Cisco Routing Protocol Metrics

Routing protocols are a set of rules which are used to define an optimal route between sources and destination systems in a network. Basically routing works on the network layer of the OSI reference model.

Purpose of Routing Protocol Metrics:

To determine the best route when multiple updates are received for the same route or we can say when various routes are learned so a router could receive routing updates from several other different routers all about the same network and it needs a mechanism to determine the best one well that’s where the metrics come into play and a metric is a numeric value and it’s different between each routing protocol it could be a small value or an immense value and routing protocol will look at the metric of all the similar route and it will use a lower metric to determine the best route so the general rule of thumb is the lower the metric the better the route and so a routing protocol will look and no compare and if it didn’t have a metric it would have all of these updates about the same network but it would not know which one to use it would have any basis to determine the best route so that’s the purpose to determine the best route.

Routing Protocol Metrics:

Each Possible path will be assigned as a ‘metric’ value by the routing protocol which indicates how preferred the path is. Always the lowest metric value is preferred. Distance vector routers advertise to each other the networks they know about and their metrics to get to each of them.

• Link state routers advertise all the links in their area of the network to each other.
• Each router will take this information and then make independent calculations of its own best path to get to each destination.
• If the best  path to a destination is lost
• For example: because a link went down it will be removed from the routing table and replaced with the next best route.

RIP Metric Hop Count:

• RIP uses hop count as the metric.
• The maximum hop count by default is 15. Paths that are more than 15 hops away are marked as unreadable.
• RIP is typically used only in small or test environments.

S.NO	Routing Protocol	Metric
1.	RIP	Hop count
2.	EIGRP	bandwidth, delay
3.	OSPF	cost

Administrative Distance:

Administrative distance is a way to find reliable routing information, and a lower administrative distance means a route is more accurate. Administrative Distance is a default value assigned to each routing protocol that will favor the best route from one protocol over another one.

Example: Router receives multiple routes over to the 10.10.10.0/24 network from both the OSPF and RIP.

 You can display the Administrative Distance in the router by typing the show IP route command in CLI:

Relationship between Administrative Distance and Routing Metric:

S.No.	Administrative distance	Routing Metric
1.	Administrative Distance is used to choose the distance among multiple paths learned through different routing protocols	The metric is used to choose among multiple paths learned through the same protocol
2.	It is considered to be the first to narrow the choice down to the single best routing protocol.	It is considered to choose the best path or paths which  make it into the routing table.