本示例的基本拓扑结构如图所示，现网中有3台路由器通过IS-IS协议实现路由互通，且Router-A和Router-B之间通过一台二层交换机及实现互联。现要求当Router-A与Router-B之间出现链路故障时，能快速检测到。
由于本实例中Router-A和Router-C中间隔了一个Router-B，所以不能建立IS-IS邻居，不能通过IS-IS与BFD的联动检测Router-A到Router-C之间的多跳链路(因为IS-IS还只能建立单跳邻居关系)，只能正在Router-A和Router-B之间检测单跳链路。
1) 配置各路由器的接口IP地址
【Router-A上的配置】
interface GigabitEthernet1/0/0
ip address 10.1.1.1 255.255.255.0
#
【Router-B上的配置】
interface GigabitEthernet1/0/0
ip address 10.1.1.2 255.255.255.0
#
interface GigabitEthernet2/0/0
ip address 10.2.1.1 255.255.255.0
#
【Router-C上的配置】
interface GigabitEthernet1/0/0
ip address 10.2.1.2 255.255.255.0
#
2) 配置各路由器的IS-IS基本功能
【Router-A上的配置】
isis 1
is-level level-2
network-entity aa.1111.1111.1111.00
#
interface GigabitEthernet1/0/0
ip address 10.1.1.1 255.255.255.0
isis enable 1
#
【Router-B上的配置】
isis 1
is-level level-2
network-entity aa.2222.2222.2222.00
#
interface GigabitEthernet1/0/0
ip address 10.1.1.2 255.255.255.0
isis enable 1
#
interface GigabitEthernet2/0/0
ip address 10.2.1.1 255.255.255.0
isis enable 1
【Router-C上的配置】
isis 1
is-level level-2
network-entity aa.3333.3333.3333.00
#
interface GigabitEthernet1/0/0
ip address 10.2.1.2 255.255.255.0
isis enable 1
【在RouterA上执行display isis peer命令，即可以看到Router-A和Router-B建立邻居关系】
<Router-A>display isis peer
Peer information for ISIS(1)
System Id Interface Circuit Id State HoldTime Type PRI
-------------------------------------------------------------------------------
2222.2222.2222 GE1/0/0 1111.1111.1111.01 Up 26s L2 64
Total Peer(s): 1
<Router-A>
【在RouterA上执行display isis route命令，即可以看到Router-A和Router-B建立邻居关系】
<Router-A>display isis route
Route information for ISIS(1)
-----------------------------
ISIS(1) Level-2 Forwarding Table
--------------------------------
IPV4 Destination IntCost ExtCost ExitInterface NextHop Flags
---------------------------------------------------------------------------
10.1.1.0/24 10 NULL GE1/0/0 Direct D/-/L/-
10.2.1.0/24 20 NULL GE1/0/0 10.1.1.2 A/-/-/-
Flags: D-Direct, A-Added to URT, L-Advertised in LSPs, S-IGP Shortcut,
U-Up/Down Bit Set
<Router-A>
3) 在Router-A和Router-B上分别配置IS-IS与静态BFD联动，包括创建BFD会话
【Router-A上的配置】
bfd
#
bfd atob bind peer-ip 10.1.1.2 interface GigabitEthernet1/0/0
discriminator local 1
discriminator remote 2
commit
#
interface GigabitEthernet1/0/0
ip address 10.1.1.1 255.255.255.0
isis enable 1
isis bfd static
【Router-B上的配置】
bfd
#
#
bfd btoa bind peer-ip 10.1.1.1 interface GigabitEthernet1/0/0
discriminator local 2
discriminator remote 1
commit
#
interface GigabitEthernet1/0/0
ip address 10.1.1.2 255.255.255.0
isis enable 1
isis bfd static
#
4) 配置验证
l 以上配置后后，通过在Router-A或Router-B上执行display bfd session命令可以看到它们之间的BFD会话的状态为Up。
<Router-A>display bfd session all
---------------------------------------------------------------------------
Local Remote PeerIpAddr State Type InterfaceName
---------------------------------------------------------------------------
1 2 10.1.1.2 Up S_IP_IF GigabitEthernet1/0/0
---------------------------------------------------------------------------
Total UP/DOWN Session Number : 1/0
<Router-A>
在Router-A上打开终端显示信息中心发送的日志信息功能。
<Router-A>terminal logging
Info: Current terminal logging is on.
<Router-A>terminal monitor
Info: Current terminal monitor is on.
<Router-A>
l 对Router-B的G1/0/0接口执行shutdown命令，模拟链路故障。
<Router-A>
May 15 2023 13:08:27-08:00 Router-A %BFD/4/STACHG_TODWN(l)[1]:BFD session changed to Down. (SlotNumber=0, Discriminator=16777216, Diagnostic=DetectDown, Applications=None, ProcessPST=False, BindInterfaceName=GigabitEthernet1/0/0, InterfacePhysicalState=Up, InterfaceProtocolState=Up)
<Router-A>
May 15 2023 13:08:27-08:00 Router-A %ISIS/4/PEER_DOWN_BFDDOWN(l)[2]:ISIS 256 neighbor 2222.2222.2222 was Down on interface GE1/0/0 because the BFD node was down. The Hello packet was received at 13:05:43 last time; the maximum interval for sending Hello packets was 438763520; the local router sent 1980500224 Hello packets and received 419758080 packets; the type of the Hello packet was Lan Level-2.
<Router-A>
May 15 2023 13:08:27-08:00 Router-A %ISIS/4/ADJ_CHANGE_LEVEL(l)[3]:The neighbor of ISIS was changed. (IsisProcessId=256, Neighbor=2222.2222.2222, InterfaceName=GE1/0/0, CurrentState=up, ChangeType=L2_BFD_DOWN, Level=Level-2)
<Router-A>
May 15 2023 13:08:27-08:00 Router-A %ISIS/4/ADJ_CHANGE_LEVEL(l)[4]:The neighbor of ISIS was changed. (IsisProcessId=256, Neighbor=2222.2222.2222, InterfaceName=GE1/0/0, CurrentState=down, ChangeType=L2_HOLDTIMER_EXPIRED, Level=Level-2)
<Router-A>
l 再在Router-A上执行display isis route或display isis peer命令，没有显示任何信息，表明Router-A与Router-B之间的IS-IS邻居关系已经拆除。
<Router-A>display isis peer
<Router-A>
