Cisco IE 4000, Software Configuration Manual

Page: 888 / 1066

884
Configuring IP Unicast Routing
Configuring BFD
To create a BFD session, you must configure BFD on both systems (BFD peers). Enabling BFD at the interface and routing
protocol level on BFD peers creates a BFD session. BFD timers are negotiated and the BFD peers send control packets
to each other at the negotiated intervals. If the neighbor is not directly connected, BFD neighbor registration is rejected.
Figure 102 on page 884 shows a simple network with two routers running OSPF and BFD. When OSPF discovers a
neighbor (1), it sends a request to the BFD process to initiate a BFD neighbor session with the neighbor OSPF router (2),
establishing the BFD neighbor session (3).
Figure 102 Establishing a BFD Session
Figure 103 on page 884 shows what happens when a failure occurs in the network (1). The BFD neighbor session with
the OSPF neighbor closes (2). BFD notifies the OSPF process that the BFD neighbor is no longer reachable, and the OSPF
process breaks the OSPF neighbor relationship (4). If an alternative path is available, the routers start converging on it.
Figure 103 Breaking an OSPF Neighbor Relationship
BFD clients are routing protocols that register neighbors with BFD. The switch supports IS-IS, OSPF v1 and v2, BGP,
EIGRP, and HSRP clients. You can use one BFD session for multiple client protocols. For example, if a network is running
OSPF and EIGRP across the same link to the same peer, you need to create only one BFD session, and information is
shared with both routing protocols.
The switch supports BFD version 0 and version 1. BFD neighbors automatically negotiate the version and the protocol
always runs at the higher version. The default version is version 1.
By default, BFD neighbors exchange both control packets and echo packets for detecting forwarding failures. The switch
sends echo packets at the configured BFD interval rate (from 50 to 999 ms), and control packets at the BFD slow-timer
rate (from 1000 to 3000 ms).
Failure-rate detection can be faster in BFD echo mode, which is enabled by default when you configure BFD session. In
this mode, the switch sends echo packets from the BFD software layer, and the BFD neighbor responds to the echo
packets through its fast-switching layer. The echo packets do not reach the BFD neighbor software layer, but are
reflected back over the forwarding path for failure detection. You configure the rate at which each BFD interface sends
BFD echo packets by entering the
bfd interval interface configuration command.
To reduce bandwidth consumption, you can disable the sending of echo packets by entering the no bfd echo interface
configuration command. When echo mode is disabled, control packets are used to detect forwarding failures. Control
packets are exchanged at the configured slow-timer rate, which could result in longer failure-detection time. You
configure this rate by entering the
bfd slow-timer global configuration command. The range is from 1000 to 3000 ms;
the default rate is every 1000 ms.
172.16.10.2 172.16.10.1
172.17.0.1 172.18.0.1
BFD
BFD neighbors
3
OSPF
2
BFD
OSPF
2
127844
OSPF neighbors
1
Router A Router B
172.16.10.2 172.16.10.1
172.17.0.1 172.18.0.1
BFD
BFD neighbors
2
1
X
X
X
OSPF
3
BFD
Router A Router B
OSPF
3
127845
OSPF neighbors
4