一台 Router 有幾個 OSPF Router ID ?

作者:


在設定 OSPF 的時候，Router ID 是 OSPF 用來識別 Router 用的。要成功的讓 OSPF 交換路由，必須先建立 neighbor relationship。要成功的建立 OSPF neighbor，每個 Router 必須要有唯一的 Router ID，是其中的一個條件。

我們來複習 OSPF Router ID (RID) 的選擇方式：

  1. OSPF process 會檢查是否有使用 router-id 這個指令手動設定 RID。
  2. 如果沒有手動設定 RID，OSPF process 會看是否有 active (up/up) 的 Loopback 介面。如果有，則挑選 Loopback interface 中最高的 IP address，將之設定為 RID。
  3. 如果 Router 中沒有 Loopback interface (或都被 shutdown)，則會選擇 active (up/up) 的實體介面(Physical interfaces)中最高的 IP address，將之設定為 RID。
  4. 如果也都沒有 active 的實體介面，或 active 的實體介面都沒有設定 IP address，那麼 OSPF 將無法設定。(如下例)
Router#show ip int brief
Interface IP-Address OK? Method Status Protocol

FastEthernet0/0 unassigned YES manual administratively down down

FastEthernet0/1 unassigned YES manual administratively down down

Vlan1 unassigned YES manual administratively down down
Router#conf t
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#router ospf 100
OSPF process 100 cannot start. There must be at least one “up” IP interface
Router(config-router)#end
%SYS-5-CONFIG_I: Configured from console by console
Router#show ip ospf
%OSPF: Router process 100 is not running, please configure a router-id
Router#
我們再來看這個例子：
2009-09-20_101306
R1:

interface Loopback1
ip address 1.1.1.1 255.255.255.255
!
interface Loopback2
ip address 2.2.2.1 255.255.255.255
!
interface Loopback3
ip address 3.3.3.1 255.255.255.255
!
interface FastEthernet0/0
ip address 10.1.1.1 255.255.255.0
!

router ospf 100
log-adjacency-changes
network 10.1.1.0 0.0.0.255 area 0


R2:

interface Loopback1
ip address 1.1.1.2 255.255.255.255
!
interface Loopback2
ip address 2.2.2.2 255.255.255.255
!
interface Loopback3
ip address 3.3.3.2 255.255.255.255
!
interface FastEthernet0/0
ip address 10.1.1.2 255.255.255.0
!

router ospf 100
log-adjacency-changes
network 10.1.1.0 0.0.0.255 area 0

當 OSPF 成功建立起 neighbor 之後我們可以從 R2 看到，R1 的 RID 是 3.3.3.1，合乎上述的選擇方式。
R2#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
3.3.3.1 1 FULL/BDR 00:00:30 10.1.1.1 FastEthernet0/0
R2#

但也因此，很多人都以為每一台 Router 都只有“一個”唯一的 Router ID。
如果，我想在 R2 上面看到 R1 的 RID 是 2.2.2.1，而且不能使用 router-id 指令，也不能將 Loopback 介面 shutdown，該怎麼做呢？
要解這個問題，必須先了解：在一台 Router 之中，可以啟動多個 OSPF process。而每一個 OSPF process 都會使用一個唯一的 Router ID。
所以上面這個問題，我只需要在 R1 上面先設定一個 OSPF process，讓它選定 3.3.3.1 為 RID，但不下任何 network 指令。再設定一個要與 R2 建 neighbor 的 OSPF process，讓它選定 2.2.2.1 為 RID。

R1:
R1#show run
router ospf 100
log-adjacency-changes
!
router ospf 200
log-adjacency-changes
network 10.1.1.0 0.0.0.255 area 0

R1#show ip ospf
Routing Process “ospf 100” with ID 3.3.3.1
Supports only single TOS(TOS0) routes
Supports opaque LSA
SPF schedule delay 5 secs, Hold time between two SPFs 10 secs
Minimum LSA interval 5 secs. Minimum LSA arrival 1 secs
Number of external LSA 0. Checksum Sum 0x000000
Number of opaque AS LSA 0. Checksum Sum 0x000000
Number of DCbitless external and opaque AS LSA 0
Number of DoNotAge external and opaque AS LSA 0
Number of areas in this router is 0. 0 normal 0 stub 0 nssa
External flood list length 0
Routing Process “ospf 200” with ID 2.2.2.1
Supports only single TOS(TOS0) routes
Supports opaque LSA
SPF schedule delay 5 secs, Hold time between two SPFs 10 secs
Minimum LSA interval 5 secs. Minimum LSA arrival 1 secs
Number of external LSA 0. Checksum Sum 0x000000
Number of opaque AS LSA 0. Checksum Sum 0x000000
Number of DCbitless external and opaque AS LSA 0
Number of DoNotAge external and opaque AS LSA 0
Number of areas in this router is 1. 1 normal 0 stub 0 nssa
External flood list length 0
Area BACKBONE(0)
Number of interfaces in this area is 1
Area has no authentication
SPF algorithm executed 2 times
Area ranges are
Number of LSA 4. Checksum Sum 0x01969f
Number of opaque link LSA 0. Checksum Sum 0x000000
Number of DCbitless LSA 0
Number of indication LSA 0
Number of DoNotAge LSA 0
Flood list length 0

R2:
R2#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
2.2.2.1 1 FULL/BDR 00:00:35 10.1.1.1 FastEthernet0/0