Integrated IS-IS

This version of Integrated Intermediate System to Intermediate System (IS-IS) is compliant with ISO 10589.

IS-IS provides an Interior Gateway Protocol (IGP). Integrated IS-IS provides support for update of routing information between multiple protocol stacks such as IP and OSI.

Because IS-IS is an ISO standard, many of the terms used in this chapter are based on ISO terminology.

IS-IS uses the term domain to refer to any group of routers that are administered by a single organization. For example, the network owned and operated by a single carrier would be a domain. This is analogous to the autonomous system in OSPF.

An area is a subset of a domain. Areas simplify management by breaking a domain into smaller, simpler chunks.

Routing protocols that operate within the domain are know as intra-area routing protocols (interior gateway protocols in the IP world). Routing protocols that operate between domains are known as inter-area routing protocols. IS-IS handles routing information within a domain and is therefore known as an intra-area routing protocol or IGP.

The IS-IS protocol runs separate copies of the routing algorithm for each level of routing configured on a system. If there is only one area running IS-IS in your network, the router can be configured as a Level 1 router; a router that handles traffic within an area.

Routers that handle traffic between areas are known as Level 2 routers.

Routers that handle traffic both within and between areas are known as Level 1 and 2 routers. Level 1 and 2 routers run two copies of the routing algorithm.

Related Information

Administrators should be very familiar with IS-IS operation and theory before configuring their system to run IS-IS. The following texts are recommended as further reading.

• Internetworking with TCP/IP, Comer, Douglas E. 1995, Prentice-Hall, Inc. ISBN0-13-216987-8
  
  
• Interconnections (Bridges and Routers), Radia Perlman, 1992, Addison-Wesley Publishing Group, ISBN 0-201-56332-0
  
  

Messages Exchanged Between Intermediate Systems

IS-IS is a link-state routing protocol. That is, each intermediate system (router) in a domain is represented as being in a particular state at any given time. Depending on the state of the intermediate system, different messages are expected from neighboring routers or from that intermediate system before the router can transition to the next state and ultimately exchange routing information and process its routing tables.

The packets used in IS-IS routing protocol fall into the following classes:

• Hello packets
  
  
• Link State PDUs (LSPs),
  
  
• Sequence Number PDUs (SNPs).
  
  

Hello Packets

Hello packets are used to initialize and maintain adjacencies between neighboring routers. There are three types of IS-IS Hello packets:

• Level 1 LAN IS to IS Hello PDUs are used by Level 1 routers on broadcast LANs.
  
  
• Level 2 LAN IS to IS Hello PDUs are used by Level 2 routers on broadcast LANs.
  
  
• Point-to-Point IS to IS Hello PDUs are used on non-broadcast media, such as point-to-point links.
  
  

Link-State PDUs

Link State PDUs (LSPs) contain link state information. There are two types of LSPs:

• Level 1 routers transmit Level 1 LSPs
  
  
• Level 2 routers transmit Level 1 and Level 2 LSPs
  
  

Sequence Number PDUs

Sequence number PDUs are used to ensure that neighboring routers have the same notion of what is the most recent LSP from every other router. The sequence number PDUs serve a similar function to acknowledgement packets. There are two types of sequence number packets for both Level 1 and Level 2 routers:

• Partial sequence-number PDUs are used to request LSPs in broadcast networks and to acknowledge LSP receipt on point-to-point networks.
  
  
• A complete sequence number PDU contains a description of all LSPs in the database. A complete sequence number packet may therefore be used to ensure synchronization of the database between adjacent routers either periodically, or when an adjacency first comes up.
  
  

Establishing an Adjacency

Routers running IS-IS establish an adjacency by passing hello packets to each other. Based on the responses to those packets, a router determines whether it can establish this adjacency, whether the adjacency is a Level 1 adjacency, a Level 2 adjacency or a Level 1 and 2 adjacency.

Addressing in IS-IS

Because Integrated IS-IS was originally designed for OSI and then evolved to include IP routing protocols, it represents routers using OSI addressing.

Each node (router or end system) in OSI is known by a unique identifier known as the Network Service Access Point (NSAP).

This address is divided into two equal parts:

• The initial domain part (IDP)
  
  
• The domain specific part (DSP).
  
  

The initial domain part is further broken into the authority and format indicator (AFI), which specifies the format of the IDP, and the initial domain identifier (IDI). The AFI is always one octet in length.

Figure 3-1. The Network Layer Address

The DSP is further broken into the High Order Domain Specific part (HO-DPS) and the system ID and selector (SEL). The system id for the Avici router is always 6 octets. The SEL is one octet in length and is always 00.

The AFI, IDI and HO-DSP make up the area address which is variable in length, depending on the value of the AFI.

Network Entity Titles

The network entity title (NET) of an IS-IS router is the NSAP with a SEL byte set to 0.

Configuring IS-IS

PROCEDURE: The following steps are used to configure IS-IS on the Avici router.

Step 1 Enable Integrated IS-IS on the router.

Step 2 Configure one or more NET.

Step 3 Optionally, configure/modify administrative distance.

Step 4 Enable IS-IS on appropriate interface(s).

Step 5 Optionally, configure authentication.

Step 6 Optionally, enable route summarization.

Step 7 Optionally, select metric styles.

Step 8 Optionally, configure interface parameters:

• Set parameters for tuning the network to control LSP generation and retransmission.
  
  
• Set parameters to manage the transmission of sequence number packets.
  
  
• Enable IS-IS on an interface.
  
  
• Establish a routing level on the circuit.
  
  
• Set a metric on an interface.
  
  
• Set parameters to manage the transmission of Hello packets.
  
  

  Step 9 Optionally, configure this router as overloaded, either on start-up only, or always.

  Step 10 Display information about IS-IS process, link-state database and all neighbor adjacencies.

Enabling Integrated IS-IS on the Router

The router isis command defines a tag for the routing process and enters router configuration mode.

At this time, IPriori supports one IS-IS routing process. In the event that multiple instances of IS-IS processes are supported in a future release, this string will distinguish the instances. This one process handles the routing for all areas in which a router participates in IS-IS routing. Once a process string is defined with the router isis command, the same string must be used any time you reenter router configuration mode to configure that IS-IS process. In the examples in this chapter, we will use the process string 13. The number 13 has no special significance. However, whenever we re-enter router configuration mode, we must specify the number 13 until we take down that routing process with the no router isis command and define a new IS-IS process with a new string.

Additionally, when IS-IS is configured on an interface, it must be configured with the same tag. See "Configuring Interface Parameters for more information on configuring IS-IS on an interface.

Configuring a NET

The network entity title of an IS-IS router is the NSAP with a SEL byte set to 0. IS-IS uses the identifier portion of the NET as its router ID.

By default, each router can be configured with three NETs. However, if you enter the max-areas command, you can increase the number of areas supported for this IS-IS process. When a router is configured with a NET, an area is also defined as part of that NET. See "Network Entity Titles" for details of the meaning and contents of a NET.

The IS-IS process does not start until at least one NET is configured.

Configure Multiple Areas

In Figure 3-2, Router A participates in a single-area domain. The area is area 49.0027.0000.0000 and the router identifier is 0000.0011.1010.

Figure 3-2. A Single-area IS-IS Domain

The following lines are configured on Router A:

router(config)#router isis 13 router(config-router)#net 49.0027.0000.0011.1010.00 router(config-router)#

In Figure 3-3, Router A participates in two separate areas.

Figure 3-3. Multiple Area IS-IS Domain

The following lines are configured on Router A:

router(config)#router isis net2 router(config-router)#net 49.0001.0000.0000.0001.00

Notice that with each new NET, the area portion changes and the system identifier portion remains the same. If you enter a group of NETs for a single router with different system IDs, the original system ID remains in effect for the system. This can be dangerous because if the original NET is later removed, the system ID reverts to the previously defined system identifier. This can result in routing delays for IS-IS.

For example:

router isis net2 net 49.0001.0000.0000.0001.00 net 49.0003.0000.0000.0002.00

In this example, the router is defined as being in area 49.0001 and having the system identifier of 1. Then a second net command adds a new area 49.0002. If the user were to subsequently remove the original NET:

no net 49.0001.0000.0000.0001.00

The system identifier would revert to 2 and the area address to 49.0003.

Configuring Administrative Distance

Administrative distance is a tool used to prefer a route learned from one protocol over the same route learned from another protocol.

The route with the lowest administrative distance is installed in the IP routing table and propagated to neighbors.

Routes with distance values of 255 are not installed in the routing table.

Use the distance weight command in Router configuration command mode to set the distance of IS-IS routes to the specified value.

Use the no distance command to set the distance to the default value of 115.

Example: In the following example:

• The router isis command enables IS-IS on the router, specifies an IS-IS instance, and changes the command mode to Router configuration.
  
  
• The distance weight command configures an administrative distance of 200.
  
  
• The show isis command displays the setting:
  
  

router#configure terminal Enter configuration commands, one per line. End with CNTL/Z. router(config)#router isis tsr router(config-router)#distance 200 router(config-router)#end router#show isis Global ISIS information ISIS process tag: tsr System ID: 0001.0001.0040 NET: 49.0001.0001.0001.0040.00 Maximum number of areas: 3 There is 1 manual area address 49..0001 There is 1 active area address 49..0001 ISIS level-2 ISIS is enabled on 4 interfaces Distance : 200 Maximum of 4 paths per route Number of SPF runs, L1: 2, L2: 668

Enabling IS-IS on an Interface

Not all interfaces on a router participate in IS-IS routing. Because IS-IS is a link-state routing protocol, routing information is not exchanged unless at least one adjacency is formed. In IS-IS, enabling of the IS-IS protocol on an interface begins the adjacency formation process. Therefore, no routing information is exchanged until IS-IS is enabled on at least one interface.

Before enabling the IS-IS routing protocol on an interface, use the interface command to define the interface on which to route IS-IS packets.

Example: In the following example, module 1/1 is a 4-port router and three of the available interfaces are enabled for running IS-IS.

interface pos 1/1/1 ip address 13.1.1.1 255.255.255.0 ip router isis net2 ! interface pos 1/1/2 ip address 13.1.2.1 255.255.255.0 ip router isis net2 ! interface pos 1/1/3 ip address 13.1.3.1 255.255.255.0 ip router isis net2 router isis net2 net 49.0001.0000.0000.0001.00 net 49.0003.0000.0000.0002.00

NOTE The tag used in the router isis command is the same as the tag used in the ip router isis command. The interface will not come up if the two tags do not match.

Enabling Route Summarization

In IS-IS, routes are leaked from Level 1 to Level 2, on routers running both Level 1 and Level 2. The summary address command aggregates the addresses that are leaked from Level 1 and Level 2.

Figure 3-4. Summarizing Addresses

In the IS-IS example above, router A is running Level 1 and Level 2. Router B is running Level 1 and router C is running Level 2. Router A learns networks 192.1.1.0 and 192.1.2.0 through Level 1 IS-IS from router B. Without a route summary, they are leaked to Level 2 and flooded to router C as 192.1.1.0 and 192.1.2.0.

The following lines are configured on Router A:

router isis net1 net 49.0001.0000.0000.0020.00 summary-address 192.1.0.0 255.255.0.0 2

Now router A will leak summary 192.1.0.0 to router C but will not flood routes 192.1.1.0 and 192.1.2.0.

A summary is only flooded if there is at least one Level 1 route that falls into the configured summary address range.

Configuring Interface Parameters

Once routing is configured on the router, fine-tuning of IS-IS routing operation can be achieved by modifying a number of interface parameters. For each interface, you can:

• Establish a routing level on the circuit. (isis circuit-type)
  
  
• Set a metric on an interface (isis metric)
  
  
• Set parameters to manage the transmission of Hello packets (hello-multiplier, isis hello-interval)
  
  

The default values configured in IPriori's Integrated IS-IS should be sufficient for most networks. Additionally, many interface parameters must be changed on both ends of an interface or for all interfaces in an area.

Changing these parameters is not recommended unless you are very familiar with troubleshooting of IS-IS configurations.

The default values for the interface parameters are as follows:

Table 3-1. IS-IS Interface Default Values  

Command/Parameter	Default Value
isis circuit-type	Level 1-2 adjacency.
isis csnp-interval	10 seconds for both Level 1 and Level 2 CSNP packets.
isis hello-interval	10 seconds.
isis hello-multiplier	10 times hello interval
isis hello-padding	Disabled.
isis max-broadcast-pkts	10 packets
isis metric	10
isis min-broadcast-interval	250 milliseconds
isis network-type	Network type is determined by the underlying interface type.
isis password	Authentication disabled
isis priority	64
isis retransmit-interval	5 seconds
isis wait-timer	20 seconds (2 times the hello timer)

Understanding IS-IS Route Leaking

Route Leaking, also known as domain-wide prefix distribution, helps reduce sub-optimal routing by providing a mechanism for redistributing, or leaking, Level 2 (L2) information into Level 1 (L1) areas. The IS-IS routing protocol allows for a two-level hierarchy of routing information. There can be multiple L1 areas interconnected by a contiguous L2 backbone. A router can belong to L1, L2, or both. The L1 link-state database only contains information about the L1 area. The L2 link-state database contains information about L2 as well as each of the L1 areas. A L1/L2 router contains both L1 and L2 databases. It advertises information about the L1 area, to which it belongs, into L2.

Each L1 area is essentially a stub area. Packets destined for an address that is outside of the L1 area are routed to the closest L1/L2 router to be forwarded on to the destination area. Because destination information is incomplete, routing to the closest L1/L2 router can lead to sub-optimal routing when the shortest path to the destination is through a different L1/L2 router. Route leaking helps reduce sub-optimal routing by providing a mechanism for leaking, or redistributing, L2 information into L1 areas. By having more detail about inter-area routes, a L1 router is able to make a better choice with regard to which L1/L2 router to forward the packet.

In order to prevent routing loops, prefixes that are derived from L2 routing must not be redistributed from L1 back into L2. Setting the Up/Down flag identifies such prefixes. These prefixes will be identified in IS-IS statistic displays as having the Up/Down flag set.

There is a scalability consideration inherent in using route leaking. Standard IS-IS summarization of prefix information aids scalability by limiting the number of data items transported and routes computed. Use of route leaking affects this aspect of IS-IS scalability. Introduction of route leaking into a formerly flat, single area network is a clear benefit to the network, but it ultimately compromises maximum scalability normally available in IS-IS.

Route leaking is enabled on a L1/L2 router using the redistribute isis ip command, specifying the direction of redistribution and a distribute list. Redistribution direction can be from Level-2 into Level-1 or Level-1 into Level-2. Applying a distribute-list to the command specifies the routes affected.

Example 1: In the following example:

• The router isis command enables IS-IS on the router, specifies an IS-IS instance, and changes the command mode to Router configuration.
  
  
• The is-type level-1-2 command configures the router to operate as a L1/L2 router.
  
  
• Sets this L1/L2 router to redistribute L2 routes into L1 using the distList1 distribute-list to specify the affected routes.
  
  

router#configure terminal Enter configuration commands, one per line. End with CNTL/Z. router(config)#router isis npi router(config-router)#is-type level-1-2 router(config-router)#redistribute isis ip level-2 into level-1 distribute-list distList1 router(config-router)#end router#

Example 2: The following example displays the results of a show ip route and a show isis database detail command from a Level-1 directly attached to the L1/L2 router. The commands display L2 into L1 routes learned from the route leaking enabled L1/L2 router. Routes are identified with UD label (Up/Down bit set to 1):

router#show ip route Codes: C - connected O - OSPF i - IS-IS S - static UD - Up/Down bit 1 L1 - level-1 B - BGP E1 - external type 1 L2 - level-2 M - MPLS E2 - external type 2 * - candidate default m - route's metric d - administrative distance S * 0.0.0.0/0 via 10.200.0.1 [d:1 m:0] S 9.9.9.9/32 via 127.0.0.1 [d:1 m:0] C 10.200.0.0/16 directly connected to Ethernet 0 i UD 12.2.41.19/32 via 55.55.55.1 [d:115 m:35] i UD 12.2.41.20/32 via 55.55.55.1 [d:115 m:35] i UD 12.2.41.21/32 via 55.55.55.1 [d:115 m:35] i UD 12.2.41.22/32 via 55.55.55.1 [d:115 m:35] i UD 12.2.41.23/32 via 55.55.55.1 [d:115 m:35] i UD 12.2.41.24/32 via 55.55.55.1 [d:115 m:35] . . . router#show isis database detail ISIS Level-1 Link State Database LSPID LSP Seq Num LSP Checksum LSP Holdtime ATT/P/OL ha191.00-00 0x00000092 0xa1c2 1149 0/0/0 Area Address: 25.0001 NLPID: 0xCC Hostname: ha191 ha191.00-01 0x00000092 0x4ac7 1149 0/0/0 Metric: 10 IS 0001.0001.0193.01, Extended ha191.00-02 0x00000092 0xe3a9 1149 0/0/0 IP Address: 191.191.191.191 IP Address: 191.194.1.2 IP Address: 55.55.55.1 IP Address: 194.10.1.2 Metric: 10 IP 12.2.41.19/32, Extended, Up/Down bit: 1 Metric: 10 IP 12.2.41.20/32, Extended, Up/Down bit: 1 Metric: 10 IP 12.2.41.21/32, Extended, Up/Down bit: 1 Metric: 10 IP 12.2.41.22/32, Extended, Up/Down bit: 1 Metric: 10 IP 12.2.41.23/32, Extended, Up/Down bit: 1 Metric: 10 IP 12.2.41.24/32, Extended, Up/Down bit: 1 . . .

Metric: 10 IP 191.191.191.191/32, Extended, Up/Down bit: 0 Metric: 10 IP 191.194.1/24, Extended, Up/Down bit: 0 Metric: 10 IP 55.55.55/24, Extended, Up/Down bit: 0 Metric: 10 IP 194.10.1/24, Extended, Up/Down bit: 0 ha193.00-00 0x00000096 0xa9b4 1070 0/0/0 Area Address: 25.0001 NLPID: 0xCC Hostname: ha193 ha193.00-01 0x00000096 0x30dd 1070 0/0/0 Metric: 10 IS 0001.0001.0193.01, Extended ha193.00-02 0x00000096 0xdc9c 1070 0/0/0 IP Address: 55.55.55.3 IP Address: 194.10.2.2 Metric: 10 IP 55.55.55/24, Extended, Up/Down bit: 0 Metric: 10 IP 194.10.2/24, Extended, Up/Down bit: 0 ha193.01-00 0x00000079 0x1819 1070 0/0/0 NLPID: 0xCC Metric: 0 IS 0001.0001.0193.00, Extended Metric: 0 IS 0001.0001.0191.00, Extended

Clearing the IS-IS Database

The IS-IS Link State DataBase (LSDB) does not dynamically reflect policy changes on a router. This and other circumstances when the LSDB does not reflect current IS-IS needs can require the clearing and regenerating of the LSDB. Clearing the LSDB causes LSP/PLSPs to be removed. New LSP/PLSPs are regenerated and LSP/PLSPs are received from all adjacencies. During the process of regenerating the LSDB, optionally setting the overload bit prevents adjacencies from using this router to forward traffic for the length of the overload bit timer.

Use the clear isis database command in global configuration mode to manually force a clearing of the IS-IS LSDB for this router.

Use the clear isis database set-overload-bit command, specifying the timer in seconds, to manually force a clearing of the IS-IS LSDB and prevent adjacencies from forwarding traffic through this router for the length of the timer.

Example: The following example manually clears the IS-IS database and instructs adjacencies to refrain from forwarding traffic through this router for 15 seconds:

router#configure terminal Enter configuration commands, one per line. End with CNTL/Z. router(config)#clear isis database set-overload-bit 15 router(config)#end router#

Configuring Multiple Parallel Routes

You can set multiple parallel routes installed in the IP routing table using the maximum-paths command. During packet switching, load balancing is performed among the multiple paths. A maximum of four paths is supported. By default, only one path is installed to the IP routing table. During load balancing, a source/destination pair always uses the same path. Use the no option to reset the number of parallel paths to 1.

You must be in router configuration mode to use this command.

Example: The following example allows a maximum of three paths to a destination:

router(config-router)#maximum-paths 3

Configuring Passwords

The following two types of passwords and their commands have been deprecated:

• The domain password configured using the domain-password command.
  
  
• The area password configured using the area-password command.
  
  

Interface passwords are used in the corresponding level IS-IS Hello PDUs. If the password is set, it must be the same for each end of that interface. The interface-level password prevents unauthorized routers from forming an adjacency with this interface.

Example: In the following example:

• The interface pos n/n/n identifies a specific POS interface and changes the command mode to Interface configuration.
  
  
• The ip router isis command enables the IS-IS process specified by the global router isis command on the interface.
  
  
• The isis password password level-1 command configures a password and enables authentication for level-2 routing for the interface.
  
  
• The show running-config interface command displays the setting:
  
  

router(config)#router isis 0 router(config-router)#net 49.0001.0000.0000.1111.00 router(config-router)#interface pos 1/5/1 router(config-if)#ip router isis 0 router(config-if)#isis password BigPOP level-1 router(config-if)#end router#show running-config interface . . ! interface pos 1/5/1 sonet path-trace NPI1 1/5/1 ip address 10.100.53.2 255.255.255.0 crc 16 ip router isis 0  isis password BigPoP level-1 . . .

Configuring Multiple Passwords for IS-IS Authentication

IPriori supports authentication of control PDUs (LSPs, PSNPs and CSNPs) and ISIS hello PDUs for either HMAC-MD5 or clear-text. Multiple passwords can be configured on an IS-IS interface by configuring a password-list. A maximum of 252 passwords can be configured.

For interfaces, password-list is used for authentication. The valid password with the least password-id is used for outgoing hello PDUs and all the valid passwords are used for authenticating incoming PDUs.

Passwords are configured by:

1. Entering password-list mode for a specified password-list.
   
   
2. Entering a password-id mode for a specified ID value for this password-list.
   
   
3. Specify the password associated with the password-id. The start and end time for this password can be configured.
   
   

Multiple password-IDs can be associated with a single password-list.

Use the password-list command to enter password-list configuration mode for the specified list.

Use the no password-list command to delete a specified password-list.

Use the password-id command to enter password-id configuration mode.

Use the no password-id command to remove the password, if any, associated with this password-id.

Use the password command to specify the password for this password-id.

Use the lifetime command to specify both the start time and the time for which this password will loose its validity.

Use the isis authentication password-list for enabling authentication of hello PDUs using the specified list.

Use the no isis authentication password-list for disabling authentication of hello PDUs for the specified list.

Example: In the following example:

1. Configuration mode for password-list list1 is entered.
   
   
2. Configuration mode for password-id is entered for password-IDs 1 and 2
   
   
3. In each password-ID mode a password is assigned and a lifetime of now and infinity is configured.
   
   
4. The password-list list1 is enabled using the authentication password-list command.
   
   
5. The show isis password-list command displays the configuration.
   
   

router#configure terminal Enter configuration commands, one per line. End with CNTL/Z. router(config)#router isis 100 router(config-router)#password-list list1 router(config-router-password-list)#password-id 1 router(config-router-password-record)#password pass1 router(config-router-password-record)#lifetime now infinity router(config-router-password-record)#exit router(config-router-password-list)#password-id 2 router(config-router-password-record)#password pass2 router(config-router-password-record)#lifetime now infinity router(config-router-password-record)#exit router(config-router-password-list)#exit router(config-router)#authentication password-list list1 hmac-md5 level-1 router(config-router)#end router#show isis password-list list1 password-list list1 password-id 1 password pass1 start-time 14:33:16 december 2 2004 end-time infinity password-id 2 password pass2 start-time 14:33:38 december 2 2004 end-time infinity

Configuring the Router as Overloaded

There are some circumstances in which it is advantageous to have an IS-IS router not fully participate in forwarding traffic. For example:

• During startup this router may be temporarily too busy with the tasks associated with convergence to forward traffic.
  
  
• The router is in a test network that has connections to a production network. The overload bit prevents traffic from moving between the two networks.
  
  

You can configure the router to not forward traffic by enabling the overload bit.

When enabled, the overload bit instructs other routers not to use this router as an intermediate hop in their SPF computations. No paths through this router are visible to other routers in the domain. IP and CLNS prefixes directly connected to this router are reachable. The overload bit is enabled/disabled using the set-overload-bit command in Router configuration command mode.

Use the set-overload-bit on-startup seconds command to configure the router to temporarily set the overload bit during startup.

Use the set-overload-bit command to enable the overload bit.

Use the no set-overload-bit command to disable the overload bit.

In the following example:

• The router isis command enables IS-IS on the router, specifies an IS-IS instance, and changes the command mode to Router configuration,
  
  
• The set-overload-bit on-startup 1000 command configures the router to enable the overload bit for 1000 seconds after the router is started.
  
  
• The show isis database detail command displays the setting:
  
  

  NOTE A "1" in the OL (overload) field indicates the overload bit is enabled.

router#configure terminal Enter configuration commands, one per line. End with CNTL/Z. router(config)#router isis tsr router(config-router)#set-overload-bit on-startup 1000 router(config-router)#end router#show isis database detail ISIS Level-1 Link State Database LSPID LSP Seq Num LSP Checksum LSP Holdtime ATT/P/OL 0007.7007.0007.00-00 0x00000014 0x28b0 559 0/0/1 . . .

IS-IS LSP-Full State

When entering IS-IS LSP-full state, IPriori sets the overload bit, increments the sequence number, and withdraws external and interlevel routes from the LSDB. Withdrawn LSP are resent on all interfaces for this IS-IS level. LSP full state is entered when a local router generates the 256th LSP fragment (LSP ID 255), or if the number of redistributed routes into this IS-IS level from another IS-IS level or external protocol (BGP, OSPF) exceeds the value configured using the redistribute maximum-prefixes command.

There are only two ways of exiting LSP-full state. A manual exit using the clear isis lsp-full command or the expiration of a fixed 30 minute timer. A policy change that clears the circumstances that led to the full state condition will not cause an exit of LSP-full state. This is also true in the case of an NSR® fail-over. A policy change configured on the backup route controller will not affect an already existing LSP-full state condition after a NSR® fail-over. If a fail-over takes place while the router is in LSP-full state, the expiration timer is reset to 30 minutes when the new primary is fully operational. Unless manually exited using the clear isis lsp-full command, in addition to the time in LSP-full state prior to the fail-over, LSP-full state will exit 30 minutes after the NSR® fail-over.

The show isis command reports when a router is in LSP-full state. The show running-config command displays the current setting for maximum number of prefixes.

Example: In the following example the show isis command displays this router in LSP-full state. The clear isis lsp-full command causes the router to exit LSP full-state:

router#show isis Global ISIS information ISIS process tag: 1 System ID: 0003.0003.0006 NET: 49.0001.0000.0001.00 Maximum number of areas: 3 There is 1 manual area address 47..0001 There is 1 active area address 47..0001 ISIS level-1 ISIS is enabled on 5 interfaces Distance : 115 Hostname : Enabled Maximum of 1 path per route Number of SPF runs, L1: 9, L2: 1 ISIS Level:L1 is in LSP FULL STATE router#clear isis lsp-full router#

Example: In the following example the redistribute maximum-prefix command sets the maximum number of external prefixes injected into IS-IS at 5000:

router(config)#router isis 1 router(config-router)#redistribute maximum-prefix 5000 router(config-router)#do show running-config isis ! router isis 1 net 49.0001.0000.0001.00 redistribute maximum-prefixes 5000

Link Down Events Processing Delay Command

A delay in the processing of link down events reduces the probability of a false link down indication when IS-IS fragments exist and TLVs in other routers move among fragments. The mpls-isis-link-deletion-delay command provides for up to a 60 second delay in link down event processing.

The format for the command is:

mpls-isis-link-deletion-delay [disabled | 0-60000ms]

The default is 200ms. If the command is disabled, the timer is not used at all. The command is configured in the mpls te command mode.

Configuring IS-IS Metric Style

IS-IS LSPs consist of a fixed header and a number of tuples commonly known as TLVs. The traditional IS-reachability TLV contains information about IS neighbors, including one octet devoted to information about the default metric. Only 6 bits of this octet are used to store the actual metric, which limits the value of the metric to a range from 0 to 63. This IS-IS metric configuration is referred to as narrow configuration. Traffic engineering extensions expand this limitation from the traditional 6 bits to 24 bits. This IS-IS metric configuration is referred to as wise configuration.

NOTE The metric-style command both sends IS-IS metrics in link state packets and includes them in the building of the IS-IS route table. When used, the metric-style command replaces both the send-metric-style and use-metric-style commands. These two metric command styles are mutually exclusive. The last entered command style is used.

NOTE When using IS-IS as the IGP in an MPLS network, the metric style MUST not be set to narrow, or the tunnels will not come up.

Configuring metric style determines which metric behavior received from other IS-IS routers will be processed for this router's SPF calculation, and which metric behavior will be transmitted to other IS-IS routers by this router. The metric style used by an IS-IS router can be either narrow or wide.

Use the metric-style narrow command to transmit only narrow IS-IS metrics and to only process received narrow metrics from other routers for this router's SPF calculation.

Use the metric-style wide command to transmit only wide IS-IS metrics, and to only process received wide metrics from other routers for this router's SPF calculation.

Use the metric-style transition command to transmit both narrow and wide metrics, and if both metrics are received, to process the narrow metrics for this router's SPF calculation. If only wide metrics are received, wide metrics will be used for this router's SPF calculation.

Even though both narrow and wide metrics are transmitted when setting this command to transition, only narrow metric values (0 - 63) may be configured for this router's interface metrics in order to avoid routing errors and loops if another router is using wide metrics instead of narrow. The transition setting facilitates the transitioning of a network from narrow to wide metrics.

Use the no metric-style command to configure this router for the default value.

Example 1: In the following example, the metric style is set to wide for interface pos 1/1/:

router#configure terminal Enter configuration commands, one per line. End with CNTL/Z. router(config)#interface pos 1/1/1 router(config-if)#metric-style wide router(config-if)#end router#

Example 2: In the following example, the metric style is reset to the default of narrow for interface pos 1/1/1:

router#configure terminal Enter configuration commands, one per line. End with CNTL/Z. router(config)#interface pos 1/1/1 router(config-if)#no metric-style router(config-if)#end router#

Enabling IS-IS TLV Exception Checking

IS-IS TLV exception checking performs a series of semantic and syntax checks on IS-IS packets. Any malformed packets are logged and dropped. The overhead associated with enabling exception checking will negatively affect IGP convergence time due to the delay in installing an LSP and triggering an SPF. The syntax check before flooding will cause some delay in network convergence.

The syntax check determines the syntactic correctness of incoming LSPs for all TLVs by code, length and value.

The following semantic checks are made:

• Presence of NLPID in fragment 0 and that it supports IP.
  
  
• Local interface address is checked to assure that it is 32 bits with 0 not allowed in last byte and can not be our address. A check is performed for martian and multicast address.
  
  
• For internal reachability the default metric field must be set to internal - bit 7 of default metric set to 0. A check is performed for martian and multicast address.
  
  
• For external reachability a check is performed for martian and multicast address.
  
  
• The router ID is checked to assure that it is not ours.
  
  
• Area address is checked for correctness based on the AFI. For every AFI, the length of the address must have bounds.
  
  
• The overload bit must be set in the 0th fragment only. If set, IP external reachability must not be present in subsequent LSPs from that router.
  
  
• Host name is checked to assure that an LSP with a different system ID does not have our hostname.
  
  

Use the sanity-check-lsp command in IS-IS router configuration mode to enable IS-IS TLV exception checking on this router.

Use the no sanity-check-lsp command in IS-IS router configuration mode to disable IS-IS TLV exception checking.

Example 1: In the following example IS-IS TLV exception checking is enabled on IS-IS router tsr:

router#configure terminal Enter configuration commands, one per line. End with CNTL/Z. router(config)#router isis tsr router(config-router)#sanity-check-lsp router(config-router)#end router#

Example 2: In the following example IS-IS TLV exception checking is disabled on IS-IS router tsr:

router#configure terminal Enter configuration commands, one per line. End with CNTL/Z. router(config)#router isis tsr router(config-router)#no sanity-check-lsp router(config-router)#end router#

Configuring IS-IS Default Route Origination

IS-IS default route origination provides for the generation of a default route when the router is either a Level-2 or Level-1-2 router. Default route origination has no affect on IS-IS Level-1 routers. The Level-1 router determines the default route by examining the nearest Level-1-2 router with the ATT bit set in its Level-1 LSP. If the IS-IS type is changed from Level-1 to Level-2 or Level-1-2, the router will originate the default route based on the route-map. If a router is configured for default route origination, IS-IS will originate an advertisement 0.0.0.0 in its Level-2 link state packets.

A route-map can be used to advertise 0/0 conditionally. With a match IP address standard access-list command, specify one or more IP routes that must exist for the router to advertise 0/0.

If a route-map is specified and one or more routes in the routing table qualifies any of the access-list or prefix-list conditions, the default route is originated. If the router receives an LSP with the default route, it will install the default route unconditionally. In the event of a default route being redistributed from other routing protocols, default route origination will take the precedence and replace the export entry.

Enabling IS-IS default route origination is recommended for scenarios such as the following:

• The router is an ASBR router and running BGP with external peers.
  
  
• As a result of network policy this border router is chosen for default traffic.
  
  

Use the default-information originate command to enable default route origination on this router.

Use the no default-information originate command to disable default route origination on this router. Specifying any Option with the no command only changes the option to its default value leaving the remainder of the configuration unchanged.

Example 1: The following example forces the generation of a default route into the IS-IS domain if the route-map criteria are met:

router#configure terminal Enter configuration commands, one per line. End with CNTL/Z. router(config)#router isis 1 router(config-router)#net 35.0001.0000.0011.1010.00 router(config-router)#is-type level-2 router(config-router)#default-information originate route-map r1 router(config-router)#show ! router isis 1 net 35.0001.0000.0011.1010.00 is-type level-2-only default-information originate route-map r1 ! router#

Example 2: The following example removes the route map configured in example 1 from the default route origination configuration:

router#configure terminal Enter configuration commands, one per line. End with CNTL/Z. router(config)#router isis 1 router(config-router)#net 35.0001.0000.0011.1010.00 router(config-router)#is-type level-2 router(config-router)#default-information originate router(config-router)#show ! router isis 1 net 35.0001.0000.0011.1010.00 is-type level-2-only default-information originate ! router#

Example 3: The following example removes the default route origination configuration for this router:

router#configure terminal Enter configuration commands, one per line. End with CNTL/Z. router(config)#router isis 1 router(config-router)#no default-information originate ! router isis 1 net 35.0001.0000.0011.1010.00 is-type level-2-only ! router#

Configuring IGP Redistribution

IPriori supports the redistribution of OSPF routes into IS-IS and the redistribution of IS-IS routes into OSPF. The following is supported:

• OSPF process ID to IS-IS route-tag and IS-IS route-tag to OSPF process ID
  
  
• Level based redistribution
  
  
• Metric-type based redistribution
  
  
• Route-map based redistribution
  
  

Use the redistribute isis command in router ospf command mode to redistribute IS-IS routes into OSPF specifying the routing-tag and route level to be redistributed.

Use the show command in the appropriate router configuration mode to display the current redistribution configuration for this router.

Displaying Information About IS-IS

The following commands are available for displaying IS-IS behavior and status.

Table 3-2. IS-IS Show Commands  

Show Command	Description
show isis	Displays general system-level information about the IS-IS process running on this router
show isis database	Displays information the IS-IS routing database
show isis interface	Displays information about the IS-IS process on all interfaces.
show isis neighbor	Displays information about IS-IS neighbors.
show isis topology	Displays a list of all connected routers in all areas.

Copyright © 2005 Avici Systems Inc.
Avici® and TSR® is a registered trademark of Avici Systems Inc.
IPriori™, Composite Links™, SSR™, QSR, and NSR® are trademarks of Avici Systems Inc.

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