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BFD diagnostic messages and session states
RFC 5880
defines diagnostic codes that specify the local system's reason for the last
change in the Bidirectional Forwarding Detection (BFD) session state.
Diagnostic messages can be helpful indicators
when troubleshooting. You can see these messages or codes when
viewing Cloud Router status.
The following table lists BFD diagnostic messages, their numeric error codes,
and suggested follow-up actions.
BFD diagnostic message
Numeric code
Suggested action
NO_DIAGNOSTIC
0
No diagnostic information is available. For more information, see the
BFD
State Machine section of RFC 5880 and the BFD session state table
in the next section.
CONTROL_DETECTION_TIME_EXPIRED
1
Local BFD did not receive a control packet from the remote side within
the (num_rx_interval * bfd multiplier) time period. This
could happen when the forwarding link is down or BFD on the remote system
is not functioning properly.
Check connectivity to the remote system, and check the remote
system's BFD session transmit counters.
ECHO_FUNCTION_FAILED
2
—
NEIGHBOR_SIGNALED_SESSION_DOWN
3
BFD neighbor signaled DOWN. Check the remote system BFD
configuration and operational state.
FORWARDING_PLANE_RESET
4
—
PATH_DOWN
5
—
CONCATENATED_PATH_DOWN
6
—
ADMINISTRATIVELY_DOWN
7
BFD in the local system has been set to DOWN by the administrator due to
a BFD software upgrade. Check that the state is back up after the
upgrade.
REVERSE_CONCATENATED_PATH_DOWN
8
—
BFD session states
The following table lists BFD session states and their numeric error codes.
[[["Easy to understand","easyToUnderstand","thumb-up"],["Solved my problem","solvedMyProblem","thumb-up"],["Other","otherUp","thumb-up"]],[["Hard to understand","hardToUnderstand","thumb-down"],["Incorrect information or sample code","incorrectInformationOrSampleCode","thumb-down"],["Missing the information/samples I need","missingTheInformationSamplesINeed","thumb-down"],["Other","otherDown","thumb-down"]],["Last updated 2025-08-29 UTC."],[],[],null,["# BFD diagnostic messages and session states\n==========================================\n\n| **Important:** BFD can only be enabled on provisioned VLAN\n| attachments that use Dataplane version 2. You can view the `dataplaneVersion` for a\n| VLAN attachment by viewing the attachment details in one of the following ways:\n|\n| - **API** : Use the [interconnectAttachments.get](/compute/docs/reference/rest/v1/interconnectAttachments/get) method. You can use this method on VLAN attachments for either Dedicated Interconnect or Partner Interconnect.\n| - **Google Cloud CLI** : Use the `gcloud compute interconnects attachments describe\n| `\u003cvar translate=\"no\"\u003eATTACHMENT-NAME\u003c/var\u003e command where \u003cvar translate=\"no\"\u003eATTACHMENT-NAME\u003c/var\u003e is the name of the VLAN attachment for either Dedicated Interconnect or Partner Interconnect.\n|\n| In the output of the API or gcloud CLI command, look for the\n| `dataplaneVersion` field with a value of `2`. If the field is absent\n| from the output, the VLAN attachment is using version 1 and cannot be used with BFD. For more\n| information, contact your customer account team.\n|\n| The Cloud Interconnect Dataplane v2 is unrelated to\n| [Google Kubernetes Engine (GKE) Dataplane V2](/kubernetes-engine/docs/concepts/dataplane-v2)\n\n[RFC 5880](https://datatracker.ietf.org/doc/html/rfc5880#section-4.1)\ndefines diagnostic codes that specify the local system's reason for the last\nchange in the Bidirectional Forwarding Detection (BFD) session state.\nDiagnostic messages can be helpful indicators\nwhen troubleshooting. You can see these messages or codes when\n[viewing Cloud Router status](/network-connectivity/docs/router/how-to/viewing-router-details#viewing-router-status).\n\nThe following table lists BFD diagnostic messages, their numeric error codes,\nand suggested follow-up actions.\n\n\u003cbr /\u003e\n\n### BFD session states\n\nThe following table lists BFD session states and their numeric error codes.\n\n### BFD status messages\n\nYou can view BFD status messages under the `bfdStatus` field in command\noutput from the [`gcloud compute routers\nget-status` command](/sdk/gcloud/reference/compute/routers/get-status).\n\nFor field definitions and descriptions, see the\n[Cloud Router API documentation](/compute/docs/reference/rest/v1/routers/getRouterStatus).\n\nWhat's next\n-----------\n\n- To modify BFD settings on a BGP session, see\n [Update or disable BFD](/network-connectivity/docs/router/how-to/updating-bfd).\n\n- For information about BGP session states, see\n [BGP session states](/network-connectivity/docs/router/concepts/bgp-states).\n\n- To troubleshoot issues with Cloud Router, see\n [Troubleshooting](/network-connectivity/docs/router/resources/troubleshooting)."]]
