RIP-v2 behavior

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Jan 14th, 2009
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I defined 3 routers connected between them with serial links and I connected to each router one LAN subnet:

LAN R1: 1.1.1.0/24

LAN R2: 2.2.2.0/24

LAN R3: 3.3.3.0/24


R1-R2: 12.12.12.0/24

R1-R3: 13.13.13.0/24

R2-R3: 23.23.23.0/24


I configured RIP-v2 and for each router I set to be advertised only its LAN subnet:

R1: network 1.0.0.0

R2: network 2.0.0.0

R3: network 3.0.0.0


But I didn't see any RIP update at all from neither of the routers on neither of their interfaces (nor on their 2 serial itf. nor on the fastethernet itf.).


Q1a: Why the LAN subnets were not advertised ?


Only after I added the other 2 networks to be advertised:

R1: network 1.0.0.0, network 12.0.0.0, network 13.0.0.0

R2: network 2.0.0.0, network 12.0.0.0, network 23.0.0.0

R3: network 3.0.0.0, network 13.0.0.0, network 23.0.0.0


the RIP updates started to be sent (they contained as well the LAN subnets).

Q1b: So any idea why this behavior ?


Also I observed by using both:

'show ip route' and 'debug ip rip events'

that the routes advertised through RIP were all with /8, not with /24.


Q2: Why this behavior, as version 2 of RIP is set on all the routers (and the RIP updates are correctly sent to 224.0.0.9 confirming so that indeed RIP-v2 is used) ?


Next I changed the LAN subnets as follows:

LAN R1: 1.1.1.0/26

LAN R2: 1.1.1.128/25

LAN R3: 1.1.1.64/26

and set as well in R2 and R3 the network 1.0.0.0 to be advertised.

I observed that still the updates and the routes were with /8 as before.

However, surprisingly for me, the ping was working between the LANs...and I don' understand how it could work.

In R1 for instance I have the following routes for 1.0.0.0 network:


R 1.0.0.0/8 [120/1] via 13.13.13.3, 00:00:18, Serial3/0

[120/1] via 12.12.12.2, 00:00:04, Serial2/0

C 1.1.1.0/26 is directly connected, FastEthernet0/0


Q3: So how the router R1 knows where to route a packet with a destination for ex. 1.1.1.253 ? To Serial 3/0 or to Serial 2/0 ?

There's ambiguity in-here, isn't there?

I even disabled the interface between R2 and R3, but ping from LAN1 was still working to LAN2 and LAN3.

Could anyone help me uncover this mystery?


PS: All the tests were done with PacketTracer 5, so on a simulated environment.


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Overall Rating: 5 (2 ratings)
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Jon Marshall Wed, 01/14/2009 - 01:01
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Petru


Q1 - The network statement when used with RIP/RIPv2/IGRP/EIGRP/OSPF does not tell the router which networks to advertise. So


router rip

network 1.0.0.0


does not tell R1 to advertise 1.0.0.0. It tells R1 to start RIP running on any interface that falls into the 1.x.x.x range. So until you added the serial subnets RIP was not running on those interfaces and hence the routers were not exchanging routes.


As for the /8 networks did you disable auto-summarisation on your routers ie.


router rip

no auto-summary


if you didn't then RIP will summarise across classful boundaries and because your serial links are 12.x.x.x and 13.x.x.x then the LAN subnets will be summarised to 1.0.0.0/8.


Jon

badalam_nt Wed, 01/14/2009 - 01:38
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Jon, thanks for your reply.

If I understood correctly what you mentioned, then the routers should have sent RIP updates on the fastethernet interface, but not on the serial interfaces. However I didn't see any RIP updates at all.

Is there an explanation for this?


Also, regarding Ping question, do you have any hint why it worked, whereas my common sense would have bet that it would never work?

Jon Marshall Wed, 01/14/2009 - 05:21
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Petru


"If I understood correctly what you mentioned, then the routers should have sent RIP updates on the fastethernet interface, but not on the serial interfaces. However I didn't see any RIP updates at all."


You should have seen something happening on the router. When i tested this by enabling RIP on just one interface on a router i was getting advertisement suppressed messages.


On your router


debug ip rip


and you should see messages. But if the advertisement is being suppressed you won't capture it with a packet sniffer on a PC.


"Also, regarding Ping question, do you have any hint why it worked, whereas my common sense would have bet that it would never work?"


R1 has 2 routes to the same destination so it will use both routes. This could cause a problem but if you draw out your topology you will see that in fact it doesn't matter whether R1 sends it to R2 or R3 because R2 & R3 are interconnected so the packet can just be forwarded between the two to get to the right destination.


Jon




Giuseppe Larosa Wed, 01/14/2009 - 08:28
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Hello Jon,


>> You should have seen something happening on the router. When i tested this by enabling RIP on just one interface on a router i was getting advertisement suppressed messages


This is a very Good point.


Because the list of routes to be advertised would be empty for the distance vector nature of RIP: the list csnnot contain the subnet of the outgoing interface and there no other networks to advertise.

Because RIP has no neighbor state machine ( no hello protocol) it has no reasons to send out the empty update that so it is suppressed.


Hope to help

Giuseppe




Jon Marshall Wed, 01/14/2009 - 08:54
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Giuseppe


"the list csnnot contain the subnet of the outgoing interface and there no other networks to advertise."


And that deserves +5. I remember thinking at the time "now why is it being suppressed" but then i got called off to do something else and forgot about it. Makes perfect sense.


Jon

fjcardenas-1 Wed, 01/14/2009 - 07:11
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In your scenario, for the routers to send/receive RIP updates to/from other routers the subnets of the serial interfaces need to be added to the RIP configuration, otherwise they won't advertise anything. If you only add the LAN subnet but you don't add the serial subnets then the LAN subnet won't be advertised because the serial interfaces are not in the RIP configuration and because of that they don't advertise.

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