Subject: [OSPF] Attached hosts question

From: Joakim Tjernlund <[email-masked]>
Date: Wed, 11 Mar 2009 18:39:32 +0100

Last in "12.4.1.  Router-LSAs" one can read:

            After consideration of all the router interfaces, host links
            are added to the router-LSA by examining the list of
            attached hosts belonging to Area A.  A host route is
            represented as a Type 3 link (stub network) whose Link ID is
            the host's IP address, Link Data is the mask of all ones
            (0xffffffff), and cost the host's configured cost (see
            Section C.7).

C.7 Host route parameters

        Host routes are advertised in router-LSAs as stub networks with
        mask 0xffffffff.  They indicate either router interfaces to
        point-to-point networks, looped router interfaces, or IP hosts
        that are directly connected to the router (e.g., via a SLIP
        line).  For each host directly connected to the router, the
        following items must be configured:

        Host IP address
            The IP address of the host.

        Cost of link to host
            The cost of sending a packet to the host, in terms of the
            link state metric.  However, since the host probably has
            only a single connection to the internet, the actual
            configured cost in many cases is unimportant (i.e., will
            have no effect on routing).

        Area ID
            The OSPF area to which the host belongs.

I read this as one can manually configure a list of attached hosts in the
router and these are then announced in the router LSA.
What I don't get is if one should do any validation that the configured
hosts are reachable or not? The spec doesn't mention this and I am getting
the feeling that this should just be a stupid list of hosts that gets 
announced
regardless if they are reachable or not.

 Jocke

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From: Acee Lindem <[email-masked]>
Date: Wed, 11 Mar 2009 19:14:31 -0400

Hi Joakim,
Even though RFC 2328 doesn't specify it, I wouldn't advertise these  
hosts unless the corresponding link is up. If we were starting from  
scratch with the protocol, I'd leave the host configuration out and  
add a "passive" interface parameter to advertise subnets on which you  
don't want to actually run OSPF.

Thanks,
Acee
On Mar 11, 2009, at 1:39 PM, Joakim Tjernlund wrote:

> _______________________________________________
> OSPF mailing list
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From: Joakim Tjernlund <[email-masked]>
Date: Thu, 12 Mar 2009 18:00:01 +0100

I see, I was planning to use this function to announce a host in every
area that the router is attached to. Passive won't let me do that. Perhaps
there is a better way?

 Jocke

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From: Acee Lindem <[email-masked]>
Date: Fri, 13 Mar 2009 08:02:53 -0400

If you advertise a prefix (host or subnet) in one area, it will be  
advertised as a Summary-LSA in the other attached areas. I don't see  
a good reason to advertise the same prefix as an intra-area route in  
multiple areas.
Thanks,
Acee