Need explanation on router behaviour: RIPv1.

Answered Question
Apr 18th, 2007
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Equip: 3600 (core_ro) to 2600 (wg_ro_a). Serial connection HDLC, core_ro se1/0, 10.140.1.1 255.255.255.0 to wg_ro_a se0/0, 10.140.1.2 255.255.255.0. All defaults. Turn on RIPv1 and get following, expected result.


wg_ro_a:

10.0.0.0/24 is subnetted, 5 subnets

R 10.3.3.0 [120/2] via 10.140.1.1, 00:00:05, Serial0/0

R 10.1.1.0 [120/1] via 10.140.1.1, 00:00:05, Serial0/0

C 10.2.2.0 is directly connected, FastEthernet0/0

R 10.140.2.0 [120/1] via 10.140.1.1, 00:00:05, Serial0/0

C 10.140.1.0 is directly connected, Serial0/0


Then change subnet mask on wg_ro_a to 255.255.0.0 and get following results:

wg_ro_a:

10.0.0.0/8 is variably subnetted, 5 subnets, 3 masks

R 10.3.3.0/32 [120/2] via 10.140.1.1, 00:00:19, Serial0/0

R 10.1.1.0/32 [120/1] via 10.140.1.1, 00:00:19, Serial0/0

C 10.2.2.0/24 is directly connected, FastEthernet0/0

R 10.140.2.0/32 [120/1] via 10.140.1.1, 00:00:19, Serial0/0

C 10.140.0.0/16 is directly connected, Serial0/0


I was expecting 10.0.0.0/16 because of 255.255.0.0 mask? Why are routes learned from RIP showing /32 prefix? Need more info please ask. I have tried to find some docs on this, so if there are papers or books, please point me. Thanks.


PS. Then I tried mask of 255.0.0.0 on wg_ro_a to see result, and I get message "%10.0.0.0 overlaps with Fastethernet 0/0." The Fa0/0 port on wg_ro_a is 10.2.2.3 255.255.255.0.


Correct Answer by mohammedmahmoud about 10 years 1 week ago

Hi James,


It is the part "If the advertised network has a host bit set in the host portion of the update, Router 2 applies the host mask (/32)." above that applies for your situation for subnets "10.3.3.0" and "10.1.1.0" as after you've changed the subnetmask to /16 now the router sees that the network has a bit set in the host portion and thus the /32 issue.


HTH,

Mohammed Mahmoud.

Correct Answer by Richard Burts about 10 years 1 week ago

James


Yes a /32 is frequently referred to as a host mask, since it defines the address space as exactly one IP address.


HTH


Rick

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jamesd058 Wed, 04/18/2007 - 10:10
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The document link is great. I've already downloaded and read several. What I still don't understand is why the updates coming across the serial link from the core_ro into wg_ro_a are getting the prefix of /32. I can't recreate the link now as I am working on something else, but I'll try later.


Thanks.


PS. Do you have a link for docs on EIGRP and OSPF? Where would I start a search for such docs myself? I have difficulty finding things.

Richard Burts Wed, 04/18/2007 - 10:56
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James


The explanation is fairly simple. Using an example from your post:

R 10.3.3.0/32 [120/2] via 10.140.1.1, 00:00:19, Serial0/0

according to your post the interface was configured with /16. This means that the first two octets are network/subnet addressing and the last two octets are host addressing. Since the third octet was non-zero, RIP treats it as a host address and gives it /32.


HTH


Rick

mohammedmahmoud Wed, 04/18/2007 - 11:12
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Hi James,


I think that Rick has explained it nice and simple, as stated from the document above:


When RIP or IGRP receive an update, they perform certain checks before they accept the update and apply the subnet mask. This is the sequence of events that occurs before Router 2 accepts an update from Router 1:


* Is the subnet received in the update on the same major net as the interface that received the update?

o Yes: Router 2 applies the mask of the interface that received the update. If the advertised network has a host bit set in the host portion of the update, Router 2 applies the host mask (/32). In the case of RIP, it continues to advertise the /32 route to the subsequent router, but IGRP does not.

o No: Do any subnets of this major net already exist in the routing table, known from interfaces other than the one that received the update? The network in this update should be a major net unless the link between the two routers is an unnumbered link, in which case it is possible for the update to contain subnet information.



As for the OSPF and EIGRP documents:


http://www.cisco.com/en/US/tech/tk365/tk480/tsd_technology_support_sub-protocol_home.html

http://www.cisco.com/en/US/tech/tk365/tk207/tsd_technology_support_sub-protocol_home.html


And as a reference you would benefit a lot by having Jeff Doyle's books "Routing TCP/IP" Volume 1 and 2.


HTH,

Mohammed Mahmoud.


jamesd058 Wed, 04/18/2007 - 11:13
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Is the prefix /32 referred to as a Host Mask? In the document, Behavior of RIP and IGRP When Sending and Receiving Updates, under heading, Receive Updates, there is a reference to ...If the update came across an unnumbered link and contains subnet information, then Router 2 applies a Host Mask. Is this what you are talking about?


I'll need to read, Understanding and Configuring the ip unnumbered command.

Correct Answer
Richard Burts Wed, 04/18/2007 - 11:18
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James


Yes a /32 is frequently referred to as a host mask, since it defines the address space as exactly one IP address.


HTH


Rick

Correct Answer
mohammedmahmoud Wed, 04/18/2007 - 11:18
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Hi James,


It is the part "If the advertised network has a host bit set in the host portion of the update, Router 2 applies the host mask (/32)." above that applies for your situation for subnets "10.3.3.0" and "10.1.1.0" as after you've changed the subnetmask to /16 now the router sees that the network has a bit set in the host portion and thus the /32 issue.


HTH,

Mohammed Mahmoud.

jamesd058 Wed, 04/18/2007 - 12:16
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Got it! The step-by-step docs are great. Thanks both for all your input. Will look for recommended books. Now to do some live testing.

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