Routing – Understanding Routing and Forwarding Decisions

ip-forwarding-tablerouterrouting

We need to build a routing table, based on routes from neighbors..

Route A receives three routes from different Routing Process: 

EIGRP - 192.168.100.0/26
OSPF - 192.168.100.0/19
RIP - 192.168.100.0/24

We have multiple paths to the same destination, How will we decide which will be installed on routing table?

Consider three elements: Administrative Distance, Metrics and Prefix Lenght

Question 1: Sorted by priority, which element will use first?

I think winner based on Administrative Distance, but I've multiple paths, I can't discart them, so these routes shows different prefix lenght.

Question 2: Are they considered different destinations, therefore will all be installed?

Question 3: Who are the Tie-Break? I mean, if I have two paths to the same destination, same AD value I'll consider Metric value and if they have equal values the load balancing is actived?

Question 4: Prefix Lenght is used only for Forwarding Decision?

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Best Answer

Routes with different prefix lengths are really different routes so they end up in the routing table. The route with the longest prefix will be used for the routing decision.

Metrics are used by a routing protocol to determine the best route to be offered as a candidate to the routing table.

Administrative distance will be used by the routing table as the tie-breaker for candidate routes offered by the routing protocols.

The various routing protocols keep or discard routes for the routing protocol's table (not the same as the routing table) based on the rules for the particular routing protocol.

Different routing protocols handle ties and load balancing differently. As I recall, EIGRP will, by Default, load balance across up to four routes, but that can be modified. Other routing protocols have different rules for load balancing, or not.

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