Subject: [OSPF] Adding host routes in the router LSA

From: Joakim Tjernlund <[email-masked]>
Date: Sun, 18 Jan 2009 14:36:12 +0100

I RFC 2328, last in chap "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).

If I read this correctly one should add in the router LSA all local
interfaces belonging
to the area as host routes. For example if eth0 is included in the area and
have IP 192.168.1.2 and a /24 subnet one should add two stub routes:
192.168.1.0/24 and a 192.168.1.2/32 to the router LSA. To me this also
suggests
that the host route should be announced even if the ethernet cable is
pulled(i.e the
interface is UP but not RUNNING).

I guess unnumbered interfaces should be omitted as these doesn't have an IP
address?

 Jocke

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From: Acee Lindem <[email-masked]>
Date: Sun, 18 Jan 2009 13:43:19 -0500

Hi Jocke,

> interface is UP but not RUNNING).

No - you've misinterpreted RFC 2328. This is refers to "Hosts  
attached directly to routers .." as described in section 2.1. Based  
on the specification, these are configured (refer to section C.7).  
I've never worked on an implementation that supported these directly  
attached hosts as described in RFC 2328.

> I guess unnumbered interfaces should be omitted as these doesn't  

No. I think we've already covered this :^)

Acee

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From: "Vishwas Manral" <[email-masked]>
Date: Sun, 18 Jan 2009 11:29:42 -0800

Hi Acee,

> No - you've misinterpreted RFC 2328. This is refers to "Hosts attached
> directly to routers .." as described in section 2.1. Based on the
> specification, these are configured (refer to section C.7). I've never
> worked on an implementation that supported these directly attached hosts as
> described in RFC 2328.
>
I mostly agree with Acee. It clearly says 1 stub entry with /32 route.
Another use of the host route could be to be able to give different
paths to some critical machines compared to others (so even hosts
directly connected over a network could be used).

To add we (IPInfusion Inc) support the host route, so there is atleast
1 implementation. Maybe Zebra does the same too. I myself got a
customer trying to use it.

Thanks,
Vishwas

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From: Joakim Tjernlund <[email-masked]>
Date: Sun, 18 Jan 2009 20:29:52 +0100

Acee Lindem <[email-masked]> wrote on 18/01/2009 19:43:19:
> Subject: Re: [OSPF] Adding host routes in the router LSA
> 
> Hi Jocke,
> 

I see, thanks for clearing this up for me.
 
> No. I think we've already covered this :^)

Yeah :)

  Jocke