Configuring OSPF:
OSPF classifies different media into the following three types of networks
by default:
+ Broadcast networks (Ethernet, Token Ring, FDDI)
+ Nonbroadcast multiaccess networks (SMDS, Frame Relay, X.25)
+ Point-to-point networks (HDLC, PPP)
Configure Your OSPF Network Type
You have the choice of configuring your OSPF network type as either
broadcast or nonbroadcast multiaccess, regardless of the default media
type. Using this feature, you can configure broadcast networks as
nonbroadcast multiaccess networks when, for example, you have routers in your
network that do not support multicast addressing. You also can configure
nonbroadcast multiaccess networks (such as X.25, Frame Relay, and SMDS) as
broadcast networks. This feature saves you from having to configure
neighbors, as described in the section "Configure OSPF for Nonbroadcast
Networks."
Can't answer #2, as it's really early and I'm not awake yet, but it seems
that the dead timer should be taken into account.
On Mon, 27 Nov 2000, Chuck Larrieu wrote:
> 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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> >
>
>
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>
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