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A router receives two identical routes from different routing protocols. Which route is put into the routing table?

I have been asked this question.

A router can only have one protocol active at a time no?

My answer to this question would be that he will register whichever route is from the protocol he is currently using.

However I fear I might be misunderstanding something?

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we talking cheap home router or cisco enterprise class ? Maybe you should say it will Open the Shortest Path First based on lowest metric. –  Knuckle-Dragger Dec 16 '13 at 3:07
    
@Knuckle-Dragger Cisco, it's part of my networking class. However the question doesn't state which protocols are running so I can't really say it will use the shortest path as we don't know if it's running OSPF. Is this not a trick question? Can a router be running more than one protocol at the same time on the same interface? –  user2018084 Dec 16 '13 at 3:10
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2 Answers

up vote 3 down vote accepted

Two identical routes from different protocols will enter the table based on their administrative distance.

Here are the defaults.

Route Source    Default Distance Values
Connected interface 0
Static route    1
Enhanced Interior Gateway Routing Protocol (EIGRP) summary route    5
External Border Gateway Protocol (BGP)  20
Internal EIGRP  90
IGRP    100
OSPF    110
Intermediate System-to-Intermediate System (IS-IS)  115
Routing Information Protocol (RIP)  120
Exterior Gateway Protocol (EGP) 140
On Demand Routing (ODR) 160
External EIGRP  170
Internal BGP    200
Unknown*    255
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A router can only have one protocol active at a time no?

That would be pretty limiting. What if you have both an exterior routing protocol (to talk to networks under control by other administrators) and an interior routing protocol (to talk to networks under your control)? What if you have to link two existing networks that already use different interior routing protocols?

In the Cisco world, you typically configure an "administrative distance" for each routing protocol. The defaults generally give interior routing protocols priority over exterior routing protocols and static routes the highest priority second only to interface routes.

In any event, even if you only had one dynamic routing protocol active, you can still have this issue. Say you have an interface in a particular network but also receive a route to that network from a router you reach over a different interface. You want to use your interface in that network when it's up, but when it's down, you want to fail over to use the router that still has a link to that network.

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+1 What David said. –  Knuckle-Dragger Dec 16 '13 at 3:35
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