CCNP BSCI Exam: Introduction to the Intermediate System-to-Intermediate System (IS-IS) Protocol

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CCNP Self-Study: Building Scalable Cisco Internetworks (BSCI)

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This chapter is from the book

This chapter is from the book 

CCNP Self-Study: Building Scalable Cisco Internetworks (BSCI)

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Basic Integrated IS-IS Router Configuration

This section covers the commands used to configure and troubleshoot Integrated IS-IS on a Cisco router.

Integrated IS-IS Configuration

This section identifies the steps and the basic commands used to configure Integrated IS-IS on a Cisco router.

Integrated IS-IS Configuration Steps

The following steps should be taken when configuring Integrated IS-IS:

Basic Integrated IS-IS Configuration Commands

To enable Integrated IS-IS on a router for IP routing, you need only three commands, as described in this section. Note that there are many more commands to tune the IS-IS processes, but only three are required to start Integrated IS-IS.

The router is-is [tag] global configuration command enables Integrated IS-IS on the router. The optional tag can be used to identify multiple IS-IS processes by giving a meaningful name for a routing process. If it is not specified, a null tag (0) is assumed and the process is referenced with a null tag. This name must be unique among all IP router processes for a given router.

After the Integrated IS-IS process is enabled, the router must be identified for IS-IS by assigning a NET to the router with the net network-entity-title router configuration command. In this command, network-entity-title is the NET that specifies the area address and the system ID for the IS-IS routing process. This argument can be either an address or a name.

Finally, interfaces that are to use IS-IS to distribute their IP information (and additionally might be used to establish IS-IS adjacencies) must be configured using the ip router isis [tag] interface configuration command. If there is more than one IS-IS process on the router (as specified using the router isis command), interfaces must state which IS-IS process they belong to by specifying the appropriate tag.

Other Integrated IS-IS Configuration Commands

By default, Cisco IOS Software enables both Level 1 and Level 2 operations on IS-IS routers. If a router is to operate only as an area router or only as a backbone router, this can be specified by entering the is-type {level-1 | level-1-2 | level-2-only} router configuration command. This command is described in Table 5-4. To specify that the router will act only as an area (or Level 1) router, use level-1. To specify that the router will act only as a backbone (or Level 2) router, use level-2-only.

Table 5-4 is-type Command Description

Similarly, although the router might be a Level 1–2 router, it might be required to establish Level 1 adjacencies only over certain interfaces and Level 2 adjacencies over other interfaces. The isis circuit-type {level-1 | level-1-2 | level-2-only} interface configuration command can be used to specify either Level 1– or Level 2–only interfaces. This command is described in Table 5-5. Because the default is Level 1–2, Cisco IOS Software attempts to establish both types of adjacency over the interface if this command is not specified.

Table 5-5 isis circuit-type Command Description

Unlike some other IP protocols, IS-IS takes no account of line speed or bandwidth when setting its link metrics. All interfaces are assigned a metric of 10 by default. To change this value, you need to use the isis metric default-metric {level-1 | level-2} interface configuration command. The metric can have different values for Level 1 and Level 2 over the same interface. This command is described in Table 5-6.

Table 5-6 isis metric Command Description

To define a name-to-NSAP mapping that can then be used with commands requiring NSAPs, use the clns host name nsap global configuration command. The assigned NSAP name is displayed, where applicable, in show and debug EXEC commands. This command is described in Table 5-7.

Table 5-7 clns host Command Description

Use the summary-address address mask {level-1 | level-1-2 | level-2} prefix mask router configuration command to create aggregate addresses for IS-IS or OSPF. The no summary-address command restores the default. This command is described in Table 5-8.

Table 5-8 summary-address Command Description

To configure the priority of designated routers, use the isis priority value {level-1 | level-2} interface configuration command. To reset the default priority, use the no form of this command. This command is described in Table 5-9.

Table 5-9 isis priority Command Description

Integrated IS-IS Configuration Examples

This section includes some example configurations of Integrated IS-IS. The first example shows the minimum commands required to run Integrated IS-IS, while the second example shows a two-area configuration.

Basic Integrated IS-IS Configuration Example

Example 5-10 shows a simple Integrated IS-IS configuration, specifying only the IS-IS process and the NET, and enabling IS-IS on the interfaces. The router with this configuration acts as an IP-only Level 1–2 router.

Example 5-10 Basic Integrated IS-IS Configuration

interface ethernet 0
ip address 10.1.1.1 255.255.255.0
ip router isis
!
interface serial 0
ip address 10.1.2.1 255.255.255.0
ip router isis

!
router isis
net 01.0001.0000.0000.0002.00

Two-Area Integrated IS-IS Configuration Example

This example shows how to configure a simple two-area IS-IS network, optimizing the Level 1 and Level 2 operations of the links and routers. Figure 5-20 shows the network used in this example.

Figure 5-20 Two-Area Integrated IS-IS Network

In Figure 5-20, router R1 is in area 49.0001 with no links outside that area and, therefore, needs to operate only as a Level 1 router. The configuration for R1 is shown in Example 5-11.

Example 5-11 Configuration of Router R1 in Figure 5-20

hostname R1

!
interface Serial0
ip address 192.168.120.1 255.255.255.0
ip router isis
!
router isis
net 49.0001.1921.6800.1005.00

is-type level-1

The is-type level-1 command under router isis in Example 5-11 ensures that the router creates only a Level 1 database and takes part only in Level 1 adjacencies over its interfaces. Note that it is not necessary to also specify the isis circuit-type command on the interfaces in this case—Level 1 on the interfaces is implied by setting the IS-IS process as Level 1 only.

Router R2 in Figure 5-20 is a member of area 49.0001 but also connects that area with the neighboring area 49.0002. Thus, R2 is required to act as both a Level 1 and a Level 2 router. This is the default operation of the is-type command, so this command is not required in the router definition. The configuration of R2 is shown in Example 5-12.

Example 5-12 Configuration of Router R2 in Figure 5-20

hostname R2
interface Ethernet0
ip address 192.168.220.2 255.255.255.0
ip router isis
isis circuit-type level-2-only
!
interface Serial0
ip address 192.168.120.2 255.255.255.0
ip router isis
isis circuit-type level-1
!
router isis
net 49.0001.1921.6800.1006.00

To optimize the operation of the interfaces to its two neighbors, router R2 in this example includes configuration to specify the type of adjacency to be established. Serial 0, the interface toward router R1 (in the same area), specifies isis circuit-type level-1, while the Ethernet 0 interface, the interface toward router R3 (in a different area, and therefore Level 2), has isis circuit-type level-2-only specified.

Troubleshooting Integrated IS-IS

This section identifies commands that can be used to troubleshoot Integrated IS-IS on a Cisco router.

CLNS Troubleshooting Commands

Troubleshooting Integrated IS-IS, even in an IP-only network, requires some investigation of CLNS data. For example, the IS-IS neighbor relationships are established over OSI, not over IP, so showing IS-IS neighbors requires using the show clns neighbors command (as described later in this section). Indeed, two ends of a CLNS adjacency can actually have IP addresses on different subnets, with no impact to the operation of IS-IS (although IP next-hop resolution could be an issue).

Some of the CLNS troubleshooting commands are shown in the context of an earlier example, in the section, "CLNS Troubleshooting Commands Used in This Example." In this section, more CLNS show commands are described.

The show clns EXEC command displays general information about the CLNS network.

The show clns protocol [domain | area-tag] EXEC command displays information for the specific IS-IS processes in the router. This command is described in Table 5-10.

Table 5-10 show clns protocol Command Description

The show clns interface [type number] EXEC command displays CLNS-specific information about the interfaces running IS-IS. This command is described in Table 5-11.

Table 5-11 show clns interface Command Description

The show clns neighbors [type number] [detail] EXEC command is very useful because it displays the neighbor ISs—that is, the routers with which this router has IS-IS adjacencies. (ES neighbors, if there are any, are also displayed.) This command is described in Table 5-12.

Table 5-12 show clns neighbors Command Description

The optional keyword detail in the show clns neighbors command displays comprehensive information about the neighbors rather than listing a summary of the neighbors, as is the case without that keyword specified. The list of neighbors can be narrowed to those neighbors across a particular interface by specifying the interface type and number parameters in the command.

To display IS-IS–related information for IS-IS router adjacencies, use the show clns is-neighbors [type number] [detail] EXEC command. Neighbor entries are sorted according to the area in which they are located. This command is described in Table 5-13.

Table 5-13 show clns is-neighbors Command Description

CLNS and IS-IS Troubleshooting Commands

Further commands to troubleshoot the Integrated IS-IS network are described in this section.

The show isis route EXEC command displays the IS-IS Level 1 routing table (that is, routes to all other system IDs in the area). An example output from this command is shown earlier, in the section "Example of OSI (CLNS) Intra-Area and Interarea Routing."

The show clns route [nsap] EXEC command displays the IS-IS Level 2 routing table (as well as static and ISO-IGRP–learned prefix routes). More details on this command and an example output are shown earlier, in the section, "Example of OSI (CLNS) Intra-Area and Interarea Routing."

The show isis database [level-1] [level-2] [l1] [l2] [detail] [lspid] EXEC command displays the contents of the IS-IS link-state database. This command is described in Table 5-14.

Table 5-14 show isis database Command Description

To force IS-IS to refresh its link-state database and recalculate all routes, issue the clear isis [tag | *] command, specifying the IS-IS process tag or * to clear all IS-IS entries.

To display how often and why the router has run a full SPF calculation, use the show isis spf-log EXEC command.

IP Troubleshooting Commands

To troubleshoot the IP functionality of the Integrated IS-IS network, you can use standard IP show commands.

The show ip protocols EXEC command displays the active routing protocols, what interfaces they are active on, what networks they are routing for, and other parameters related to the routing protocols.

The show ip route [address [mask] [longer-prefixes]] | [protocol [process-id]] command displays the IP routing table. The detail for a particular route or a list of all routes in the routing table from a particular process can be specified. This command is described in detail earlier in the section, "Example of OSI (CLNS) Intra-Area and Interarea Routing."

Integrated IS-IS Troubleshooting Command Output Examples

The network used to obtain these example outputs is the same as the one used for the earlier configuration examples shown in Figure 5-20. The network is shown again in Figure 5-21, with the IP addresses added.

Figure 5-21 Two-Area Integrated IS-IS Network with IP Addressing

The system IDs of the routers in Figure 5-21 are as follows:

• Router R1: 1921.6800.1005

• Router R2: 1921.6800.1006

• Router R3: 1921.6800.1007

• Router R4: 1921.6800.1008

Example 5-13 shows the show clns protocol command output from router R2 in Figure 5-21.

Example 5-13 show clns protocol Command Output from Router R2 in Figure 5-21

R2#show clns protocol
IS-IS Router: <Null Tag>
System Id: 1921.6800.1006.00 IS-Type: level-1-2
Manual area address(es):
49.0001
Routing for area address(es):
49.0001
Interfaces supported by IS-IS:
Serial0 - IP
Ethernet0 - IP
Redistributing:
static
Distance: 110

Example 5-13 shows the following:

• The Integrated IS-IS process tag (if present). Router R2 has a null tag.

• The system ID, level type(s), and area ID for this router. Router R2's system ID is 1921.6800.1006.00, it is a Level 1–2 router, and it is in area 49.0001. (Note that in this command, the System ID includes both the System ID field and the NSEL field, in this case 00).

• The interfaces using Integrated IS-IS for routing (including whether that is for IP or CLNS or both). On router R2, interfaces Serial 0 and Ethernet 0 are using Integrated IS-IS for IP routing only.

• Any redistribution of other route sources. Router R2 is redistributing static routes, by default.

• The administrative distance for CLNS routes. This is similar to the administrative distances used for IP routing. For IS-IS in a CLNS environment, the default administrative distance is 110, as shown for router R2 in this example. (Use the distance value [clns] router configuration command to change this default.)

Example 5-14 shows the show clns neighbors command output from routers R1 and R2.

Example 5-14 show clns neighbors Command Output from Routers R1 and R2 in Figure 5-21

R1#show clns neighbors
System Id      Interface SNPA           State Holdtime Type Protocol
1921.6800.1006 Se0       *HDLC*         Up    28       L1   IS-IS
R2#show clns neighbors
System Id      Interface SNPA           State Holdtime Type Protocol
1921.6800.1007 Et0       0010.7b81.d6ec Up    24       L2   IS-IS
1921.6800.1005 Se0       *HDLC*         Up    21       L1   IS-IS

Example 5-14 indicates the following:

• The IS-IS neighbors. Router R1 has one neighbor, router R2; Router R2 has two neighbors, routers R1 and R3.

• The neighbors SNPA and state.

• The hold time, the number of seconds before this adjacency entry times out. It indicates the remaining time to wait for receipt of a hello before the neighbor is declared down.

• The neighbors' level and type. Router R1 sees R2 as a Level 1 router. Router R2 sees R1 as a Level 1 router and R3 as a Level 2 router.

Example 5-15 shows the show clns interface command output from router R2.

Example 5-15 show clns interface Command Output from Router R2 in Figure 5-21

R2#show clns interface s0
Serial0 is up, line protocol is up
Checksums enabled, MTU 1500, Encapsulation HDLC
ERPDUs enabled, min. interval 10 msec.
RDPDUs enabled, min. interval 100 msec., Addr Mask enabled
Congestion Experienced bit set at 4 packets
CLNS fast switching enabled
CLNS SSE switching disabled
DEC compatibility mode OFF for this interface
Next ESH/ISH in 21 seconds
Routing Protocol: IS-IS
Circuit Type: level-1
Interface number 0x1, local circuit ID 0x100
Level-1 Metric: 10, Priority: 64, Circuit ID: 1921.6800.1006.00
Number of active level-1 adjacencies: 1
Next IS-IS Hello in 7 seconds

Example 5-15 indicates the following:

• That the Serial 0 interface is running IS-IS and is attempting to establish only Level 1 adjacencies

• The interface numbers and circuit ID for IS-IS purposes

• The metric(s) for the interface and a priority for DIS negotiation (not relevant in this case because it is a serial interface)

• Information about hello timers and the number of adjacencies that have been established

Example 5-16 shows the show ip protocols command output from router R2.

Example 5-16 show ip protocols Command Output from Router R2 in Figure 5-21

R2#show ip protocols
Routing Protocol is "isis"
Sending updates every 0 seconds
Invalid after 0 seconds, hold down 0, flushed after 0
Outgoing update filter list for all interfaces is
Incoming update filter list for all interfaces is
Redistributing: isis
Address Summarization:
None
Routing for Networks:
Ethernet0
Serial0
Routing Information Sources:
Gateway       Distance Last Update
192.168.120.1 115      00:04:53
192.168.220.1 115      00:04:58
Distance: (default is 115)

Example 5-16 shows that Integrated IS-IS is running on router R2. It also shows the interfaces taking part in Integrated IS-IS and the sources of routing information (the neighbor routers). The default IP administrative distance for Integrated IS-IS is 115.

Example 5-17 shows the show ip route isis command output from routers R1 and R2. This command displays only the IS-IS routes from the IP routing table.

Example 5-17 show ip route isis Command Output from Routers R1 and R2 in Figure 5-21

R1#show ip route isis
i*L1 0.0.0.0/0 [115/10] via 192.168.120.2,
Serial0
R2#show ip route isis
i L2 192.168.230.0/24 [115/20] via 192.168.220.1,
Ethernet0

The route in Example 5-17 on router R1 is from Level 1, as indicated by the i L1 tag; this is a default route to router R2. The route in Example 5-17 on router R2 is from Level 2, as indicated by the i L2 tag. As for all IP routes, the administrative distance and the metric are shown in square brackets, as in [115/20]. Integrated IS-IS has an IP administrative distance of 115, by default. The metric shown for each route is the IS-IS cost to the destination.

Example 5-18 shows the show clns command output from router R2.

Example 5-18 show clns Command Output from Router R2 in Figure 5-21

R2#show clns
Global CLNS Information:
2 Interfaces Enabled for CLNS
NET: 49.0001.1921.6800.1006.00
Configuration Timer: 60, Default Holding Timer: 300,
Packet Lifetime 64
ERPDU's requested on locally generated
packets
Intermediate system operation enabled (forwarding
allowed)
IS-IS level-1-2 Router:
Routing for Area: 49.0001

Example 5-18 shows that router R2 has two interfaces enabled for CLNS and that it is a Level 1–2 router in area 49.0001.

Example 5-19 shows the show clns is-neighbors command output from routers R1 and R2.

Example 5-19 show clns is-neighbors Command Output from Routers R1 and R2 in Figure 5-21

R1#show clns is-neighbors
System Id Interface State Type Priority Circuit Id Format
1921.6800.1006 Se0 Up L1 0 00 Phase V
R2#show clns is-neighbors

System Id Interface State Type Priority Circuit Id Format
1921.6800.1007 Et0 Up L2 64 1921.6800.1006.0Phase V
1921.6800.1005 Se0 Up L1 0 00 Phase V

As shown in Example 5-19, router R1 has one IS neighbor, router R2. Router R2 has two IS neighbors, routers R1 and R3.

Example 5-20 shows the show isis databse command output from router R2.

Example 5-20 show isis database Command Output from Router R2 in Figure 5-21

R2#show isis database
IS-IS Level-1 Link State Database
LSPID LSP Seq Num LSP Checksum LSP Holdtime ATT/P/OL
1921.6800.1005.00-00 0x00000004 0x485B 936 0/0/0
1921.6800.1006.00-00* 0x00000005 0x2E18 1155 1/0/0
1921.6800.1006.01-00* 0x00000001 0xFC74 462 0/0/0

IS-IS Level-2 Link State Database
LSPID LSP Seq Num LSP Checksum LSP Holdtime ATT/P/OL
1921.6800.1006.00-00* 0x00000003 0x28FA 1180 0/0/0
1921.6800.1006.01-00* 0x00000002 0x7C36 1196 0/0/0
1921.6800.1007.00-00 0x00000003 0xF3BF 462 0/0/0

Example 5-20 shows that router R2, a Level 1–2 router, has two separate databases, one for Level 1 and another for Level 2.

Example 5-21 shows the show isis spf-log command output from router R2. This command displays how often and why the router has run a full SPF calculation.

Example 5-21 show isis spf-log Command Output from Router R2 in Figure 5-21

R2#show isis spf-log

Level 1 SPF log
When     Duration Nodes Count Last trigger    LSP Triggers
00:17:52 0        1     4     1921.6800.1006.00-00 NEWAREA NEWADJ NEWLSP TLVCONTENT
00:17:47 4 2 1 1921.6800.1005.00-00 TLVCONTENT
00:12:24 4 3 2 1921.6800.1006.01-00 NEWLSP TLVCONTENT
00:12:13 4 3 2 1921.6800.1006.00-00 ATTACHFLAG LSPHEADER
00:04:32 4 3 1 PERIODIC

Level 2 SPF log
When Duration Nodes Count Last trigger LSP Triggers
00:17:53 0 1 1 1921.6800.1006.00-00 NEWLSP
00:12:24 4 2 3 1921.6800.1006.01-00 NEWADJ NEWLSP TLVCODE
00:12:19 4 3 1 1921.6800.1007.00-00 NEWLSP
00:04:33 8 3 1 PERIODIC

As Example 5-21 shows, router R2 keeps separate logs for the Level 1 and Level 2 SPF algorithm.