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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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  • Please do your own homework. These sorts of questions are specifically off-topic for this forum. Please see the Help Center for which types of questions are allowed, and which types are not. – Ron Maupin Oct 3 '15 at 18:19
  • That's not homework :\ I was studying about Route Decision, I am sorry if I exaggerated on the questions. Usually I post these sorts questions on Cisco Forum. – TMoraes Oct 3 '15 at 18:56
  • OK, I will provide and answer, but this is very close to being off-topic. The Help Center says that all education questions are off topic here, but are welcome in the chat. – Ron Maupin Oct 3 '15 at 19:18
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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.

  • When you say The route with the longest prefix will be used for the routing decision. you mean forwarding? I thought which Metrics was used as the tie-breaker, so, Metrics are used to calculate the best path, if it learns multiples path to the same destination, Administrative Distance is used IF these paths are different routing protocols? – TMoraes Oct 3 '15 at 19:39
  • I think we mean the same thing by "the routing decision" and "forwarding"; the decision is made to forward to the route with the longest match. Metrics are not really a tie-breaker since they determine the best path for a particular routing protocol, and there may actually be a metric tie for routes in the routing protocol's table. Administrative Distance is a tie breaker for the routing table for the route candidates from the various sources (routing protocols, connected routes, or statically defined routes). – Ron Maupin Oct 3 '15 at 19:47
  • Administrative Distance is a tie breaker for the . . . Ok, but "If there are multiple paths to the same destination from a single routing protocol, then the multiple paths would have the same administrative distance and the best path is selected based on the metrics" That can happens, all this depends on the what administrator is handling.. Can I say that? – TMoraes Oct 3 '15 at 19:58
  • OK. This is why we don't like these sorts of question in this forum. We can't keep chatting like this - it belongs in the chat room. We are going to be shut down here. – Ron Maupin Oct 3 '15 at 20:01
  • Thanks Ron, I appreciate your help, You always help me. . . – TMoraes Oct 3 '15 at 20:03

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