Suboptimal Path in IS-IS routing

http://www.juniper.net/techpubs/en_US/junose10.0/information-products/topic-collections/swconfig-ip-ipv6/redistributing-routes-between-levels.html

The two-level routing hierarchy of IS-IS can lead to suboptimal path selection in certain situations. Because a level 1 router by default has knowledge only of level 1 routes, traffic from a level 1 router to a router in another area passes through the nearest level 1-2 router as its next hop. Consider the topology shown in Figure 20.

Figure 20: Example of Level 1 and Level 2 Routing

In this example, Router 4 in Area 1 considers Router 2 to be its next hop for interarea traffic, and Router 5 considers Router 3 to be its next hop for interarea traffic. Traffic from Router 4 to Router 8 passes through Router 2, requiring a total of five hops to the destination: Routers 2, 1, 3, 9, and 8. Similarly, five hops are required for traffic from Router 5 to Router 7.

Neither of these paths is optimal. For example, it would be shorter for traffic from Router 4 to take the four-hop path: Routers 5, 3, 9, and 8.

You can configure IS-IS to redistribute routes between the routing levels; this is sometimes known as route leaking between levels. The redistribute isis ip command enables you to specify a route filter (an access list) and the direction of leakage, as shown in the following example:

host1(config)#access-list leakList permit ip 100.0.0.0 0.255.255.255 any

host1(config)#router isis 1

host1(config-router)#redistribute isis ip level-1 into level-2 distribute-list leakList

host1(config-router)#redistribute isis ip level-2 into level-1 distribute-list leakList

When you issue the redistribute isis ip command and include the route-map keyword, you can use the map to set a route tag for a route redistributed from one IS-IS level to another. Make sure the route map you specify includes the set tag command that defines a tag value for the IS-IS routes to be redistributed. For details about configuring and using route maps, see JUNOSe IP Services Configuration Guide.

To redistribute IPv6 routes from one IS-IS level to another, use the redistribute isis command from within the IS-IS IPv6 address family.

redistribute isis

• Use to redistribute IS-IS IPv6 routes from level 1 to level 2 or from level 2 to level 1.
• Use the route-map keyword to specify the route map to be applied. You can use the route map to set a route tag for redistributed routes.
• Example

  host1(config-router-af)#redistribute isis level-1 into level-2

• Use the no version to stop redistribution of IPv6 routes between the specified levels.
• See redistribute isis

redistribute isis ip

• Use to redistribute IS-IS IP routes from level 1 to level 2 or from level 2 to level 1.
• Specify one of the following:
  • Use the distribute-list keyword to specify the IP access list used to filter routes between levels. Issue the access list command to create a route filter to apply to the redistribution.
  • Use the route-map keyword to specify the route map to be applied. You can use the route map to set a route tag for redistributed routes.
• Example 1—Redistributes IS-IS IP routes between levels, filtered by an access list.

  host1(config-router)#redistribute isis ip level-1 into level-2 distribute-list leakList

• Example 2—Redistributes IS-IS IP routes between levels, filtered by a route map.

  host1(config-router)#redistribute isis ip level-2 into level-1 route-map boston01

• Use the no version to stop redistribution of IP routes between the specified levels.
• See redistribute isis ip