使用Standby Preempt和Standby Track命令

简介

本文档介绍standby preempt和standby track命令如何协同工作以及如何使用。

先决条件

要求

本文档没有任何特定的要求。

使用的组件

本文档中的信息基于以下软件和硬件版本：

• 思科IOS^®软件版本12.2(10b)

• Cisco 2503 路由器

本文档中的信息都是基于特定实验室环境中的设备编写的。本文档中使用的所有设备最初均采用原始（默认）配置。如果您的网络处于活动状态，请确保您了解所有命令的潜在影响。

规则

有关文档规则的详细信息，请参阅 Cisco 技术提示规则。

背景信息

standby preemptcommand使具有最高优先级的热备份路由器协议(HSRP)路由器立即成为活动路由器。优先级的确定首先取决于所配置的优先级值，然后取决于 IP 地址。在每种情形下，都是更高的值具有更高的优先级。当一个更高优先级的路由器优先于一个较低优先级的路由器时，该路由器会发送一条 Coup 消息。当优先级较低的活动路由器从一个更高优先级的活动路由器接收到 Coup 消息或 Hello 消息时，该路由器会更改为对话状态，并发送放弃消息。

standby track 命令允许您在路由器上为 HSRP 进程指定另一个要监控的接口，以便更改指定组的 HSRP 优先级。如果指定接口的线路协议关闭，则会降低 HSRP 优先级。这意味着，如果另一台优先级较高的HSRP路由器启用了备用抢占，则该路由器可以成为活动路由器。

此图表显示一个将 standby preempt 命令与 standby track 命令一起使用的示例。

备用抢占和备用跟踪配置

配置示例

在网络图中，HSRP 是以这种方式配置的：

• R1是活动路由器，跟踪R1 Serial 0接口状态。当R1是活动路由器时，从主机（主机1、主机2、主机3）到服务器的所有流量都通过R1路由。

• R2是备用路由器并跟踪R2 Serial 1接口状态。

• 如果R1 Serial 0接口关闭，则R1 HSRP优先级将降低10。此时R2 HSRP优先级高于R1，并且R2将接管活动路由器的角色。

• 当R2成为活动路由器时，从主机到服务器的所有流量都会通过R2路由。

注意：使用 HSRP 虚拟 IP 地址（在本例中为 172.16.6.100）配置主机 1、2 和 3 的默认网关。在路由器上配置一个路由协议（例如 RIP），可实现主机和服务器之间的连接。

这是每台路由器的配置：

interface Ethernet0 
 ip address 172.16.6.5 255.255.255.0

!--- Assigns an IP address to the interface.

 no ip redirects 
 standby 1 ip 172.16.6.100

!--- Assigns a standby group and standby IP address

 standby 1 priority 105

!--- Assign a priority (105 in this case) to the router interface (e0) 
!--- for a particular group number (1). The default is 100.

 standby 1 preempt

!--- Allows the router to become the active router when the priority 
!--- is higher than all other HSRP-configured routers in the hot standby group. 
!--- If you do not use the standby preempt command in the configuration 
!--- for a router, that router does not become the active router, even if 
!--- the priority is higher than all other routers. 

 standby 1 track Serial0

!--- Indicates that HSRP tracks Serial0 interface. 
!--- The interface priority can also be configured, which indicates the 
!--- amount by which the router priority decreases when 
!--- the interface goes down. The default is 10.

interface Serial0 
 ip address 172.16.2.5 255.255.255.0

interface Ethernet0
 ip address 172.16.6.6 255.255.255.0

!--- Assigns an IP address to the interface.

 no ip redirects
 standby 1 ip

!--- Indicates the hot standby group. Here the IP address of the virtual 

router !--- is not configured. See the note after this table.

standby 1 preempt

!--- Allows the router to become the active router when the priority 
!--- is higher than all other HSRP-configured routers in the hot standby group. 
!--- If you do not use the standby preempt command in the configuration 
!--- for a router, that router does not become the active router, even if 
!--- the priority is higher than all other routers. 

standby 1 track Serial1

!--- Indicates that HSRP tracks Serial1 interface. 
!--- The interface priority can also be configured, which indicates the 
!--- amount by which the router priority decreases when 
!--- the interface goes down. The default is 10. 
!--- The priority is also not configured and hence the default 
!--- priority value of 100 is applied.

interface Serial1
 ip address 172.16.7.6 255.255.255.0

注意：R2 没有配置备用的 IP 地址。这样做是特意为了显示出这是有效配置。当R1和R2交换HSRP hello时，R2从R1获取备用IP地址。要为R2配置备用IP地址（在R1上配置相同的备用地址），这也是有效配置

R1#show standby
Ethernet0 - Group 1
Local state is Active, priority 105, may preempt
Hellotime 3 sec, holdtime 10 sec
Next hello sent in 1.458
Virtual IP address is 172.16.6.100 configured
Active router is local
Standby router is 172.16.6.6 expires in 8.428
Virtual mac address is 0000.0c07.ac01
2 state changes, last state change 02:09:49
IP redundancy name is "hsrp-Et0-1" (default)
Priority tracking 1 interface, 1 up:
Interface    Decrement  State
Serial0         10      Up
 
R2#show standby
Ethernet0 - Group 1
Local state is Standby, priority 100, may preempt
Hellotime 3 sec, holdtime 10 sec
Next hello sent in 1.814
Virtual IP address is 172.16.6.100
Active router is 172.16.6.5, priority 105 expires in 9.896
Standby router is local
3 state changes, last state change 00:10:21
IP redundancy name is "hsrp-Et0-1" (default)
Priority tracking 1 interface, 1 up:
Interface   Decrement  State
Serial1        10      Up

虽然R2没有配置HSRP优先级，但输出显示R2优先级为HSRP默认值100。两台路由器的输出都表示路由器跟踪串行接口的状态。R1 的状态是活动的，而 R2 的状态为备用。最后，两个路由器均使用 standby preempt 命令配置。

如果 R1 的 serial 0 接口断开，将会如何？show standby 命令的输出如下所示：

R1#show standby
Ethernet0 - Group 1
Local state is Standby, priority 95 (confgd 105), may preempt
Hellotime 3 sec, holdtime 10 sec
Next hello sent in 2.670
Virtual IP address is 172.16.6.100 configured
Active router is 172.16.6.6, priority 100 expires in 8.596
Standby router is local
4 state changes, last state change 00:01:45
IP redundancy name is "hsrp-Et0-1" (default)
Priority tracking 1 interface, 0 up:
Interface    Decrement  State
Serial0         10      Down
   
R2#show standby
Ethernet0 - Group 1
Local state is Active, priority 100, may preempt
Hellotime 3 sec, holdtime 10 sec
Next hello sent in 0.810
Virtual IP address is 172.16.6.100
Active router is local
Standby router is 172.16.6.5 expires in 9.028
Virtual mac address is 0000.0c07.ac01
4 state changes, last state change 00:01:38
IP redundancy name is "hsrp-Et0-1" (default)
Priority tracking 1 interface, 1 up:
Interface   Decrement  State
Serial1        10      Up

请注意，在输出中，R1的HSRP优先级降低了10 — 至95。此更改会使R2的优先级增加100。当R2的优先级变高时，R2被配置为standby preempt，因此R2成为活动路由器，R1成为备用路由器。此时，如果 R1 serial 0 接口恢复，则 R1 优先级会重新变为 105。在这种情况下，R1 会抢占优先并再次成为 HSRP 活动路由器。

注意：如果R2上未配置standby preempt，则R2不会向R1发送Coup消息，这会导致R2变为活动状态。这样的话，R1 将仍为活动路由器。

例如，请看以下配置：

standby priority 120
standby track serial 0
standby track serial 1

使用 standby priority命令配置HSRP优先级120，并将HSRP配置为跟踪两个接口（Serial0和Serial1）的状态。由于 standby track命令中未指定递减值值，因此当被跟踪的接口关闭时，HSRP优先级将递减为默认值10。最初，两个接口均启用，并且接口的 HSRP 优先级为 120，正如 show standby 命令输出中所示：

R1#show standby
Ethernet0 - Group 1
  Local state is Active, priority 120, may preempt
  Hellotime 3 sec, holdtime 10 sec
  Next hello sent in 1.034
  Virtual IP address is 10.0.0.5 configured
  Active router is local
  Standby router is unknown
  Virtual mac address is 0000.0c07.ac01
  2 state changes, last state change 00:00:04
  IP redundancy name is "hsrp-Et0-1" (default)
  Priority tracking 2 interfaces, 2 up:
    Interface                  Decrement  State
    Serial0                       10      Up
    Serial1                       10      Up

此时，接口 Serial 0 断开。

R1#
1w0d: %LINK-3-UPDOWN: Interface Serial0, changed state to down
1w0d: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0, changed state to down
R1#

这会将HSRP优先级从120减少到110。您可以使用show standby命令进行验证：

R1#show standby
Ethernet0 - Group 1
  Local state is Active, priority 110 (confgd 120), may preempt
  Hellotime 3 sec, holdtime 10 sec
  Next hello sent in 2.544
  Virtual IP address is 10.0.0.5 configured
  Active router is local
  Standby router is unknown
  Virtual mac address is 0000.0c07.ac01
  2 state changes, last state change 00:00:48
  IP redundancy name is "hsrp-Et0-1" (default)
  Priority tracking 2 interfaces, 1 up:
    Interface                  Decrement  State
    Serial0                       10      Down  (line protocol down)
    Serial1                       10      Up

接下来，第二个所跟踪的接口 Serial 1 断开：

R1#
1w0d: %LINK-3-UPDOWN: Interface Serial1, changed state to down
1w0d: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial1, changed state to down
R1#

这再次将HSRP优先级降低了10 — 从110降低到值100。您可以使用show standby命令验证这一点：

R1#show standby
Ethernet0 - Group 1
  Local state is Active, priority 100 (confgd 120), may preempt
  Hellotime 3 sec, holdtime 10 sec
  Next hello sent in 1.846
  Virtual IP address is 10.0.0.5 configured
  Active router is local
  Standby router is unknown
  Virtual mac address is 0000.0c07.ac01
  2 state changes, last state change 00:01:06
  IP redundancy name is "hsrp-Et0-1" (default)
  Priority tracking 2 interfaces, 0 up:
    Interface                  Decrement  State
    Serial0                       10      Down  (line protocol down)
    Serial1                       10      Down  (line protocol down)

当被跟踪的任一串行接口（serial 0或serial 1）关闭时，结果优先级为110。当这些被跟踪的串行接口（serial 0和serial 1）都关闭时，结果优先级为100。

注意：在某些早期版本的 Cisco IOS 中，如果未在 standby track 命令中明确定义递减值，则 HSRP 优先级的递减值不会累加。有关详细信息，请参阅Cisco Bug ID CSCdp72834。只有注册的思科用户才能访问内部Bug信息。

相关信息

• HSRP 支持页
• 技术支持和文档 - Cisco Systems