This documentation is for the extended support release (ESR) version of Cumulus Linux. We will continue to keep this content up to date until 21 February, 2023, when ESR support ends. For more information about ESR, please read this knowledge base article.

If you are using the current version of Cumulus Linux, the content on this page may not be up to date. The current version of the documentation is available here. If you are redirected to the main page of the user guide, then this page may have been renamed; please search for it there.

Prescriptive Topology Manager - PTM

In data center topologies, right cabling is a time-consuming endeavor and is error prone. Prescriptive Topology Manager (PTM) is a dynamic cabling verification tool to help detect and eliminate such errors. It takes a Graphviz-DOT specified network cabling plan (something many operators already generate), stored in a file, and couples it with runtime information derived from LLDP to verify that the cabling matches the specification. The check is performed on every link transition on each node in the network.

You can customize the file to control ptmd at both the global/network level and the node/port level.

PTM runs as a daemon, named ptmd.

For more information, see man ptmd(8).

Supported Features

  • Topology verification using LLDP. ptmd creates a client connection to the LLDP daemon, lldpd, and retrieves the neighbor relationship between the nodes/ports in the network and compares them against the prescribed topology specified in the file.
  • Only physical interfaces, like swp1 or eth0, are currently supported. Cumulus Linux does not support specifying virtual interfaces like bonds or subinterfaces like eth0.200 in the topology file.
  • Forwarding path failure detection using Bidirectional Forwarding Detection (BFD); however, demand mode is not supported. For more information on how BFD operates in Cumulus Linux, read the Bidirectional Forwarding Detection - BFD chapter and read man ptmd(8).
  • Integration with FRRouting (PTM to FRRouting notification).
  • Client management: ptmd creates an abstract named socket /var/run/ptmd.socket on startup. Other applications can connect to this socket to receive notifications and send commands.
  • Event notifications: see Scripts below.
  • User configuration via a file; see below.

Configure PTM

ptmd verifies the physical network topology against a DOT-specified network graph file, /etc/ptm.d/

PTM supports undirected graphs.

At startup, ptmd connects to lldpd, the LLDP daemon, over a Unix socket and retrieves the neighbor name and port information. It then compares the retrieved port information with the configuration information that it read from the topology file. If there is a match, then it is a PASS, else it is a FAIL.

PTM performs its LLDP neighbor check using the PortID ifname TLV information. Previously, it used the PortID port description TLV information.

Basic Topology Example

This is a basic example DOT file and its corresponding topology diagram. You should use the same file on all switches, and don’t split the file per device; this allows for easy automation by pushing/pulling the same exact file on each device!

graph G {
    "spine1":"swp1" -- "leaf1":"swp1";
    "spine1":"swp2" -- "leaf2":"swp1";
    "spine2":"swp1" -- "leaf1":"swp2";
    "spine2":"swp2" -- "leaf2":"swp2";
    "leaf1":"swp3" -- "leaf2":"swp3";
    "leaf1":"swp4" -- "leaf2":"swp4";
    "leaf1":"swp5s0" -- "server1":"eth1";
    "leaf2":"swp5s0" -- "server2":"eth1";

ptmd Scripts

ptmd executes scripts at /etc/ptm.d/if-topo-pass and /etc/ptm.d/if-topo-fail for each interface that goes through a change, running if-topo-pass when an LLDP or BFD check passes and running if-topo-fails when the check fails. The scripts receive an argument string that is the result of the ptmctl command, described in the ptmd commands section below.

You should modify these default scripts as needed.

Configuration Parameters

You can configure ptmd parameters in the topology file. The parameters are classified as host-only, global, per-port/node and templates.

Host-only Parameters

Host-only parameters apply to the entire host on which PTM is running. You can include the hostnametype host-only parameter, which specifies whether PTM should use only the host name (hostname) or the fully-qualified domain name (fqdn) while looking for the self-node in the graph file. For example, in the graph file below, PTM will ignore the FQDN and only look for switch04, since that is the host name of the switch it’s running on:

It’s a good idea to always wrap the hostname in double quotes, like “”. Otherwise, ptmd can fail if you specify a fully-qualified domain name as the hostname and do not wrap it in double quotes.

Further, to avoid errors when starting the ptmd process, make sure that /etc/hosts and /etc/hostname both reflect the hostname you are using in the file.

graph G {
          "cumulus":"swp44" -- "":"swp20"
          "cumulus":"swp46" -- "":"swp22"

However, in this next example, PTM will compare using the FQDN and look for, which is the FQDN of the switch it’s running on:

graph G {
         "cumulus":"swp44" -- "":"swp20"
         "cumulus":"swp46" -- "":"swp22"

Global Parameters

Global parameters apply to every port listed in the topology file. There are two global parameters: LLDP and BFD. LLDP is enabled by default; if no keyword is present, default values are used for all ports. However, BFD is disabled if no keyword is present, unless there is a per-port override configured. For example:

graph G {
          "cumulus":"swp44" -- "qct-ly2-04":"swp20"
          "cumulus":"swp46" -- "qct-ly2-04":"swp22"

Per-port Parameters

Per-port parameters provide finer-grained control at the port level. These parameters override any global or compiled defaults. For example:

graph G {
          "cumulus":"swp44" -- "qct-ly2-04":"swp20" [BFD="upMinTx=150,requiredMinRx=250,afi=both"]
          "cumulus":"swp46" -- "qct-ly2-04":"swp22"


Templates provide flexibility in choosing different parameter combinations and applying them to a given port. A template instructs ptmd to reference a named parameter string instead of a default one. There are two parameter strings ptmd supports:

  • bfdtmpl, which specifies a custom parameter tuple for BFD.
  • lldptmpl, which specifies a custom parameter tuple for LLDP.

For example:

graph G {
          "cumulus":"swp44" -- "qct-ly2-04":"swp20" [BFD="bfdtmpl=BFD1", LLDP="lldptmpl=LLDP1"]
          "cumulus":"swp46" -- "qct-ly2-04":"swp22" [BFD="bfdtmpl=BFD2", LLDP="lldptmpl=LLDP2"]
          "cumulus":"swp46" -- "qct-ly2-04":"swp22"

In this template, LLDP1 and LLDP2 are templates for LLDP parameters while BFD1 and BFD2 are templates for BFD parameters.

Supported BFD and LLDP Parameters

ptmd supports the following BFD parameters:

  • upMinTx: the minimum transmit interval, which defaults to 300ms, specified in milliseconds.

  • requiredMinRx: the minimum interval between received BFD packets, which defaults to 300ms, specified in milliseconds.

  • detectMult: the detect multiplier, which defaults to 3, and can be any non-zero value.

  • afi: the address family to be supported for the edge. The address family must be one of the following:

    • v4: BFD sessions will be built for only IPv4 connected peer. This is the default value.
    • v6: BFD sessions will be built for only IPv6 connected peer.
    • both: BFD sessions will be built for both IPv4 and IPv6 connected peers.

The following is an example of a topology with BFD applied at the port level:

graph G {
          "cumulus-1":"swp44" -- "cumulus-2":"swp20" [BFD="upMinTx=300,requiredMinRx=100,afi=v6"]
          "cumulus-1":"swp46" -- "cumulus-2":"swp22" [BFD="detectMult=4"]

ptmd supports the following LLDP parameters:

  • match_type, which defaults to the interface name (ifname), but can accept a port description (portdescr) instead if you want lldpd to compare the topology against the port description instead of the interface name. You can set this parameter globally or at the per-port level.
  • match_hostname, which defaults to the host name (hostname), but enables PTM to match the topology using the fully-qualified domain name (fqdn) supplied by LLDP.

The following is an example of a topology with LLDP applied at the port level:

graph G {
         "cumulus-1":"swp44" -- "cumulus-2":"swp20" [LLDP="match_hostname=fqdn"]
         "cumulus-1":"swp46" -- "cumulus-2":"swp22" [LLDP="match_type=portdescr"]

When you specify match_hostname=fqdn, ptmd will match the entire FQDN, like in the example below. If you do not specify anything for match_hostname, ptmd will match based on hostname only, like cumulus-3 below, and ignore the rest of the URL:

graph G {
          "cumulus-1":"swp44" -- "":"swp20" [LLDP="match_hostname=fqdn"]
          "cumulus-1":"swp46" -- "cumulus-3":"swp22" [LLDP="match_type=portdescr"]

Bidirectional Forwarding Detection (BFD)

BFD provides low overhead and rapid detection of failures in the paths between two network devices. It provides a unified mechanism for link detection over all media and protocol layers. Use BFD to detect failures for IPv4 and IPv6 single or multihop paths between any two network devices, including unidirectional path failure detection. For information about configuring BFD using PTM, see the BFD topic

The FRRouting routing suite enables additional checks to ensure that routing adjacencies are formed only on links that have connectivity conformant to the specification, as determined by ptmd.

You only need to do this to check link state; you do not need to enable PTM to determine BFD status.

When the global ptm-enable option is enabled, every interface has an implied ptm-enable line in the configuration stanza in the interfaces file.

To enable the global ptm-enable option, run the following FRRouting command:

cumulus@switch:~$ sudo vtysh

switch# configure terminal
switch(config)# ptm-enable
switch(config)# end
switch# write memory
switch# exit

To disable the checks, delete the ptm-enable parameter from the interface. For example:

cumulus@switch:~$ net del interface swp51 ptm-enable
cumulus@switch:~$ net pending
cumulus@switch:~$ net commit

If you need to reenable PTM for that interface, run:

cumulus@switch:~$ net add interface swp51 ptm-enable
cumulus@switch:~$ net pending
cumulus@switch:~$ net commit

With PTM enabled on an interface, the zebra daemon connects to ptmd over a Unix socket. Any time there is a change of status for an interface, ptmd sends notifications to zebra. Zebra maintains a ptm-status flag per interface and evaluates routing adjacency based on this flag. To check the per-interface ptm-status:

cumulus@switch:~$ net show interface swp1
Interface swp1 is up, line protocol is up
Link ups:       0    last: (never)
  Link downs:     0    last: (never)
  PTM status: disabled
  vrf: Default-IP-Routing-Table
  index 3 metric 0 mtu 1550
  HWaddr: c4:54:44:bd:01:41

ptmd Service Commands

PTM sends client notifications in CSV format.

cumulus@switch:~$ sudo systemctl start|restart|force-reload ptmd.service: Starts or restarts the ptmd service. The file must be present in order for the service to start.

cumulus@switch:~$ sudo systemctl reload ptmd.service: Instructs ptmd to read the file again without restarting, applying the new configuration to the running state.

cumulus@switch:~$ sudo systemctl stop ptmd.service: Stops the ptmd service.

cumulus@switch:~$ sudo systemctl status ptmd.service: Retrieves the current running state of ptmd.

ptmctl Commands

ptmctl is a client of ptmd; it retrieves the operational state of the ports configured on the switch and information about BFD sessions from ptmd. ptmctl parses the CSV notifications sent by ptmd.

See man ptmctl for more information.

ptmctl Examples

The examples below contain the following keywords in the output of the cbl status column, which are described here:

cbl status KeywordDefinition
passThe interface is defined in the topology file, LLDP information is received on the interface, and the LLDP information for the interface matches the information in the topology file.
failThe interface is defined in the topology file, LLDP information is received on the interface, and the LLDP information for the interface does not match the information in the topology file.
N/AThe interface is defined in the topology file, but no LLDP information is received on the interface. The interface may be down or disconnected, or the neighbor is not sending LLDP packets.
The N/A and fail statuses may indicate a wiring problem to investigate.
The N/A status is not shown when using the -l option with ptmctl. If you specify the -l option, ptmctl displays only those interfaces that are receiving LLDP information.

For basic output, use ptmctl without any options:

cumulus@switch:~$ sudo ptmctl
port  cbl     BFD     BFD                  BFD    BFD       
      status  status  peer                 local  type      
swp1  pass    pass             N/A    singlehop
swp2  pass    N/A     N/A                  N/A    N/A        
swp3  pass    N/A     N/A                  N/A    N/A  

For more detailed output, use the -d option:

cumulus@switch:~$ sudo ptmctl -d
port  cbl    exp     act      sysname  portID  portDescr  match  last    BFD   BFD    
      status nbr     nbr                                  on     upd     Type  state  
swp45 pass   h1:swp1 h1:swp1  h1       swp1    swp1       IfName 5m: 5s  N/A   N/A    
swp46 fail   h2:swp1 h2:swp1  h2       swp1    swp1       IfName 5m: 5s  N/A   N/A    
#continuation of the output
BFD   BFD       det_mult  tx_timeout  rx_timeout  echo_tx_timeout  echo_rx_timeout  max_hop_cnt
peer  DownDiag
N/A   N/A       N/A       N/A         N/A         N/A              N/A              N/A
N/A   N/A       N/A       N/A         N/A         N/A              N/A              N/A

To return information on active BFD sessions ptmd is tracking, use the -b option:

cumulus@switch:~$ sudo ptmctl -b
port  peer        state  local         type       diag

swp1    Up     N/A           singlehop  N/A  
N/A  Up    multihop   N/A    

To return LLDP information, use the -l option. It returns only the active neighbors currently being tracked by ptmd.

cumulus@switch:~$ sudo ptmctl -l
port  sysname  portID  port   match  last
                           descr  on     upd
swp45 h1       swp1    swp1   IfName 5m:59s
swp46 h2       swp1    swp1   IfName 5m:59s

To return detailed information on active BFD sessions ptmd is tracking, use the -b and -d options (results are for an IPv6-connected peer):

cumulus@switch:~$ sudo ptmctl -b -d
port  peer                 state  local  type       diag  det   tx_timeout  rx_timeout  
swp1  fe80::202:ff:fe00:1  Up     N/A    singlehop  N/A   3     300         900
swp1  3101:abc:bcad::2     Up     N/A    singlehop  N/A   3     300         900
#continuation of output
echo        echo        max      rx_ctrl  tx_ctrl  rx_echo  tx_echo
tx_timeout  rx_timeout  hop_cnt
0           0           N/A      187172   185986   0        0
0           0           N/A      501      533      0        0

ptmctl Error Outputs

If there are errors in the topology file or there isn’t a session, PTM will return appropriate outputs. Typical error strings are:

Topology file error [/etc/ptm.d/] [cannot find node cumulus] -
please check /var/log/ptmd.log for more info
Topology file error [/etc/ptm.d/] [cannot open file (errno 2)] -
please check /var/log/ptmd.log for more info
No Hostname/MgmtIP found [Check LLDPD daemon status] -
please check /var/log/ptmd.log for more info
No BFD sessions . Check connections
No LLDP ports detected. Check connections
Unsupported command

For example:

cumulus@switch:~$ sudo ptmctl
cmd         error
get-status  Topology file error [/etc/ptm.d/] 
            [cannot open file (errno 2)] - please check /var/log/ptmd.log 
            for more info

If you encounter errors with the file, you can use dot (included in the Graphviz package) to validate the syntax of the topology file.

By simply opening the topology file with Graphviz, you can ensure that it is readable and that the file format is correct.

If you edit file from a Windows system, be sure to double check the file formatting; there may be extra characters that keep the graph from working correctly.

Caveats and Errata

  • Prior to version 2.1, Cumulus Linux stored the ptmd configuration files in /etc/cumulus/ptm.d. When you upgrade to version 2.1 or later, all the existing ptmd files are copied from their original location to /etc/ptm.d with a dpkg-old extension, except for, which gets copied to /etc/ptm.d.

    If you customized the if-topo-pass and if-topo-fail scripts, they are also copied to dpkg-old, and you must modify them so they can parse the CSV output correctly.

    Sample if-topo-pass and if-topo-fail scripts are available in /etc/ptm.d. A sample file is available in /usr/share/doc/ptmd/examples.

  • When PTMD is incorrectly in a failure state and the Zebra interface is enabled, PIF BGP sessions are not establishing the route, but the subinterface on top of it does establish routes.

    If the subinterface is configured on the physical interface and the physical interface is incorrectly marked as being in a PTM FAIL state, routes on the physical interface are not processed in FRRouting, but the subinterface is working.

  • If an LLDP neighbor advertises a PortDescr that contains commas, ptmctl -d splits the string on the commas and misplaces its components in other columns. Do not use commas in your port descriptions.