IS-IS

Intermediate System to Intermediate System (IS-IS) is a link-state IGP that uses the Shortest Path First (SPF) algorithm to determine routes. Routing decisions are made using the link-state information. IS-IS evaluates topology changes and, if necessary, performs SPF recalculations.

Entities within IS-IS include networks, intermediate systems, and end systems. In IS-IS, a network is an Autonomous System (AS), or routing domain, with end systems and intermediate systems. A router is an intermediate system. End systems are network devices that send and receive protocol data units (PDUs). Intermediate systems send, receive, and forward PDUs.

End system and intermediate system protocols allow routers and nodes to identify each other. IS-IS sends out link-state updates periodically throughout the network, so each router can maintain current network topology information.

IS-IS supports large ASs by using a two-level hierarchy. A large AS can be administratively divided into smaller, more manageable areas. A system logically belongs to one area. Level 1 routing is performed within an area. Level 2 routing is performed between areas. Routers can be configured as Level 1, Level 2, or both Level 1/2.

On SR Linux, you can configure a single named IS-IS instance per network instance. The following summarizes SR Linux support for IS-IS:

  • Level 1, Level 2, and Level 1/2 IS types

  • Configurable network entity title (NET) per IS-IS instance.

  • Support for IPv4/v6 routing

  • ECMP with up to 128 next hops per destination

  • IS-IS export policies (redistribution of other types of routes into IS-IS)

  • Authentication of CSNP, PSNP, and IIH PDUs with authentication type and key, configurable per instance and per instance level. Authentication type and key for IIH PDUs also configurable per interface and level.

  • Support for authentication keychains

  • Purge Originator ID TLV (RFC 6232)

  • Options to ignore and suppress the attached bit

  • Ability to set the overload bit immediately or to set the bit after each subsequent restart of the IS-IS manager application and leave it on for a configurable duration each time

  • Control over the Link-state PDU (LSP) MTU size, with range from 490 bytes to 9490 bytes

  • Configuration control over timers related to LSP lifetime, LSP refresh interval, SPF calculation triggers, and LSP generation

  • Support for hello padding (strict, loose and adaptive modes)

  • Support for graceful restart, but only acting as a helper of the restarting router

  • Level 1 to Level 2 route summarization

  • BFD for fast failure detection

  • Configurable hello timer/multiple per interface and level

  • Support for wide metrics (configurable per level)

  • Configurable route preference for each route type, Level 1-internal, Level 1-external, Level 2-internal and Level 2-external.

The info detail command displays default values for an IS-IS instance on SR Linux. For example:

--{ * candidate shared default }--[ network-instance default protocols isis ]--
# info detail
    instance i1 {
        admin-state disable
        level-capability L2
        max-ecmp-paths 1
        poi-tlv false
        attached-bit {
            ignore false
            suppress false
        }
        overload {
            advertise-interlevel false
            advertise-external false
            immediate {
                set-bit false
                max-metric false
            }
            on-boot {
                set-bit false
                max-metric false
            }
        }
        timers {
            lsp-lifetime 1200
            lsp-refresh {
                interval 600
                half-lifetime true
            }
            spf {
                initial-wait 1000
                second-wait 1000
                max-wait 10000
            }
            lsp-generation {
                initial-wait 1000
                second-wait 1000
                max-wait 5000
            }
        }
        transport {
            lsp-mtu-size 1492
        }
        ipv4-unicast {
            admin-state enable
        }
        ipv6-unicast {
            admin-state enable
            multi-topology false
        }
        graceful-restart {
            helper-mode false
        }
        auto-cost {
        }
        authentication {
            csnp-authentication false
            psnp-authentication false
            hello-authentication false
        }
        inter-level-propagation-policies {
            level1-to-level2 {
            }
        }
    }

Basic IS-IS configuration

To configure IS-IS, perform the following tasks:

  • Enable an IS-IS instance

  • If necessary, modify the level capability on the global IS-IS instance level

  • Define area addresses

  • Configure IS-IS interfaces

Enabling an IS-IS instance

SR Linux supports a single IS-IS instance within a network instance. The following example enables an IS-IS instance within the default network instance.

--{ * candidate shared default }--[ network-instance default protocols ]--
# info isis
 isis {
        instance i1 {
        }
    }

Configuring the router level

When IS-IS is enabled, the default level-capability value is Level 1/2. This means that the router operates with both Level 1 and Level 2 routing capabilities. To change the default value in order for the router to operate as a Level 1 router or a Level 2 router, you must explicitly modify the level value.

The level-capability value can be configured on the global IS-IS instance level and also on the interface level. The level-capability value determines which level values can be assigned on the router level or on an interface-basis.

In order for the router to operate as a Level 1 only router or as a Level 2 only router, you must explicitly specify the level-number value.

  • Specify Level 1 to route only within an area

  • Specify Level 2 to route to destinations outside an area, toward other eligible Level 2 routers

The following example configures the level capability for an IS-IS instance to Level 2.

--{ * candidate shared default }--[ network-instance default protocols ]--
# info isis
 isis {
        instance i1 {
            level-capability L2
        }
    }

Configuring the Network Entity Title

SR Linux supports a configurable network entity title (NET) per IS-IS instance. The NET is 8-20 octets long and consists of 3 parts: the area address (1-13 octets), the system ID (6 octets), and the n-selector (1 octet, must be 00)

The area address portion of the NET defines the IS-IS area to which the router belongs. At least one area address should be configured on each router participating in IS-IS.

The area address portion of the NET identifies a point of connection to the network, such as a router interface. The routers in an area manage routing tables about destinations within the area. The NET value is used to identify the IS-IS area to which the router belongs.

The NET value is divided into three parts. Only the Area ID portion is configurable.

1. Area ID — A variable length field between 1 and 13 bytes. This includes the Authority and Format Identifier (AFI) as the most significant byte and the area ID.

2. System ID — A 6-byte system identifier. This value is not configurable. The system ID is derived from the system or router ID.

3. Selector ID — A 1-byte selector identifier that must contain zeros when configuring a NET. This value is not configurable. The selector ID is always 00.

The following example configures a NET for an IS-IS instance:

--{ * candidate shared default }--[ network-instance default protocols ]--
# info isis
 isis {
        instance i1 {
            net 49.0001.1921.6800.1002.00
            }
        }
    }

Configuring global parameters

Commands and parameters configured on the global IS-IS instance level are inherited by the interface levels. Parameters specified in the interface and interface-level configurations take precedence over global configurations.

The following example shows the command usage to configure global-level IS-IS. The authentication setting references a keychain defined at the system level (see Protocol authentication).

--{ * candidate shared default }--[ network-instance default protocols ]--
# info isis
    isis {
        instance i1 {
            level-capability L2
            overload {
                on-boot {
                    timeout 90
                }
            }
            authentication {
                keychain isisglobal
            }
        }
    }

Configuring interface parameters

There are no interfaces associated with IS-IS by default. An interface belongs to all areas configured on a router. Interfaces cannot belong to separate areas. There are no default interfaces applied to the router IS-IS instance. You must configure at least one IS-IS interface in order for IS-IS to work.

You can configure both the Level 1 parameters and the Level 2 parameters on an interface. The level-capability value determines which level values are used.

The following example configures interface parameters for an IS-IS instance:

--{ * candidate shared default }--[ network-instance default protocols isis ]--
# info instance i1
    instance i1 {
        interface ethernet-1/2.1 {
            circuit-type point-to-point
            ipv4-unicast {
                admin-state enable
            }
            level 1 {
                authentication {
                    hello-authentication true
                }
            }
        }
        level 1 {
        }
    }

Displaying IS-IS information

Use the following show commands to display the following information for an IS-IS instance running in a specified network instance.

  • Interface information

  • Adjacency information

  • IS-IS link state database information

To display summary information for an IS-IS instance:

# show network-instance default protocols isis interface
--------------------------------------------------------------------------------------------------
Network-instance    : default
IS-IS instance      : global
--------------------------------------------------------------------------------------------------
Interface      Oper-State Level   Circuit-Id Circuit-Type  L1/L2 Metric  
lo0.0          up         L2      1          p2p           -/0          
ethernet-1/1.0 up         L1L2    2          p2p           10/100
--------------------------------------------------------------------------------------------------
Interfaces: 2
--------------------------------------------------------------------------------------------------

To display details for each interface:

# show network-instance default protocols isis interface detail
--------------------------------------------------------------------------------------------------
Network-instance    : default
IS-IS instance      : global
--------------------------------------------------------------------------------------------------
Interface: lo0.0
  Status          : IS-IS is admin enabled, oper up
  Circuit id/type : id 1, passive
  Hello auth      : disabled
  Hello padding   : disabled
  CSNP interval   : n/a
  LSP pacing      : n/a
  ------------------------------------------------------------------------------------------------
  Level 1 is disabled
  ------------------------------------------------------------------------------------------------
  Level 2 is enabled
  Adjacencies         : 0
  Designated IS       : n/a
  Hello auth          : disabled
  DIS priority        : 64
  Hello interval      : 9
  Hello multiplier    : 3
  Metric              : 0
  IPv6-unicast metric : 0
  ------------------------------------------------------------------------------------------------
--------------------------------------------------------------------------------------------------
Interface: ethernet-1/1.0
  Status          : IS-IS is admin enabled, oper up
  Circuit id/type : id 2, point-to-point
  Hello auth      : enabled using keychain "test"
  Hello padding   : strict
  CSNP interval   : 10 seconds
  LSP pacing      : 100 ms
  ------------------------------------------------------------------------------------------------
  Level 1 is enabled
  Adjacencies         : 1
  Designated IS       : n/a
  Hello auth          : enabled using keychain "test"
  DIS priority        : 64
  Hello interval      : 9
  Hello multiplier    : 3
  Metric              : 10
  IPv6-unicast metric : 0
  ------------------------------------------------------------------------------------------------
  Level 2 is enabled
  Adjacencies         : 1
  Designated IS       : n/a
  Hello auth          : enabled using keychain "test"
  DIS priority        : 64
  Hello interval      : 9
  Hello multiplier    : 3
  Metric              : 100
  IPv6-unicast metric : 0
  ------------------------------------------------------------------------------------------------
--------------------------------------------------------------------------------------------------
Interfaces: 2
--------------------------------------------------------------------------------------------------

To display IS-IS adjacency information:

# show network-instance default protocols isis adjacency
--------------------------------------------------------------------------------------------------
Network-instance    : default
IS-IS instance      : global
--------------------------------------------------------------------------------------------------
System-Id      Adj-Level  Interface        IPv4-Address  State  Uptime       Rem-Hold  
<hostname1>    L1         ethernet-1/1.0   10.0.0.1      Up     0d 00:46:43  19s
<hostname1>    L2         ethernet-1/1.0   10.0.0.1      Up     0d 00:46:43  19s
--------------------------------------------------------------------------------------------------
Adjacencies: 2
--------------------------------------------------------------------------------------------------

To display information from the IS-IS link state database:

# show network-instance default protocols isis database
--------------------------------------------------------------------------------------------------
Network-instance    : default
IS-IS instance      : global
--------------------------------------------------------------------------------------------------
IS-IS level 1 link state database
LSP count: 3
--------------------------------------------------------------------------------------------------
LSP ID       Sequence   Checksum   Lifetime   Attributes
R1.00-00     0x12       0x58b6     708        L1 L2 ATT OL
R2.00-00     0x140x39a6     834        L1 L2
R2.03-00     0x10       0xdee2     915        L1 L2
--------------------------------------------------------------------------------------------------
IS-IS level 2 link state database
LSP count: 3
--------------------------------------------------------------------------------------------------
LSP ID       Sequence   Checksum   Lifetime   Attributes
R1.00-00     0x49       0x8850     810        L1 L2 OL   
R2.00-00     0x490x9d04     906        L1 L2
R2.03-00     0x10       0xdbba     937        L1 L2
--------------------------------------------------------------------------------------------------

Clearing IS-IS information

To clear information for an IS-IS instance, use the tools commands below:

To clear statistics for an IS-IS instance running in a specified network instance:

# tools network-instance default protocols isis instance i1 statistics clear

To clear link state database information for a level:

# tools network-instance default protocols isis instance i1 link-state-database clear

To clear IS-IS adjacency information for an interface:

# tools network-instance default protocols isis instance i1 interface ethernet 1/2 adjacencies clear