Explore the role of Router-ID in the OSPF Routing Protocol

As we all know, the role of Router-ID in OSPF-like Link State Routing Protocols is enormous, and the election of DR/BDR relies on RID, but today, I found that the Router-ID exists in the same way as the roeigrp-ID, but it has never been used at ordinary times. What is its role?

After some research, we can draw the following two conclusions:

1. Same as OSPF, the routing rules for Router-ID election in OSPF have the Loopback interface selected first. If not, select the largest active interface. Of course, the router-id specified by the command of the OSPF Router-ID takes precedence over the specified router-id.

2. the Router-ID in the VPC also plays a marking role, which plays an important role in solving the loop problem caused by redistribution routing. It can be understood that when an external route update is received by a vroeigrp that runs the VPN gateway, the Router-ID of the route update is the same as its own Router-ID, this route update will be ignored to avoid loops.

The following uses an experiment to observe this phenomenon:

[Lab environment]

The C3640-IK9O3S-M Version 12.4 (10)

[Experiment topology]

[Experiment description]

R1 loopback interface 1.1.1.1

R2 and R3 loopback interfaces 2.2.2.2

R1 and R2 run RIP

R2 and R3 run the kernel 110

Redistributes RIP on R2.

Lab procedure]

1. Basic R1 Configuration

!
Interface Loopback0
Ip address 1.1.1.1 255.255.255.255
!
Interface FastEthernet0/0
Ip address 12.12.1 255.255.255.0
!
Router rip
Version 2
Network 1.0.0.0
Network 12.0.0.0
!

2. Basic R2 Configuration

!
Interface Loopback0
Ip address 2.2.2.2 255.255.255.255
!
Interface FastEthernet0/0
Ip address 12.12.2 255.255.255.0
!
Interface FastEthernet1/0
Ip address 23.23.23.2 255.255.255.0
!
Router VPN 110
Redistribute rip metric 100000 1000 255 1 1500
Network 23.23.23.0 0.0.255
No auto-summary
!
Router rip
Version 2
Network 12.0.0.0
!

3. Basic R3 Configuration

!
Interface Loopback0
Ip address 2.2.2.2 255.255.255.255
!
Interface FastEthernet0/0
Ip address 23.23.23.3 255.255.255.0
!
Router VPN 110
Network 23.23.23.0 0.0.255
No auto-summary
!

[Experiment results]

R3 # sh ip route
2.0.0.0/32 is subnetted, 1 subnets
C 2.2.2.2 is directly connected, Loopback0
23.0.0.0/24 is subnetted, 1 subnets
C 23.23.23.0 is directly connected, FastEthernet0/0

On R3, no information about the re-distributed RIP route is received (about the 1.0.0.0 network segment)

R2 # sh ip: VPN topology
IP-EIGRP Topology Table for AS (110)/ID (2.2.2.2)

Codes: P-Passive, A-Active, U-Update, Q-Query, R-Reply,
R-reply Status, s-sia Status

P 1.0.0.0/8, 1 successors, FD is 281600
Via Redistributed (281600/0)
P 12.12.12.0/24, 1 successors, FD is 281600
Via Redistributed (281600/0)
P 23.23.23.0/24, 1 successors, FD is 28160
Via Connected, FastEthernet1/0

We can see that the route 1.0.0.0/8 is updated in the topology table of R2, and it is also re-distributed.

R2 # sh ip maid
IP-EIGRP (AS 110): Topology entry for 1.0.0.0/8
State is Passive, Query origin flag is 1, 1 Successor (s), FD is 281600
Routing Descriptor Blocks:
12.12.12.1, from Redistributed, Send flag is 0x0
Composite metric is (281600/0), Route is External
Vector metric:
Minimum bandwidth is 100000 Kbit
Total delay is 10000 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 0
External data:
Originating router is 2.2.2.2 (this system)
AS number of route is 0
External protocol is RIP, external metric is 1
Administrator tag is 0 (0x00000000)

Observe the route update carefully and you can see that its original RID is 2.2.2.2 (actually itself)

Next let's take a look at what happened on R3:

R3 # sh ip subnet event
Event information for AS 110:
1 00:00:08. 691 Ignored route, metric: 1.0.0.0 284160
2 00:00:08. 691 Ignored route, neighbor info: 23.23.23.2 Loopback0
3 00:00:08. 691 Ignored route, dup router: 2.2.2.2
4 00:00:08. 691 Ignored route, metric: 12.12.12.0 284160
5 00:00:08. 691 Ignored route, neighbor info: 23.23.23.2 Loopback0
6 00:00:08. 691 Ignored route, dup router: 2.2.2.2

As you can see, R3 ignores route updates for 1.0.0.0 and 12.12.12.0 because these route updates have the same source RID as their own RID. To avoid loops, R3 ignores them.

Then we can see the role of the Router-ID in the network.