CCNP - ENTERPRISE - Routing Protocol Metrics

Continuing the theme of grouping together common attributes/characteristics (see my posts on Summarisation and Authentication), in this article I'm going to set out the Metric for each routing protocol tested on the BSCI.


RIPv2 :  Distance Vector protocol
Metric = Hop count, 1-15, with 16 being 'infinity' or unreachable


EIGRP: Advanced Distance Vector protocol
Metric = Calculation based on Bandwidth (K1), Load(K2), Delay(K3), Reliability(K4), MTU (K5) although MTU is tracked through the path to find the smallest MTU - it is NOT used in the metric calculation.


Calculation is:  Metric = 256*([K₁*Bw + K₂*Bw/(256-Load) + K₃*Delay]*[K₅/(Reliability + K₄)])


Where:
 [K₅/(Reliability + K₄)] is disregarded if K5 = 0


Default K-values in use are K1 and K3 therefore is you use the default settings the default metric is based on Bandwidth and Delay.


OSPF: Link-State routing protocol
Metric = Cost where cost is calculated by - cost= 10000 0000/bandwith in bps

• The Cost is an indication of the overhead required to send a packet over a specified interface. 
• Cost is inversely affected by the bandwidth, the greater the bandwidth the lower the cost
• The Cost of the outbound interface is used
• To change the Cost of a given interface and therefore influence path selection you apply the command #ip ospf cost [cost value] to the outbound interface concerned

IS-IS: Link-State routing protocol
Metric = Arbitrary value between 0 -63, you decide what it means. Default value is 10

• To fine tune IS-IS you manually assign a metric value to each interface configured for IS-IS
• Similar to the OSPF bandwidth
• use the cmd #isis metric [metric] [level1| level2] to change the metric and assign it the appropriate routing level.

BGP: Distance Vector exterior routing protocol

Metric - is the Multi-Exit Discriminator value.

• the Lower the MED the better
• Used to decided how to enter an AS
• Default is 0
• Optional, Non-transitive
• Usually only shared between 2 AS's that have multiple eBGP connections with each other