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317 lines
12 KiB
ReStructuredText
317 lines
12 KiB
ReStructuredText
.. _sbfd:
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****
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SBFD
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****
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:abbr:`SBFD (Seamless Bidirectional Forwarding Detection)` is:
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Seamless Bidirectional Forwarding Detection, a simplified mechanism for using BFD with a large
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proportion of negotiation aspects eliminated, thus providing benefits
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such as quick provisioning, as well as improved control and
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flexibility for network nodes initiating path monitoring.
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-- :rfc:`7880`
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It is described and extended by the following RFCs:
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* :rfc:`7880`
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* :rfc:`7881`
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.. _sbfd-sate-machine:
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SBFD state machine
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==================
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SBFD takes the same data packet format as BFD, but with a much simpler state machine.
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According to RFC7880, SBFD has a stateless SBFDReflector and a stateful SBFDInitiator with the state machine as below:
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::
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+--+
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ADMIN DOWN, | |
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TIMER | V
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+------+ UP +------+
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| |-------------------->| |----+
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| DOWN | | UP | | UP
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| |<--------------------| |<---+
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+------+ ADMIN DOWN, +------+
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TIMER
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Figure 1: SBFDInitiator Finite State Machine
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* If SBFDInitiator doesn't receive the response packet in time, session is DOWN.
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* If SBFDInitiator receives the response packet in time: reponse state is ADMINDOWN, session goes DOWN; reponse state is UP, session goes UP.
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.. note::
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SBFDReflector is stateless, it just transmit a packet in response to a received S-BFD packet having a valid S-BFD Discriminator in the Your Discriminator field.
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.. _sbfd-extention:
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SBFD extension - SRv6 encapsulation
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===================================
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SBFDInitiator periodically send packets to monitor the connection to SBFDReflector. We set up an SBFD connection between the source and the destination node of a path,
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with the source node serving as Initiator and the destination node as Reflector. The communicated SBFD packets should also follow every exact hop in the path,
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from the source to the destination, which could be achieved by segment routing. This requirement extends the node verification to the path verification.
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In the following example, we set up a sbfd session to monitor the path A-B-D (all nodes in the topo are SRv6 ready, which can decap and forward SRv6 packets).
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::
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+------------C-----------+
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/ \
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A---------------B---------------D
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^ ^ ^
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End: 100::A End: 100::B End: 100::D
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Loopback: 200::A Loopback: 200::D
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BFD Discrim: 123 BFD Discrim: 456
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A is the SBFDInitiator, and D is the SBFDReflector, A will trasmit the SBFD packet to B as the format:
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::
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IPv6(src="200::A", dst="100::B", nh=43)/IPv6ExtHdrSegmentRouting(addresses=["100::D"], nh=41, segleft=1)/IPv6(src="200::A", dst="200::D")/UDP(dport=7784)/BFD(my_dis=123, your_disc=456, state=UP)
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Upon receiving the packet, B will take the Srv6 End action since the dst ip 100::B is the End address, B will the shift the dst address according to Srv6 spec, then trasmit the SBFD packet to D as the format:
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::
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IPv6(src="200::A", dst="100::D", nh=41)/IPv6(src="200::A", dst="200::D")/UDP(dport=7784)/BFD(my_dis=123, your_disc=456, state=UP)
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After D receive the packet, It will decap the outer IPv6 header since the dst ip 100::D is the End address, the decapped packet is:
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::
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IPv6(src="200::A", dst="200::D")/UDP(dport=7784)/BFD(my_dis=123, your_disc=456, state=UP)
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This packet will be routed to kernel stack of D since its dst is 200::D. Then the SBFDReflector service on D will get the packet and Reflect it. The response packet will be:
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::
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IPv6(src="200::D", dst="200::A")/UDP(sport=7784)/BFD(my_dis=456, your_disc=123, state=UP)
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This packet will be routed in the topo according to the dst ip 200::A, it will go back to A by D-B-A or D-C-A in this case.
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In this example, Command used to configure the SBFDInitiator on A is:
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.. clicmd:: peer 200::D bfd-mode sbfd-init bfd-name a-b-d multihop local-address 200::A remote-discr 456 srv6-source-ipv6 200::A srv6-encap-data 100::B 100::D
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Command used to configure the SBFDReflector on D is:
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.. clicmd:: sbfd reflector source-address 200::D discriminator 456
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.. _sbfd-echo:
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Echo SBFD with SRv6 encapsulation
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=================================
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The SBFD Initiator-Reflector mode requires the configuration on both source and destination nodes. It can not work if the remote node has no SBD feature supported, especial on some third-party devices.
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The Echo SBFD can solve this kind of deployment issue since it only requires the configuration on source node. This is also known as One-Arm BFD Echo or unaffiliated BFD Echo.
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For example, we use Echo SBFD session to protect Srv6 path: A-B-D
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::
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+------------C-----------+
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/ \
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A---------------B---------------D
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^ ^ ^
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End: 100::A End: 100::B End: 100::D
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Loopback: 200::A Loopback: 200::D
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BFD Discrim: 123
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A is also the SBFDInitiator, and B, C, D is Srv6 ready nodes, A will trasmit the SBFD packet to B as the format:
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::
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IPv6(src="200::A", dst="100::B", nh=43)/IPv6ExtHdrSegmentRouting(addresses=["100::D"], nh=41, segleft=1)/IPv6(src="200::A", dst="200::A")/UDP(dport=3785)/BFD(my_dis=123, your_disc=123, state=UP)
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Upon receiving the packet, B will take the Srv6 End action since the dst ip 100::B is the End address, B will the shift the dst address according to Srv6 spec, then trasmit the SBFD packet to D as the format:
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::
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IPv6(src="200::A", dst="100::D", nh=41)/IPv6(src="200::A", dst="200::A")/UDP(dport=3785)/BFD(my_dis=123, your_disc=123, state=UP)
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After D receive the packet, It will decap the outer IPv6 header since the dst ip 100::D is the End address, the decapped packet is:
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::
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IPv6(src="200::A", dst="200::A")/UDP(dport=3785)/BFD(my_dis=123, your_disc=123, state=UP)
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This packet will be routed in the topo according to the dst ip 200::A, it will go back to A by D-B-A or D-C-A in this case.
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In this example, Command used to configure the SBFDInitiator on A is:
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.. clicmd:: peer 200::A bfd-mode sbfd-echo bfd-name a-b-d local-address 200::A srv6-source-ipv6 200::A srv6-encap-data 100::B 100::D
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no configuration needed on D.
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.. _sbfd-normal:
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normal SBFD with no SRv6 encapsulation
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======================================
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We can also configure a SBFD Initiator-Reflector session based on simple IPv6/IPv4 packet, no Srv6 involved in this case.
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::
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+------------C-----------+
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/ \
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A---------------B---------------D
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^ ^ ^
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Loopback: 200::A Loopback: 200::D
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BFD Discrim: 123 BFD Discrim: 456
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A is the SBFDInitiator, and D is the SBFDReflector, A will trasmit the SBFD packet to B or C as the format:
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::
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IPv6(src="200::A", dst="200::D")/UDP(dport=7784)/BFD(my_dis=123, your_disc=456, state=UP)
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Upon receiving the packet, B/C will route the packet to D according to the dst ip 200::D.
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After D receive the packet, packet will be sent to kernel stack of D since its dst is 200::D. Then the SBFDReflector service on D will get the packet and reflect it. The response packet will be:
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::
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IPv6(src="200::D", dst="200::A")/UDP(sport=7784)/BFD(my_dis=456, your_disc=123, state=UP)
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This packet will be routed in the topo according to the dst ip 200::A, it will go back to A by D-B-A or D-C-A in this case.
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In this example, Command used to configure the SBFDInitiator on A is:
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.. clicmd:: peer 200::D bfd-mode sbfd-init bfd-name a-d local-address 200::A remote-discr 456
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Command used to configure the SBFDReflector on D is:
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.. clicmd:: sbfd reflector source-address 200::D discriminator 456
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.. note::
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Currently some features are not yet implemented:
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1) SBFD in IPv4 only packet
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2) The ADMIN DOWN logic
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3) SBFD echo function in a initiator session
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4) SBFD over MPLS
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.. _sbfd-show:
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show command
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============
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The exsiting bfd show command is also appliable to SBFD sessions, for example:
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This command will show all the BFD and SBFD sessions in the bfdd:
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.. clicmd:: show bfd peers
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::
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BFD Peers:
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peer 200::D bfd-mode sbfd-init bfd-name a-d multihop local-address 200::A vrf default remote-discr 456
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ID: 1421669725
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Remote ID: 456
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Active mode
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Minimum TTL: 254
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Status: up
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Uptime: 5 hour(s), 48 minute(s), 39 second(s)
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Diagnostics: ok
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Remote diagnostics: ok
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Peer Type: sbfd initiator
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Local timers:
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Detect-multiplier: 3
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Receive interval: 300ms
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Transmission interval: 1000ms
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Echo receive interval: 50ms
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Echo transmission interval: disabled
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Remote timers:
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Detect-multiplier: -
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Receive interval: -
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Transmission interval: -
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Echo receive interval: -
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This command will show all the BFD and SBFD session packet counters:
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.. clicmd:: show bfd peers counters
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BFD Peers:
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peer 200::A bfd-mode sbfd-echo bfd-name a-b-d local-address 200::A vrf default srv6-source-ipv6 200::A srv6-encap-data 100::B 100::D
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Control packet input: 0 packets
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Control packet output: 0 packets
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Echo packet input: 23807 packets
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Echo packet output: 23807 packets
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Session up events: 1
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Session down events: 0
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Zebra notifications: 1
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Tx fail packet: 0
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peer 200::D bfd-mode sbfd-init bfd-name a-d local-address 200::A vrf default remote-discr 456
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Control packet input: 25289 packets
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Control packet output: 51812 packets
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Echo packet input: 0 packets
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Echo packet output: 0 packets
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Session up events: 5
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Session down events: 4
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Zebra notifications: 9
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Tx fail packet: 0
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we also implemented a new show command to display BFD sessions with a bfd-name, the bfd-name is the key to search the sessioon.
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.. clicmd:: show bfd bfd-name a-b-d
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BFD Peers:
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peer 200::A bfd-mode sbfd-echo bfd-name a-b-d local-address 200::A vrf default srv6-source-ipv6 200::A srv6-encap-data 100::B 100::D
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ID: 123
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Remote ID: 123
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Active mode
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Status: up
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Uptime: 5 hour(s), 39 minute(s), 34 second(s)
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Diagnostics: ok
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Remote diagnostics: ok
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Peer Type: echo
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Local timers:
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Detect-multiplier: 3
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Receive interval: 300ms
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Transmission interval: 300ms
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Echo receive interval: 300ms
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Echo transmission interval: 1000ms
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Remote timers:
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Detect-multiplier: -
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Receive interval: -
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Transmission interval: -
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Echo receive interval: -
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