IS-IS中间系统到中间系统的路由选择协议是由 ISO 提出的一种路由选择协议。它是一种链路状态协议。在该协议中，IS路由器负责交换基于链路开销的路由信息并决定网络拓扑结构。IS-IS 类似于 TCP/IP 网络的开放最短路径优先OSPF协议。
网络环境
NE80E_A、NE80E_B与NE40三台设备都运行在IS-IS Level-2区域中。为了引导NE40的上行流量负载分担，NE80E_A与NE80E_B同时对NE40下发了IS-IS缺省路由，出现路由环路。
故障分析
步骤 1 在NE80E_A 、NE80E_B和NE40设备上分别执行命令display current-configuration查看接口上IS-IS的配置情况，发现NE80E_A 、 NE80E_B和NE40设备的IS-IS配置正确，如下所示。
NE80E_Au00a0displayu00a0current-configuration
sysnameu00a0RouterA
apsu00a0fast-intervalu00a00
isisu00a01
is-levelu00a0level-2
network-entityu00a010..
default-route-advertiseu00a0always
interfaceu00a0Serial1/0/0
link-protocolu00a0ppp
ipu00a0addressu00a01.1.1.1u00a0255.255.255.0
isisu00a0enableu00a01
interfaceu00a0Serial1/0/1
link-protocolu00a0ppp
ipu00a0addressu00a03.3.3.1u00a0255.255.255.0
isisu00a0enableu00a01
interfaceu00a0Serial1/0/2
link-protocolu00a0ppp
ipu00a0addressu00a010.10.10.1u00a0255.255.255.0
isisu00a0enableu00a01
NE80E_Bu00a0displayu00a0current-configuration
sysnameu00a0RouterB
apsu00a0fast-intervalu00a00
isisu00a01
is-levelu00a0level-2
network-entityu00a010..
default-route-advertiseu00a0always
interfaceu00a0Serial1/0/0
link-protocolu00a0ppp
ipu00a0addressu00a02.2.2.1u00a0255.255.255.0
isisu00a0enableu00a01
interfaceu00a0Serial1/0/1
link-protocolu00a0ppp
ipu00a0addressu00a04.4.4.1u00a0255.255.255.0
isisu00a0enableu00a01
interfaceu00a0Serial1/0/2
link-protocolu00a0ppp
ipu00a0addressu00a010.10.10.2u00a0255.255.255.0
isisu00a0enableu00a01
NE40u00a0displayu00a0current-configuration
sysnameu00a0RouterC
apsu00a0fast-intervalu00a00
isisu00a01
is-levelu00a0level-2
network-entityu00a010..
interfaceu00a0Serial1/0/0
link-protocolu00a0ppp
ipu00a0addressu00a03.3.3.2u00a0255.255.255.0
isisu00a0enableu00a01
interfaceu00a0Serial1/0/1
link-protocolu00a0ppp
ipu00a0addressu00a04.4.4.2u00a0255.255.255.0
isisu00a0enableu00a01
步骤 2 在NE40设备上执行命令display isis peer，查看邻居状况，发现NE40已 分别与NE80E_A 和NE80E_B建立邻居关系。
NE40u00a0displayu00a0isisu00a0peer
Peeru00a0informationu00a0foru00a0ISIS1
Systemu00a0Idu00a0Interfaceu00a0Circuitu00a0Idu00a0Stateu00a0HoldTimeu00a0Typeu00a0PRI
-------------------------------------------------------------------------------
.0001u00a0S1/0/0u00a002u00a0Upu00a023su00a0L2u00a0--
.0002u00a0S1/0/1u00a002u00a0Upu00a029su00a0L2u00a0--
步骤 3 在NE40设备上执行命令display isis route，发现存在NE80E_A 和 NE80E_B的缺省路由。在NE80E_A 和NE80E_B设备上分别执行命令 display isis route发现NE80E_A 和NE80E_B能互相学习到对方的 缺 省路由，因此而形成了路由环路，如下所示。
NE40u00a0displayu00a0isisu00a0route
Routeu00a0informationu00a0foru00a0ISIS1
-----------------------------
ISIS1u00a0Level-2u00a0Forwardingu00a0Table
--------------------------------
IPV4u00a0Destinationu00a0IntCostu00a0ExtCostu00a0ExitInterfaceu00a0NextHopu00a0Flags
----------------------------------------------------------------------------
0.0.0.0/0u00a010u00a0NULLu00a0S1/0/0u00a03.3.3.1u00a0A/-/-/-
S1/0/1u00a04.4.4.1u00a04.4.4.0/24u00a010u00a0NULLu00a0S1/0/1u00a0Directu00a0D/-/L/-
10.10.10.0/24u00a020u00a0NULLu00a0S1/0/0u00a03.3.3.1u00a0A/-/-/-
S1/0/1u00a04.4.4.1
3.3.3.0/24u00a010u00a0NULLu00a0S1/0/0u00a0Directu00a0D/-/L/-
2.2.2.0/24u00a020u00a0NULLu00a0S1/0/1u00a04.4.4.1u00a0A/-/-/-
1.1.1.0/24u00a020u00a0NULLu00a0S1/0/0u00a03.3.3.1u00a0A/-/-/-
Flags:u00a0D-Direct，u00a0A-Addedu00a0tou00a0URT，u00a0L-Advertisedu00a0inu00a0LSPs，u00a0S-IGPu00a0Shortcut，
U-Up/Downu00a0Bitu00a0Set
NE80E_Au00a0displayu00a0isisu00a0route
Routeu00a0informationu00a0foru00a0ISIS1
-----------------------------
ISIS1u00a0Level-2u00a0Forwardingu00a0Table
----------
