Cisco 7206 - VXR Router User Manual

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5-3
Cisco Nexus 7000 Series NX-OS Interfaces Configuration Guide, Release 5.x
OL-23435-03
Chapter 5 Configuring Bidirectional Forwarding Detection
Information About BFD
Figure 5-2 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 immediately start converging on it.
Note The BFD failure detection occurs in less than a second, which is much faster than OSPF Hello messages
could detect the same failure.
Figure 5-2 Tearing Down an OSPF Neighbor Relationship
Distributed Operation
Cisco NX-OS can distribute the BFD operation to compatible modules that support BFD. This process
offloads the CPU load for BFD packet processing to the individual modules that connect to the BFD
neighbors. All BFD session traffic occurs on the module CPU. The module informs the supervisor when
a BFD failure is detected.
BFD Echo Function
The BFD echo function sends echo packets from the forwarding engine to the remote BFD neighbor. The
BFD neighbor forwards the echo packet back along the same path in order to perform detection; the BFD
neighbor does not participate in the actual forwarding of the echo packets. The echo function and the
forwarding engine are responsible for the detection process. BFD can use the slow timer to slow down
the asycnhronous session when the echo function is enabled and reduce the number of BFD control
packets that are sent between two BFD neighbors. Also, the forwarding engine tests the forwarding path
on the remote (neighbor) system without involving the remote system, so there is less interpacket delay
variability and faster failure detection times.
The echo function is without asymmetry when both BFD neighbors are running echo function.
Note Unicast Reverse Path Forwarding check (uRPF) is disabled by default. If you need to enable it on an
interface functioning with BFD, the BFD echo function must be disabled.
172.16.10.2 172.16.10.1
172.17.0.1172.18.0.1
BFD
BFD neighbors
2
1
X
X
X
OSPF
3
BFD
Router A Router B
OSPF
3
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OSPF neighbors
4

Cisco 7206 - VXR Router Specifications

General

Brand	Cisco
Model	7206 - VXR Router
Category	Network Router
Language	English