rip version behaviour

Answered Question
Mar 14th, 2009

Hi every body!

I just want to confirm if i am correct.

Case # 1 ra s0----------------s0 rb

ra config:

loopback 172.172.172.1/24

s0 172.172.173.1/24

==============

rb config:

s0 172.172.173.2/24

ra and rb are running ripv1

Now routing table at rb looks like as:

R 172.172.172.0/24[120/1]via 172.172.172.1

ra passes the route 172.172.172.0 since no mask is sent, r2 assumes the mask configured on receiving interface. This is true if the subnet belongs to same major net otherwise ripv1 auto summarize 172.172.172.0 as 172.172.0.0 in update.

====================================

case# 2

ra s0---------------s0 rb

ra config:

loopback 1

172.172.172.17/28

s0 172.172.172.5/30

==================

rb config

s0 172.172.172.6/30

Both are running ripv1.

I just implemented the above in my lab using my precious 2500 series routers.I find the routing table at rb as:

rb# show ip route

172.172.0.0/30 is subnetted, 2 subnets

C 172.172.172.4 is directly connected, Serial0

R 172.172.172.16 [120/1] via 172.172.172.5, 00:00:15, Serial0

The importantthing is rb receives the update 172.172.172.16 and assumes the mask of receiving interface s0.

If my memory is not playing games with me, i remember during my ccna study, ra does not advertises the update for a subnet which has different mask from the mask configured on the interface being used to advertise the route. For example 172.172.172.17 (loopback 1) has 28 prefix while so on ra has 30 prefix length.Since both are in same major net (172.172.0.0) but have different mask, ra should not have( according to my memory) advertised it. Is this true or i am wrong? or this behavior is ios dependent?

thanks a lot and have a nice weekend!

I have this problem too.
0 votes
Correct Answer by Joseph W. Doherty about 7 years 8 months ago

Oh, sure . . .

Instead of:

case# 2

ra s0---------------s0 rb

ra config:

loopback 1

172.172.172.17/28

s0 172.172.172.5/30

try:

case# 2

ra s0---------------s0 rb

ra config:

loopback 1

172.172.172.30/28

s0 172.172.172.5/30

Correct Answer by Joseph W. Doherty about 7 years 8 months ago

172.172.0.0/16 is the correct classful subnet for 172.172.172.17/28, but you're assuming, since you can't see it (except for 172.172.0.0/30 is subnetted,), rb is using the /30 mask from 172.172.172.6/30 for 172.172.172.17/28.

On ra try:

172.172.172.30/28 for loopback1. (This avoids 172.172.172.17 being valid in both /28 and /30.)

I suspect you still see

R 172.172.172.16 [120/1] via 172.172.172.5, 00:00:15, Serial0

but depending whether you can ping 172.172.172.30 from router rb, it should confirm the subnet as /28 or /30 (from rb).

[edit]

About a month ago, I was doing some lab testing with RIPv2 (since I've never used it). I had noticed, even with v2 inactive, it was passing classful subnets, such as your example knowing there's another subnet on rb, but I don't recall (or more likely noticed) the subnet mask restriction you do, but I wasn't allocating subnets the same way, either. Also I was using 2801s and 3750 with very late code. (I'm curious what your ping test might show, if you try my suggestion.)

Correct Answer by Giuseppe Larosa about 7 years 8 months ago

Hello Sarah,

rip version 1 supports FLSM = fixed subnet mask.

in your example the real problem is that RA will treat 172.172.172.16 as a /30.

So host 172.172.172.21 that is part of 172.172.172.16/28 cannot be reached by RA.

I'm not sure but I think that if the base subnet address can fit with the different subnet mask in the link between the two routers the subnet is advertised.

I mean if the link between RA and RB has a less speciific mask like

172.172.172.125 /25 can the subnet 172.172.172.4 be advertised with mask /25 ?

In this case or the subnet is not advertised because base address doesn't fit with the mask or it should be changed to 172.172.172.0 /25 but this would be a form or route summarization

I would suggest you to test this second case and to report what happens

Hope to help

Giuseppe

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Correct Answer
Giuseppe Larosa Sun, 03/15/2009 - 00:46

Hello Sarah,

rip version 1 supports FLSM = fixed subnet mask.

in your example the real problem is that RA will treat 172.172.172.16 as a /30.

So host 172.172.172.21 that is part of 172.172.172.16/28 cannot be reached by RA.

I'm not sure but I think that if the base subnet address can fit with the different subnet mask in the link between the two routers the subnet is advertised.

I mean if the link between RA and RB has a less speciific mask like

172.172.172.125 /25 can the subnet 172.172.172.4 be advertised with mask /25 ?

In this case or the subnet is not advertised because base address doesn't fit with the mask or it should be changed to 172.172.172.0 /25 but this would be a form or route summarization

I would suggest you to test this second case and to report what happens

Hope to help

Giuseppe

sarahr202 Sun, 03/15/2009 - 09:03

Thanks Giuseppe! I need some detail before i implement your suggested lab. i will quote you below:

"I mean if the link between RA and RB has a less speciific mask like

172.172.172.125 /25 can the subnet 172.172.172.4 be advertised with mask /25 ? "

Please elaborate on 172.172.172.4. what mask are we assuming here? are you assuming /30 for 172.172.172.4?

If you are assuming;

172.172.172.4/30 ,

range of valid ip: 172.172.172.5-172.172.172.6

172.172.172.125/25 ;

subnet number: 172.172.172.0

range of valid ip :172.172.172.1-172.172.172.126

Both ranges overlap (for example 172.172.172.5, 172.172.172.6 are also in the range of 172.172.172.0/25). The router will generate error and won't accept the ip address assignment

Thanks a lot!

Correct Answer
Joseph W. Doherty Sun, 03/15/2009 - 06:41

172.172.0.0/16 is the correct classful subnet for 172.172.172.17/28, but you're assuming, since you can't see it (except for 172.172.0.0/30 is subnetted,), rb is using the /30 mask from 172.172.172.6/30 for 172.172.172.17/28.

On ra try:

172.172.172.30/28 for loopback1. (This avoids 172.172.172.17 being valid in both /28 and /30.)

I suspect you still see

R 172.172.172.16 [120/1] via 172.172.172.5, 00:00:15, Serial0

but depending whether you can ping 172.172.172.30 from router rb, it should confirm the subnet as /28 or /30 (from rb).

[edit]

About a month ago, I was doing some lab testing with RIPv2 (since I've never used it). I had noticed, even with v2 inactive, it was passing classful subnets, such as your example knowing there's another subnet on rb, but I don't recall (or more likely noticed) the subnet mask restriction you do, but I wasn't allocating subnets the same way, either. Also I was using 2801s and 3750 with very late code. (I'm curious what your ping test might show, if you try my suggestion.)

sarahr202 Sun, 03/15/2009 - 09:15

Thanks Joseph.

The routing table will indicate what mask is being used for a certain route.

In my case, at rb routing table shows some things like this.

172.172.0.0 is subnettted /30, two subnets.

C 172.172.172.4 s0

R 172.172.172.16 [120/1] via 172.172.172.5 s0

===================

So we know what mask is used for 172.172.172.16. It is /30 as stated at the beginning of routing table.

Now the point if we could ping 172.172.172.16 from rb.

I think yes here is my reasons( i would run the test to see my theory matches).

ping 172.172.172.17 is issued on rb,

Rb using the subnet mask, matches the entry R 172.172.172.16 via 172.172.172.5

ra receives the ping and find a match 172.172.172.16/28 in its routing table.

Now return path:

This time destination address is 172.172.172.6 , ip address of so of rb,

Again the ra find a match and forwards the packet to rb.

==================================

thanks and have a nice day! I will come back shortly with my results.

Joseph W. Doherty Sun, 03/15/2009 - 09:26

Yes, I noticed (and commented) on 172.172.0.0/30, but neither subnet falls within 172.172.0.0/30. The question is, whether this implies both subnets are being treated as /30 or not. The question is, whether it treats both of the actual subnets as /30s.

If you ping 172.172.172.17 and it responds, since it also falls within a /30, we don't know whether the /28 is understood by rb. Again, that's why I suggested 172.172.172.30. (NB: any address higher in the /28 beyond the /30 will do for this test -- take note of Giuseppe's "in your example the real problem is that RA will treat 172.172.172.16 as a /30.

So host 172.172.172.21 that is part of 172.172.172.16/28 cannot be reached by RA." -- that's what we want to verify.)

Looking with interest upon your upcoming results.

sarahr202 Sun, 03/15/2009 - 10:07

Thanks joseph . But i am afraid i am at loss.

I have this setup

Two 2500 series routers.

ra s0--------------s0 rb

would you please give me the ip address and subnet mask , you want me to configure? May be by implementing i would better understand your point.

thanks and waiting.

badalam_nt Sun, 03/15/2009 - 10:51

Hi Sarah,

To quote you:

"Since both are in same major net (172.172.0.0) but have different mask, ra should not have( according to my memory) advertised it."

This is my understanding as well, so I'm really surprised that Ra advertised to Rb that route.

I have never seen this behaviour in neither of the tests I performed in GNS/Dynamips. I used 2600, 3700 and 7200 routers and for your example Ra never advertised any route to Rb.

What IOS are you using, maybe there is a bug there???

sarahr202 Sun, 03/15/2009 - 12:59

Hi Badalam!

Below is the output of show version:

Cisco Internetwork Operating System Software

IOS (tm) 2500 Software (C2500-I-L), Version 12.1(9), RELEASE SOFTWARE (fc1)

Copyright (c) 1986-2001 by cisco Systems, Inc.

Compiled Wed 13-Jun-01 15:12 by kellythw

Image text-base: 0x03041334, data-base: 0x00001000

ROM: System Bootstrap, Version 5.2(8a), RELEASE SOFTWARE

BOOTLDR: 3000 Bootstrap Software (IGS-RXBOOT), Version 10.2(8a), RELEASE SOFTWAR

E (fc1)

RouterA uptime is 3 minutes

System returned to ROM by power-on

System image file is "flash:c2500-i-l.121-9.bin"

cisco 2500 (68030) processor (revision D) with 16384K/2048K bytes of memory.

Processor board ID 03902165, with hardware revision 00000000

Bridging software.

X.25 software, Version 3.0.0.

1 Ethernet/IEEE 802.3 interface(s)

2 Serial network interface(s)

32K bytes of non-volatile configuration memory.

8192K bytes of processor board System flash (Read ONLY)

output omitted!

thanks a lot and have a nice weekend!

Correct Answer
Joseph W. Doherty Sun, 03/15/2009 - 11:46

Oh, sure . . .

Instead of:

case# 2

ra s0---------------s0 rb

ra config:

loopback 1

172.172.172.17/28

s0 172.172.172.5/30

try:

case# 2

ra s0---------------s0 rb

ra config:

loopback 1

172.172.172.30/28

s0 172.172.172.5/30

sarahr202 Sun, 03/15/2009 - 13:11

Hi Joseph!

here is the result:

Router a config:

interface Loopback1

ip address 172.172.172.30 255.255.255.240

!

!

interface Serial0

ip address 172.172.172.5 255.255.255.252

clockrate 56000

!

outer rip

version 1

network 172.172.0.0

============================

router b config:

nterface Serial0

ip address 172.172.172.6 255.255.255.252

!

outer rip

version 1

network 172.172.0.0

!==========================

routing table at rb:

outerB#show ip route

172.172.0.0/30 is subnetted, 1 subnets

C 172.172.172.4 is directly connected, Serial0

=================

This time ra did not advertise the subnet 172.172.172.16/28

My question is why?

thanks a lot!

sarahr202 Sun, 03/15/2009 - 13:45

Hi to all of you!

Let me apologize first for the oversight.

I rerun the lab.

First using ripv1.

ras0-------------s0rb

ra config:

so 172.172.172.5/30

loopback 1 172.172.172.17/28

rb config:

so 172.172.172.6/30

Both running ripv1.

The routing table at rb only shows one directly connected route to so.

When i run ripv2.

routing table shows at rb " R 172.172.172.16/28"

I must have been asleep first time when i ran this lab.

thanks a lot and have a nice day!

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