Subject: Re: Type 7 LSA question

From: "Pat Murphy - (650)329-4044" <[email-masked]>
Date: Fri, 17 Dec 2004 08:26:50 -0700

>In reviewing RFC 3101, the recommended configuration is to provide
>control over the P LSA option bit when redistributing (aka importing)
>external  routes into an NSSA. There is no requirement on an implementation
>to provide a way to set the forwarding address to 0 - although a non-zero
>forwarding address is only required when the P bit is set.

When the P-bit is clear RFC 3101 encourages that the forwarding address be 
set just as discussed in RFC 2328 Section 12.4.4.1 where it can indeed be 0. 
Picking a forwarding address from one of the NSSA's interfaces can result in 
inefficient forwarding. It was my hope that implementations would code it 
this way. When the P-bit is set a non-zero forwarding address must be picked 
from one of the NSSA's interfaces, as it is required for translation from 
Type 7 to Type 5.

If an implemention uses the recommended approach a simple static route that 
points at an address external to the OSPF domain will result in a 0 
forwarding address when it is redistributed into OSPF with the P-bit clear.

RFC 3101 recommends that the default setting of the P-bit be clear (See 
Appendix A). Corporate OSPF backbone routing tables are a lot cleaner when 
the default setting of the P-bit is clear. This does require a means of 
toggling the P-bit via configuration. The intent to flood Type-7s into the 
full OSPF routing domain should always be a concious act given its potential 
adverse impact (See Appendix E). 

Pat