between routers in an IS-IS network.
Content 4.4 Configuring Basic Integrated IS-IS 4.4.1 Integrated IS-IS in an CLNS Environment The commands required to configure Integrated IS-IS are slightly different than those of the other IP routing protocols that we have covered so far. It is important to understand how to enable IS-IS processes. Additionally, the default settings for IS-IS can result in an inefficient use of router and network resources and suboptimal routing, so a network administrator needs to know how to tune IS-IS to achieve optimum performance. Even when IS-IS is used to support IP exclusively, network devices must still be configured to use the OSI CLNS protocol. Because Integrated IS-IS depends on the support of CLNS routing, each IS-IS router must have a NET address configured. IS-IS packets are directly encapsulated onto the data link layer instead of traveling inside IP packets. A NET address identifies a device (an intermediate system [IS] or end system [ES]) and not an interface. This is a critical difference between a NET address and an IP address. A device identifies other devices within its own area based on matching area addresses in their NET. The OSI protocols (hello PDUs) are used to form the neighbor relationship between routers, and the shortest path first (SPF) calculations rely on a configured NET address to identify the routers. When using IS-IS to route IP traffic, IP subnets are treated as leaf objects associated with an IS-IS area, and default routes are injected into areas by the Level 1–2 router. To route traffic, an IS-IS router looks up the destination network in its routing table. If the other device’s NET address area matches its own, the router does not forward traffic to the default route. If the area addresses do not match, the network belongs to a different area and the router forwards that traffic to its nearest Level 1–2 router. Note
This lesson focuses on how to configure IS-IS over IPv4. For an example of configuring IS-IS over IPv6, refer to:
http://www.cisco.com/en/US/tech/
tk365/technologies_configuration_
example09186a00801875b7.shtml
Content 4.4 Configuring Basic Integrated IS-IS 4.4.2 Building a Routing Table IS-IS requires a two-step process to build the routing table: When LSPs are exchanged between ISs, they contain NET addresses and IP information. IS-IS routers refer to their LSDB to calculate the SPF tree to the NET addresses of the various ISs and then record this information in the CLNS routing table. The output of the show isis database command in Figure displays the contents and format of a sample IS-IS LSDB. Notice that Level 1 and Level 2 routes have separate LSDB databases. Therefore, routers may run the SPF algorithm twice, once for each level, and create separate SPF trees for each level. Once the LSDB databases are synchronized, routers insert the shortest paths in the CLNS routing table. The total of the link metrics along each path determines the shortest path to any given destination. The sample output of the show clns route in Figure displays the CLNS destinations to which this router can route packets. Integrated IS-IS also includes IP information in the LSPs consisting of a 4-octet IP address plus a 4-octet subnet mask. However, the IP information simply identifies an ES or a leaf connection in the SPF tree. It takes no part in the calculation of the SPF tree. Updating the IP reachability requires only a partial route calculation (PRC), similar to ES reachability in an OSI network. The PRC generates best-path choices for IP routes and offers the routes to the IP routing table, where they are accepted based on normal IP routing table rules. For example, if there is more than one routing protocol running, the router compares administrative distance. When the IS-IS routes are entered in the IP routing table, they are shown as Level 1 or Level 2, as appropriate. This separation of IP reachability from the core IS-IS network architecture provides Integrated IS-IS better scalability than OSPF. OSPF sends LSAs for individual IP subnets. If an IP subnet fails, the LSA floods through the network, and all routers must run a full SPF calculation, which is extremely CPU-intensive. Integrated IS-IS builds the SPF tree from CLNS information. If an IP subnet fails, the IS-IS LSP floods through the network, which is the same for OSPF. However, if this is a leaf (stub) IP subnet (that is, if the loss of the subnet does not affect the underlying CLNS architecture), the SPF tree is unaffected, and only a PRC occurs.
Content 4.4 Configuring Basic Integrated IS-IS 4.4.3 Steps to Configuring Integrated IS-IS The basic setup of IS-IS requires four steps. Figure lists the commands for configuring Integrated IS-IS on a router for IP routing. Step 1 Map out the areas, plan the addressing, and determine which interfaces are to participate in the IS-IS routing process. Step 2 Enable IS-IS on the router with the router isis global configuration command. Step 3 Configure the NET address with the net router configuration command. Step 4 Enable Integrated IS-IS on the appropriate interfaces with the ip router isis interface configuration command. Additional commands are available for fine-tuning the configuration. Web Links Configuring Integrated IS-IS
http://www.cisco.com/en/US/products/sw/
iosswrel/ps1828/products_configuration_
guide_chapter09186a00800ca56f.html

Cisco IOS ISO CLNS Command Reference, Release 12.4
http://www.cisco.com/en/US/products/
ps6350/products_command_reference_
book09186a008042df10.html
Content 4.4 Configuring Basic Integrated IS-IS 4.4.4 Step 1: Define Area and Addressing Because all intra-area traffic in IS-IS must traverse the Level 2 backbone area, CLNS addresses must be planned to execute a two-level hierarchy. You must decide which routers will be backbone (Level 2) routers, which routers will be Level 1–2, and which will be internal area (Level 1) routers. If some routers must do both Level 1 and Level 2 routing, you should identify the specific interfaces that will participate in each type of routing. The CLNS address of a router (the NET) consists of three main parts: Along with planning the IS-IS area addressing, the IP addressing should be based on a hierarchical addressing design. IP addressing must be scalable and allow for summarization of addresses. This helps minimize routing update traffic and resource utilization. Be particularly careful when you configure the IP addressing on the router, since it is more difficult to troubleshoot IP address misconfigurations with IS-IS. The IS-IS neighbor relationships are established over OSI CLNS, not over IP. Because of this approach, two ends of a CLNS adjacency can have IP addresses on different subnets, with no impact to IS-IS operation.
Content 4.4 Configuring Basic Integrated IS-IS 4.4.5 Step 2: Enable IS-IS on