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Configuring IS-IS Protocol

Chapter Description

This supplemental material provides an overview of Intermediate System-to-Intermediate System (IS-IS) technology and its structures and protocols, along with basic configuration examples.

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:

Step 1

Define areas, prepare an addressing plan for the routers (including defining the NETs), and determine interfaces that will run Integrated IS-IS.

Step 2

Enable IS-IS as an IP routing protocol on the routers, and assign a tag to the process (if required).

Step 3

Configure the NETs on the routers. This identifies the routers for IS-IS.

Step 4

Enable Integrated IS-IS on the proper interfaces on the routers. Do not forget interfaces to stub IP networks, such as loopback interfaces (although there will not be any CLNS neighbors on these interfaces).


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.

NOTE

When routing of CLNS packets is also required, use the clns routing global configuration command. (This command was on by default on the routers used for testing, but the Cisco IOS Software documentation says that it is not on by default.)

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.

NOTE

Configuring Integrated IS-IS to run on an interface is slightly different than configuring interfaces for most other IP routing protocols. In most other protocols, the interfaces are defined by network commands in the router configuration mode. There is no network command under the router isis command.

When routing of CLNS packets is also required, use the clns router isis [tag] interface configuration command.

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 S-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 S-4 is-type Command Description

is-type Command

Description

level-1

Router acts as a station router. This router will learn about destinations only inside its area. For interarea routing, it depends on the closest Level 1–2 router.

level-1-2

Router acts as both a station router and an area router. This router will run two instances of the routing algorithm. This is the default.

level-2-only

Router acts as an area router only. This router is part of the backbone and does not talk to Level 1–only routers in its own area.


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 S-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 S-5 isis circuit-type Command Description

isis circuit-type Command

Description

level-1

A Level 1 adjacency might be established if there is at least one area address in common between this system and its neighbors. Level 2 adjacencies will never be established over this interface.

level-1-2

A Level 1 and Level 2 adjacency is established if the neighbor is also configured as level-1-2 and there is at least one area in common. If there is no area in common, a Level 2 adjacency is established. This is the default.

level-2-only

Level 2 adjacencies are established if the other routers are Level 2 or Level 1–2 routers and their interfaces are configured for Level 1–2 or Level 2. Level 1 adjacencies will never be established over this interface.


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 S-6.

Table S-6 isis metric Command Description

isis metric Command

Description

default-metric

Specifies the metric assigned to the link and used to calculate the cost from each other router through the links in the network to other destinations. You can configure this metric for Level 1 or Level 2 routing. The range is from 0 to 63. The default value is 10.

level-1

Specifies that this metric should be used only in the SPF calculation for Level 1 (intra-area) routing.

level-2

Specifies that this metric should be used only in the SPF calculation for Level 2 (interarea) routing.


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 S-7.

Table S-7 clns host Command Description

clns host Command

Description

name

Desired name for the NSAP. The first character can be either a letter or a number, but if you use a number, the operations that you can perform are limited.

nsap

NSAP to which that the name maps.


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 S-8.

Table S-8 summary-address Command Description

summary-address Command

Description

address

Summary address designated for a range of addresses.

mask

IP subnet mask used for the summary route.

level-1

Only routes redistributed into Level 1 are summarized with the configured address/mask value.

level-1-2

The summary route is applied both when redistributing routes into Level 1 and Level 2 IS-IS, and when Level 2 IS-IS–advertised Level 1 routes reachable in its area.

summary-address Command

Description

level-2

Routes learned by Level 1 routing are summarized into the Level 2 backbone with the configured address/mask value, and redistributed routes into Level 2 IS-IS are summarized also.

prefix

IP route prefix for the destination.

mask

IP subnet mask used for the summary route.


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 S-9.

Table S-9 isis priority Command Description

Isis priority Command

Description

value

Sets the priority of a router and is a number from 0 to 127. The default value is 64.

level-1

Sets the priority for Level 1 independently.

level-2

Sets the priority for Level 2 independently.


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 S-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 S-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 S-20 shows the network used in this example.

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

In Figure S-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 S-11.

Example S-11 Configuration of Router R1 in Figure S-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 S-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 S-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 S-12.

Example S-12 Configuration of Router R2 in Figure S-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.

NOTE

The interfaces on both routers R1 and R2 could also have had the isis metric command configured, to reflect the different metrics on the serial and Ethernet interfaces.

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 S-10.

Table S-10 show clns protocol Command Description

show clns protocol Command

Description

domain

(Optional) Particular ISO IGRP routing domain

area-tag

(Optional) Particular IS-IS area


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

Table S-11 show clns interface Command Description

show clns interface Command

Description

type

(Optional) Interface type

number

(Optional) Interface number


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 S-12.

Table S-12 show clns neighbors Command Description

show clns neighbors Command

Description

type

(Optional) Interface type.

number

(Optional) Interface number.

detail

(Optional) When specified, the area addresses advertised by the neighbor in the hello messages are displayed. Otherwise, a summary display is provided.


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 S-13.

Table S-13 show clns is-neighbors Command Description

show clns is-neighbors Command

Description

type

(Optional) Interface type.

number

(Optional) Interface number.

detail

(Optional) When specified, the areas associated with the intermediate systems are displayed. Otherwise, a summary display is provided.


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 S-14.

Table S-14 show isis database Command Description

show isis database Command

Description

level-1

(Optional) Displays the IS-IS link-state database for Level 1.

level-2

(Optional) Displays the IS-IS link-state database for Level 2.

l1

(Optional) Abbreviation for the option level-1.

l2

(Optional) Abbreviation for the option level-2.

show isis database Command

Description

detail

(Optional) When specified, the contents of each LSP are displayed. Otherwise, a summary display is provided.

lspid

(Optional) The link-state PDU identifier. When specified, the contents of a single LSP are displayed by its ID number.


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.

NOTE

The clear isis command is not documented in the Cisco IOS Software documentation, but it does work on the routers.

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 S-20. The network is shown again in Figure S-21, with the IP addresses added.

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

The system IDs of the routers in Figure S-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 S-13 shows the show clns protocol command output from router R2 in Figure S-21.

Example S-13 show clns protocol Command Output from Router R2 in Figure S-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 S-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.

  • 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 S-14 shows the show clns neighbors command output from routers R1 and R2.

Example S-14 show clns neighbors Command Output from Routers R1 and R2 in Figure S-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 S-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 S-15 shows the show clns interface command output from router R2.

Example S-15 show clns interface Command Output from Router R2 in Figure S-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 S-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 S-16 shows the show ip protocols command output from router R2.

Example S-16 show ip protocols Command Output from Router R2 in Figure S-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 S-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 S-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 S-17 show ip route isis Command Output from Routers R1 and R2 in Figure S-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 S-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 S-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.

NOTE

There are two different administrative distances. The CLNS administrative distance, shown in the show clns protocol command, defaults to 110; the IP administrative distance for IS-IS routes, shown in the show ip route command, defaults to 115.

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

Example S-18 show clns Command Output from Router R2 in Figure S-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 S-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 S-19 shows the show clns is-neighbors command output from routers R1 and R2.

Example S-19 show clns is-neighbors Command Output from Routers R1 and R2 in Figure S-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     1921.6800.1006.0      Phase V
1921.6800.1005 Se0      Up   L1  0      00                    Phase V

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

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

Example S-20 show isis database Command Output from Router R2 in Figure S-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 S-20 shows that router R2, a Level 1–2 router, has two separate databases, one for Level 1 and another for Level 2.

Example S-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 S-21 show isis spf-log Command Output from Router R2 in Figure S-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 S-21 shows, router R2 keeps separate logs for the Level 1 and Level 2 SPF algorithm.

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