# ISIS

ISIS is a routing protocol which is described in ISO10589, RFC 1195, RFC 5308. ISIS is an IGP. Compared with RIP, ISIS can provide scalable network support and faster convergence times like OSPF. ISIS is widely used in large networks such as ISP and carrier backbone networks.

## Configuring isisd

There are no isisd specific options. Common options can be specified (Common Invocation Options) to isisd. isisd needs to acquire interface information from zebra in order to function. Therefore zebra must be running before invoking isisd. Also, if zebra is restarted then isisd must be too.

Like other daemons, isisd configuration is done in ISIS specific configuration file `isisd.conf`.

## ISIS router

To start the ISIS process you have to specify the ISIS router. As of this writing, isisd does not support multiple ISIS processes.

router isis WORD [vrf NAME]

  Enable or disable the ISIS process by specifying the ISIS domain with 'WORD'. isisd does not yet support multiple ISIS processes but you must specify the name of ISIS process. The ISIS process name 'WORD' is then used for interface (see command `ip router isis WORD`).

net XX.XXXX. ... .XXX.XX

  Set/Unset network entity title (NET) provided in ISO format.

hostname dynamic

  Enable support for dynamic hostname.

area-password [clear | md5] <password>

domain-password [clear | md5] <password>

  Configure the authentication password for an area, respectively a domain, as clear text or md5 one.

attached-bit [receive ignore | send]

  Set attached bit for inter-area traffic:

  - receive If LSP received with attached bit set, create default route to neighbor

  - send If L1|L2 router, set attached bit in LSP sent to L1 router

log-adjacency-changes

  Log changes in adjacency state.

log-pdu-drops

  Log any dropped PDUs.

metric-style [narrow | transition | wide]

  Set old-style (ISO 10589) or new-style packet formats:

  - narrow Use old style of TLVs with narrow metric

  - transition Send and accept both styles of TLVs during transition

  - wide Use new style of TLVs to carry wider metric. FRR uses this as a default value

advertise-high-metrics

  Advertise high metric value on all interfaces to gracefully shift traffic off the router. Reference: RFC 3277

  For narrow metrics, the high metric value is 63; for wide metrics, 16777215; for transition metrics, 62.

set-overload-bit

  Set overload bit to avoid any transit traffic.

set-overload-bit on-startup (0-86400)

  Set overload bit on startup for the specified duration, in seconds. Reference: RFC 3277

purge-originator

  Enable or disable RFC 6232 purge originator identification.

lsp-mtu (128-4352)

  Configure the maximum size of generated LSPs, in bytes.

advertise-passive-only

  Advertise prefixes of passive interfaces only.

## ISIS Timer

lsp-gen-interval [level-1 | level-2] (1-120)

  Set minimum interval in seconds between regenerating same LSP, globally, for an area (level-1) or a domain (level-2).

lsp-refresh-interval [level-1 | level-2] (1-65235)

  Set LSP refresh interval in seconds, globally, for an area (level-1) or a domain (level-2).

max-lsp-lifetime [level-1 | level-2] (360-65535)

  Set LSP maximum LSP lifetime in seconds, globally, for an area (level-1) or a domain (level-2).

spf-interval [level-1 | level-2] (1-120)

  Set minimum interval between consecutive SPF calculations in seconds.

## ISIS Fast-Reroute

Unless stated otherwise, commands in this section apply to all LFA flavors (local LFA, Remote LFA and TI-LFA).

spf prefix-priority [critical | high | medium] WORD

  Assign a priority to the prefixes that match the specified access-list.

  By default loopback prefixes have medium priority and non-loopback prefixes have low priority.

fast-reroute priority-limit [critical | high | medium] [level-1 | level-2]

  Limit LFA backup computation up to the specified prefix priority.

fast-reroute lfa tiebreaker [downstream | lowest-backup-metric | node-protecting] index (1-255) [level-1 | level-2]

  Configure a tie-breaker for multiple local LFA backups. Lower indexes are processed first.

fast-reroute load-sharing disable [level-1 | level-2]

  Disable load sharing across multiple LFA backups.

fast-reroute remote-lfa prefix-list [WORD] [level-1 | level-2]

  Configure a prefix-list to select eligible PQ nodes for remote LFA backups (valid for all protected interfaces).

redistribute <ipv4 | ipv6> table (1-65535) <level-1 | level-2> [metric (0-16777215)|route-map WORD]

  Redistribute routes from a given routing table into the given ISIS level database.

## ISIS region

is-type [level-1 | level-1-2 | level-2-only]

  Define the ISIS router behavior:

  - level-1 Act as a station router only

  - level-1-2 Act as both a station router and an area router

  - level-2-only Act as an area router only

## ISIS interface

<ip|ipv6> router isis WORD

  Activate ISIS adjacency on this interface. Note that the name of ISIS instance must be the same as the one used to configure the ISIS process (see command `router isis WORD`). To enable IPv4, issue `ip router isis WORD`; to enable IPv6, issue `ipv6 router isis WORD`.

isis circuit-type [level-1 | level-1-2 | level-2]

  Configure circuit type for interface:

  - level-1 Level-1 only adjacencies are formed

  - level-1-2 Level-1-2 adjacencies are formed

  - level-2-only Level-2 only adjacencies are formed

isis csnp-interval (1-600) [level-1 | level-2]

  Set CSNP interval in seconds globally, for an area (level-1) or a domain (level-2).

isis hello padding

  Add padding to IS-IS hello packets.

isis hello padding during-adjacency-formation

  Add padding to IS-IS hello packets during adjacency formation only.

isis hello-interval (1-600) [level-1 | level-2]

  Set Hello interval in seconds globally, for an area (level-1) or a domain (level-2).

isis hello-multiplier (2-100) [level-1 | level-2]

  Set multiplier for Hello holding time globally, for an area (level-1) or a domain (level-2).

isis metric [(0-255) | (0-16777215)] [level-1 | level-2]

  Set default metric value globally, for an area (level-1) or a domain (level-2). Max value depend if metric support narrow or wide value (see command `metric-style [narrow | transition | wide]`).

isis network point-to-point

  Set network type to 'Point-to-Point' (broadcast by default).

isis passive

  Configure the passive mode for this interface.

isis password [clear | md5] <password>

  Configure the authentication password (clear or encoded text) for the interface.

isis priority (0-127) [level-1 | level-2]

  Set priority for Designated Router election, globally, for the area (level-1) or the domain (level-2).

isis psnp-interval (1-120) [level-1 | level-2]

  Set PSNP interval in seconds globally, for an area (level-1) or a domain (level-2).

isis three-way-handshake

  Enable or disable RFC 5303 Three-Way Handshake for P2P adjacencies. Three-Way Handshake is enabled by default.

isis fast-reroute lfa [level-1 | level-2]

  Enable per-prefix local LFA fast reroute link protection.

isis fast-reroute lfa [level-1 | level-2] exclude interface IFNAME

  Exclude an interface from the local LFA backup nexthop computation.

isis fast-reroute remote-lfa tunnel mpls-ldp [level-1 | level-2]

  Enable per-prefix Remote LFA fast reroute link protection. Note that other routers in the network need to be configured to accept LDP targeted hello messages in order for RLFA to work.

isis fast-reroute remote-lfa maximum-metric (1-16777215) [level-1 | level-2]

  Limit Remote LFA PQ node selection within the specified metric.

isis fast-reroute ti-lfa [level-1|level-2] [node-protection [link-fallback]]

  Enable per-prefix TI-LFA fast reroute link or node protection. When node protection is used, option link-fallback enables the computation and use of link-protecting LFAs for destinations unprotected by node protection.

## Showing ISIS information

show isis [vrf <NAME|all>] summary [json]

  Show summary information about ISIS.

show isis hostname

  Show information about ISIS node.

show isis [vrf <NAME|all>] interface [detail] [IFNAME] [json]

  Show state and configuration of ISIS specified interface, or all interfaces if no interface is given with or without details.

show isis [vrf <NAME|all>] neighbor [detail] [SYSTEMID] [json]

  Show state and information of ISIS specified neighbor, or all neighbors if no system id is given with or without details.

show isis [vrf <NAME|all>] database [detail] [LSPID] [json]

  Show the ISIS database globally, for a specific LSP id without or with details.

show isis topology [level-1|level-2] [algorithm (128-255)]

  Show topology IS-IS paths to Intermediate Systems, globally, in area (level-1) or domain (level-2).

show isis route [level-1|level-2] [prefix-sid|backup] [algorithm (128-255)]

  Show the ISIS routing table, as determined by the most recent SPF calculation.

show isis fast-reroute summary [level-1|level-2]

  Show information about the number of prefixes having LFA protection, and network-wide LFA coverage.

## Traffic Engineering

Note

IS-IS-TE supports RFC 5305 (base TE), RFC 6119 (IPv6) and RFC 7810 / 8570 (Extended Metric) with or without Multi-Topology. All Traffic Engineering information are stored in a database formally named TED. However, best acccuracy is provided without Multi-Topology due to inconsistency of Traffic Engineering Advertisement of 3rd party commercial routers when MT is enabled. At this time, FRR offers partial support for some of the routing protocol extensions that can be used with MPLS-TE. FRR does not currently support a complete RSVP-TE solution.

mpls-te on

  Enable Traffic Engineering LSP flooding.

mpls-te router-address <A.B.C.D>

  Configure stable IP address for MPLS-TE.

mpls-te router-address ipv6 <X:X::X:X>

  Configure stable IPv6 address for MPLS-TE.

mpls-te export

  Export Traffic Engineering DataBase to other daemons through the ZAPI Opaque Link State messages.

show isis mpls-te interface

show isis mpls-te interface INTERFACE

  Show MPLS Traffic Engineering parameters for all or specified interface.

show isis mpls-te router

  Show Traffic Engineering router parameters.

show isis [vrf <NAME|all>] mpls-te database [detail|json]

show isis [vrf <NAME|all>] mpls-te database vertex [WORD] [detail|json]

show isis [vrf <NAME|all>] mpls-te database edge [A.B.C.D|X:X::X:X] [detail|json]

show isis [vrf <NAME|all>] mpls-te database subnet [A.B.C.D/M|X:X::X:X/M] [detail|json]

  Show Traffic Engineering Database

See also

Traffic Engineering

## Segment Routing

This is an EXPERIMENTAL support of Segment Routing as per RFC8667 for MPLS dataplane. It supports IPv4, IPv6 and ECMP and has been tested against Cisco & Juniper routers.

Known limitations:

  - No support for level redistribution (L1 to L2 or L2 to L1)

  - No support for binding SID

  - No support for SRMS

  - No support for SRLB

  - Only one SRGB and default SPF Algorithm is supported

segment-routing on

  Enable Segment Routing.

segment-routing global-block (16-1048575) (16-1048575) [local-block (16-1048575) (16-1048575)]

  Set the Segment Routing Global Block i.e. the label range used by MPLS to store label in the MPLS FIB for Prefix SID. Note that the block size may not exceed 65535. Optionally sets also the Segment Routing Local Block. The negative command always unsets both.

segment-routing node-msd (1-16)

  Set the Maximum Stack Depth supported by the router. The value depend of the MPLS dataplane. E.g. for Linux kernel, since version 4.13 the maximum value is 32.

segment-routing prefix <A.B.C.D/M|X:X::X:X/M> [algorithm (128-255)] <absolute (16-1048575)|index (0-65535) [no-php-flag|explicit-null] [n-flag-clear]

  prefix. The 'no-php-flag' means NO Penultimate Hop Popping that allows SR node to request to its neighbor to not pop the label. The 'explicit-null' flag allows SR node to request to its neighbor to send IP packet with the EXPLICIT-NULL label. The 'n-flag-clear' option can be used to explicitly clear the Node flag that is set by default for Prefix-SIDs associated to loopback addresses. This option is necessary to configure Anycast-SIDs.

show isis segment-routing node [algorithm (128-255)]

  Show detailed information about all learned Segment Routing Nodes.

## Flex-Algos (Flex-Algo)

isisd supports some features of RFC 9350 on an MPLS Segment-Routing dataplane. The compatibility has been tested against Cisco.

IS-IS uses by default the Shortest-Path-First algorithm that basically calculates paths based on the shortest total metric to the destinations. Flex-Algo allows new algorithms to run in parallel to compute paths in different manners, based on metrics (IGP metric or a new type of metrics such as Traffic Engineering (TE) metric and minimum delay...) and constraints. New metric types are not yet implemented but constraints are already operational. Constraints can restrict paths to links with specific affinities or avoid links with specific affinities. Combinations of these are also possible.

The administrator can configure up to 128 Flex-Algos in an IS-IS area. To do so, it defines a set of Flex-Algo Definitions (FAD) which have the following characteristics:

- a numeric identifier (ID) between 128 and 255 inclusive

- a set of constraints (basically, include or exclude a certain given set of

    links, designated by a admin-group)

- the calculation type (only the Shortest-Path-First is currently supported)

- the metric type (only the IGP inherited metric type is currently supported)

- some additional flags (not supported for the moment).

A subset of routers advertises the Flex-Algo Definitions (FAD) to the other routers within an area. In order to use a common set of FADs, each router runs a FAD election process for each locally configured algorithm, using the following rules:

- If a locally configured FAD is not advertised to the area, the router does not

    participate in the particular flex algorithm.

- If a given flex algorithm is running, the participation in this particular

    flex algorithm stops when its advertisements are over.

- A router includes its own FAD in the election process if and only if it is

    advertised to the other routers.

- If only one router advertises the FAD, the FAD is elected.

- If several FADs are advertised with different priorities, the one with the

    highest priority value is selected.

- If there are multiple advertisements of the FAD with the same highest

    priority, the FAD of the router with the highest IS-IS system-ID is selected.

Routers only use the specifications of the elected FAD regardless of the locally configured definitions. If a router does not support one of the FAD characteristics, it stops participating in the Flex-Algo.

For each running Flex-Algo, the Segment-Routing SIDs must be configured with values unique to the algorithm. It allows routers to identify which flex algorithm they must use for a given packet.

The following commands configure Flex-Algo at the 'router isis' configuration level. Segment-Routing prefixes must be configured for the Flex-Algo.

flexible-algorithm (128-255)

  Add a Flex-Algo Definition (FAD) and enter the FAD configuration level. The algorithm ID value is in the range of 128 to 255 inclusive.

no flexible-algorithm (128-255)

  Unconfigure a Flex-Algo Definition.

affinity-map NAME bit-position (0-255)

  Add the specified 'affinity-map'. Affinity-map definitions are used in FADs and in interfaces admin-group definition.

  Affinity-maps format in advertisement TLVs use the extended admin-group format defined in the RFC7308 section 2.2. The extended admin-group uses a 256 bits field. If an affinity-map is set, the bit at the extended admin-group 'bit-position' is set 1, else it is set to 0.

The following commands configure Flex-Algo at the 'router isis' and 'flexible-algorithm (128-255)' configuration level.

advertise-definition

  Advertise the current FAD to other IS-IS routers by using specific IS-IS TLVs. By default, the definition is is not shared with other routers.

A router can advertise a FAD without participating in the Flex-Algo.

priority (0-255)

  Set the specified 'priority' in the current FAD advertisements .

metric-type [igp|te|delay]

  Set the 'metric-type' for the current FAD. 'igp' is the default value and refers to the classic 'Shortest-Path-First' algorithm. If the 'te' or the 'delay' metric is selected, the value is advertised but the flex algorithm is disabled locally because these types are not currently supported.

no metric-type

  Reset the 'metric-type' to the default 'igp' metric.

affinity exclude-any NAME

  Add the specified affinity to the list of exclude-any affinities. The Flex-Algo will compute paths that exclude the segments with any of the specified affinities.

no affinity exclude-any NAME

  Remove the specified affinity to the list of exclude-any affinities.

affinity include-all NAME

  Add the specified affinity to the list of include-all affinities. The Flex-Algo will compute paths that include the segments with all the specified affinities.

no affinity include-all NAME

  Remove the specified affinity to the list of include-all affinities.

affinity include-any NAME

  Add the specified affinity to the list of include-any affinities. The Flex-Algo will compute paths that include the segments with any of the specified affinities.

no affinity include-any NAME

  Remove the specified affinity to the list of include-any affinities.

The following commands configure Flex-Algo at the 'interface' configuration level.

isis affinity flex-algo NAME

  Add the specified affinity to the interface.

no isis affinity flex-algo NAME

  Remove the specified affinity from the interface.

The following command show Flex-Algo information:

show isis flex-algo [(128-255)]

  Show information about the elected FADs

'show isis route', 'show isis topology' and 'show isis segment-routing node' includes an 'algorithm (128-255)' optional argument. See Showing ISIS information and Segment Routing.

## Segment Routing over IPv6 (SRv6)

This feature enables extensions in IS-IS to support Segment Routing over IPv6 data plane (SRv6) as per RFC 9352.

segment-routing srv6

  Enable Segment Routing over IPv6 data plane (SRv6).

locator NAME

  Specify the SRv6 locator to use for SRv6. The locator must be configured in Zebra. Once the locator is configured, IS-IS automatically allocates prefix SID and adjacency SIDs, creates local SID entries in the data plane, and advertises them in the IGP domain.

interface NAME

  Specify the dummy interface used to install SRv6 SIDs in the Linux data plane. The interface must be created manually. By default, the interface is 'sr0'. The interface can be created using the iproute2 utility:

      ip link add sr0 type dummy
      ip link set sr0 up

show isis segment-routing srv6 node

  Show detailed information about all learned SRv6 Nodes.

## Debugging ISIS

debug isis adj-packets

  IS-IS Adjacency related packets.

debug isis checksum-errors

  IS-IS LSP checksum errors.

debug isis events

  IS-IS Events.

debug isis local-updates

  IS-IS local update packets.

debug isis packet-dump

  IS-IS packet dump.

debug isis protocol-errors

  IS-IS LSP protocol errors.

debug isis route-events

  IS-IS Route related events.

debug isis snp-packets

  IS-IS CSNP/PSNP packets.

debug isis spf-events

debug isis spf-statistics

debug isis spf-triggers

  IS-IS Shortest Path First Events, Timing and Statistic Data and triggering events.

debug isis update-packets

  Update related packets.

debug isis te-events

  IS-IS Traffic Engineering events

debug isis sr-events

  IS-IS Segment Routing events.

debug isis lfa

  IS-IS LFA events.

show debugging isis

  Print which ISIS debug level is activate.

## ISIS Configuration Examples

A simple example, with MD5 authentication enabled:

    !
    interface eth0
     ip router isis FOO
     isis network point-to-point
     isis circuit-type level-2-only
    !
    router isis FOO
    net 47.0023.0000.0000.0000.0000.0000.0000.1900.0004.00
     metric-style wide
     is-type level-2-only

A Traffic Engineering configuration, with Inter-ASv2 support.

First, the `zebra.conf` part:

    hostname HOSTNAME
    password PASSWORD
    log file /var/log/zebra.log
    !
    interface eth0
     ip address 10.2.2.2/24
     link-params
      max-bw 1.25e+07
      max-rsv-bw 1.25e+06
      unrsv-bw 0 1.25e+06
      unrsv-bw 1 1.25e+06
      unrsv-bw 2 1.25e+06
      unrsv-bw 3 1.25e+06
      unrsv-bw 4 1.25e+06
      unrsv-bw 5 1.25e+06
      unrsv-bw 6 1.25e+06
      unrsv-bw 7 1.25e+06
      admin-grp 0xab
    !
    interface eth1
     ip address 10.1.1.1/24
     link-params
      enable
      metric 100
      max-bw 1.25e+07
      max-rsv-bw 1.25e+06
      unrsv-bw 0 1.25e+06
      unrsv-bw 1 1.25e+06
      unrsv-bw 2 1.25e+06
      unrsv-bw 3 1.25e+06
      unrsv-bw 4 1.25e+06
      unrsv-bw 5 1.25e+06
      unrsv-bw 6 1.25e+06
      unrsv-bw 7 1.25e+06
      neighbor 10.1.1.2 as 65000

Then the `isisd.conf` itself:

    hostname HOSTNAME
    password PASSWORD
    log file /var/log/isisd.log
    !
    !
    interface eth0
     ip router isis FOO
    !
    interface eth1
     ip router isis FOO
    !
    !
    router isis FOO
     isis net 47.0023.0000.0000.0000.0000.0000.0000.1900.0004.00
      mpls-te on
      mpls-te router-address 10.1.1.1
    !
    line vty

A Segment Routing configuration, with IPv4, IPv6, SRGB and MSD configuration.

    hostname HOSTNAME
    password PASSWORD
    log file /var/log/isisd.log
    !
    !
    interface eth0
     ip router isis SR
     isis network point-to-point
    !
    interface eth1
     ip router isis SR
    !
    !
    router isis SR
     net 49.0000.0000.0000.0001.00
     is-type level-1
     topology ipv6-unicast
     lsp-gen-interval 2
     segment-routing on
     segment-routing node-msd 8
     segment-routing prefix 10.1.1.1/32 index 100 explicit-null
     segment-routing prefix 2001:db8:1000::1/128 index 101 explicit-null
    !

An SRv6 configuration:

    hostname HOSTNAME
    password PASSWORD
    log file /var/log/isisd.log
    !
    !
    interface eth0
    ipv6 router isis FOO
    ip router isis FOO
    isis hello-interval 5
    !
    interface eth1
    ip router isis FOO
    !
    !
    router isis FOO
    net 49.0001.1111.1111.1111.00
    is-type level-2-only
    metric-style wide
    segment-routing srv6
       locator loc1
    !
    line vty

## ISIS Vrf Configuration Examples

A simple vrf example:

    !
    interface eth0 vrf RED
     ip router isis FOO vrf RED
     isis network point-to-point
     isis circuit-type level-2-only
    !
    router isis FOO vrf RED
     net 47.0023.0000.0000.0000.0000.0000.0000.1900.0004.00
     metric-style wide
     is-type level-2-only