Bidirectional Forwarding Detection - BFD
Bidirectional Forwarding Detection (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.
Cumulus Linux does not support:
- BFD demand mode
- Dynamic BFD timer negotiation on an existing session. Any change to the timer values takes effect only when the session goes down and comes back up.
BFD Multihop Routed Paths
BFD multihop sessions are built over arbitrary paths between two systems, which results in some complexity that does not exist for single hop sessions. Here are some best practices for using multihop paths:
To avoid spoofing with multihop paths, configure the maximum hop count (
max_hop_cnt
) for each peer, which limits the number of hops for a BFD session. All BFD packets exceeding the maximum hop count are dropped.Because multihop BFD sessions can take arbitrary paths, demultiplex the initial BFD packet based on the source/destination IP address pair. Use FRRouting, which monitors connectivity to the peer, to determine the source/destination IP address pairs.
Cumulus Linux supports multihop BFD sessions for both IPv4 and IPv6 peers.
Configure BFD
You can configure BFD by either using FRRouting (with NCLU or vtysh
commands) or by specifying the configuration in the PTM `topology.dot` file. However, the topology file has some limitations:
- The topology file supports BFD IPv4 and IPv6 single hop sessions only; you cannot specify IPv4 or IPv6 multihop sessions in the topology file.
- The topology file supports BFD sessions for only link-local IPv6 peers; BFD sessions for global IPv6 peers discovered on the link are not created.
Use FRRouting to register multihop peers with PTM and BFD as well as to monitor the connectivity to the remote BGP multihop peer. FRRouting can dynamically register and unregister both IPv4 and IPv6 peers with BFD when the BFD-enabled peer connectivity is established or de-established. Also, you can configure BFD parameters for each BGP or OSPF peer.
The BFD parameter configured in the topology file is given higher precedence over the client-configured BFD parameters for a BFD session that has been created by both the topology file and FRRouting.
BFD requires an IP address for any interface on which it is configured. The neighbor IP address for a single hop BFD session must be in the ARP table before BFD can start sending control packets.
When you configure BFD, you can set the following parameters for both IPv4 and IPv6 sessions. If you do not set these parameters, the default values are used.
- The required minimum interval between the received BFD control packets. The default value is 300ms.
- The minimum interval for transmitting BFD control packets. The default value is 300ms.
- The detection time multiplier. The default value is 3.
BFD in BGP
When you configure BFD in BGP, neighbors are registered and deregistered with PTM dynamically.
To configure BFD in BGP, run the following commands.
You can configure BFD for a peer group or for an individual neighbor.
The following example configures BFD for swp1 and uses the default intervals.
cumulus@switch:~$ net add bgp neighbor swp1 bfd
cumulus@switch:~$ net pending
cumulus@switch:~$ net commit
The following example configures BFD for the peer group fabric
and sets the interval multiplier to 4, the minimum interval between received BFD control packets to 400, and the minimum interval for sending BFD control packets to 400.
cumulus@switch:~$ net add bgp neighbor fabric bfd 4 400 400
cumulus@switch:~$ net pending
cumulus@switch:~$ net commit
The following example configures BFD for swp1 and uses the default intervals:
cumulus@switch:~$ sudo vtysh
switch# configure terminal
switch(config)# router bgp 65000
switch(config-router)# neighbor swp1 bfd
switch(config-router)# exit
switch(config)# exit
switch# write mem
switch# exit
cumulus@switch:~$
The following example configures BFD for the peer group fabric
and sets the interval multiplier to 4, the minimum interval between received BFD control packets to 400, and the minimum interval for sending BFD control packets to 400.
cumulus@switch:~$ sudo vtysh
switch# configure terminal
switch(config)# router bgp 65000
switch(config-router)# neighbor fabric bfd 4 400 400
switch(config-router)# exit
switch(config)# exit
switch# write mem
switch# exit
cumulus@switch:~$
The NCLU and vtysh
commands save the configuration in the /etc/frr/frr.conf
file. For example:
...
router bgp 65000
neighbor fabric bfd 4 400 400
...
To see neighbor information in BGP, including BFD status, run the NCLU net show bgp neighbor <interface>
command or the vtysh
show ip bgp neighbor <interface>
command. For example:
cumulus@switch:~$ net show bgp neighbor swp1
...
BFD: Type: single hop
Detect Mul: 3, Min Rx interval: 300, Min Tx interval: 300
Status: Down, Last update: 0:00:00:08
...
BFD in OSPF
When you enable or disable BFD in OSPF, neighbors are registered and de-registered dynamically with PTM. When BFD is enabled on the interface, a neighbor is registered with BFD when two-way adjacency is established and deregistered when adjacency goes down. The BFD configuration is per interface and any IPv4 and IPv6 neighbors discovered on that interface inherit the configuration.
To configure BFD in OSPF, run the following commands.
The following example configures BFD in OSPFv3 for interface swp1 and sets interval multiplier to 4, the minimum interval between received BFD control packets to 400, and the minimum interval for sending BFD control packets to 400.
cumulus@switch:~$ net add interface swp1 ospf6 bfd 4 400 400
cumulus@switch:~$ net pending
cumulus@switch:~$ net commit
The following example configures BFD in OSPFv3 for interface swp1 and sets interval multiplier to 4, the minimum interval between received BFD control packets to 400, and the minimum interval for sending BFD control packets to 400.
cumulus@switch:~$ sudo vtysh
switch# configure terminal
switch(config)# interface swp1
switch(config-if)# ipv6 ospf6 bfd 4 400 400
switch(config-if)# exit
switch(config)# exit
switch# write mem
switch# exit
cumulus@switch:~$
The NCLU and vtysh
commands save the configuration in the /etc/frr/frr.conf
file. For example:
...
interface swp1
ipv6 ospf6 bfd 4 400 400
...
You can run different commands to show neighbor information in OSPF, including BFD status.
To show IPv6 OSPF interface information, run the NCLU
net show ospf6 interface <interface>
command or thevtysh
show ip ospf6 interface <interface>
command.To show IPv4 OSPF interface information, run the NCLU
net show ospf interface <interface>
command or thevtysh
show ip ospf interface <interface>
command.The following example shows IPv6 OSPF interface information.
cumulus@switch:~$ net show ospf6 interface swp2s0 swp2s0 is up, type BROADCAST Interface ID: 4 Internet Address: inet : 11.0.0.21/30 inet6: fe80::4638:39ff:fe00:6c8e/64 Instance ID 0, Interface MTU 1500 (autodetect: 1500) MTU mismatch detection: enabled Area ID 0.0.0.0, Cost 10 State PointToPoint, Transmit Delay 1 sec, Priority 1 Timer intervals configured: Hello 10, Dead 40, Retransmit 5 DR: 0.0.0.0 BDR: 0.0.0.0 Number of I/F scoped LSAs is 2 0 Pending LSAs for LSUpdate in Time 00:00:00 [thread off] 0 Pending LSAs for LSAck in Time 00:00:00 [thread off] BFD: Detect Mul: 3, Min Rx interval: 300, Min Tx interval: 300
To show IPv6 OSPF neighbor details, run the NCLU
net show ospf6 neighbor detail
command or thevtysh
show ip ospf6 neighbor detail
command.To show IPv4 OSPF interface information, run the NCLU
net show ospf neighbor detail
command or thevtysh
show ip ospf neighbor detail
command.The following example shows IPv6 OSPF neighbor details.
cumulus@switch:~$ net show ospf6 neighbor detail Neighbor 0.0.0.4%swp2s0 Area 0.0.0.0 via interface swp2s0 (ifindex 4) His IfIndex: 3 Link-local address: fe80::202:ff:fe00:a State Full for a duration of 02:32:33 His choice of DR/BDR 0.0.0.0/0.0.0.0, Priority 1 DbDesc status: Slave SeqNum: 0x76000000 Summary-List: 0 LSAs Request-List: 0 LSAs Retrans-List: 0 LSAs 0 Pending LSAs for DbDesc in Time 00:00:00 [thread off] 0 Pending LSAs for LSReq in Time 00:00:00 [thread off] 0 Pending LSAs for LSUpdate in Time 00:00:00 [thread off] 0 Pending LSAs for LSAck in Time 00:00:00 [thread off] BFD: Type: single hop Detect Mul: 3, Min Rx interval: 300, Min Tx interval: 300 Status: Up, Last update: 0:00:00:20
Scripts
ptmd
executes scripts at /etc/ptm.d/bfd-sess-down
when BFD sessions go down and /etc/ptm.d/bfd-sess-up
when BFD sessions goes up. Modify these default scripts as needed.
Echo Function
Cumulus Linux supports the echo function for IPv4 single hops only, and with the asynchronous operating mode only (Cumulus Linux does not support demand mode).
Use the echo function to test the forwarding path on a remote system. To enable the echo function, set echoSupport
to 1 in the topology file.
After the echo packets are looped by the remote system, the BFD control packets can be sent at a much lower rate. You configure this lower rate by setting the slowMinTx
parameter in the topology file to a non-zero value in milliseconds.
You can use more aggressive detection times for echo packets because the round-trip time is reduced; echo packets access the forwarding path. You can configure the detection interval by setting the echoMinRx
parameter in the topology file. The minimum setting is 50 milliseconds. After configured, BFD control packets are sent out at this required minimum echo Rx interval. This indicates to the peer that the local system can loop back the echo packets. Echo packets are transmitted if the peer supports receiving echo packets.
About the Echo Packet
BFD echo packets are encapsulated into UDP packets over destination and source UDP port number 3785. The BFD echo packet format is vendor-specific and has not been defined in the RFC. BFD echo packets that originate from Cumulus Linux are 8 bytes long and have the following format:
0 | 1 | 2 | 3 |
---|---|---|---|
Version | Length | Reserved | Reserved |
My Discriminator |
Where:
- Version is the version of the BFD echo packet.
- Length is the length of the BFD echo packet.
- My Discriminator is a non-zero value that uniquely identifies a BFD session on the transmitting side. When the originating node receives the packet after being looped back by the receiving system, this value uniquely identifies the BFD session.
Transmit and Receive Echo Packets
BFD echo packets are transmitted for a BFD session only when the peer has advertised a non-zero value for the required minimum echo Rx interval (the echoMinRx
setting) in the BFD control packet when the BFD session starts. The transmit rate of the echo packets is based on the peer advertised echo receive value in the control packet.
BFD echo packets are looped back to the originating node for a BFD session only if locally the echoMinRx
and echoSupport
are configured to a non-zero values.
Echo Function Parameters
You configure the echo function by setting the following parameters in the topology file at the global, template and port level:
- echoSupport enables and disables echo mode. Set to 1 to enable the echo function. It defaults to 0 (disable).
- echoMinRx is the minimum interval between echo packets the local system is capable of receiving. This is advertised in the BFD control packet. When the echo function is enabled, it defaults to 50. If you disable the echo function, this parameter is automatically set to 0, which indicates the port or the node cannot process or receive echo packets.
- slowMinTx is the minimum interval between transmitting BFD control packets when the echo packets are being exchanged.
Troubleshooting
To troubleshoot BFD, run the NCLU net show bfd sessions
or net show bfd sessions detail
command.
cumulus@switch:~$ net show bfd sessions detail
----------------------------------------------------------------------------------------
port peer state local type diag det tx_timeout rx_timeout
mult
----------------------------------------------------------------------------------------
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
You can also run the Linux ptmctl -b
command.