Redistributing Routes Between Levels; Figure 20: Example Of Level 1 And Level 2 Routing - Juniper IGP - CONFIGURATION GUIDE V11.1.X Configuration Manual

JUNOSe 11.0.x IP, IPv6, and IGP Configuration Guide

Redistributing Routes Between Levels

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
on page 362.

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.
362
Configuring Global IS-IS Parameters
Optionally, you can specify a route map and use it to set a route tag for routes
redistributed to IS-IS.
Example 1 Redistributing static IP routes with a route map
host1(config-router)#redistribute static ip route-map 10
Example 2 Redistributing IPv6 routes from OSPF into IS-IS level 1 and level 2
host1(config-router-af)#redistribute ospf level-1-2
Use the no version to disable redistribution.
See redistribute