VxLAN Underlay Fabric Convergence

VxLan underlay fabric convergence is based on 3 factors

• IGP Convergence 
• PIM Convergence 
• BGP Convergence 

Factors must be addressed separately to achieve high availability for VxLan overlay flow

Generally 4 factors affect IGP convergence time

1. Failure Detection time           : is the neighbor down ?
2. Event propagation time          : Tell neighbor about the change
3. Recalculation Time                 : Run SPF/DUAL/ etc calculation 
4. Forwarding table update time : Install new paths

• Failure Detection Time 
• How long does it take me to realize there is a failure ?
• Example failure detection 
• Link up/down event
• Routing protocol hello/dead timers
• IP SLA & EEM
• Bidirectional Forwarding Detection (BFD) 

• Event Propagation Time
• How long does it take to tell everyone else ?
• Example event propagation 
• Eigrp Query/Reply
• OSPF LSA Flooding Procedure 
• BGP update / withdraw

• Recalculation Time
• How long does it take me to decide on the new topology
• Example recalculation Time 
• Eigrp DUAL/ OSPF SPF / BGP Best path selection 

• Forwarding Table update Time
• How long does it take me to install the changes 
• Example update time 
• Eigrp topology to RIB download 
• RIB to S/W FIB download
• S/W FIB to H/W TCAM download 

Example ospf re convergence 

ospf failure detection 

   - Neighbor dead interval expires

ospf event propagation 

   - LSA flooding procedure 

ospf recalculation time 

    - SPF runtime 

forwarding table update time

    - ospf database to RIB installation , RIB to FIB , FIB to TCAM

How do we affect convergence 

Some factors are s/w & configuration dependent 

e.g smaller eigrp query domain is better 

e.g ospf stub areas are better 

e.g unnumbered fabric links or prefix-suppresison is better 

Some factor are h/w dependent 

e.g SPF runtime is function of CPU Size

e.g TCAM download is function of the line card

Methods of modifying convergence time 

Can be both reactive & proactive .

Reactive optimizations:

e.g Carrier delay & link de bounce timer

e.g Fast Hellos & BFD

e.g ospf LSA & SPF pacing

e.g FIB prefix prioritization 

Proactive optimizations 

Eigrp feasible successors

ospf loop free alternate (LSA)

BGP prefix independent convergence 

MPLS traffic engineering fast reroute ( TE FRR)

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