How to enable automatically an interface disable by BFD?

Created: Jan 22, 2020 18:24:44Latest reply: Jan 26, 2020 06:59:01 82 4 0 0
  Rewarded Hi-coins: 0 (problem resolved)

Hello,


I used the documentation to configure a BFD session to trigger down an interface, but I got a deadlock, because that was the interface used by the BFD link. (The documentation talked about the deadlock...), but I just have that interface to shutdown. Is possible to forcifully enable a interface bfd down, using bfd just to shutdown and other kind of configuration to "no shut"?


Is there any workaround to get rid of that deadlock situation?

I´m using an s5720-EI....


https://support.huawei.com/enterprise/en/doc/EDOC1000142090/7c8ed80e

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Featured Answers
chenhui
Admin Created Jan 23, 2020 03:44:43 Helpful(0) Helpful(0)

Hi @jess99
well, there isn't such command could enable the shutdown interface automatically.
But you can try configuring association between a BFD session and an interface, but not trigger the associated interface in down state, so that when the BFD sesseion failed, it would set the interface to BFD status down, when the BFD seseesion turns up, the BFD status down will be removed automatically.
Actually, you are kindly advised to describe your requirement detailedly, so that we could help to recommend you the most appropriate configuration.
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All Answers
wissal
wissal MVE Created Jan 22, 2020 19:48:03 Helpful(0) Helpful(0)

Hello,

Please find below below the answer for your question.


The display bfd session command displays information about BFD sessions.
Format
display bfd session { all | static } [ for-ip | for-lsp | for-te | for-pw | for-vsi-pw ] [ verbose ]
display bfd session { discriminator discr-value | peer-ip peer-ip [ vpn-instance vpn-instance-name ] | static-auto } [ verbose ]
display bfd session { ldp-lsp peer-ip peer-ip [ nexthop nexthop-ip [ interface interface-type interface-number ] ] | mpls-te interface interface-type interface-number [ te-lsp ] | pw interface interface-type interface-number [ secondary ] } [ verbose ]
display bfd session peer-ipv6 peer-ipv6 [ vpn-instance vpn-instance-name ] [ verbose ]
display bfd session { all | static } for-ipv6 [ verbose ]
display bfd session passive-dynamic [ peer-ip peer-ip remote-discriminator discr-value ] [ verbose ]
display bfd session vsi-pw vsi vsi-name peer peer-address [ vc-id vc-id ] [ verbose ]
display bfd session peer-ip { peer-address [ vpn-instance vpn-instance-name ] | default-ip } [ verbose ]
display bfd session mpls-te interface {interface-type interface-number }[ te-lsp ] [ verbose ]
display bfd session dynamic [ verbose ]
display bfd session ldp-lsp peer-ip peer-ip [ nexthop nexthop-ip [ interface {interface-type interface-number } ] ]
display bfd session { all | static } [ for-lsp] [ verbose ]
display bfd session bgp-tunnel peer-ip bgp-tunnel-address [ verbose ]
display bfd session sr-be-tunnel peer-ip peer-ip [ verbose ]
Parameters
Parameter Description Value
all Displays information about all BFD sessions. -
static Displays information about static BFD sessions. -
passive-dynamic Displays information about the BFD sessions that are passively created. -
dynamic Displays information about dynamic and statically self-negotiated BFD sessions. -
discriminator discr-value Displays information about the BFD sessions with a specified local discriminator. The value is an integer ranging from 1 to 4294967295.
remote-discriminator discr-value Displays information about the BFD sessions with a specified remote discriminator. The value is an integer ranging from 1 to 4294967295.
peer-ip peer-ip Displays information about the BFD session with a specified peer IP address. The value is in dotted decimal notation.
peer-ipv6 peer-ipv6 Displays information about the BFD session with a specified peer IPv6 address. The value is a 32-digit hexadecimal number, in the format of X:X:X:X:X:X:X:X.
nexthop nexthop-ip Displays information about the BFD session with a specified next-hop IP address. The value is in dotted decimal notation.
interface interface-type interface-number Displays information about the BFD session with a specified interface. -
vpn-instance vpn-instance-name Displays information about the BFD session bound to a specified VPN instance.
static-auto Displays information about the BFD session with automatically negotiated discriminators. -
for-ip Displays information about the BFD session for detecting faults in IP links. -
for-ipv6 Displays information about the BFD session for detecting faults in IPv6 links. -
for-lsp Displays information about the BFD, LDP LSP session for detecting faults in LSPs. -
for-te Displays information about the BFD, TE TUNNLE, TE LSP session for detecting faults in TE tunnels. -
for-pw Displays information about the BFD session for detecting faults in a PW. -
for-vsi-pw Displays information about the BFD session for detecting faults in a VSI PW. -
ldp-lsp Displays information about the BFD session for detecting faults in an LDP LSP. -
mpls-te Displays information about the BFD session for detecting faults in MPLS TE tunnels. -
te-lsp Displays information about the BFD session for detecting faults in the primary CR-LSP bound to the TE tunnel. -
pw Displays information about the BFD session for detecting faults in a PW. -
secondary Displays statistics about a BFD session for monitoring a secondary PW. -
vsi-pw Displays information about the BFD session for detecting faults in a VSI PW. -
vsi vsi-name Specifies the VSI name. The value is a string of 1 to 31 characters.
peer peer-address Specifies the IPv4 address of the peer of the VSI PW. The value is in dotted decimal notation.
vc-id vc-id Specifies the ID of a Layer 2 VC. The value is an integer ranging from 1 to 4294967295.
peer-ip default-ip Displays statistics about BFD sessions with the default multicast address. The value is in dotted decimal notation.
verbose Displays detailed information about BFD sessions. -
bgp-tunnel Displays information about a BFD session that monitors a BGP Tunnel. -
peer-ip bgp-tunnel-address Specifies the peer IP address of a BGP tunnel. The value is in dotted decimal notation.
sr-be-tunnel Displays information about a BFD session that monitors an SR-BE tunnel. -
Views
All views
Default Level
1: Monitoring level
Task Name and Operations
Task Name Operations
bfd read
Usage Guidelines
Usage Scenario
To view information about BFD sessions, run the display bfd session command with one or more of the following parameters:
• If only all is configured, information about all BFD sessions is displayed. You can also configure the parameter all and one of the following parameters:
If for-ip is configured, information about all BFD sessions for detecting faults in IP links is displayed.
If for-ipv6 is configured, information about all BFD sessions for detecting faults in IPv6 links is displayed.
If for-lsp is configured, information about all BFD sessions for detecting faults in LSPs is displayed.
If for-te is configured, information about all BFD sessions for detecting faults in TE tunnels is displayed.
• If only static is configured, information about static BFD sessions is displayed. You can also configure the parameter static and one of the following parameters:
If for-ip is configured, information about the static BFD sessions for detecting faults in IP links is displayed.
If for-ipv6 is configured, information about all BFD sessions for detecting faults in IPv6 links is displayed.
If for-lsp is configured, information about the static BFD sessions for detecting faults in LSPs is displayed.
If for-te is configured, information about the static BFD sessions for detecting faults in TE tunnels is displayed.
• If only passive-dynamic is configured, information about the passively created BFD sessions is displayed.
• If only peer-ip peer-ip discriminator discr-value is configured, information about the BFD sessions with the specified remote IP address and local discriminator is displayed.
• If only dynamic is configured, information about dynamic BFD sessions is displayed.
• If only discriminator discr-value is configured, information about the BFD sessions with the specified local discriminator is displayed.
• If only peer-ip peer-ip is configured, information about the BFD sessions with the specified peer IP address is displayed.
• If only peer-ipv6 peer-ipv6, information about the BFD sessions with the specified peer IPv6 address is displayed.
• If only peer-ip peer-ip [ vpn-instance vpn-instance-name ] is configured, information about the BFD sessions with the specified remote IP address and bound to the specified VPN instance is displayed.
• If only static-auto is configured, information about the BFD sessions with automatically negotiated discriminators is displayed.
• If only ldp-lsp peer-ip peer-ip is configured, information about the BFD sessions for detecting faults in LDP LSPs is displayed.
• If only mpls-te interface interface-type interface-number is configured, information about the BFD sessions for detecting faults in TE tunnels is displayed.
• If only segment-list segment-list-id is configured, information about the BFD sessions with the specified SR Policy is displayed.
Prerequisites
BFD has been globally enabled using the bfd command in the system view.
Precautions
• When the BFD session changes from Down to Up, and the WTR time is not zero, the notification that the BFD session goes Up can be received only after the WTR time expires.
• If a static BFD session with automatically negotiated discriminators and a dynamic BFD session have the same configurations, you can view that the shared BFD session belongs to both the static session with automatically negotiated discriminators and the dynamic session in terms of the BFD session type.
• The display bfd session dynamic command displays information about both dynamic and static sessions. The statically self-negotiated sessions are treated as dynamic sessions.
Example
# Display brief information about all BFD sessions.
<HUAWEI> display bfd session all
(w): State in WTR
(*): State is invalid
------------------------------------------------------------------------------
Local Remote PeerIpAddr State Type InterfaceName
------------------------------------------------------------------------------
111 222 10.10.10.2 Up S_IP_IF GigabitEthernet2/0/0
------------------------------------------------------------------------------
Total UP/DOWN Session Number : 1/0
Table 1 Description of the display bfd session all command output
Item Description
Local Local discriminator of the BFD session
Remote Remote discriminator of the BFD session
PeerIpAddr Remote IP address of the BFD session
State Status of the BFD session:
• Up: The BFD session is in the Up state.
• Down: The BFD session is in the Down state.
• AdmDown: The BFD session is in the AdminDown state. After the shutdown (BFD session view) command is run, the BFD session becomes AdminDown.
• Up(w): indicates that the BFD session is in the Up state and during the WTR time.
• Up(*)/Down(*): indicates that the BFD session is in the invalid state.
Type Type of the BFD session:
• S_IP_PEER: indicates a session created statically and bound to an IP link.
• S_IP_IF: indicates a session created statically and bound to an interface.
• D_IP_PEER: indicates a session created dynamically and bound to an IP link.
• D_IP_IF: indicates a session created dynamically and bound to an interface.
• E_Dynamic: indicates the entirely dynamic session, after the egress of an LSP is enabled with the function of passively creating a BFD session.
• S_AUTO_IF: indicates the static single-hop BFD session with automatically negotiated discriminators.
• S_AUTO_PEER: indicates the static multi-hop BFD session with automatically negotiated discriminators.
• D_SR_BE_TNL: indicates a session created dynamically and bound to an SR-BE tunnel.
• S_TE_LSP: indicates a session created statically and bound to a TE LSP.
• S_TE_TNL: indicates a session created statically and bound to a TE tunnel.
• D_TE_LSP: indicates a session created statically and bound to a TE LSP.
• S_LDP_LSP: indicates a session created dynamically and bound to a dynamic LSP.
• D_LDP_LSP: indicates a session created dynamically and bound to a dynamic LSP.
• D_TE_TNL: indicates a session created dynamically and bound to a TE tunnel.
InterfaceName Outbound interface to which the BFD session is bound. For a multi-hop BFD session, "-" is displayed because no interface is bound to the BFD session.
Total UP/DOWN Session Number UP BFD Session Number/DOWN BFD Session Number
# Display detailed information about all BFD sessions.
<HUAWEI> display bfd session all verbose
(w): State in WTR
(*): State is invalid
--------------------------------------------------------------------------------
(One Hop) State : Up Name : atob
------------------------------------------------------------------------------
Local Discriminator : 111 Remote Discriminator : 222
Session Detect Mode : Asynchronous Mode Without Echo Function
BFD Bind Type : Interface(GigabitEthernet2/0/0)
Bind Session Type : Static
Bind Peer IP Address : 10.10.10.2
Bind Interface : GigabitEthernet2/0/0
FSM Board Id : 1 TOS-EXP : 7
Min Tx Interval (ms) : 10 Min Rx Interval (ms) : 10
Actual Tx Interval (ms): 2000 Actual Rx Interval (ms): 2000
Local Detect Multi : 3 Detect Interval (ms) : 30
Echo Passive : Disable Acl Number : -
Destination Port : 4784 TTL : 254
Proc Interface Status : Disable Process PST : Disable
WTR Interval (ms) : 0 Config PST : Enable
Active Multi : 0
Last Local Diagnostic : No Diagnostic
Bind Application : No Application Bind
Session TX TmrID : 2369967508 Session Detect TmrID : -
Session Init TmrID : - Session WTR TmrID : -
Session Echo Tx TmrID : -
Session Description : -
------------------------------------------------------------------------------

Total UP/DOWN Session Number : 1/0
Table 2 Description of the display bfd session all verbose command output
Item Description
State Status of the BFD session:
• Up: The BFD session is in the Up state.
• Down: The BFD session is in the Down state.
• AdminDown: The BFD session is in the AdminDown state. After the shutdown (BFD session view) command is run, the BFD session becomes AdminDown.
• Up(w): indicates that the BFD session is in the Up state and during the WTR time.
• Up(*)/Down(*): indicates that the BFD session is in the invalid state.
Name Name of the BFD session
Local Discriminator Local discriminator of the BFD session
Remote Discriminator Remote discriminator of the BFD session
Session Detect Mode Detection mode of the BFD session:
• Asynchronous Mode Without Echo Function
• Asynchronous Mode With Echo Function
• Asynchronous One-arm-echo Mode
• Seamless Mode without Echo Function
BFD Bind Type Binding type of the BFD session:
• Peer Ip Address: The BFD session detects a multi-hop IP link.
• For the single-hop IP link to be detected, the interface and the bound local interface are displayed.
• Static_Auto: A static BFD session is established through automatically negotiated discriminators.
• SR_BE_TUNNEL: The BFD session detects an SR-BE tunnel.
• TE_TUNNEL: The BFD session detects a TE tunnel.
• TE_LSP: The BFD session detects an LSP that is bound to a TE tunnel.
• LDP_LSP: The BFD session detects an LDP LSP.
Bind Session Type Establishment type of the BFD session:
• Static: A BFD session is established through static configuration.
• Dynamic: The establishment of a BFD session is triggered dynamically.
• Static_Auto: A static BFD session is established through automatically negotiated discriminators.
• Entire_Dynamic: The establishment of a BFD session is triggered dynamically, after the egress of an LSP is enabled with the function of passively creating a BFD session.
Bind Peer Ip Address Remote IP address of the BFD session
Bind Interface Outbound interface to which the BFD session is bound
Bind Source Ip Address Source IP address of the local interface bound to the BFD session, which is displayed only when the source IP address is specified for the BFD session using the bfd bind peer-ip command

FSM-Board-Id Slot ID of the main control board of the BFD state machine
TOS-EXP Priority of BFD packets
Min Tx Interval(ms) Configured minimum interval between sending BFD packets
Min Rx Interval(ms) Configured minimum interval between receiving BFD packets
Actual Tx Interval(ms) Indicates the actual minimum interval between sending BFD packets.
Actual Rx Interval(ms) Actual minimum interval between receiving BFD packets
Local Detect Multi Local detection multiplier
Detect Interval(ms) Detection interval of the BFD session
Echo Passive Whether the BFD passive Echo function is enabled:
• Enable
• Disable
Acl Number ACL number applied to the BFD session
Destination Port Destination port number of BFD packets
TTL TTL of BFD packets
Proc interface status Status of the association between the status of a BFD session and an interface:
• Enable: enables the association between the status of the BFD session and the interface.
• Disable: disables the association between the status of the BFD session and the interface.
If the process-interface-status command has been configured, the field is displayed as Enable; otherwise, the field is displayed as Disable.
Process PST Flag of processing the PST:
• Enable: permits the BFD session to modify the PST.
• Disable: prohibits the BFD session from modifying the PST.
If the process-pst command has been configured, the field is displayed as Enable; otherwise, the field is displayed as Disable.
WTR Interval(ms) WTR time of the BFD session
Active Multi Active detection multiplier of the BFD session
Last Local Diagnostic Local diagnostic reason for the last Down state of the BFD session:
• Control Detection Time Expired
• No Diagnostic
• Neighbor Signaled Session Down
• Administratively Down
• Neighbor Signaled Session Down (Receive AdminDown)
• Concatenated Path Down
• Reverse Concatenated Path Down
Bind Application Application to which the BFD session is bound:
• No Application Bind is displayed if no application is bound to the BFD session.
• A specific application name is displayed if the application is bound to the BFD session. For example, if an interface is bound to the BFD session, IFNET is displayed.
Session TX TmrID Timer used by the BFD session to send packets
Session Detect TmrID Timer for BFD session detection
Session Init TmrID Timer for the BFD session state machine initialization
Session WTR TmrID WTR timer for the BFD session
Session Echo Tx TmrID Timer used by the BFD session to send Echo packets
Session Not Up Reason Cause for a session negotiation failure, displayed when a session is in the following state:
• Board selecting failed
• In negotiation
Session Description Description of the BFD session





Thanks
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ejcastriver
ejcastriver Created Jan 22, 2020 23:57:35 Helpful(0) Helpful(0)

Hi Jess,

The physical interface associated with BFD cannot be the interface of the BFD link. Otherwise, link deadlock is unavoidable.

I suggest you instead to configure link flapping protection on interfaces. With this you can detect cable faults or active/standby switchovers which cause an interface to frequently alternate between Up and Down. If the number of interface flappings within the specified period reaches the configured limit, the switch shuts down the interface. After do this, you will need to configure automatic recovery of the interface within a specified interval.
error-down auto-recovery cause link-flap interval 30
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chenhui
chenhui Admin Created Jan 23, 2020 03:44:43 Helpful(0) Helpful(0)

Hi @jess99
well, there isn't such command could enable the shutdown interface automatically.
But you can try configuring association between a BFD session and an interface, but not trigger the associated interface in down state, so that when the BFD sesseion failed, it would set the interface to BFD status down, when the BFD seseesion turns up, the BFD status down will be removed automatically.
Actually, you are kindly advised to describe your requirement detailedly, so that we could help to recommend you the most appropriate configuration.
  • x
  • convention:

lubna
lubna Created Jan 26, 2020 06:59:01 Helpful(0) Helpful(0)

hy jess
may it can help you

Bidirectional Forwarding Detection
This document describes how to enable the Bidirectional Forwarding Detection (BFD) protocol. BFD is a detection protocol designed to provide fast forwarding path failure detection times for all media types, encapsulations, topologies, and routing protocols. In addition to fast forwarding path failure detection, BFD provides a consistent failure detection method for network administrators. Because the network administrator can use BFD to detect forwarding path failures at a uniform rate, rather than the variable rates for different routing protocol hello mechanisms, network profiling and planning will be easier, and reconvergence time will be consistent and predictable.

Finding Feature Information in This Module

Your Cisco IOS software release may not support all of the features documented in this module. To reach links to specific feature documentation in this module and to see a list of the releases in which each feature is supported, use the "Feature Information for Bidirectional Forwarding Detection" section.

Finding Support Information for Platforms and Cisco IOS and Catalyst OS Software Images

Use Cisco Feature Navigator to find information about platform support and Cisco IOS and Catalyst OS software image support. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.

Contents
•Prerequisites for Bidirectional Forwarding Detection

•Restrictions for Bidirectional Forwarding Detection

•Information About Bidirectional Forwarding Detection

•How to Configure Bidirectional Forwarding Detection

•Configuration Examples for Bidirectional Forwarding Detection

•Additional References

•Feature Information for Bidirectional Forwarding Detection

•Glossary

Prerequisites for Bidirectional Forwarding Detection
•Cisco Express Forwarding (CEF) and IP routing must be enabled on all participating routers.

•You must enable Cisco Parallel eXpress Forwarding (PXF) on the Cisco 10720 Internet router in order for BFD to operate properly. PXF is enabled by default and is generally not turned off.

•One of the IP routing protocols supported by BFD must be configured on the routers before BFD is deployed. You should implement fast convergence for the routing protocol that you are using. See the IP routing documentation for your version of Cisco IOS software for information on configuring fast convergence. See the "Restrictions for Bidirectional Forwarding Detection" section for more information on BFD routing protocol support in Cisco IOS software.

Restrictions for Bidirectional Forwarding Detection
•For the current Cisco implementation of BFD for Cisco IOS Releases 12.2(18)SXE, 12.0(31)S, 12.4(4)T, 12.0(32)S, 12.2(33)SRA, and 12.2(33)SRB, only asynchronous mode is supported. In asynchronous mode, either BFD peer can initiate a BFD session.

•For the current Cisco implementation of BFD for Cisco IOS Releases 12.2(18)SXE, 12.0(31)S, 12.4(4)T, 12.0(32)S, 12.2(33)SRA, and 12.2(33)SRB, BFD is supported only for IPv4 networks.

•For Cisco IOS Release 12.2(33)SRB, the Cisco implementation of BFD supports only the following routing protocols: BGP, EIGRP, IS-IS, and OSPF.

•For Cisco IOS Release 12.2(33)SRA, the Cisco implementation of BFD supports only the following routing protocols: BGP, IS-IS, and OSPF.

•For Cisco IOS Release 12.4(4)T, the Cisco implementation of BFD supports only the following routing protocols: Border Gateway Protocol (BGP), Enhanced Interior Gateway Routing Protocol (EIGRP), Intermediate System-to-Intermediate System (IS-IS), and Open Shortest Path First (OSPF).

•For Cisco IOS Release 12.4(11)T, the Cisco implementation of BFD introduced support for the Hot Standby Router Protocol (HSRP). BFD support is not available for all platforms and interfaces. In Cisco IOS Release 12.4(11)T, this feature was introduced on Cisco 7200 series, Cisco 7600 series, and Cisco 12000 series routers.

•For Cisco IOS Releases 12.0(31)S and 12.0(32)S, the Cisco implementation of BFD supports only the following routing protocols: BGP, IS-IS, and OSPF.

•For Cisco IOS Release 12.2(18)SXE, the Cisco implementation of BFD supports only the following routing protocols: EIGRP, IS-IS, and OSPF

•For Cisco IOS Release 12.2(18)SXH, the Cisco implementation of BFD supports the following routing protocols: BGP, EIGRP, IS-IS, and OSPF.

•BFD works only for directly connected neighbors. BFD neighbors must be no more than one IP hop away. Multihop configurations are not supported.

•BFD support is not available for all platforms and interfaces. To confirm BFD support for a specific platform or interface and obtain the most accurate platform and hardware restrictions, see the Cisco IOS software release notes for your software version.

•For the following Cisco IOS Releases, BFD on PortChannel is not a supported configuration: 12.2 SXH, 12.2 SXF, 12.2 SRC, and 12.2 SRB.

•On the Cisco 10720 Internet router, BFD is supported only on Fast Ethernet, Gigabit Ethernet, and RPR-IEEE interfaces. BFD is not supported on Spatial Reuse Protocol (SRP) and Packet-over-SONET (POS) interfaces.

•When you configure the BFD session parameters on a Cisco 10720 interface using the bfd command (in interface configuration mode), the minimum configurable time period supported for the milliseconds argument in both the interval milliseconds and min_rx milliseconds parameters is 50 milliseconds.

•A maximum of 100 BFD sessions are supported on the Cisco 10720 Internet router. When BFD tries to set up a connection between routing protocols and establish a 101th session between a Cisco 10720 Internet router and adjacent routers, the following error message is displayed:

00:01:24: %OSPF-5-ADJCHG: Process 100, Nbr 10.0.0.0 on RPR-IEEE1/1 from LOADING to
FULL, Loading Done
00:01:24: D-5-SESSIONLIMIT: Attempt to exceed session limit of 100 neighbors.

•The Cisco 10720 Internet router does not support the following BFD features:

–Demand mode

–Echo packets

–BFD over IP Version 6

•On the Cisco 12000 series router, asymmetrical routing between peer devices may cause a BFD control packet to be received on a line card other than the line card that initiated the session. In this special case, the BFD session between the routing peers will not be established.

•A maximum 100 sessions per line card are supported for the distributed Cisco 12000 series Internet router. The minimum hello interval is 50 ms with up to three Max retries for a BFD control packet to be received from a remote system before a session with a neighbor is declared down.


Note For the most accurate platform and hardware restrictions, see the Cisco IOS software release notes for your software version.

Information About Bidirectional Forwarding Detection
Before you configure BFD, you should become familiar with the information in the following sections:

•BFD Operation

•Benefits of Using BFD for Failure Detection

BFD Operation
BFD provides a low-overhead, short-duration method of detecting failures in the forwarding path between two adjacent routers, including the interfaces, data links, and forwarding planes. BFD is a detection protocol that you enable at the interface and routing protocol levels. Cisco supports the BFD asynchronous mode, which depends on the sending of BFD control packets between two systems to activate and maintain BFD neighbor sessions between routers. Therefore, in order for a BFD session to be created, you must configure BFD on both systems (or BFD peers). Once BFD has been enabled on the interfaces and at the router level for the appropriate routing protocols, a BFD session is created, BFD timers are negotiated, and the BFD peers will begin to send BFD control packets to each other at the negotiated interval.

BFD provides fast BFD peer failure detection times independently of all media types, encapsulations, topologies, and routing protocols BGP, EIGRP, IS-IS, and OSPF. By sending rapid failure detection notices to the routing protocols in the local router to initiate the routing table recalculation process, BFD contributes to greatly reduced overall network convergence time. Figure 1 shows a simple network with two routers running OSPF and BFD. When OSPF discovers a neighbor (1) it sends a request to the local BFD process to initiate a BFD neighbor session with the OSPF neighbor router (2). The BFD neighbor session with the OSPF neighbor router is established (3).

Figure 1 Establishing a BFD Neighbor Relationship


Figure 2 shows what happens when a failure occurs in the network (1). The BFD neighbor session with the OSPF neighbor router is torn down (2). BFD notifies the local OSPF process that the BFD neighbor is no longer reachable (3). The local OSPF process tears down the OSPF neighbor relationship (4). If an alternative path is available the routers will immediately start converging on it.

Figure 2 Tearing Down an OSPF Neighbor Relationship


BFD Detection of Failures
Once a BFD session has been established and timer negations are complete, BFD peers send BFD control packets that act in the same manner as an IGP hello protocol to detect liveliness, except at a more accelerated rate. The following information should be noted:

•BFD is a forwarding path failure detection protocol. BFD detects a failure, but the routing protocol must take action to bypass a failed peer.

•Typically, BFD can be used at any protocol layer. However, the Cisco implementation of BFD for Cisco IOS Releases 12.2(18)SXE, 12.0(31)S, and 12.4(4)T supports only Layer 3 clients, in particular, the BGP, EIGRP, IS-IS, and OSPF routing protocols.

•Cisco devices will use one BFD session for multiple client protocols in the Cisco implementation of BFD for Cisco IOS Releases 12.2(18)SXE, 12.0(31)S, and 12.4(4)T. For example, if a network is running OSPF and EIGRP across the same link to the same peer, only one BFD session will be established, and BFD will share session information with both routing protocols.

BFD Version Interoperability
Cisco IOS Release 12.4(9)T supports BFD Version 1 as well as BFD Version 0. All BFD sessions come up as Version 1 by default and will be interoperable with Version 0. The system automatically performs BFD version detection, and BFD sessions between neighbors will run in the highest common BFD version between neighbors. For example, of one BFD neighbor is running BFD Version 0 and the other BFD neighbor is running Version 1, the session will run BFD Version 0. The output from the show bfd neighbors [details] command will verify which BFD version a BFD neighbor is running.

See the "Configuring BFD in an EIGRP Network with Echo Mode Enabled by Default: Example" section for an example of BFD version detection.

BFD Support on Cisco 12000 Routers
The Cisco 12000 series routers support distributed BFD to take advantage of its distributed Route Processor (RP) and line card (LC) architecture. The BFD tasks will be divided and assigned to the BFD process on RP and LC as described in the following sections:

•BFD Process on the RP

•BFD Process on the LC

BFD Process on the RP
Client Interaction

The BFD process on the RP will handle the interaction with clients, which create and delete BFD sessions.

Session Management for BFD Process on the RP

The BFD RP process will primarily own all BFD sessions on the router. It will pass the session creation and deletion requests to the BFD processes on all LCs. BFD LC sessions will have no knowledge of sessions being added or deleted by the clients. Only the BFD RP process will send session addition and deletion commands to the BFD LC process.

Session Database Management

The BFD RP process will maintain a database of all the BFD sessions on the router. This database will contain only the minimum required information.

Process EXEC Commands

The BFD RP process services the BFD show commands.

BFD Process on the LC
Session Management for BFD Process on the LC

The BFD LC process manages sessions, adds and deletes commands from the BFD RP process, and creates and deletes new sessions based on the commands. In the event of transmit failure, receive failure, or session down detection, the LC BFD instance will immediately notify the BFD RP process. It will also update transmit and receive counters. The BFD session is maintained completely on the LC. BFD control packets are received and processed, as well as sent, from the LC itself.

Database Management

The BFD LC process maintains a database of all the BFD sessions hosted on the LC.

Receive and Transmit

The BFD LC process is responsible for transmitting and receiving BFD packets for the sessions on the LC.

Benefits of Using BFD for Failure Detection
When you deploy any feature, it is important to consider all the alternatives and be aware of any trade-offs being made.

The closest alternative to BFD in conventional EIGRP, IS-IS, and OSPF deployments is the use of modified failure detection mechanisms for EIGRP, IS-IS, and OSPF routing protocols.

If you set EIGRP hello and hold timers to their absolute minimums, the failure detection rate for EIGRP falls to within a one- to two-second range.

If you use fast hellos for either IS-IS or OSPF, these Interior Gateway Protocol (IGP) protocols reduce their failure detection mechanisms to a minimum of one second.

There are several advantages to implementing BFD over reduced timer mechanisms for routing protocols:

•Although reducing the EIGRP, IS-IS, and OSPF timers can result in minimum detection timer of one to two seconds, BFD can provide failure detection in less than one second.

•Because BFD is not tied to any particular routing protocol, it can be used as a generic and consistent failure detection mechanism for EIGRP, IS-IS, and OSPF.

•Because some parts of BFD can be distributed to the data plane, it can be less CPU-intensive than the reduced EIGRP, IS-IS, and OSPF timers, which exist wholly at the control plane.

How to Configure Bidirectional Forwarding Detection
You start a BFD process by configuring BFD on the interface. When the BFD process is started, no entries are created in the adjacency database, in other words, no BFD control packets are sent or received. BFD echo mode, which is supported in BFD Version 1 for Cisco IOS 12.4(9)T, is enabled by default. BFD echo packets are sent and received in addition to BFD control packets. The adjacency creation takes places once you have configured BFD support for the applicable routing protocols. This section contains the following procedures:

•Configuring BFD Session Parameters on the Interface (required)

•Configuring BFD Support for Routing Protocols (required)

•Configuring BFD Echo Mode (optional)

•Monitoring and Troubleshooting BFD (optional)
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