Subject: OSPF Questions

From: Ali Alwan <[email-masked]>
Date: Wed, 21 Sep 2005 13:32:01 -0400

Reference to Jeff Doyle's book (Routing TCP/IP Volume I); we can configur 
a primary and multiple secondary addresses on a physical interface of a 
router (which creates one primary and multiple secondary sub-interfaces). 
When we configure OSPF on secondary addresses (sub-interfaces), it follows 
the following two rules:
1. The OSPF must be configured on the primary sub-interface. 
2. The OSPF treats secondary sub-interfaces as stubnetworks, that is it 
does not send Hello messages on the secondary sub-interface and 
consequently does not build adjacencies.

Questions are: 

1. Is the above a standard behaviour of OSPF? because I couldn't find this 
issued discussed in John Moy's book. 

2. What prevents OSPF from configuring sub-interfaces as a network of 
choice, for example a broadcast network; other than the messaging overhead 
in building multiple adjacencies on the single physical interface?

 I will really appreicate your insight on this issue. Thanks.

---

From: Acee Lindem <[email-masked]>
Date: Wed, 21 Sep 2005 18:07:11 -0400

Hi Ali,

>router (which creates one primary and multiple secondary sub-interfaces). 
Actually, this is incorrect. The configuration of a secondary address 
doesn't create a
sub-interface (at least not in most vendor's terminology). What it 
creates is a subnet.

>issued discussed in John Moy's book. 
Some implementations have chosen to limit an OSPF transit interface to 
using the
primary address. Secondary addresses are then advertised as stub networks.

>in building multiple adjacencies on the single physical interface.
>
RFC 2328 doesn't preclude this.

> I will really appreicate your insight on this issue. Thanks.
Hope this helps,
Acee