ROUTE FORWARDING DECISION BY DEFAULT (3.2)

 

DETERMINE WHO A ROUTE MAKES A FORWARDING DECISION BY DEFAULT.

3.2.a    Longest Prefix Match.

3.2.b    Administrative Distance.

3.2.c     Routing Protocol Metric.

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LONGEST PREFIX MATCH

LONGEST PREFIX MATCH (LPM) IS AN ALGORITHM USED BY ROUTERS TO SELECT THE MOST SPECIFIC ROUTE TO A DESTINATION IN THEIR ROUTING TABLE. THE ALGORITHM WORKS BY COMPARING THE DESTINATION ADDRESS OF A PACKET TO THE PREFIXES IN THE ROUTING TABLE. THE ROUTE WITH THE LONGEST MATCHING PREFIX IS THE ONE THAT IS CHOSEN.

FOR EXAMPLE, CONSIDER A ROUTER WITH THE FOLLOWING ROUTING TABLE:

Destination Prefix Length Next Hop 192.168.1.0/24 24 192.168.1.1 192.168.2.0/24 24 192.168.2.1 192.168.3.0/24 24 192.168.3.1

IF A PACKET ARRIVES WITH A DESTINATION ADDRESS OF 192.168.1.2, THE ROUTER WILL COMPARE THE DESTINATION ADDRESS TO THE PREFIXES IN THE ROUTING TABLE. THE FIRST PREFIX THAT MATCHES IS 192.168.1.0/24, WHICH HAS A PREFIX LENGTH OF 24. THIS IS THE LONGEST MATCHING PREFIX, SO THE ROUTER WILL FORWARD THE PACKET TO THE NEXT HOP OF 192.168.1.1.

IF TWO OR MORE PREFIXES HAVE THE SAME PREFIX LENGTH, THE ROUTER WILL CHOOSE THE ONE WITH THE LOWEST ADMINISTRATIVE DISTANCE. ADMINISTRATIVE DISTANCE IS A VALUE ASSIGNED BY A ROUTER TO INDICATE THE RELIABILITY OF A ROUTE SOURCE. IT RANGES FROM 0 TO 255, WITH A LOWER VALUE INDICATING A MORE TRUSTWORTHY SOURCE.

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ADMINISTRATVIE DISTANCE

ADMINISTRATIVE DISTANCE (AD) IS A VALUE USED IN ROUTING PROTOCOLS TO DETERMINE WHICH ROUTE TO USE WHEN MULTIPLE ROUTES EXIST FOR THE SAME DESTINATION. THE LOWER THE AD, THE MORE PREFERRED THE ROUTE. ADS ARE ASSIGNED BY THE ROUTING PROTOCOL VENDOR AND CAN RANGE FROM 0 TO 255.

FOR EXAMPLE, OSPF ROUTES HAVE AN AD OF 110, WHILE RIP ROUTES HAVE AN AD OF 120. THIS MEANS THAT OSPF ROUTES WILL ALWAYS BE PREFERRED OVER RIP ROUTES, EVEN IF THE RIP ROUTE HAS A SHORTER METRIC.

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ROUTING PROTOCOL METRIC

A ROUTING PROTOCOL METRIC IS A VALUE THAT IS USED TO MEASURE THE COST OF A ROUTE. ROUTING PROTOCOLS USE METRICS TO DETERMINE THE BEST PATH TO A DESTINATION WHEN THERE ARE MULTIPLE PATHS AVAILABLE. THE LOWER THE METRIC, THE BETTER THE ROUTE. ROUTING MATRICS ARE ASSIGN TO ROUTES BY ROUTING PROTOCOL.

MANY ROUTING PROTOCOL HAVE USED IN ONE METRIC AND MANY USED MORE THAN ONE METRIC.

RIP IS USED HOP COUNT AS A METRIC TO DETERMINE THE BEST PATH THROUGH THE NETWORK. OSPF IS USED COST AS A MATRIC.

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