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Beginner

RIPv1

Capture.PNGRIPv1 is configured in 3 routers. All configurations are correct.

Why R3 doesn't have a route for 10.10.0.0/16 in its routing table(R3 to R1)?

1 ACCEPTED SOLUTION

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Highlighted
Beginner

Re: RIPv1

Hello, gowdakssujan,

I would like to extend the answers above a little bit as this topic has really vanished from modern books because the classful routing protocols are pretty old.

The idea behind RIPv1 is that subnets are advertised without masks. However, it doesn't mean that you cannot divide a classful network into subnets. The mask in case of classful IGPs is derived from connected interfaces in the same classful network. Consider the following network:
10.0.1.0/24 --- (R1) --- 10.0.12.0/24 --- (R2) --- 10.0.2.0/24
All subnets inside the domain have the same masks, that is why routing is correct.
10.0.1.0/28 --- (R1) --- 10.0.12.0/24 --- (R2) --- 10.0.2.0/27
In this case, however, routers get confused what mask to use for subnets of classful networks and do not advertise anything at all! That is why FLSM wastes so much address space because contiguous masks have to be used throughout the whole classful network for routing to be correct.
In case of interoperation between RIPv1 and RIPv2 the same behaviour stands: RIPv2 router would send to RIPv1 only the routes that have the same mask as the outgoing interface:
(R1 - RIPv2) --- 10.0.12.0/24 --- (R2 - RIPv1)
R1 would send 10.0.24.0/24, for example, but 10.1.0.0/16 or 10.2.2.0/28 would not be present in the update.

5 REPLIES 5
VIP Advisor

Re: RIPv1

Hi there,

RIPv1 is a classful routing protocol, ie it does not support CIDR or VLSM, therefore it will not see routes more specific than 10.0.0.0/8

 

cheers,

Seb.

Beginner

Re: RIPv1

Capture.PNGOkay, then how, in this case, R2 will have a route to 10.10.0.0/16?

Hall of Fame Expert

Re: RIPv1

Hello gowdakssujan,

yes in this second case you are using the same subnet mask on all links so RIPv1 that does not carry the subnet mask in updates can work correctly.

This is called FLSM = Fixed Length Subnet Mask.

RiPv1 is really a legacy protocol together with IGRP.

RIPv2 supports subnet masks and supports VLSM that is required in modern networks.

 

Hope to help

Giuseppe

 

VIP Advisor

Re: RIPv1

You will have two connected subnets with /16 masks and the route received via RIPv1 will still be for 10.0.0.0/8 .

 

I assume this is more an academic question than a production design? As @Giuseppe Larosa says, RIPv1 is a legacy protocol, and for good reason! :)

 

cheers,

Seb.

Highlighted
Beginner

Re: RIPv1

Hello, gowdakssujan,

I would like to extend the answers above a little bit as this topic has really vanished from modern books because the classful routing protocols are pretty old.

The idea behind RIPv1 is that subnets are advertised without masks. However, it doesn't mean that you cannot divide a classful network into subnets. The mask in case of classful IGPs is derived from connected interfaces in the same classful network. Consider the following network:
10.0.1.0/24 --- (R1) --- 10.0.12.0/24 --- (R2) --- 10.0.2.0/24
All subnets inside the domain have the same masks, that is why routing is correct.
10.0.1.0/28 --- (R1) --- 10.0.12.0/24 --- (R2) --- 10.0.2.0/27
In this case, however, routers get confused what mask to use for subnets of classful networks and do not advertise anything at all! That is why FLSM wastes so much address space because contiguous masks have to be used throughout the whole classful network for routing to be correct.
In case of interoperation between RIPv1 and RIPv2 the same behaviour stands: RIPv2 router would send to RIPv1 only the routes that have the same mask as the outgoing interface:
(R1 - RIPv2) --- 10.0.12.0/24 --- (R2 - RIPv1)
R1 would send 10.0.24.0/24, for example, but 10.1.0.0/16 or 10.2.2.0/28 would not be present in the update.

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