BFD
Solves the problem of broken unidirectional links neatly, by putting packets into the dataplane then watching them return.
Terms
BFD Async
- Test the BFD control plane
BFD Echo
- Test the dataplane
BOB — BFD over Bundle
BLB — BFD over Logical Bundle
- VLANS & Sub-interfaces
- This requires multipath to be enabled
- Multipath doesn’t inject BFD packets into the HP queue
Ports
BFD is UDP, to an application on the network device
BFD Control is sent as SRC UDP 49512 –> Destination 3784
BFD Payload is sent as SRC UDP 3785 –> Destination 3785
BFD Async without Echo
- Control plane oriented
“Please respond to this packet with the control plane of the far device.”
BFD Async with Echo
- Data plane oriented
“Just loop the BFD packets back onto the link, please.”
Peer B’s BFD process does not handle these packets, they are forwarded as data traffic.
BFD State Machine
Courtesy of the RFC
RFC 5880 Bidirectional Forwarding Detection June 2010
(removed)
The following diagram provides an overview of the state machine.
Transitions involving AdminDown state are deleted for clarity (but
are fully specified in sections 6.8.6 and 6.8.16). The notation on
each arc represents the state of the remote system (as received in
the State field in the BFD Control packet) or indicates the
expiration of the Detection Timer.
┌──┐
│ │ UP, ADMIN DOWN, TIMER
│ ▼
DOWN ┌─┴────┐ INIT
┌────────────┤ ├────────────┐
│ │ DOWN │ │
│ ┌────────►│ │◄────────┐ │
│ │ └──────┘ │ │
│ │ │ │
│ │ ADMIN DOWN,│ │
│ │ADMIN DOWN, DOWN,│ │
│ │TIMER TIMER│ │
▼ │ │ ▼
┌────┴─┐ ┌─┴────┐
┌────┤ │ │ ├────┐
DOWN│ │ INIT │--------------------->│ UP │ │INIT, UP
└───►│ │ INIT, UP │ │◄───┘
└──────┘ └──────┘
IOS-XR Commands
Multipath
multipath include location 0/1/CPU0
bundle coexistence bob-blb logical
show tech-support routing bfd file
Take The Session Down If Latency Grows To 150ms For A Single Echo Packet
bfd fast detect
bfd multiplier 50
echo latency detect
Take The Session Down If Latency Grows To 300ms For A Single Echo Packet
bfd fast detect
bfd multiplier 50
bfd echo latency detect percentage 200
Take The Session Down If The Latency Grows To 150ms For 3 Consequitive Echo Packets
bfd fast detect
bfd multiplier 50
bfd echo latency detect percentage 100 count 3
Disable Echo Mode
bfd
interface g0/0/0/0
echo disable
Protecting The BFD Data-Plane Packets From QoS
192.168.100.1 <-> 192.168.100.2
!
! Config for 192.168.100.1
!
ipv4 access-list BFD-TRAFFIC
5 permit udp host 192.168.100.1 any range 3784 3785
10 permit udp host 192.168.100.2 any range 3784 3785
!
class-map match-any BFD-CLASS
match access-group ipv4 BFD-TRAFFIC
!
policy-map OUT
class BFD-CLASS
priority level 1
police rate 10 kbps
!
interface TenGig <>
service-policy output OUT
bfd address-family ipv4 multiplier 3
bfd address-family ipv4 destination 192.168.100.1
bfd address-family ipv4 fast-detect
bfd address-family ipv4 minimum-interval 100
!
Enabling BFD On RSVP (IOS)
A Config
ip rsvp signalling bfd hello
!
! this very dangerous because CPU load will affect processing of BFD control packets
!
int f0/0.45
ip rsvp signalling hello bfd
bfd interval 50 min_rx 50 multiplier 3
Verification
show ip rsvp hello bfd nbr