# PATH

PATH is a daemon that handles the installation and deletion of Segment Routing (SR) Policies. Based on MPLS (This means that your OS of choice must support MPLS), SR add a stack of MPLS labels to ingress packets so these packets are egress through the desired path.

The SR policies and Segment Lists can be configured either locally by means of vtysh or centralized based on a SDN controller (ODL, Cisco, ...) communicating using the PCEP protocol (RFC 5440).

## Configuration

### Explicit Segment Lists

This is the simplest way of configuration, no remote PCE is necessary. In order to create a config that match the graphics used in this documentation, we will create a segment list (SL) called SL1 with an element for each hop and that element will be assigned a MPLS label. Then the SL1 will be used in the policy `example1`, please note also the preference as in the case of multiple segment list it will be used with the criteria of bigger number more preference. Let see now the final configuration that match the graphics shown above.

    segment-routing
     traffic-eng
      segment-list SL1
       index 10  mpls label 16001
       index 20  mpls label 16002
      !
      policy color 1 endpoint 192.0.2.4
       name example1
       binding-sid 1111
       candidate-path preference 100 name CP1 explicit segment-list SL1

### Explicit Segment Lists and Traffic Engineering Database (TED)

Sometimes is difficult to know the values of MPLS labels (adjacency changes,...). Based on the support of IS-IS or OSPF we can activate TED support what will allow pathd to resolve MPLS based in different types of segments (:rfc: draft-ietf-spring-segment-routing-policy-07). The supported types are Type C (prefix and local interface), Type E (prefix and algorithm), Type F (a pair of IP's). So the configuration would change to this

    segment-routing
     traffic-eng
      mpls-te on
      mpls-te import ospfv2
      segment-list SL1
       index 10  nai prefix 10.1.2.1/32 iface 1
       index 20  nai adjacency 10.1.20.1 10.1.20.2
      !
      policy color 1 endpoint 192.0.2.4
       name example1
       binding-sid 1111
       candidate-path preference 100 name CP1 explicit segment-list SL1

In this case no MPLS are provided but the pathd TED support will resolve the configuration provided to corresponding MPLS labels.

Note

Please note the `mpls-te` configuration added that activate the TED support and points to `ospfv2` so the ospfv2 (Traffic Engineering) daemon must be also running and configure to export TED information.

Note

It would be the same for isis (Traffic Engineering) but in the moment of writing it's not fully tested.

### Dynamic Segment Lists

One of the useful options to configure is the creation of policies with the dynamic option. In this case based on a given endpoint the SL will be ,first calculated, and then sent by means of PCEP protocol by the configured PCE.

    traffic-eng
     !
     pcep
      !
      pce PCE1
       address ip 192.0.2.10
      !
      pcc
       peer PCE1 precedence 10
     !
     policy color 1 endpoint 192.0.2.4
      name example
      binding-sid 1111
      candidate-path preference 100 name CP2 dynamic

Note

Please note the configuration for the remote pce which allows pathd to connect to the given PCE and act as a PCC (PCEP Client)

Note

If the TED support feature is active, the data obtained from PCE will be validated, so in a SL from PCEP/PCE the IP and MPLS will be checked against local TED obtained and built from the igp configured in that case.

### Pce Initiated

We can step forward in the use of our controller not only by asking to calculate paths to an endpoint but also to create the whole policies in the controller and obtain those by means of the PCEP protocol.

    traffic-eng
     !
     pcep
      !
      pce PCE1
       address ip 192.0.2.10
       pce-initiated
      !
      pce PCE2
       address ip 192.0.2.9
       pce-initiated
      !
      pcc
       peer PCE1 precedence 10
       peer PCE2 precedence 20
     !

Note

Now there is no locally created policies in the config as they will be obtain from the configured pce. Please check command `show sr-te policy` in `vtysh` to see the obtained policies.

Note

Another interesting command is `show mpls table` to check the installed mpls configuration based in those obtained policies.

Note

SR Policies could be a mix of local, remote obtained from PCE and delegated to a PCE (but while testing Pce Initiated with Cisco PCE, happens that controller sends PCE initiated delete commands to delete the locally created configuration related to that PCE).

## Starting

Default configuration file for pathd is `pathd.conf`. The typical location of `pathd.conf` is /etc/frr/pathd.conf.

If the user is using integrated config, then `pathd.conf` need not be present and the `frr.conf` is read instead.

PATH supports all the common FRR daemon start options (Common Invocation Options).

## PCEP Support

A pceplib is included in the frr source tree and build by default.

To start pathd with pcep support the extra parameter -M pathd_pcep should be passed to the pathd daemon.

An example of command line with pcep module could be this

    pathd -u root -g root -f pathd.conf -z /tmp/zebra-demo1.sock --vty_socket=/var/run/demo1.vty -i /tmp/pathd-demo1.pid -M frr/modules/pathd_pcep.so --log file:/tmp/kk.txt

## Pathd Configuration

Example:

    debug pathd pcep basic
    segment-routing
     traffic-eng
      mpls-te on
      mpls-te import ospfv2
      segment-list SL1
       index 10 mpls label 16010
       index 20 mpls label 16030
      !
      segment-list SL2
       index 10  nai prefix 10.1.2.1/32 iface 1
       index 20  nai adjacency 10.1.20.1 10.1.20.2
       index 30  nai prefix 10.10.10.5/32 algorithm 0
       index 40  mpls label 18001
      !
      policy color 1 endpoint 192.0.2.1
       name default
       binding-sid 4000
       candidate-path preference 100 name CP1 explicit segment-list SL1
       candidate-path preference 200 name CP2 dynamic
        affinity include-any 0x000000FF
        bandwidth 100000
        metric bound msd 16 required
        metric te 10
        objective-function mcp required
      !
      pcep
       pce-config GROUP1
        source-address 192.0.2.1
        tcp-md5-auth secret
        timer keep-alive 30
       !
       pce PCE1
        config GROUP1
        address ip 192.0.2.10
       !
       pce PCE2
        config GROUP1
        address ip 192.0.2.9
       !
       pcc
        peer PCE1 precedence 10
        peer PCE2 precedence 20
       !
      !
     !
    !

### Configuration Commands

segment-routing

  Configure segment routing.

traffic-eng

  Configure segment routing traffic engineering.

mpls-te <on|off>

  Activate/Deactivate use of internal Traffic Engineering Database

mpls-te import <ospfv2|ospfv3|isis>

  Load data from the selected igp

segment-list NAME

  Delete or start a segment list definition.

index INDEX mpls label LABEL

index INDEX nai adjacency A.B.C.D A.B.C.D

index INDEX nai prefix A.B.C.D/M algorithm <0|1>

index INDEX nai prefix A.B.C.D/M iface (0-65535)

  Delete or specify a segment in a segment list definition.

policy color COLOR endpoint ENDPOINT

  Delete or start a policy definition.

name NAME

  Specify the policy name.

binding-sid LABEL

  Specify the policy SID.

candidate-path preference PREFERENCE name NAME explicit segment-list SEGMENT-LIST-NAME

  Delete or define an explicit candidate path.

candidate-path preference PREFERENCE name NAME dynamic

  Delete or start a dynamic candidate path definition.

affinity <exclude-any|include-any|include-all> BITPATTERN

  Delete or specify an affinity constraint for a dynamic candidate path.

bandwidth BANDWIDTH [required]

  Delete or specify a bandwidth constraint for a dynamic candidate path.

metric [bound] METRIC VALUE [required] [computed]

  Delete or specify a metric constraint for a dynamic candidate path.

  The possible metrics are:

    - igp: IGP metric

    - te: TE metric

    - hc: Hop Counts

    - abc: Aggregate bandwidth consumption

    - mll: Load of the most loaded link

    - igp: Cumulative IGP cost

    - cte: Cumulative TE cost

    - igp: P2MP IGP metric

    - pte: P2MP TE metric

    - phc: P2MP hop count metric

    - msd: Segment-ID (SID) Depth

    - pd: Path Delay metric

    - pdv: Path Delay Variation metric

    - pl: Path Loss metric

    - ppd: P2MP Path Delay metric

    - pdv: P2MP Path Delay variation metric

    - ppl: P2MP Path Loss metric

    - nap: Number of adaptations on a path

    - nlp: Number of layers on a path

    - dc: Domain Count metric

    - bnc: Border Node Count metric

objective-function OBJFUN1 [required]

  Delete or specify a PCEP objective function constraint for a dynamic candidate path.

  The possible functions are:

    - mcp: Minimum Cost Path [RFC5541]

    - mlp: Minimum Load Path [RFC5541]

    - mbp: Maximum residual Bandwidth Path [RFC5541]

    - mbc: Minimize aggregate Bandwidth Consumption [RFC5541]

    - mll: Minimize the Load of the most loaded Link [RFC5541]

    - mcc: Minimize the Cumulative Cost of a set of paths [RFC5541]

    - spt: Shortest Path Tree [RFC8306]

    - mct: Minimum Cost Tree [RFC8306]

    - mplp: Minimum Packet Loss Path [RFC8233]

    - mup: Maximum Under-Utilized Path [RFC8233]

    - mrup: Maximum Reserved Under-Utilized Path [RFC8233]

    - mtd: Minimize the number of Transit Domains [RFC8685]

    - mbn: Minimize the number of Border Nodes [RFC8685]

    - mctd: Minimize the number of Common Transit Domains [RFC8685]

    - msl: Minimize the number of Shared Links [RFC8800]

    - mss: Minimize the number of Shared SRLGs [RFC8800]

    - msn: Minimize the number of Shared Nodes [RFC8800]

debug pathd pcep [basic|path|message|pceplib]

  Enable or disable debugging for the pcep module:

  > - basic: Enable basic PCEP logging

  > - path: Log the path structures

  > - message: Log the PCEP messages

  > - pceplib: Enable pceplib logging

pcep

  Configure PCEP support.

pce-config NAME

  Define a shared PCE configuration that can be used in multiple PCE declarations.

pce NAME

  Define or delete a PCE definition.

config WORD

  Select a shared configuration. If not defined, the default configuration will be used.

address <ip A.B.C.D | ipv6 X:X::X:X> [port (1024-65535)]

  Define the address and port of the PCE.

  If not specified, the port is the standard PCEP port 4189.

  This should be specified in the PCC peer definition.

source-address [ip A.B.C.D | ipv6 X:X::X:X] [port PORT]

  Define the address and/or port of the PCC as seen by the PCE. This can be used in a configuration group or a PCC peer declaration.

  If not specified, the source address will be the router identifier selected by zebra, and the port will be the standard PCEP port 4189.

  This can be specified in either the PCC peer definition or in a configuration group.

tcp-md5-auth WORD

  Enable TCP MD5 security with the given secret.

  This can be specified in either the PCC peer definition or in a configuration group.

sr-draft07

  Specify if a PCE only support segment routing draft 7, this flag will limit the PCC behavior to this draft.

  This can be specified in either the PCC peer definition or in a configuration group.

pce-initiated

  Specify if PCE-initiated LSP should be allowed for this PCE.

  This can be specified in either the PCC peer definition or in a configuration group.

timer [keep-alive (1-63)] [min-peer-keep-alive (1-255)] [max-peer-keep-alive (1-255)] [dead-timer (4-255)] [min-peer-dead-timer (4-255)] [max-peer-dead-timer (4-255)] [pcep-request (1-120)] [session-timeout-interval (1-120)] [delegation-timeout (1-60)]

  Specify the PCEP timers.

  This can be specified in either the PCC peer definition or in a configuration group.

pcc

  Disable or start the definition of a PCC.

msd (1-32)

  Specify the maximum SID depth in a PCC definition.

no msd [(1-32)]

  Default the maximum SID depth to 4.

peer WORD [precedence (1-255)]

  Specify a peer and its precedence in a PCC definition.

### Debugging

debug pathd policy

  Enable or disable Pathd policy information.

### Introspection Commands

show sr-te policy [detail]

  Display the segment routing policies.

    router# show sr-te policy

     Endpoint  Color  Name     BSID  Status
     ------------------------------------------
     192.0.2.1   1      default  4000  Active

    router# show sr-te policy detail

    Endpoint: 192.0.2.1  Color: 1  Name: LOW_DELAY  BSID: 4000  Status: Active
        Preference: 100  Name: cand1  Type: explicit  Segment-List: sl1  Protocol-Origin: Local
      * Preference: 200  Name: cand1  Type: dynamic  Segment-List: 32453452  Protocol-Origin: PCEP

The asterisk (*) marks the best, e.g. active, candidate path. Note that for segment-lists which are retrieved via PCEP a random number based name is generated.

show sr-te pcep counters

  Display the counters from pceplib.

show sr-te pcep pce-config [NAME]

  Display a shared configuration. if no name is specified, the default configuration will be displayed.

show sr-te pcep pcc

  Display PCC information.

show sr-te pcep session [NAME] [json]

  Display the information of a PCEP session, if not name is specified all the sessions will be displayed.

### Utility Commands

clear sr-te pcep session [NAME]

  Reset the pcep session by disconnecting from the PCE and performing the normal reconnection process. No configuration is changed.

## Usage with BGP route-maps

It is possible to steer traffic 'into' a segment routing policy for routes learned through BGP using route-maps:

    route-map SET_SR_POLICY permit 10
     set sr-te color 1
    !
    router bgp 1
     bgp router-id 192.0.2.2
     neighbor 192.0.2.1 remote-as 1
     neighbor 192.0.2.1 update-source lo
     !
     address-family ipv4 unicast
      neighbor 192.0.2.1 next-hop-self
      neighbor 192.0.2.1 route-map SET_SR_POLICY in
      redistribute static
     exit-address-family
     !
    !

In this case, the SR Policy with color 1 and endpoint 192.0.2.1 is selected.

## Sample configuration

    ! Default pathd configuration sample
    !
    password frr
    log stdout

    segment-routing
     traffic-eng
      segment-list test1
       index 10 mpls label 123
       index 20 mpls label 456
      !
      segment-list test2
       index 10 mpls label 321
       index 20 mpls label 654
      !
      policy color 1 endpoint 192.0.2.1
       name one
       binding-sid 100
       candidate-path preference 100 name test1 explicit segment-list test1
       candidate-path preference 200 name test2 explicit segment-list test2
      !
      policy color 2 endpoint 192.0.2.2
       name two
       binding-sid 101
       candidate-path preference 100 name def explicit segment-list test2
       candidate-path preference 200 name dyn dynamic
        bandwidth 12345
        metric bound abc 16 required
        metric te 10
       !
      !
      pcep
       pcc-peer PCE1
        address ip 127.0.0.1
        sr-draft07
       !
       pcc
        peer PCE1
       !
     !
    !