• For IPv4 on other physical interfaces whose minimum interval is less than two seconds.
When BFD is running asynchronously without echo packets (Figure 35), the following occurs:
• Each system periodically sends BFD control packets to one another. Packets sent by BFD router “Peer
A” to BFD router “Peer B” have a source address from Peer A and a destination address for Peer B.
• Control packet streams are independent of each other and do not work in a request/response model.
• If a number of packets in a row are not received by the other system, the session is declared down.
Figure 8: BFD Asynchronous Mode Without Echo Packets
When BFD is running asynchronously with echo packets (Figure 36), the following occurs:
• BFD echo packets are looped back through the forwarding path only of the BFD peer and are not processed
by any protocol stack. So, packets sent by BFD router “Peer A” can be sent with both the source and
destination address of Peer A.
• BFD echo packets are sent in addition to BFD control packets.
Figure 9: BFD Asynchronous Mode With Echo Packets
For more information about control and echo packet intervals in asynchronous mode, see the BFD Packet
Intervals and Failure Detection.
BFD Packet Information
BFD Source and Destination Ports
BFD payload control packets are encapsulated in UDP packets, using destination port 3784 and source port
49152. Even on shared media, like Ethernet, BFD control packets are always sent as unicast packets to the
BFD peer.
Echo packets are encapsulated in UDP packets, as well, using destination port 3785 and source port 3785.
The BFD over bundle member feature increments each byte of the UDP source port on echo packets with
each transmission. UDP source port ranges from 0xC0C0 to 0xFFFF. For example:
1st echo packet: 0xC0C0
2nd echo packet: 0xC1C1
3rd echo packet: 0xC2C2
Routing Configuration Guide for Cisco NCS 6000 Series Routers, IOS XR Release 6.4.x
130
Implementing BFD
BFD Packet Information
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