Configuring IPv4 IS-IS to Import External Routes

If devices in an IS-IS routing domain need to learn external routes, configure IS-IS on a border device of this routing domain to import external routes.

Context

If a border device is configured to advertise the default route, all traffic destined beyond the routing domain is forwarded by this border device because the other devices in the IS-IS routing domain do not have external routes. As a result, this border device will be overloaded.

If multiple border devices are deployed, optimal routes to other routing domains need to be selected. To ensure optimal routes are selected, all the other devices in the IS-IS routing domain must learn all or some external routes.

Routing policies can be configured to import or advertise external routes that meet specified conditions to the IS-IS routing domain.

Procedure

Configure IS-IS to import external routes.
1. Run system-view
  The system view is displayed.
2. Run isis [ process-id ]
  The IS-IS view is displayed.
3. Configure IS-IS to import external routes.
  - To import external routes and set a cost for them, run the import-route { direct | static | unr | { ospf | rip | isis } [ process-id ] | bgp [ permit-ibgp ] } [ cost-type { external | internal } | cost cost | tag tag | route-policy route-policy-name | [ level-1 | level-2 | level-1-2 ] ] ^* command or run the import-route { ospf | isis } [ process-id ] [ cost-type { external | internal } | cost cost | tag tag | route-policy route-policy-name | [ level-1 | level-2 | level-1-2 ] | no-sid ] ^* command to import external routes to IS-IS..
  - To import external routes and keep the original costs, run the import-route { { ospf | rip | isis } [ process-id ] | bgp [ permit-ibgp ] | direct } inherit-cost [ tag tag | route-policy route-policy-name | [ level-1 | level-2 | level-1-2 ] ] ^* command or run the import-route { ospf | isis } [ process-id ] inherit-cost [ { level-1 | level-2 | level-1-2 } | tag tag | route-policy route-policy-name | no-sid ] ^* command to import external routes to IS-IS. When configuring IS-IS to retain the original costs of imported routes, the source routes cannot be static.
  IS-IS advertises all imported external routes to an IS-IS routing domain by default.
  
  If only some imported external routes need to be advertised to the IS-IS routing domain, run the filter-policy export command to set a filtering policy.
4. Run commit
  The configuration is committed.
Configure IS-IS to advertise some external routes to an IS-IS routing domain.
1. Run system-view
  The system view is displayed.
2. Run isis [ process-id ]
  The IS-IS view is displayed.
3. Configure IS-IS to advertise specified external routes to the IS-IS routing domain.
  Run any of the following commands as required:
  - Based on the basic ACL:
    1. Run filter-policy { acl-number | acl-name acl-name } export [ protocol [ process-id ] ]
      
      IS-IS is configured to filter the routes to be advertised.
    2. Run quit
      
      Return to the system view.
    3. Run acl { name basic-acl-name { basic | [ basic ] number basic-acl-number } | [ number ] basic-acl-number } [ match-order { config | auto } ]
      
      The ACL view is displayed.
    4. Run rule [ rule-id ] [ name rule-name ] { deny | permit } [ fragment-type { fragment | non-fragment | non-subseq | fragment-subseq | fragment-spe-first } | source { source-ip-address { source-wildcard | 0 | src-netmask } | any } | time-range time-name | vpn-instance vpn-instance-name ] ^*
      
      The rule for the ACL is configured.
      
      When the rule command is run to configure rules for a named ACL, only the source address range specified by source and the time period specified by time-range are valid as the rules.
      When a filtering policy of a routing protocol is used to filter routes:
      If the action specified in an ACL rule is permit, a route that matches the rule will be received or advertised by the system.
      
      If the action specified in an ACL rule is deny, a route that matches the rule will not be received or advertised by the system.
      
      If a route has not matched any ACL rules, the route will not be received or advertised by the system.
      
      If an ACL does not contain any rules, all routes matching the route-policy that references the ACL will not be received or advertised by the system.
      
      In the configuration order, the system first matches a route with a rule that has a smaller number and then matches the route with a rule with a larger number. Routes can be filtered using a blacklist or a whitelist:
      
      Route filtering using a blacklist: Configure a rule with a smaller number and specify the action deny in this rule to filter out the unwanted routes. Then, configure another rule with a larger number in the same ACL and specify the action permit in this rule to receive or advertise the other routes.
      
      Route filtering using a whitelist: Configure a rule with a smaller number and specify the action permit in this rule to permit the routes to be received or advertised by the system. Then, configure another rule with a larger number in the same ACL and specify the action deny in this rule to filter out unwanted routes.
  - Based on the IP prefix:
    
    Run filter-policy ip-prefix ip-prefix-name export [ protocol [ process-id ] ]
  - Based on the route policy:
    
    Run filter-policy route-policy route-policy-name export [ protocol [ process-id ] ]
  After this command is run, only external routes that meet the specified conditions can be advertised to the IS-IS routing domain.
4. Run commit
  The configuration is committed.