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Bidirectional Forwarding Detection (BFD) for MPLS

Configuring Bidirectional Forwarding Detection for MPLS (CLI Procedure)

You can configure the Bidirectional Forwarding Detection (BFD) protocol on EX8200 standalone switches and EX8200 Virtual Chassis to detect failures in the MPLS label-switch path (LSP). The BFD protocol is a simple hello mechanism that detects failures in a network. Hello packets are sent at a specified, regular interval. A neighbor failure is detected when the routing device stops receiving a reply from the neighbor after a specified interval. BFD works with a wide variety of network environments and topologies. The failure detection timers for BFD have shorter time limits than those of the failure detection mechanisms for static routes, and thus provide faster detection. These timers are also adaptive. For example, a timer can adapt to a higher value if an adjacency fails, or a neighbor can negotiate a higher value than the one configured.

This topic describes configuring the provider edge (PE) switches and the provider switches to support for LDP-based LSPs and RSVP-based LSPs.

This topic includes:

Configuring BFD on Provider Edge and Provider Switches for an LDP-Based LSP

You can enable BFD for the LDP-based LSPs or RSVP-based LSPs associated with a specific forwarding equivalence class (FEC). Alternatively, you can configure an Operation Administration and Maintenance (OAM) ingress policy to enable BFD on a range of FEC addresses.

Before you configure BFD for an LDP-based based LSP, you must configure the basic components for an MPLS network:

To configure BFD on PE and provider switches:

  1. Define an OAM policy:
  2. Specify the FEC on which you want to enable OAM:
  3. Specify the minimum transmit and receive interval for the BFD configuration:
    Note:

    If you configure the minimum-interval statement, you do not need to configure the minimum-receive-interval statement or the minimum-transmit-interval statement.

    or

  4. Specify the detection time multiplier. The negotiated transmit interval multiplied by this value gives the detection time for the receiving system in Asynchronous mode:
  5. Specify the minimum transmit interval (or the minimum receive interval).
  6. Specify a threshold for detecting the adaptation of the detection time:
  7. Configure route and next-hop action in the event of a BFD session failure event on the LDP-based LSP:
    Note:

    When a BFD session goes down, you can configure the Junos OS to resignal the LSP path or to simply disable the LSP path. You can configure a standby LSP path to handle traffic while the primary LSP path is unavailable. The switch can automatically recover from LSP failures that can be detected by BFD. By default, if a BFD session fails, the event is simply logged.

  8. Specify how long the BFD session must be up before adding the route or next hop. Specifying a time of 0 seconds causes the route or next hop to be added as soon as the BFD session comes back up.
  9. Enable tracing of FECs for LDP-based LSPs and specify a source address for sending probes. Then, specify a wait interval, after which to send the probe packet.
  10. Specify the duration of the LSP ping interval in seconds:
  11. Specify the action to be taken for the OAM policy:
  12. Apply the BFD configurations at the MPLS hierarchy level for the configuration to inherit the statements in the configuration group:

Configuring BFD on Provider Edge and Provider Switches for an RSVP-Based LSP

When BFD is configured for an RSVP-based LSP on the ingress switch, it is enabled on the primary path and on all standby secondary paths for that LSP. You can enable BFD for all LSPs on a switch or for specific LSPs. If you configure BFD for a specific LSP, whatever values configured globally for BFD are overridden on that LSP. The BFD sessions originate only at the ingress switch and terminate at the egress switch.

Before you configure BFD for an RSVP-based LSP, you must configure the basic components for an MPLS network:

To configure BFD on PE and provider switches:

  1. Specify the minimum transmit and receive interval for the BFD configuration:
    Note:

    If you configure the minimum-interval statement, you do not need to configure the minimum-receive-interval statement or the minimum-transmit-interval statement.

    or

  2. Specify the detection time multiplier. The negotiated transmit interval multiplied by this value gives the detection time for the receiving system in Asynchronous mode:
  3. Specify the minimum transmit interval (or the minimum receive interval):
  4. Configure route and next-hop actions in the event of a BFD session failure event on the RSVP-based LSP:
    Note:

    When a BFD session goes down, you can configure the Junos OS to resignal the LSP path or to simply disable the LSP path. You can configure a standby LSP path to handle traffic while the primary LSP path is unavailable. The switch can automatically recover from LSP failures that can be detected by BFD. By default, if a BFD session fails, the event is simply logged if you do not specifically configure a failure action.

BFD-Triggered Local Repair for Rapid Convergence

Understanding BFD-Triggered Local Protection

The time it takes for a network to converge following a link or node failure can vary dramatically based on a number of factors, including network size, the protocols used, and network design. However, while each particular convergence event is different, the process of convergence is essentially consistent. The failure is detected, the failure is reported (flooded) in the network, an alternate path is found for traffic, and the forwarding plane is updated to pass traffic on a new path.

This overview discusses how Bidirectional Forwarding Detection (BFD)-triggered local repair contributes to a quicker restoration time for rapid convergence in an MPLS network.

Purpose of BFD-Triggered Local Repair

In Junos OS, general MPLS traffic protection for RSVP-signaled label-switched path (LSP) failures is provided by several complementary mechanisms. These protection mechanisms include local protection (fast reroute, link protection, and node-link protection) and path protection (primary and secondary paths). Local protection in conjunction with path protection can provide minimum packet loss for an LSP, and control the way the LSP is rerouted after a failure. Traditionally, both types of protection rely on fast detection of connectivity failure at the physical level. However, for transmission media without fast physical level detection, Junos OS supports BFD and MPLS ping for fast failure detection.

With links between routers, when a route goes down, the routing protocol process recalculates the next best path. When MPLS fast reroute (FRR) is enabled, ifl messages are flooded to all Flexible PIC Concentrators (FPCs). The edge FPC enables the bypass MPLS LSP tunnel. Lastly, all routes are repaired and sent through the bypass MPLS LSP tunnel. The amount of time it takes to repair all routes is proportional to the number of routes.

This repair scenario becomes more difficult when a switch lies between two links. See Figure 1.

Figure 1: Topology with BFD-Triggered Local RepairTopology with BFD-Triggered Local Repair

When a link goes down at the remote end, the failure is not detected at the local end until the interior gateway protocol (IGP) goes down. To wait for the routing protocol process to recalculate the next best path takes too much time.

With BFD-triggered local repair enabled, the Packet Forwarding Engine completes the repair first, using the bypass MPLS LSP tunnel (that is preconfigured and installed), then informs the routing protocol process to recalculate a new route. By doing this, when the primary MPLS LSP tunnel goes down, the FPC can intermittently and immediately divert traffic to the FPC with the bypass MPLS LSP tunnel.

Using local repair in this way achieves a faster restoration time of less than 50 ms.

Configuring BFD-Triggered Local Repair

BFD-triggered local repair is not configurable, but is part of the default configuration.

BFD-triggered local repair works within the legacy Junos OS features MPLS-FRR, BFD for IGP, and loop-free alternates (LFAs).

Disabling BFD-Triggered Local Repair

By default, BFD-triggered local repair is enabled for all routing interfaces. If desired, you can disable BFD-triggered local repair at the [edit routing-options] hierarchy level.

To explicitly disable BFD-triggered local repair:

  1. Include the no-bfd-triggered-local-repair statement at the [edit routing-options] hierarchy level:

  2. (Optional) Verify your configuration settings before committing them by using the show routing-options command.

Confirm your configuration by issuing the show routing-options command.

Note:

When you disable this feature, you must also restart routing by including the graceful-restart statement for the IGP. For example, for OSPF, this is accomplished by including the graceful-restart statement at the [edit protocols ospf] hierarchy level.

Configuring BFD for MPLS IPv4 LSPs

You can configure Bidirectional Forwarding Detection (BFD) protocol on MPLS IPv4 LSPs as outlined in the Internet draft draft-ietf-bfd-mpls-02.txt, BFD for MPLS LSPs. BFD is used as a periodic Operation, Administration, and Maintenance (OAM) feature for LSPs to detect LSP data plane faults. You can configure BFD for LSPs that use either LDP or RSVP as the signaling protocol.

Note:

BFD for MPLS IPv4 LSP is based on the Routing Engine and is not distributed. As a result, the minimum supported BFD timer interval is (100 ms * 3) per one LSP session, and for scaled LSP sessions, the minimum supported BFD timer interval is (300 ms * 3). As you increase the number of LSP sessions with BFD, you must also increase (scale) the interval timers to support the network.

For Routing Engine switchover instances with nonstop active routing (NSR) support, the minimum supported BFD timer interval is (2.5 seconds * 3).

You can also use the LSP ping commands to detect LSP data plane faults. However, BFD has a couple of benefits: it requires less computer processing than LSP ping commands and can quickly detect faults in large numbers of LSPs (LSP ping commands must be issued for each LSP individually). On the other hand, BFD cannot be used to verify the control plane against the data plane at the egress LSR, which is possible when an LSP ping echo request is associated with a forwarding equivalence class (FEC).

The BFD failure detection timers are adaptive and can be adjusted to be more or less aggressive. For example, the timers can adapt to a higher value if the adjacency fails, or a neighbor can negotiate a higher value for a timer than the configured value. The timers adapt to a higher value when a BFD session flap occurs more than three times in a span of 15 seconds. A back-off algorithm increases the receive (Rx) interval by two if the local BFD instance is the reason for the session flap. The transmission (Tx) interval is increased by two if the remote BFD instance is the reason for the session flap. You can use the clear bfd adaptation command to return BFD interval timers to their configured values. The clear bfd adaptation command is hitless, meaning that the command does not affect traffic flow on the routing device.

Starting from Junos OS Release 13.2R4, 13.3R2, and 14.1, you can set the time interval between LSP ping messages and the number of LSP ping responses, respectively, after which the Bidirectional Forwarding Detection (BFD) session is brought down. To so do, you configure the lsp-ping-interval statement and the lsp-ping-multiplier statement at the [edit protocols mpls oam] hierarchy level.

For configuration instructions for LDP-signaled LSPs, see Configuring BFD for LDP LSPs. For configuration instructions for RSVP-signaled LSPs, see the following section.

Configuring BFD for RSVP-Signaled LSPs

BFD for RSVP supports unicast IPv4 LSPs. When BFD is configured for an RSVP LSP on the ingress router, it is enabled on the primary path and on all standby secondary paths for that LSP. The source IP address for outgoing BFD packets from the egress side of an MPLS BFD session is based on the outgoing interface IP address. You can enable BFD for all LSPs on a router or for specific LSPs. If you configure BFD for a specific LSP, whatever values configured globally for BFD are overridden. The BFD sessions originate only at the ingress router and terminate at the egress router.

An error is logged whenever a BFD session for a path fails. The following example shows how BFD for RSVP LSP log messages might appear:

You can configure BFD for all of the RSVP LSPs on the router, a specific LSP, or the primary path of a specific LSP. To configure BFD for RSVP LSPs, include the oam and bfd-liveness-detection statements.

You can configure this statement at the following hierarchy levels:

The bfd-liveness-detection statement includes the following options:

  • minimum-interval—Specifies the minimum transmit and receive interval.

  • minimum-receive-interval—Specifies the minimum receive interval. The range is from 1 through 255,000 milliseconds.

  • minimum-transmit-interval—Specifies the minimum transmit interval. The range is from 1 through 255,000 milliseconds.

  • lsp-ping-multiplier—Specifies the detection time multiplier. The range is from 1 through 255.

    Note:

    To avoid triggering false negatives, configure a BFD fault detection time that is longer than the fast reroute time.

You can also configure the lsp-ping-interval option to adjust the time interval between LSP pings. The LSP ping command for RSVP-signaled LSPs is ping mpls rsvp. For more information on the ping mpls rsvp command, see the CLI Explorer.

Configuring a Failure Action for the BFD Session on an RSVP LSP

When the BFD session for an RSVP LSP goes down, the LSP is torn down and resignaled. Traffic can be switched to a standby LSP, or you can simply tear down the LSP path. Any actions performed are logged.

When a BFD session for an RSVP LSP path goes down, you can configure the Junos OS to resignal the LSP path or to simply disable the LSP path. A standby LSP path could be configured to handle traffic while the primary LSP path is unavailable. The router can automatically recover from LSP failures that can be detected by BFD. By default, if a BFD session fails, the event is simply logged.

To enable the Junos OS to tear down an RSVP LSP path in the event of a BFD event, include the failure-action statement:

For a list of the hierarchy levels at which you can include this statement, see the statement summary section for this statement.

You can configure either the teardown or make-before-break options:

  • teardown—Causes the LSP path to be taken down and resignaled immediately.

  • make-before-break—Causes the Junos OS to attempt to signal a new LSP path before tearing down the old LSP path. You can also configure the teardown-timeout option to automatically tear down the LSP after the time period specified if the attempt to resignal the LSP fails within the teardown-timeout interval. If you specify a value of 0 for the teardown-timeout interval, the LSP is taken down and resignaled immediately (the same behavior as when you configure the teardown option).

To configure a failure action for all of the RSVP LSPs, include the failure-action statement at the [edit protocols mpls oam bfd-liveness-detection] hierarchy level. To configure a failure action for a specific RSVP LSP, include the failure-action statement at the [edit protocols mpls label-switched-path lsp-name oam bfd-liveness-detection] hierarchy level.

To configure a failure action for a specific primary path, include the failure-action statement at the [edit protocols mpls label-switched path lsp-name primary path-name oam bfd-liveness-detection] hierarchy level. To configure a failure action for a specific secondary LSP path, include the failure-action statement at the [edit protocols mpls label-switched-path lsp-name secondary path-name oam bfd-liveness-detection] hierarchy level.

Related Documentation

Change History Table

Feature support is determined by the platform and release you are using. Use Feature Explorer to determine if a feature is supported on your platform.

Release
Description
13.2R4
Starting from Junos OS Release 13.2R4, 13.3R2, and 14.1, you can set the time interval between LSP ping messages and the number of LSP ping responses, respectively, after which the Bidirectional Forwarding Detection (BFD) session is brought down.