Best path selection

1.3.b i RD, FD, FC, successor, feasible successor

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Reported Distance (RD)

General information on “EIGRP Reported Distance (RD)":

• The current metric to the destination from the neighbor routers perspective
• The second/right value when checking the EIGRP topology table

Computed Distance (CD) (not on blueprint)

General information on “EIGRP Computed Distance (CD)":

• The current metric to the destination from the local routers perspective
• The first/left value when checking the EIGRP topology table
• CD = Metric to neighbor + Reported Distance

Feasible Distance (FD)

General information on “EIGRP Feasible Distance (FD)":

• A snapshot of the best/lowest metric for the route from the local routers perspective at the last time the route went from ACTIVE to PASSIVE state
• Changes each time a route goes from ACTIVE to PASSIVE state
• Can be lower than the actual metric (CD = computed distance) of the successor/best route if the distance changed in a way that no diffusing computation was needed (eg. Local interface bandwidth/delay/… changed)
• Used to verify a routes feasibility condition

Feasibility condition (FC):

General information on “EIGRP Feasibility condition (FC)":

• Is met when the RD of a route is lower than the FD
• This automatically means that the neighbor is closer to the destination
• If condition is not met, the route can’t be guaranteed to be loop-free and is also not put in the topology table

Successor route

General information on “EIGRP Successor route”:

• The best route to a destination network
• If there are two or more routes, the route with the lowest CD will be put in the routing table
• The next-hop router is known as successor

Feasible successor route

General information on “EIGRP Feasible successor route”:

• The backup route to a destination network
• The next-hop backup router is known as feasible successor
• The RD of a backup path needs to be lower than the FD in order to get considered as backup path (feasibility condition)
• The FS route gets installed into the RIB immediately after the successor route fails BUT ONLY if it also has the lowest CD (Computed Distance) of all possible backup links to the failed destination
• If this is not the case the route will go into ACTIVE state and a possible shorter path that wasn’t considered as FS route (maybe because it didn’t pass the FC check before) could be selected as FS and will be installed into the RIB
• If that happens the FD value will be set to the current CD value of the successor route

Zero successor routes (not on blueprint)

General information on “EIGRP Zero successor routes”:

• Routes that don’t get installed into the RIB because the route was learnt from another/more trusted source (eg. routing protocol) which has a lower AD than EIGRP
• Zero successor routes won’t get advertised further because the router is not actively using it and therefor can’t pretend to others that the he’s using it

Important: All routes to a destination network are put in the topology table but only the best route (when load-balancing is NOT configured) with the lowest FD will be put in the routing table!

1.3.b ii Classic Metrics and Wide Metrics

Classic Metrics

EIGRP classic metric calculation formula:

• 256 * ([K1 * bandwidth] + [K2 * bandwidth] / [256 – Load] + K3 * Delay) * (K5 / [Reliability + K4])

EIGRP classic metric calculation formula (simplified, all K values default):

• 256 * ((10^7/bandwidth in kbps) + (delay/10))

EIGRP metric calculation parameters:

• Bandwidth:
  • Lowest interface bandwidth along the path
  • Also known as “least-bandwidth”
  • Measured in kilobits
• Load:
  • Highest link load along the path
  • A percentage value between 1-255 (1 = 0% load, 255 = 100% load)
  • Only measured at the time of a link change
• Delay:
  • The cumulative (summary) of all interface delay along the path
  • Measured in tens of microseconds
• Reliability:
  • Worst reliability along the path
  • A percentage value between 1-255 (1 = 0% reliable, 255 = 100% reliable)
  • Based on keepalives
  • Only measured at the time of a link change

EIGRP classic metric K values:

• Default values for K:
  • K1 = 1
  • K2 = 0
  • K3 = 1
  • K4 = 0
  • K5 = 0
• This results in that by default only bandwidth and delay are used for metric calculation
• If reliability and load would be used for metric calculation, there would be a frequent recalculation of the topology
• The K values must match between all EIGRP neighbors
• Important: Cisco recommends not to change the K values!

Wide Metrics

General information on “EIGRP Wide Metrics”:

• Can only and will be automatically enabled with EIGRP named mode
• Wide metrics are based on 64-bit calculations whereas classic metrics are based on 32-bit calculations
• Backwards compatible with the classic metric
• Fixes two problems of the “classic metric”:
  • Scales with interface bandwidth >1G
  • Scales with path delay if the path bandwidth is >1G
• Bandwidth is now called Throughput and the calculation formula for it changed:
  • Wide metric throughput calculation formula: (65536 x 10^7)/(interface bandwidth)
• Delay is now called Latency and measured in picoseconds instead of microseconds and the calculation formula changed:
  • Interface physically <=1G w/o bandwidth and delay command: default delay converted to picoseconds
  • Interface physically >1G w/o bandwidth and delay command: (10^13) / (interface default bandwidth)
  • Explicit bandwidth configuration without delay command: default delay converted to picoseconds
  • Explicit delay configuration regardless of physical/bandwidth command: (10^7) x (value of delay command)
• EIGRP automatically detects whether their neighbors also support wide metrics, uses the appropriate metric type (wide metrics are preferred) and switch back to classic metric on a per-link basis if the neighbor doesn’t support wide metrics
• “RIB scale” is introduced:
  • Instead of putting the CD as metric into the routing table (RIB), a scaled metric is now calculated
  • This is done because the RIB can only handle 32-bit integer values whereas the calculated wide metric values are 64-bit
  • The default RIB scale factor is 128 but can be modified if needed
  • Calculation: (CD/128) = RIB metric

“EIGRP Wide Metrics” CLI configuration commands:

## Modifying the EIGRP named mode RIB scale value
Router(config-router-af)# metric rib-scale <1-255>