Table Of Contents
Understanding ISO CLNS Routing Processes
Intermediate Systems and End Systems
ISO CLNS Configuration Task List
Configuring ISO IGRP Dynamic Routing
Configuring ISO IGRP Parameters
Enabling or Disabling Split Horizon
Configuring IS-IS Dynamic Routing
Configuring Miscellaneous IS-IS Parameters
Configuring CLNS Static Routing
Configuring Variations of the Static Route
Mapping NSAP Addresses to Media Addresses
Configuring Miscellaneous Features
Specifying Shortcut NSAP Addresses
Using the IP Domain Name System to Discover ISO CLNS Addresses
Creating Packet-Forwarding Filters and Establishing Adjacencies
Redistributing Routing Information
Configuring ES-IS Hello Packet Parameters
Configuring DECnet OSI or Phase V Cluster Aliases
Configuring Digital-Compatible Mode
Allowing Security Option Packets to Pass
Enhancing ISO CLNS Performance
Disabling Fast Switching Through the Cache
Setting the Congestion Threshold
Sending Error Protocol Data Units
Controlling Redirect Protocol Data Units
Configuring Parameters for Locally Sourced Packets
Monitoring and Maintaining the ISO CLNS Network
Enabling TARP and Configuring a TARP TID
Disabling TARP PDU Origination and Propagation
Configuring Multiple NSAP Addresses
Configuring Static TARP Adjacency and Blacklist Adjacency
Configuring Miscellaneous TARP PDU Information
Monitoring and Maintaining the TARP Protocol
Routing IP over ISO CLNS Networks
Configuring IP over a CLNS Tunnel
Monitoring and Maintaining IP over a CLNS Tunnel
ISO CLNS Configuration Examples
Dynamic Routing Within the Same Area Example
Dynamic Routing in More Than One Area Example
Dynamic Routing in Overlapping Areas Example
Dynamic Interdomain Routing Example
IS-IS Routing Configuration Examples
Static Intradomain Routing Example
Static Interdomain Routing Example
DECnet Cluster Aliases Example
Performance Parameters Example
Configuring ISO CLNS
The International Organization for Standardization (ISO) Connectionless Network Service (CLNS) protocol is a standard for the network layer of the Open System Interconnection (OSI) model. Before you can configure this protocol, you must understand addresses and routing processes. This chapter describes addresses, routing processes, and the steps you follow to configure ISO CLNS. For a complete description of the ISO CLNS commands in this chapter, refer to the "ISO CLNS Commands" chapter of the Cisco IOS Apollo Domain, Banyan VINES, DECnet, ISO CLNS, and XNS Command Reference publication. To locate documentation of other commands that appear in this chapter, use the command reference master index or search online.
To identify the hardware platform or software image information associated with a feature, use the Feature Navigator on Cisco.com to search for information about the feature or refer to the software release notes for a specific release. For more information, see the "Identifying Supported Platforms" section in the "Using Cisco IOS Software" chapter.
Understanding Addresses
Addresses in the ISO network architecture are referred to as network service access point (NSAP) addresses and network entity titles (NETs). Each node in an OSI network has one or more NETs. In addition, each node has many NSAP addresses. Each NSAP address differs from one of the NETs for that node in only the last byte. This byte is called the N-selector. Its function is similar to the port number in other protocol suites.
Our implementation supports all NSAP address formats that are defined by ISO 8348/Ad2; however, Cisco provides ISO Interior Gateway Routing Protocol (IGRP) or Intermediate System-to-Intermediate System (IS-IS) dynamic routing only for NSAP addresses that conform to the address constraints defined in the ISO standard for IS-IS (ISO 10589).
An NSAP address consists of the following two major fields, as shown in Figure 15:
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The initial domain part (IDP) is made up of 1-byte authority and format identifier (AFI) and a variable-length initial domain identifier (IDI). The length of the IDI and the encoding format for the domain specific part (DSP) are based on the value of the AFI.
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Figure 15 NSAP Address Fields
Assign addresses or NETs for your domains and areas. The domain address uniquely identifies the routing domain. All routers within a given domain are given the same domain address. Within each routing domain, you can set up one or more areas, as shown in Figure 16. Determine which routers are to be assigned to which areas. The area address uniquely identifies the routing area and the system ID identifies each node.
Figure 16 Sample Domain and Area Addresses
The key difference between the ISO IGRP and IS-IS NSAP addressing schemes is in the definition of area addresses. Both use the system ID for Level 1 routing (routing within an area). However, they differ in the way addresses are specified for area routing. An ISO IGRP NSAP address includes three separate fields for routing: the domain, area, and system ID. An IS-IS address includes two fields: a single continuous area field (comprising the domain and area fields) and the system ID.
ISO IGRP NSAP Address
The ISO IGRP NSAP address is divided into three parts: a domain part, an area address, and a system ID. Domain routing is performed on the domain part of the address. Area routing for a given domain uses the area address. System routing for a given area uses the system ID part. The NSAP address is laid out as follows:
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The Cisco ISO IGRP routing implementation interprets the bytes from the AFI up to (but not including) the area field in the DSP as a domain identifier. The area field specifies the area, and the system ID specifies the system.
Figure 17 illustrates the ISO IGRP NSAP addressing structure. The maximum address size is 20 bytes.
Figure 17 ISO IGRP NSAP Addressing Structure
IS-IS NSAP Address
An IS-IS NSAP address is divided into two parts: an area address and a system ID. Level 2 routing (routing between areas) uses the area address. Level 1 routing (routing within an area) uses the system ID address. The NSAP address is defined as follows:
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The IS-IS routing protocol interprets the bytes from the AFI up to (but not including) the system ID field in the DSP as an area identifier. The system ID specifies the system.
Figure 18 illustrates the IS-IS NSAP addressing structure. The maximum address size is 20 bytes.
Figure 18 IS-IS NSAP Addressing Structure
Addressing Rules
All NSAP addresses must obey the following constraints:
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Addressing Examples
The following examples show how to configure OSI network and Government OSI Profile (GOSIP) NSAP addresses using the ISO IGRP implementation.
The following example shows an OSI network NSAP address format:
| Domain|Area| System ID| S|47.0004.004D.0003.0000.0C00.62E6.00The following example shows an GOSIP NSAP address structure. This structure is mandatory for addresses allocated from the International Code Designator (ICD) 0005 addressing domain. Refer to the GOSIP document U.S. Government Open Systems Interconnection Profile (GOSIP), draft version 2.0, April 1989, for more information.
| Domain|Area| System ID| S|47.0005.80.ffff00.0000.ffff.0004.0000.0c00.62e6.00| | | | | |AFI IDI DFI AAI Resv RDSample Routing Table
You enter static routes by specifying NSAP prefix and next hop NET pairs (by using the clns route command). The NSAP prefix can be any portion of the NSAP address. NETs are similar in function to NSAP addresses.
If an incoming packet has a destination NSAP address that does not match any existing NSAP addresses in the routing table, Cisco IOS software will try to match the NSAP address with an NSAP prefix to route the packet. In the routing table, the best match means the longest NSAP prefix entry that matches the beginning of the destination NSAP address.
Table 4 shows a sample static routing table in which the next hop NETs are listed for completeness, but are not necessary to understand the routing algorithm. Table 5 offers examples of how the longest matching NSAP prefix can be matched with routing table entries in Table 4.
Octet boundaries must be used for the internal boundaries of NSAP addresses and NETs.
Understanding ISO CLNS Routing Processes
The basic function of a router is to forward packets: receive a packet in one interface and send it out another (or the same) interface to the proper destination. All routers forward packets by looking up the destination address in a table. The tables can be built either dynamically or statically. If you are configuring all the entries in the table yourself, you are using static routing. If you use a routing process to build the tables, you are using dynamic routing. It is possible, and sometimes necessary, to use both static and dynamic routing simultaneously.
When you configure only ISO CLNS and not routing protocols, Cisco IOS software makes only forwarding decisions. It does not perform other routing-related functions. In such a configuration, the software compiles a table of adjacency data, but does not advertise this information. The only information that is inserted into the routing table is the NSAP and NET addresses of this router, static routes, and adjacency information.
You can route ISO CLNS on some interfaces and transparently bridge it on other interfaces simultaneously. To enable this type of routing, you must enable concurrent routing and bridging by using the bridge crb command. For more information on bridging, refer to the "Configuring Transparent Bridging" chapter in the Cisco IOS Bridging and IBM Networking Configuration Guide.
Dynamic Routing
Cisco supports the following two dynamic routing protocols for ISO CLNS networks:
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When dynamically routing, you can choose either ISO IGRP or IS-IS, or you can enable both routing protocols at the same time. Both routing protocols support the concept of areas. Within an area, all routers know how to reach all the system IDs. Between areas, routers know how to reach the proper area.
ISO IGRP supports three levels of routing: system routing, area routing, and interdomain routing. Routing across domains (interdomain routing) can be done either statically or dynamically with ISO IGRP. IS-IS supports two levels of routing: station routing (within an area) and area routing (between areas).
Intermediate Systems and End Systems
Some intermediate systems (ISs) keep track of how to communicate with all the end systems (ESs) in their areas and thereby function as Level 1 routers (also referred to as local routers). Other ISs keep track of how to communicate with other areas in the domain, functioning as Level 2 routers (sometimes referred to as area routers). Cisco routers are always Level 1 and Level 2 routers when routing ISO IGRP; they can be configured to be Level 1 only, Level 2 only, or both Level 1 and Level 2 routers when routing IS-IS.
ESs communicate with ISs using the ES-IS protocol. Level 1 and Level 2 ISs communicate with each other using either ISO IS-IS or the Cisco ISO IGRP protocol.
Static Routing
Static routing is used when it is not possible or desirable to use dynamic routing. The following are some instances of when you would use static routing:
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Routing Decisions
A Connectionless Network Protocol (CLNP) packet sent to any of the defined NSAP addresses or NETs will be received by the router. Cisco IOS software uses the following algorithm to select which NET to use when it sends a packet:
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ISO CLNS Configuration Task List
To configure ISO CLNS, you must configure the routing processes, associate addresses with the routing processes, and customize the routing processes for your particular network.
To configure the ISO CLNS protocol, you must use some combination of the tasks in the following sections:
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See the "ISO CLNS Configuration Examples" section at the end of this chapter for configuration examples.
Configuring ISO IGRP Dynamic Routing
The ISO IGRP is a dynamic distance-vector routing protocol designed by Cisco for routing an autonomous system that contains large, arbitrarily complex networks with diverse bandwidth and delay characteristics.
To configure ISO IGRP, perform the tasks in the following sections. The tasks in the "Configuring ISO IGRP Parameters" section are optional, although you might be required to to perform them depending upon your specific application.
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In addition, you can configure the following miscellaneous features described later in this chapter:
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Enabling ISO IGRP
To configure ISO IGRP dynamic routing, you must enable the ISO IGRP routing process, identify the address for the router, and specify the interfaces that are to route ISO IGRP. Optionally, you can set a level for your routing updates when you configure the interfaces. CLNS routing is enabled by default on routers when you configure ISO IGRP. You can specify up to ten ISO IGRP routing processes.
To configure ISO IGRP dynamic routing on the router, use the following commands beginning in global configuration mode:
Although IS-IS allows you to configure multiple NETs, ISO IGRP allows only one NET per routing process.
You can assign a meaningful name for the routing process by using the tag option. You can also specify a name for a NET in addition to an address. For information on how to assign a name, see the "Specifying Shortcut NSAP Addresses" section later in this chapter.
You can configure an interface to advertise Level 2 information only. This option reduces the amount of router-to-router traffic by telling Cisco IOS software to send out only Level 2 routing updates on certain interfaces. Level 1 information is not passed on the interfaces for which the Level 2 option is set.
To configure ISO IGRP dynamic routing on the interface, use the following command in interface configuration mode:
Router(config-if)# clns router iso-igrp tag [level 2]
Enables ISO IGRP on specified interfaces; also sets the level type for routing updates.
See the sections "Dynamic Routing in Overlapping Areas Example," "Dynamic Interdomain Routing Example," and "ISO CLNS over X.25 Example" at the end of this chapter for examples of configuring dynamic routing.
Configuring ISO IGRP Parameters
The Cisco ISO IGRP implementation allows you to customize certain ISO IGRP parameters. You can perform the optional tasks discussed in the following sections:
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Adjusting ISO IGRP Metrics
You have the option of altering the default behavior of ISO IGRP routing and metric computations. Altering the default behavior enables, for example, the tuning of system behavior to allow for transmissions via satellite. Although ISO IGRP metric defaults were carefully selected to provide excellent operation in most networks, you can adjust the metric.
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You can use different metrics for the ISO IGRP routing protocol on CLNS. To configure the metric constants used in the ISO IGRP composite metric calculation of reliability and load, use the following command in router configuration mode
Router(config-router)# metric weights qos k1 k2 k3 k4 k5
Adjusts the ISO IGRP metric.
Two additional ISO IGRP metrics can be configured: the bandwidth and delay associated with an interface. Refer to the Cisco IOS Interface Command Reference publication for details about the bandwidth (interface) and delay interface configuration commands used to set these metrics.
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Adjusting ISO IGRP Timers
The basic timing parameters for ISO IGRP are adjustable. Because the ISO IGRP routing protocol executes a distributed, asynchronous routing algorithm, it is important that these timers be the same for all routers in the network.
To adjust ISO IGRP timing parameters, use the following command in router configuration mode:
Router(config-router)# timers basic update-interval holddown-interval invalid-interval
Adjusts the ISO IGRP timers (in seconds).
Enabling or Disabling Split Horizon
Split horizon blocks information about routes from being advertised out the interface from which that information originated. This feature usually optimizes communication among multiple routers, particularly when links are broken.
To either enable or disable split horizon for ISO IGRP updates, use the following commands in interface configuration mode:
Router(config-if)# clns split-horizon
Enables split horizon for ISO IGRP updates.
Router(config-if)# no clns split-horizon
Disables split horizon for ISO IGRP updates.
The default for all LAN interfaces is for split horizon to be enabled; the default for WAN interfaces on X.25, Frame Relay, or Switched Multimegabit Data Service (SMDS) networks is for split horizon to be disabled.
Configuring IS-IS Dynamic Routing
IS-IS is an ISO dynamic routing specification. IS-IS is described in ISO 10589. The Cisco implementation of IS-IS allows you to configure IS-IS as an ISO CLNS routing protocol.
IS-IS Configuration Task List
To configure IS-IS, perform the tasks in the following sections. Enabling IS-IS is required; the remainder of the tasks are optional, although you might be required to perform them depending upon your specific application.
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In addition, you can configure the following miscellaneous features described later in this chapter:
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Enabling IS-IS
Unlike other routing protocols, enabling IS-IS requires that you create an IS-IS routing process and assign it to a specific interface, rather than to a network. You can specify more than one IS-IS routing process per Cisco unit, using the multiarea IS-IS configuration syntax. You then configure the parameters for each instance of the IS-IS routing process.
Small IS-IS networks are built as a single area that includes all the routers in the network. As the network grows larger, it is usually reorganized into a backbone area made up of the connected set of all Level 2 routers from all areas, which is in turn connected to local areas. Within a local area, routers know how to reach all system IDs. Between areas, routers know how to reach the backbone, and the backbone routers know how to reach other areas.
Routers establish Level 1 adjacencies to perform routing within a local area (intra-area routing). Routers establish Level 2 adjacencies to perform routing between Level 1 areas (interarea routing).
Some networks use legacy equipment that supports only Level 1 routing. These devices are typically organized into many small areas that cannot be aggregated due to performance limitations. Cisco routers are used to interconnect each area to the Level 2 backbone.
A single Cisco router can participate in routing in up to 29 areas and can perform Level 2 routing in the backbone. In general, each routing process corresponds to an area. By default, the first instance of the routing process configured performs both Level 1and Level 2 routing. You can configure additional router instances, which are automatically treated as Level 1 areas. You must configure the parameters for each instance of the IS-IS routing process individually.
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For IS-IS multiarea routing, you can configure only one process to perform Level 2 routing, although you can define up to 29 Level 1 areas for each Cisco unit. If Level 2 routing is configured on any process, all additional processes are automatically configured as Level 1. You can configure this process to perform Level 1 routing at the same time. If Level 2 routing is not desired for a router instance, remove the Level 2 capability using the is-type command. Use the is-type command also to configure a different router instance as a Level 2 router.
To enable IS-IS, use the following commands beginning in global configuration mode:
You can assign a meaningful name for the routing process by using the tag option. You can also specify a name for a NET in addition to an address. For information on how to assign a name, see the "Specifying Shortcut NSAP Addresses" section later in this chapter.
See the "IS-IS Routing Configuration Examples" section at the end of this chapter for examples of configuring IS-IS routing.
Enabling Routing for an Area on an Interface
To enable CLNS routing and specify the area for each instance of the IS-IS routing process, use the following commands beginning in global configuration mode:
See the "IS-IS Routing Configuration Examples" section at the end of this chapter for examples of configuring IS-IS routing.
Assigning Multiple Area Addresses to IS-IS Areas
IS-IS routing supports the assignment of multiple area addresses on the same router. This concept is referred to as multihoming. Multihoming provides a mechanism for smoothly migrating network addresses, as follows:
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You must statically assign multiple area addresses on a router. Cisco currently supports assignment of up to three area addresses on a router. All the addresses must have the same system ID. For example, you can assign one address (area1 plus system ID), and two additional addresses in different areas (area2 plus system ID and area3 plus system ID) where the system ID is the same. The number of areas allowed in a domain is unlimited.
A router can dynamically learn about any adjacent router. As part of this process, the routers inform each other of their area addresses. If two routers share at least one area address, the set of area addresses of the two routers are merged. A merged set cannot contain more than three addresses. If there are more than three, the three addresses with the lowest numerical values are kept, and all others are dropped.
To configure multiple area addresses in IS-IS areas, use the following commands beginning in global configuration mode:
See the "NETs Configuration Examples" section at the end of this chapter for examples of configuring NETs and multiple area addresses.
Configuring IS-IS Interface Parameters
The Cisco IS-IS implementation allows you to customize certain interface-specific IS-IS parameters. You can perform the optional tasks discussed in the following sections:
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You are not required to alter any of these parameters, but some interface parameters must be consistent across all routers in the network. Therefore, be sure that if you do configure any of these parameters, the configurations for all routers on the network have compatible values.
Adjusting IS-IS Link-State Metrics
You can configure a cost for a specified interface. The default metric is used as a value for the IS-IS metric and is assigned when there is no quality of service (QoS) routing performed. The only metric that is supported by Cisco IOS software and that you can configure is the default-metric, which you can configure for Level 1 or Level 2 routing or both. The range for the default-metric is from 0 to 63. The default value is 10.
To configure the link-state metric, use the following command in interface configuration mode:
Router(config-if)# isis metric default-metric [level-1 | level-2]
Configures the metric (or cost) for the specified interface.
Setting the Advertised Hello Interval and Hello Multiplier
You can specify the length of time (in seconds) between hello packets that Cisco IOS software sends on the interface. You can also change the default hello packet multiplier used on the interface to determine the hold time sent in IS-IS hello packets (the default is 3).
The hold time determines how long a neighbor waits for another hello packet before declaring the neighbor down. This time determines how quickly a failed link or neighbor is detected so that routes can be recalculated.
To set the advertised hello interval and multiplier, use the following commands in interface configuration mode:
The hello interval can be configured independently for Level 1 and Level 2, except on serial point-to-point interfaces. (Because there is only a single type of hello packet sent on serial links, the hello packet is independent of Level 1 or Level 2.) Specify an optional level for X.25, SMDS, and Frame Relay multiaccess networks.
Use the isis hello-multiplier command in circumstances where hello packets are lost frequently and IS-IS adjacencies are failing unnecessarily. You can raise the hello multiplier and lower the hello interval (the isis hello-interval command) correspondingly to make the hello protocol more reliable without increasing the time required to detect a link failure.
Setting the Advertised Complete Sequence Number PDU Interval
Complete sequence number PDUs (CSNPs) are sent by the designated router to maintain database synchronization.
To configure the IS-IS CSNP interval for the interface, use the following command in interface configuration mode:
Router(config-if)# isis csnp-interval seconds [level-1 | level-2]
Configures the IS-IS CSNP interval for the specified interface.
The isis csnp-interval command does not apply to serial point-to-point interfaces. It does apply to WAN connections if the WAN is viewed as a multiaccess meshed network.
Setting the Retransmission Interval
You can configure the number of seconds between retransmission of Link-State PDUs (LSPs) for point-to-point links.
To set the retransmission level, use the following command in interface configuration mode:
Router(config-if)# isis retransmit-interval seconds
Configures the number of seconds between retransmission of IS-IS LSPs for point-to-point links.
The value you specify should be an integer greater than the expected round-trip delay between any two routers on the network. The setting of this parameter should be conservative, or needless retransmission will result. The value you determine should be larger for serial lines and virtual links.
Setting the Retransmission Throttle Interval
You can configure the maximum rate (number of milliseconds between packets) at which IS-IS LSPs will be re-sent on point-to-point links This interval is different from the retransmission interval, the time between successive retransmissions of the same LSP.
To set the retransmission throttle interval, use the following command in interface configuration mode:
Router(config-if)# isis retransmit-throttle-interval milliseconds
Configures the IS-IS LSP retransmission throttle interval.
This command is usually unnecessary, except when very large networks contain high point-to-point neighbor counts.
Specifying Designated Router Election
You can configure the priority to use for designated router election. Priorities can be configured for Level 1 and Level 2 individually. The designated router enables a reduction in the number of adjacencies required on a multiaccess network, which in turn reduces the amount of routing protocol traffic and the size of the topology database.
To configure the priority to use for designated router election, use the following command in interface configuration mode:
Router(config-if)# isis priority value [level-1 | level-2]
Configures the priority to use for designated router election.
Specifying the Interface Circuit Type
It is normally not necessary to configure this feature because the IS-IS protocol automatically determines area boundaries and keeps Level 1 and Level 2 routing separate. However, you can specify the adjacency levels on a specified interface.
To configure the adjacency for neighbors on the specified interface, use the following command in interface configuration mode:
If you specify Level 1, a Level 1 adjacency is established if there is at least one area address common to both this node and its neighbors.
If you specify both Level 1 and Level 2 (the default value), a Level 1 and 2 adjacency is established if the neighbor is also configured as both Level 1 and Level 2 and there is at least one area in common. If there is no area in common, a Level 2 adjacency is established.
If you specify Level 2 only, a Level 2 adjacency is established. If the neighbor router is a Level 1 router, no adjacency is established.
Configuring IS-IS Authentication Passwords
You can assign different authentication passwords for different routing levels. By default, authentication is disabled. Specifying Level 1 or Level 2 enables the password only for Level 1 or Level 2 routing, respectively. If you do not specify a level, the default is Level 1.
To configure an authentication password for an interface, use the following command in interface configuration mode:
Router(config-if)# isis password password [level-1 | level-2]
Configures the authentication password for an interface.
You can assign authentication passwords to areas and domains. An area password is inserted in Level 1 (station router) LSPs, CSNPs, and partial sequence number PDUs (PSNPs). A routing domain authentication password is inserted in Level 2 (area router) LSPs, CSNPs, and PSNPs.
To configure area or domain passwords, use the following commands in router configuration mode:
Limiting LSP Flooding
Limiting LSP flooding is important to IS-IS networks in general, and is not limited to configuring multiarea IS-IS networks. In a network with a high degree of redundancy, such as a fully meshed set of point-to-point links over a nonbroadcast multiaccess (NBMA) transport, flooding of LSPs can limit network scalability. You can reduce LSP flooding in two ways:
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The advantage of full blocking over mesh groups is that it is easier to configure and understand, and fewer LSPs are flooded. Blocking flooding on all links permits the best scaling performance, but results in a less robust network structure. Permitting flooding on all links results in poor scaling performance.
The advantage of mesh groups over full blocking is that mesh groups allow LSPs to be flooded over one hop to all routers on the mesh, while full blocking allows some routers to receive LSPs over multiple hops. This relatively small delay in flooding can have an impact on convergence times, but the delay is negligible compared to overall convergence times.
Blocking Flooding on Specific Interfaces
You can completely block flooding (full blocking) on specific interfaces, so that new LSPs will not be flooded out over those interfaces. However, if flooding is blocked on a large number of links, and all remaining links go down, routers cannot synchronize their link-state databases even though there is connectivity to the rest of the network. When the link-state database is no longer updated, routing loops usually result.
To use CSNPs on selected point-to-point links to synchronize the link-state database, configure a CSNP interval using the isis csnp-interval command on selected point-to-point links over which normal flooding is blocked. You should use CSNPs for this purpose only as a last resort.
Configuring Mesh Groups
Configuring mesh groups (a set of interfaces on a router) can help to limit redundant flooding. All routers reachable over the interfaces in a particular mesh group are assumed to be densely connected (each router has many links to other routers), where many links can fail without isolating one or more routers from the network.
Normally, when a new LSP is received on an interface, it is flooded out over all other interfaces on the router. When the new LSP is received over an interface that is part of a mesh group, the new LSP will not be flooded out over the other interfaces that are part of that same mesh group.
Mesh groups rely on a full mesh of links between a group of routers. If one or more links in the full mesh goes down, the full mesh is broken, and some routers might miss new LSPs, even though there is connectivity to the rest of the network. When you configure mesh groups to optimize or limit LSP flooding, be sure to select alternative paths over which to flood in case interfaces in the mesh group go down.
To minimize the possibility of incomplete flooding, you should allow unrestricted flooding over at least a minimal set of links in the mesh. Selecting the smallest set of logical links that covers all physical paths results in very low flooding, but less robustness. Ideally you should select only enough links to ensure that LSP flooding is not detrimental to scaling performance, but enough links to ensure that under most failure scenarios no router will be logically disconnected from the rest of the network.
Configuring Miscellaneous IS-IS Parameters
The Cisco IS-IS implementation allows you to customize certain IS-IS parameters. You can perform the optional tasks discussed in the following sections:
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Specifying Router-Level Support
It is seldom necessary to configure the IS type because the IS-IS protocol will automatically establish the IS type. However, you can configure the router to act as a Level 1 (intra-area) router, as both a Level 1 router and a Level 2 (interarea) router, or as an interarea router only.
To configure the IS-IS level, use the following command in router configuration mode:
Router(config-router)# is-type [level-1 | level-1-2 | level-2-only]
Configures the IS-IS level at which the router is to operate.
Ignoring IS-IS LSP Errors
You can configure the router to ignore IS-IS LSPs that are received with internal checksum errors, rather than purging the LSPs. LSPs are used by the receiving routers to maintain their routing tables.
The IS-IS protocol definition requires that a received LSP with an incorrect data-link checksum be purged by the receiver, which causes the initiator of the LSP to regenerate it. However, if a network has a link that causes data corruption while still delivering LSPs with correct data-link checksums, a continuous cycle of purging and regenerating large numbers of LSPs can occur, rendering the network nonfunctional.
To allow the router to ignore LSPs with an internal checksum error, use the following command in router configuration mode:
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Logging Adjacency State Changes
You can configure IS-IS to generate a log message when an IS-IS adjacency changes state (up or down). Generating a log message may be useful when monitoring large networks. Messages are logged using the system error message facility. Messages are of the following form:
%CLNS-5-ADJCHANGE: ISIS: Adjacency to 0000.0000.0034 (Serial0) Up, new adjacency%CLNS-5-ADJCHANGE: ISIS: Adjacency to 0000.0000.0034 (Serial0) Down, hold time expiredTo generate log messages when an IS-IS adjacency changes state, use the following command in router configuration mode:
Changing IS-IS LSP Maximum Transmission Unit Size
Under normal conditions, the default maximum transmission unit (MTU) size should be sufficient. However, if the MTU of a link is lowered to less than 1500 bytes, the LSP MTU must be lowered accordingly on each router in the network. If LSP MTU is not lowered, routing will become unpredictable.
The MTU size must be less than or equal to the smallest MTU of any link in the network. The default size is 1497 bytes.
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To change the MTU size of IS-IS LSPs, use the following command in router configuration mode:
Router(config-router)# lsp-mtu size
Specifies the maximum LSP packet size, in bytes.
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Enabling Partitioning Avoidance
In ISO CLNS networks using a redundant topology, it is possible for an area to become "partitioned" when full connectivity is lost among a Level 1-2 border router, all adjacent Level 1 routers, and end hosts. In such a case, multiple Level 1-2 border routers advertise the Level 1 area prefix into the backbone area, even though any one router can reach only a subset of the end hosts in the Level 1 area.
When enabled, the partition avoidance command prevents this partitioning by causing the border router to stop advertising the Level 1 area prefix into the Level 2 backbone.
Other cases of connectivity loss within the Level 1 area itself are not detected or corrected by the border router, and this command has no effect.
To enable partitioning avoidance, use the following command in router configuration mode:
Changing the Routing Level for an Area
You can change the routing level configured for an area using the is-type command. If the router instance has been configured for Level 1-2 area (the default for the first instance of the IS-IS routing process in a Cisco unit), you can remove Level 2 (interarea) routing for the area using the is-type command and change the routing level to Level 1 (intra-area). You can also configure Level 2 routing for an area using the is-type command, but the instance of the IS-IS router configured for Level 2 on the Cisco unit must be the only instance configured for Level 2.
To change the routing level for an IS-IS routing process in a given area, use the following command in router configuration mode:
Router(config-router)# is-type [level-1 | level-1-2 | level-2-only]
Configures the routing level for an instance of the IS-IS routing process.
Modifying the Output of show Commands
To customize display output when the multiarea feature is used, making the display easier to read, use the following command in EXEC mode:
Router# isis display delimiter [return cnt |char cnt]
Specifies the delimiter to be used to separate displays of information about individual IS-IS areas.
For example, the following command causes information about individual areas to be separated by 14 hyphens (-) in the display:
isis display delimiter - 14The output for a configuration with two Level 1 areas and one Level 2 area configured is as follows:
dtp-5# show clns neighbors--------------Area L2BB:System Id Interface SNPA State Holdtime Type Protocol0000.0000.0009 Tu529 172.21.39.9 Up 25 L1L2 IS-IS--------------Area A3253-01:System Id Interface SNPA State Holdtime Type Protocol0000.0000.0053 Et1 0060.3e58.ccdb Up 22 L1 IS-IS0000.0000.0003 Et1 0000.0c03.6944 Up 20 L1 IS-IS--------------Area A3253-02:System Id Interface SNPA State Holdtime Type Protocol0000.0000.0002 Et2 0000.0c03.6bc5 Up 27 L1 IS-IS0000.0000.0053 Et2 0060.3e58.ccde Up 24 L1 IS-ISConfiguring CLNS Static Routing
You need not explicitly specify a routing process to use static routing facilities. You can enter a specific static route and apply it globally, even if you have configured the router for ISO IGRP or IS-IS dynamic routing.
To configure a static route, perform the tasks in the following sections. Only enabling CLNS is required; the remaining tasks are optional, although you might be required to perform them depending upon your specific application.
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Enabling Static Routes
To configure static routing, you must enable CLNS on the router and on the interface. CLNS routing is enabled on the router by default when you configure ISO IGRP or IS-IS routing protocols. NSAP addresses that start with the NSAP prefix you specify are forwarded to the next hop node.
To configure CLNS on the router, use the following commands beginning in global configuration mode:
Note
You also must enable ISO CLNS for each interface you want to pass ISO CLNS packet traffic to end systems, but for which you do not want to perform any dynamic routing on the interface. ISO CLNS is enabled automatically when you configure IS-IS or ISO IGRP routing on an interface; however, if you do not intend to perform any dynamic routing on an interface, you m
