Decision Process

Phase 1: Calculation of Degree of Preference

Occurs upon reception of UPDATE. Adj-TRIB-in is updated with computed local_pref.

Can be peer-paralleled

• For internal routes, it takes LocalPref value
• For external routes, its locally determined by means of configuration

Phase 2: Route Selection

Occurs on TRIB update

Phase 2a: External routes and local routes

Route selection alg (external Ext-TRIBs-in, local route table) -> Ext-TRIB

Phase 2b: Ext-TRIB and internal routes

Route selection alg (internal Ext-TRIBs-in, Ext-TRIB) -> Loc-TRIB

Route selection

Overlapping routes

More specific route takes precedence

Both routes will be installed in tripd implementation

Breaking ties (route selection)

An improvement to what the RFC suggests

In order of selection

• 1. Highest degree of preference (local pref)
• 2. Originated by local LS first
• 3. Shortest ITAD-path
• 4. Highest MED
• 5. eTRIP over iTRIP (external peer route before internal peer route)
• 6. Oldest route
• 7. Highest LS ID

Phase 3: Route disemination

After Loc-TRIB is changed or new session is established.

All routes in Loc-TRIB are processed into Adj-TRIBs-out optionally applying aggregation or reductions.

Update processes

Internal update process

Upon reception of UPDATE, flood other internal peers

Send all Ext-TRIB to peer, with itself as Originator

Remove withdrawn routes from Ext-TRIB-in

If some withdrawn routes were in Ext-TRIB, insert replacement route if available or mark withdrawn in Ext-TRIB-out and UPDATE

External update process

Phase 3 has updated Adj-TRIBs-out.

Send UPDATE messages with new routes in ReachableRoutes, and those marked as withdrawned or newly unreachable routes within ITAD in WithdrawnRoutes.