[c-nsp] Difference between Partial Route Computation and iSPF

[Mark Tinka]

On Thursday, April 26, 2012 05:26:50 AM Arun Kumar wrote:

> I would like to know is there any functional difference
> between PRC and iSPF as far as both OSPF and IS-IS is
> concerned. I know IS-IS had PRC feature inherent to its
> protocol as it has separate TLV for NLRI and topology
> information and OSPF was not. Introduction of iSPF made
> OSPF the same functionality as PRC in IS-IS. But iSPF is
> introduced to IS-IS as well. So
> 
> 1. introducing iSPF to IS-IS would not make much
> difference as it already had PRC?
> 2. any functional difference between iSPF and PRC?
> 3. PRC is jargon which is enabled by default or not be
> enabled or disabled like iSPF?

iSPF deals specifically with topology changes, while PRC 
deals with route changes (NLRI, to be specific).

As you mention, this distinction is clearer with IS-IS than 
with OSPFv2, due to the TLV structure. It is also clearer 
with OSPFv3, as like IS-IS, OSPFv3 is based on the TLV 
structure.

iSPF is useful when it isn't necessary to run an entire SPF 
calculation against a change in the topology, e.g., when 
only one router is being added to or removed from the 
network. In this case, the full SPF run isn't necessary.

On the otherhand, PRC deals with NLRI changes, e.g., change 
or deletion of a metric assigned to an IP prefix. While IS-
IS and OSPF (both versions) record attributes associated 
with a route, the actual routes, themselves, have no bearing 
on an SPF calculation. Once an SPT has been identified, all 
that remains is to record what prefixes are attached to 
routers on that SPT. So if the prefix NLRI changes (but the 
topology remains the same), the neighboring routers on the 
SPT do not need to re-calculate the SPF; only the changes to 
the NLRI.

But as you rightly mention, PRC needs to be understood for 
OSPFv2 and IS-IS/OSPFv3, separately. IS-IS and OSPFv3 
separate topology from routing information. So the effects 
of PRC are immediate and obvious. However, OSPFv2 carriers 
prefix information in Type 1 and Type 2 LSA's. So a change 
in the NLRI also triggers an update in the topology 
information, even though the topology hasn't changed, 
meaning a full SPF is run within an OSPF area even if only 
NLRI has changed.

In OSPFv2, only Type 3, 4, 5 and 7 LSA's trigger true PRC, 
as their only purpose in life is to signal prefix 
information (due to belonging to external areas). Remember, 
the whole purpose of areas is to restrict topology updates 
to routers belonging to the same area, so Type 1 and Type 2 
LSA's have no business propagating to another area, hence 
the innate PRC capability of OSPFv2 when exchanging routes 
between areas.

Hope this helps.

Cheers,

Mark.