Full & Partial SPF in OSPF

LINK STATE ADVERTISEMENTS
Link - connection between routers
State - UP/Down
Advertisements - info which routers can use to share information about the n/w topology.

A lot of times i read about this term i.e. Partial SPF calculation which forced me to re-think about the SPF algorithm functionality. This post will help you understand this simple yet unknown term easily (well, i hope so :)

The SPF algorithm allows two quicker ways to calculate routes :

1. Full spf
2. Partial spf

The full spf runs when there is a topology change expressed in router lsa's. As , the name suggests, it shouts for the topology change within an area which means all the routers within an area will run SPF to calculate the new routes with the changed metrics(if any). Even a single lsa could thrust SPF to re-run and cause new updates in the network.

What happens in Full SPF ?
Each router processes each LSA it received and keep it in to the Link State Database with the help of LSU(Each LSU may contain one or more lsa's). SPF is based on Djikstra's algorithm which runs on this database to create a TREE. A best route selection process takes over and selects the best route from all the links available in the LSDB ( which follows the order as Longest Prefix match --> AD-->Metric --> Load balance ).

The SPF algorithm is CPU-intensive and is thus heavily throttled: the router will not execute the SPF algorithm until a predefined timeout has expired from the event triggering the SPF (goal: collect as many changes as possible in the topology database to prevent multiple SPF runs) and will not execute subsequent SPF run until the inter-SPF interval expires (goal: minimize CPU utilization in case of severe network instability).

Use the TIMERS THROTTLE SPF router configuration command to adjust OSPF SPF timers and the SPF-INTERVAL router configuration command to adjust IS-IS SPF timers.

Alright! i hear you saying " I understand that! ( i know this tone :)) but when does Partial SPF occur ??

Here is the scoop:
Lets say we have two areas Area 1 and Area 2. Area 1 sends its routes as an Inter-Area Summary routes (LSA Type 3) to Area 2. Any change in Area 1 topology or IP Prefix flap will never cause the Area 2 routers to run full SPF algorithm. Summary LSAs cause partial SPF to run. In OSPF, partial spf relates to Summary and External LSA's only.

Partial SPF will run in AREAS BESIDE THE ONE WHICH HAD A TOPOLOGY CHANGE VIA summary lsa or External lsa's.Full SPF creates an spf tree from the list of all ip prefixes available in the link state database. Best routes are selected by partial spf.

What's Partial SPF ?

The best route selection after the shortest path tree is built is not computationally complex and not as heavily throttled. The best route selection after the SPF tree has been built is called partial SPF in OSPF and partial route calculation (PRC) in IS-IS.

The route selection is a simple distance-vector operation where the router selects the minimum-cost IP prefixes from the set of all advertised IP prefixes