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Network running multiple Protocols

I am running multiple Protocols on different Routers as so:

  • R1-RIPv2 EIGRP OSPF
  • R2-OSPF and RIPv2
  • R3-RIPv2 and EIGRP
  • R4-EIGRP and OSPF

This is not a practical exercise, it is just to explore how different protocols may take precedence over other protocols and how to manipulate the routing table. Usually you run a single protocol, so I never thought about this until this scenario came up.

R2 learned through RIPv2 about network 3.3.3.3 on the loopback of R3. I always assumed the information came from the direct neighbor, in this case R1. The update did take that path, and R1 has a RIPv2 database of course but most of the routes on R1 were learned through EIGRP (D), but one, R2 2.2.2.2 was learned through OSPF because R2 isn't running EIGRP. This is important to my question so keep it in mind please for just a moment. R2 learned about 3.3.3.3 (R3) through RIPv2. I was under the impression the routes are exchanged by direct neighbors, so that it would have to learn about R3 3.3.3.3 from R1, but R1 has no RIPv2 routes in its table, the route was learned through the g0/0/1 interface on R2 which is 10.1.1.2 on the image. So did R3 send that information through R1 to R2 or did R1 access that from its RIP database and send the information. Which I can't fathom. If it did come from R1 I would expect it would have been exchanged with OSPF as it is a better AD than RIP, which is why I suspect it was learned from other RIP Routers or in this case R3? Can someone clarify, as I said my assumptions were based on all routers running the same protocol which made it easy to believe the neighbor router updates it's neighbors because of the path the update takes. RIP is a routing by rumour protocol is it not. SO I am confused as to how R2 learned about R3 loopback 3.3.3.3

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The only routing protocol in common between R3 and R2 is RIPv2, so that is the only way that R2 can learn the R3 loopback because there does not seem to be any route redistribution on any of the routers. R4 can learn the R3 loopback from EIGRP, but it cannot advertise it via OSPF unless you redistribute the route from EIGRP to OSPF.

Each routing protocol maintains its own, separate tables in a router. The router will choose the best routes from each of the routing protocols, taking into account AD, to install in its routing table. The routing protocols will only advertise routes they know about from their own protocol tables, and they do not look at other routing protocols or the routing table for that. R4 OSPF has no idea what is in the R4 EIGRP tables or the R4 routing table; it only knows what is in the R4 OSPF tables, which is what it learned from OSPF neighbors or R4 network statements. The R3 loopback is in the R4 EIGRP tables and R4 routing table, so R4 OSPF does not know about that route, and it cannot advertise it to R2.

You can redistribute routes between routing protocols, but you must be careful doing this because you can create routing loops.

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  • Thanks Ron Maupin, as I said I kind of suspected it had to learn it from other RIP Routers, and yeah it made me question what I knew from a single protocol running in a network. I have done a little work on redistribution of routes while studying OSPF in detail but I definitely have to go back to that soon.
    – Angel Gd
    Sep 19 '20 at 0:18
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I can't answer you question particularly exactly because .i am not understanding you question but some points to clear your understanding

Multiple routing protocols can run among different network s connecting each other by redistribution routes among various routing protocols.. Administrative distance value is used by layer3 devices to choose best routing protocols to forward packet if multiple routing protocols are used to route traffic to same destination , least the AD value have high priority to choosen by layer 3 device.

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