Content

| 7.4 |  | IP Routing with Integrated IS-IS |
| 7.4.3 |  | Intra-area and interarea Integrated IS-IS routing example |

An array of useful IS-IS commands in the context of the pictured topology will be introduced and analyzed.

The show isis topology command will be explored first. The show isis topology command displays the least-cost paths to the destination NETs. The system ID shows the NET of the destination. IOS uses Dynamic Hostname Mapping, see RFC 2763, to map this system ID to a hostname, when that hostname is available to the router. The hostname of the router is included in its outgoing LSP.

The metric shows the sum of the metrics on the least-cost path to the destination. The next hop router, IS, is shown. Also shown is the interface through which that next hop is reached and the SNPA of that next hop. HDLC is shown as the next hop across a serial line. The output for Router R2 shows that separate topology databases exist for L1 and L2.

Recall that the SNPA is taken from the following:

- MAC address on a LAN interface

- Virtual Circuit ID for X.25 or ATM

- DLCI for Frame Relay

- HDLC for High-Level Data Link Control interfaces

The show clns route and show isis route commands are the next commands to be looked at. There is a common source of confusion for those learning Integrated IS-IS. That confusion comes from the frequent use of commands directly referencing CLNS, but used for the purpose of verifying and troubleshooting IP routing.

The show clns route command displays the CLNS destinations to which this router can route packets. R1 shows only its local NET entry, because it is an L1 only router and therefore has no L2 area routes to display. The show isis route command shows the L1 routes to IS-IS neighbors. R1 has visibility of the other L1 routers in its area. The L1L2 routers appear in the L1 routing table by virtue of their L1 connection. There is a note at the end of their entry to show that they also act as L2. The closest L1L2 router also appears as the default route out of area. Again, the next-hop IS, its SNPA, is the interface over which that next hop is reached. The cumulative metric to that destination is shown for all IS routes. The neighbors show that their state is ’up’ and the Hello process has established an adjacency.

The command show clns route shows the local NET entry. This command also shows the L2 routes to its own area and the neighbor areas. Notice that L2 regards the route to the area of R2 as being through itself. This further emphasizes that the L1 and L2 processes operate separately. The command show isis routes shows the IS-IS neighbors.
Next, the which-route command in the context of L1 and L2 will be explored. The which-route command is an alternative method of finding the route to a destination NET or NSAP. The command is entered on the L1 only router, R1.The command returns the next hop to the destination and states whether the destination is reachable by L1 or by the default exit point to L2. Executing the which-route command on an L2 router specifies the next hop. It also states that the route was matched by an entry from the CLNS L2 routing table.

Building the IP Forwarding Table
So far, the process and outputs have referred to the OSI part of the IS-IS process. These are the same as for pure OSI IS-IS routing. However, in the IP world, when running Integrated IS-IS, IP information is included in the LSPs. IP reachability behaves in IS-IS as if it were ES information. IP information takes no part in the calculation of the SPF tree. It is simply information about leaf connections to the tree. Therefore, updating the IP reachability is only a PRC. This is similar to ES reachability. IP routes are generated by the PRC and offered to the routing table. Here they will be accepted based on routing table rules comparing, for example, administrative distance. When entered in the routing table, IP IS-IS routes are shown as being by way of Level 1 or Level 2, as appropriate. The separation of IP reachability from the core IS-IS network architecture gives Integrated IS-IS better scalability than, for example, OSPF. OSPF sends LSAs for individual IP subnets. If an IP subnet fails, then the LSA is flooded through the network. In all circumstances, all routers must run a full SPF calculation. In Integrated IS-IS, the SPF tree is built from CLNS information. If an IP subnet fails in Integrated IS-IS, the LSP is flooded as it is for OSPF. However, if this is a leaf IP subnet, meaning the loss of the subnet has not affected the underlying CLNS architecture, then the SPF tree is unaffected. Only a PRC will take place.

The IP routing table, a pure entity, will now be looked at. The output for the show ip route command shows the IS-IS routes chosen by the SPF algorithm, from the IS-IS LSP database, to populate the IP routing table. The “i” indicates that the route was sourced from IS-IS. "L1" and "L2" show whether the IS-IS path to these destination IP networks is by way of IS-IS L1 or L2 routing. The next-hop IP addresses are matched from the corresponding next-hop IS-IS neighbor routers. Notice the metric is ten (10) for each route because ten is the Cisco default for the IS-IS metric over a link.

| Lab Activity e-Lab Activity: IS-IS Show Commands This lab is to analyze the running IS-IS router process, using IS-IS show commands.