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
- Failure Detection time : is the neighbor down ?
- Event propagation time : Tell neighbor about the change
- Recalculation Time : Run SPF/DUAL/ etc calculation
- 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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