I think this is meant to mean that if a router-id has not been set and no configuration has taken place i.e.
router ospf 1
is the only configuration it will try to select a router id (this is when OSPF becomes active)
[the description is wrong]
The second part means, if i was to do this:
router ospf 1
network 192.168.100.1 0.0.0.0 area 0
Lets say i've established an adjacency now, my router-id being used is the highest IP address I have (i havent got a loopback yet - if i did, it would select the highest IP address of a loopback interface), in my case i only have 192.168.100.1.
It would select 192.168.100.1 as my router-id. If i was to later add the configuration a loopback and router-id like this:
int l1
ip address 1.1.1.1 255.255.255.255
conf t
router ospf 1
router-id 1.1.1.1
But when I do this i'll get:
R1(config-router)#router-id 1.1.1.1
Reload or use "clear ip ospf process" command, for this to take effect
R1(config-router)#
Hope this helps. I like Jeff Doyle's take on Router-ID's. Here's an extract.
Router ID
1. The router chooses the numerically highest IP address on any of its loopback interfaces.
2. If no loopback interfaces are configured with IP addresses, the router chooses the numerically
highest IP address on any of its physical interfaces. The interface from which the Router ID is
taken does not have to be running OSPF.
Using addresses associated with loopback interfaces has two advantages:
The loopback interface is more stable than any physical interface. It is active when the router
boots up, and it only fails if the entire router fails.
The network administrator has more leeway in assigning predictable or recognizable addresses as
the Router IDs.
Cisco's OSPF will continue to use a Router ID learned from a physical interface even if the interface
subsequently fails or is deleted (see "Case Study: Setting Router IDs with Loopback Interfaces," later in
this chapter). Therefore, the stability of a loopback interface is only a minor advantage. The primary
benefit is the ability to control the Router ID.
The OSPF router begins a neighbor relationship by advertising its Router ID in Hello packets.
Please rate useful posts & remember to mark any solved questions as answered. Thank you.
Please rate useful posts & remember to mark any solved questions as answered. Thank you.
