CCNP实战教程之 OSPF中为何将环回口通告为主机路由？

课题内容：

OSPF中为何将环回口通告为主机路由？

实验拓扑：

基本IP地址配置及OSPF宣告配置  

R1(config)#interface loopback 0
  R1(config-if)#ip address 11.1.1.1 255.255.255.0
  R1(config-if)#ip ospf 110 area 0
  R1(config-if)#interface e0/0
  R1(config-if)#ip address 12.1.1.1 255.255.255.252
  R1(config-if)#no shutdown
  R1(config-if)#ip ospf 110 area 0                 
  R1(config-if)#end

R2(config)#interface loopback 0
R2(config-if)#ip address 22.1.1.1 255.255.255.0
R2(config-if)#ip ospf 110 area 0
R2(config-if)#interface e0/0
R2(config-if)#ip address 12.1.1.2 255.255.255.252
R2(config-if)#no shutdown
R2(config-if)#ip ospf 110 area 0
R2(config-if)#end

观察现象：

R1#show ip route ospf | begin Gateway
Gateway of last resort is not set

      22.0.0.0/32 is subnetted, 1 subnets
O        22.1.1.1 [110/11] via 12.1.1.2, 00:01:28, Ethernet0/0

R2#show ip route ospf | begin Gateway
Gateway of last resort is not set

      11.0.0.0/32 is subnetted, 1 subnets
O        11.1.1.1 [110/11] via 12.1.1.1, 00:01:42, Ethernet0/0

// R1 和 R2 的环回接口配置了24位长度的子网掩码，然而在路由表中却显示为 32位的主机路由 //

众所周知，OSPF的路由是由LSDB中的计算而来，咱们来看下LSA的具体内容

 

R1#show ip ospf database router self-originate

            OSPF Router with ID (11.1.1.1) (Process ID 110)

                Router Link States (Area 0)

  LS age: 359
   Options: (No TOS-capability, DC)
   LS Type: Router Links
   Link State ID: 11.1.1.1
   Advertising Router: 11.1.1.1
   LS Seq Number: 80000003
   Checksum: 0x527B
   Length: 48
   Number of Links: 2

    Link connected to: a Stub Network
      (Link ID) Network/subnet number: 11.1.1.1
      (Link Data) Network Mask: 255.255.255.255
       Number of MTID metrics: 0
        TOS 0 Metrics: 1

    Link connected to: a Transit Network
      (Link ID) Designated Router address: 12.1.1.2
      (Link Data) Router Interface address: 12.1.1.1
       Number of MTID metrics: 0
        TOS 0 Metrics: 10

//注意下划线标注的位置，R1在将环回口的链路状态收集为LSA内容的时候就已经标记为了 32 位的子网掩码； //

那么，OSPF为何要这样作呢？

咱们在RFC 2328中寻找答案：

Section 9.1节选

Loopback
In this state, the router’s interface to the network is looped back. The interface may be looped back in hardware or software. The interface will be unavailable for regular data traffic. However, it may still be desirable to gain information on the quality of this interface, either through sending ICMP pings to the interface or through something like a bit error test. For this reason, IP packets may still be addressed to an interface in Loopback state. To facilitate this, such interfaces are advertised in router-LSAs as single host routes, whose destination is the IP interface address.

翻译：

在此状态下，路由器与网络的接口将被环回。 接口能够在硬件或软件中循环。 该接口将没法用于常规数据流量。 可是，仍然须要得到有关此接口质量的信息，方法是将ICMP ping发送到接口或经过比特错误测试。 所以，IP数据包仍能够寻址到Loopback状态的接口。 为此，这些接口在Router-LSA中做为单个主机路由通告，其目的地是IP接口地址。

看起来，OSPF仅仅把环回口看成测试须要而设计；

然而在现现在主流的MPLS+IP的解决方案中，环回口的子网掩码则很是重要。在MPLS环境中，OSPF的环回口子网掩码若是配置的不是32位，则会形成标签断裂，从而形成×××流量被丢弃。（有关详细内容，欢迎关注乾颐堂达叔的博客更新，以及CCIE理论课哦）

关于解决方案：

不一样的厂商解决方案也不一样；

在cisco的设备上，将环回口的OSPF网络类型修改成点到点，将可以在LSA中真实反应出实际配置的子网掩码；

R1>enable
R1#conf t
R1(config)#interface loopback 0
R1(config-if)#ip ospf network point-to-point
R1(config-if)#end

验证：

R1#show ip ospf database router self-originate

            OSPF Router with ID (11.1.1.1) (Process ID 110)

                Router Link States (Area 0)

  LS age: 28
   Options: (No TOS-capability, DC)
   LS Type: Router Links
   Link State ID: 11.1.1.1
   Advertising Router: 11.1.1.1
   LS Seq Number: 80000006
   Checksum: 0x4289
   Length: 48
   Number of Links: 2

    Link connected to: a Stub Network
      (Link ID) Network/subnet number: 11.1.1.0
      (Link Data) Network Mask: 255.255.255.0
       Number of MTID metrics: 0
        TOS 0 Metrics: 1

    Link connected to: a Transit Network
      (Link ID) Designated Router address: 12.1.1.2
      (Link Data) Router Interface address: 12.1.1.1
       Number of MTID metrics: 0
        TOS 0 Metrics: 10

R2#show ip route ospf | begin Gateway
Gateway of last resort is not set

     11.0.0.0/24 is subnetted, 1 subnets
O        11.1.1.0 [110/11] via 12.1.1.1, 00:00:57, Ethernet0/0

谢谢你们！我是达叔。