What is Router ID?

The router ID is a 32-bit number that identifies the OSPF router. Just like IPv4 addresses, it’s an 8-bit decimal number separated by “. (dot)” and then write four of them in a row.

The router ID is the name of the OSPF router, so to speak, and a unique router ID is always required for OSPF processing. The router ID recognizes the neighbor. The LSA is also marked with the router ID of the generated router.

The format of the notation is the same as the IPv4 address, and also, by default, the router ID is determined from the IPv4 address of the router. Therefore, it is easy to make a mistake, but the router ID and the IP address are two different things.
Even for OSPFv3 for IPv6, the router ID is 32 bits.
In addition to OSPF, EIGRP and BGP also have a router ID. Neighbor-aware routing protocols use router IDs to identify their respective routers. The method of determining router IDs in EIGRP/BGP is the same as in OSPF.

How to determine the router ID

The router ID is determined as follows.

  1. Manual configuration using the router-id command
  2. Largest IP address of the active loopback interfaces
  3. Largest IP address of any active interfaces other than loopback
Figure Determining the OSPF Router ID (Cisco)
Figure Determining the OSPF Router ID (Cisco)

Here’s how to determine the highest priority router IDs, from top to bottom. If the router ID changes, the OSPF process will have to be redone from the establishment of the neighbor. Therefore, the above priority levels are determined so that the router IDs do not change.

Manual configuration using the router-id command

You can set the router ID manually. On Cisco routers, within the OSPF routing process, use the following command to configure the router ID.

Manual configuration

(config)#router ospf <process-id>
(config-router)#router-id <router-id>

<process-id> : process ID
<router-id> : Router ID

Make sure that the router ID to be set does not conflict with the router ID of another router. If you set it manually with the router-id command, the router ID will not change unless you change the configuration.

If a new router ID is configured using the router-id command when a router ID has already been assigned, it will not be reflected immediately. Please restart the OSPF process with the clear ip ospf process to reflect the router ID configuration.

Largest IP address of the active loopback interfaces

The loopback interface is a virtual interface that you create in your configuration. If any one router interface is active, it is a stable interface that will not go down unless you explicitly shutdown it. The IP address of a stable loopback interface can be automatically used as the router ID. You can create multiple loopback interfaces. If there are multiple loopback interfaces, the largest IP address is used as the router ID.

If the router ID is determined from the IP address of the loopback interface, the possibility of the router ID changing depending on the state of a particular interface is reduced. However, if a loopback interface is added later, the router ID may change along with it.

The addition of a loopback interface does not immediately change the router ID.

Largest IP address of any active interfaces other than loopback

Creating a loopback interface is optional. For routers that do not have a loopback interface, the maximum IP address of the active interface is used as the router ID.

If the interface of the IP address being used as the router ID goes down, the router ID may change.

When an interface goes down, the router ID does not immediately change.

Router ID verification (Cisco)

The following are the main show commands for verifying the router ID on a Cisco router.

  • show ip protocols
  • show ip ospf
  • show ip ospf interface

Here is sample output for each show command

show ip protocols

R1#show ip protocols
Routing Protocol is "ospf 1"
  Outgoing update filter list for all interfaces is not set
  Incoming update filter list for all interfaces is not set
  Router ID 1.1.1.1
  Number of areas in this router is 1. 1 normal 0 stub 0 nssa
  Maximum path: 4
  Routing for Networks:
    192.168.12.0 0.0.0.255 area 0
 Reference bandwidth unit is 100 mbps
  Routing Information Sources:
    Gateway         Distance      Last Update
  Distance: (default is 110)

show ip ospf

R1#show ip ospf
 Routing Process "ospf 1" with ID 1.1.1.1
 Start time: 00:00:02.000, Time elapsed: 00:03:16.636
 Supports only single TOS(TOS0) routes
 Supports opaque LSA
 Supports Link-local Signaling (LLS)
 Supports area transit capability
 Router is not originating router-LSAs with maximum metric
 Initial SPF schedule delay 5000 msecs
 Minimum hold time between two consecutive SPFs 10000 msecs
 Maximum wait time between two consecutive SPFs 10000 msecs
 Incremental-SPF disabled
 Minimum LSA interval 5 secs
 Minimum LSA arrival 1000 msecs
 LSA group pacing timer 240 secs
 Interface flood pacing timer 33 msecs
 Retransmission pacing timer 66 msecs
 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
 Number of areas transit capable is 0
 External flood list length 0
    Area BACKBONE(0)
        Number of interfaces in this area is 2
        Area has no authentication
        SPF algorithm last executed 00:00:56.772 ago
        SPF algorithm executed 4 times
        Area ranges are
        Number of LSA 3. Checksum Sum 0x01F21D
        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

show ip ospf interface

FastEthernet0/0 is up, line protocol is up
  Internet Address 192.168.12.1/24, Area 0
  Process ID 1, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 1
  Transmit Delay is 1 sec, State BDR, Priority 1
  Designated Router (ID) 2.2.2.2, Interface address 192.168.12.2
  Backup Designated router (ID) 1.1.1.1, Interface address 192.168.12.1
  Flush timer for old DR LSA due in 00:00:27
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
    oob-resync timeout 40
    Hello due in 00:00:00
  Supports Link-local Signaling (LLS)
  Index 1/1, flood queue length 0
  Next 0x0(0)/0x0(0)
  Last flood scan length is 1, maximum is 1
  Last flood scan time is 0 msec, maximum is 0 msec
  Neighbor Count is 1, Adjacent neighbor count is 1
    Adjacent with neighbor 2.2.2.2  (Designated Router)
  Suppress hello for 0 neighbor(s)

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