Yes and no. recall that a DR is elected on a shared broadcast medium,
typically ethernet. ( dumb question - how does this play out in a token ring
environment? My supposition is that it would work out the same, even though
TR is not a broadcast medium )
So if you were to change your scenario to the ethernet port going down, then
yes.
Hhhhmmmm.......
Suppose you had:
------------------------------------- ethernet
| | |
DR BDR DR/other
| |
(---frame relay cloud ---)
DR ethernet hardware fails.
Now then, given that the DR hello is roughly 10 seconds by default, will the
BDR be promoted to DR even though there is an alternate route to the
existing DR. Recall that the SPF would most definitely be the ethernet port.
Would there be a footrace among the various router interfaces? Would the
alternate route to the DR be propagated to everyone prior to the BDR
promotion? Without doing a Q&D lab, I would venture a guess that the BDR
would be promoted because even though there is an alternative route to the
DR loopback, hellos go only to adjacent routers, and the DR is no longer
adjacent.
Any comments?
Can't answer number 2. As my real world OSPF experience is limited.
Chuck
-----Original Message-----
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] On Behalf Of
whitaker
Sent: Monday, November 27, 2000 9:43 PM
To: [EMAIL PROTECTED]
Subject: RE: question about loopback interfaces
Thanks for the response! I have two more questions:
If in an OSPF and BGP environment, I think can see the stability with
loopbacks... Correct me if I'm wrong in this theoritical scenario:
Two routers in an OSPF share the highest priority. To break the tie and
determine the DR, OSPF looks at router id, which is the highest interface
address (let's say serial interface) and determines a particular router has
the highest id. It becomes DR. Without a loopback address, if the serial
interface goes down, then the BDR comes online and a new BDR election takes
place. This requires processing power and could slow down the network.
With a loopback interface configured with the highest IP address, then if a
router is the DR and looses its serial interface, it would remain the DR.
Is this correct?
My second question involves the configuration of the loopback from
real-world experience. What do most of you use - an address from the same
subnet one of the interfaces is on or a seperate address?
-----Original Message-----
From: Tony van Ree [mailto:[EMAIL PROTECTED]]
Sent: Monday, November 27, 2000 9:28 PM
To: whitaker; [EMAIL PROTECTED]
Subject: Re: question about loopback interfaces
The notes I have here suggest that using a loopback interface provides a
more stable interface than a physical interface. As the loopback interface
is up as long as the RAM is working the chances of losing this interface are
greatly reduced.
Both OSPF and BGP use the highest active IP address as the router ID. If a
loopback address is configured they will use the loopback address.
Therefore the loopback address provides more stability.
It can also make network management and troubleshooting tasks easier.
Teunis.
On Monday, November 27, 2000 at 08:58:00 PM, whitaker wrote:
> Could someone explain the importance of using loopback interfaces? I keep
> reading that it is important when using routing protocols to use loopback
> interfaces. A consultant that is writing documentation for me suggested I
> obtain an entire class C network just for loopback addresses. Cisco says
> when configuring BGP that, "We recommend you use a loopback interface to
> guarantee reachability in networks with multiple paths." (from
> http://www.cisco.com/warp/public/459/23.html#3).
>
> Maybe I'm missing something totally obvious here, but exactly what benefit
> do I gain from using loopback addresses? I know it is supposed to provide
> reliability for routing protocols, but how?
>
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