Tuesday, August 31, 2010

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)])

 [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

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