At 10:45 AM -0800 11/29/2000, Healis, Jim wrote:
>Use loopback ports in OSPF so you can set the Router OSPF ID, otherwise it
>will take the highest IP address.
>
>Jim Healis CCNP, CCDP
>Senior Network Administrator
>Virata
Unless there have been recent IOS changes (I'm really most current in
11-something), it's even more unpredictable than just the highest IP
address:
At the time of OSPF initialization, the router ID is:
if there are multiple loopback interfaces, the highest IP address on any
loopback (i.e., not highest loopback interface number)
if there is a single loopback interface, use its address
if there are no loopback interfaces, use the highest IP address on any
active interface (i.e., if all interfaces are in shutdown, OSPF
can't initialize. Using loopbacks avoids this because a loopback
cannot be down.)
A fiendish troubleshooting scenario:
R1 comes up first, then R2. They share an Ethernet. Neither has
any loopbacks.
Scenario 1 (R1 is initially misconfigured)
R1 E0: 10.6.0.1 DOWN R2 E0: 10.1.0.1 UP/UP
E1: 10.5.0.2 UP/UP E1: 10.5.0.2 UP/UP
E2: 10.2.0.1 UP/UP E2: 10.2.0.2 UP/UP
Admin discovers that R1 E1 is misconfigured and should have been 10.5.0.1.
Our Heroine corrects that interface to 10.5.0.1.
Assuming both routers had OSPF configured with
network 10.0.0.0 0.255.255.255 area 0.0.0.1
Will a device on R1 E2 be able to ping a host on R2 E2?
>
>
> -----Original Message-----
>From: Moerdo [mailto:[EMAIL PROTECTED]]
>Sent: Wednesday, November 29, 2000 8:26 AM
>To: [EMAIL PROTECTED]
>Subject: About OSPF and Loopback port
>
>Does anyone here can explain to me, why me must use loopback port for OSPF
>configuration. Thank you for the answer for this stupid question. Thank you.
>
>moerdo.
>
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