Verification

Confirm that the configuration is working properly.

Verifying That the Static Route Is Redistributed

Purpose

Make sure that the IS-IS policy is working by checking the routing tables.

Action

inet.0: 6 destinations, 6 routes (6 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

0.0.0.0/0          *[Static/5] 00:00:45
                      Discard
10.0.0.0/30        *[IS-IS/15] 1w0d 10:14:14, metric 20
                      to 10.0.2.2 via lt-1/2/0.3
                    > to 10.0.1.2 via lt-1/2/0.5
10.0.1.0/30        *[Direct/0] 1w0d 10:15:18
                    > via lt-1/2/0.5
10.0.1.1/32        *[Local/0] 1w0d 10:15:18
                      Local via lt-1/2/0.5
10.0.2.0/30        *[Direct/0] 1w0d 10:15:18
                    > via lt-1/2/0.3
10.0.2.1/32        *[Local/0] 1w0d 10:15:18
                      Local via lt-1/2/0.3

iso.0: 1 destinations, 1 routes (1 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

49.0001.1234.1600.2231/72                
                   *[Direct/0] 1w0d 10:17:19
                    > via lo0.3

inet.0: 6 destinations, 6 routes (6 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

0.0.0.0/0          *[IS-IS/160] 00:01:38, metric 10
                    > to 10.0.2.1 via lt-1/2/0.4
10.0.0.0/30        *[Direct/0] 1w0d 10:16:11
                    > via lt-1/2/0.1
10.0.0.2/32        *[Local/0] 1w0d 10:16:11
                      Local via lt-1/2/0.1
10.0.1.0/30        *[IS-IS/15] 1w0d 10:15:07, metric 20
                    > to 10.0.0.1 via lt-1/2/0.1
                      to 10.0.2.1 via lt-1/2/0.4
10.0.2.0/30        *[Direct/0] 1w0d 10:16:11
                    > via lt-1/2/0.4
10.0.2.2/32        *[Local/0] 1w0d 10:16:11
                      Local via lt-1/2/0.4

iso.0: 1 destinations, 1 routes (1 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

49.0001.1720.1600.2002/72                
                   *[Direct/0] 1w0d 10:18:12
                    > via lo0.2

inet.0: 6 destinations, 6 routes (6 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

0.0.0.0/0          *[IS-IS/160] 00:02:01, metric 10
                    > to 10.0.1.1 via lt-1/2/0.0
10.0.0.0/30        *[Direct/0] 1w0d 10:16:34
                    > via lt-1/2/0.2
10.0.0.1/32        *[Local/0] 1w0d 10:16:34
                      Local via lt-1/2/0.2
10.0.1.0/30        *[Direct/0] 1w0d 10:16:34
                    > via lt-1/2/0.0
10.0.1.2/32        *[Local/0] 1w0d 10:16:34
                      Local via lt-1/2/0.0
10.0.2.0/30        *[IS-IS/15] 1w0d 10:15:55, metric 20
                      to 10.0.1.1 via lt-1/2/0.0
                    > to 10.0.0.2 via lt-1/2/0.2

iso.0: 1 destinations, 1 routes (1 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

49.0001.1720.1600.1001/72                
                   *[Direct/0] 1w0d 10:18:35
                    > via lo0.1

Meaning

The routing table on Logical System LS3 contains the default 0.0.0.0/0 route from protocol Static. The routing tables on Logical System LS1 and Logical System LS2 contain the default 0.0.0.0/0 route from protocol IS-IS. If Logical System LS1 and Logical System LS2 receive packets destined for networks not specified in their routing tables, those packets will be sent to Logical System LS3 for further processing. This configuration assumes that Logical System LS3 has a connection to an ISP or another external network.