I am working on network lab designing topology. I am able to configured two routing protocols which surprised me. I configured RIP and OSPF routing protocols in layer3 switch and advertised same destination address on both routing protocols. Here my doubt is from which routing protocol traffic is routed among two routing protocol RIP & OSPF

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    Commented Dec 17, 2020 at 20:46

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Routing protocols do not route traffic, they simply share route information between routers. If a router learns the same routing information from two different sources, it will use AD (Administrative Distance) to determine which source is most reliable. The information from the most reliable source is what gets installed in the routing table, but the same routing information from the other sources is kept in reserve in case the first source loses that routing information or has some other problem.

AD works like a cost or distance where a lower value is the most preferred. Cisco has defined default AD for its routers:

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

Other router vendors have their own terminology and values or other ways of choosing the preferred information source.


Cisco and other vendor layer 3 switches and routers have feasibility to configure more than one routing protocols on layer3 devices .This features enable us to advertise route with more than one routing protocols on same layer3 devices for same destination.

Layers 3 devices chooses routing protocols considering AD value (administrative distance) lesser the AD value have more priority for chossen routing protocol by layer3 devices when more than one routing protocol is configured

Default Adminstrative value of routing protocols

Directly connected route ==> 0

Static routing protocol ==> 1

RIP ===============>>120

EIGRP ==============>>90

OSPF =≠=============>> 110

Lesser the AD value more priority to choose routing protocol by layer3 devices

For example

If layer3 devices is configured with RIP and OSPF routing protocols advertising same route to same destination . Layer 3 devices use OSPF routing protocols advertise route details for passing traffic because OSPF routing protocols AD value is less than RIP

We even have feasible to changes default AD values of routing protocols according to our requirements

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