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classfull routing protocols

RIP-1 and IGRP are both classfull routing protocols, in that they send the subnet mask in the routing updates. If the router is incorrectly configured with different masks on different interfaces which mask is assumed?

i.e. Router has interfaces

Serial 0/0 172.29.1.0/24

Serial 0/1 172.29.2.0/24

Ethernet 0 10.1.0.0/16

and recieved RIP-1 updates through all three interfaces, what happens.

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Hall of Fame Super Silver

Re: classfull routing protocols

Mark

What would happen if interfaces had mismatching masks is that each interface believes that it is correct and the other one(s) is wrong. So if it receives advertisement of a subnet of the network that it is in, it will apply its mask to that subnet as it is put into the routing table. And it sends any updates for subnets within that classful network whose mask matches its own mask and drops any updates whose mask is different.

But there are a couple of things in your post that need comment. First your beginning sentence gets it backwards: RIP-1 and IGRP are classful because they do NOT send subnet mask in the routing update. Second since your serial interfaces are in class B network 172.29 and your Ethernet is in class A network 10 there is no mismatch of masks. For there to be a mismatch of masks which causes a problem the interfaces with different mask must be in the same classful network.

So in your example the router may learn various subnets of 10 through the Ethernet and learn various subnets of 172.29 through the serials. And with a classful protocol it will auto summarize and send only 10.0.0.0/8 out the serials and send only 172.19.0.0/16 out the Ethernet.

HTH

Rick

5 REPLIES
Hall of Fame Super Silver

Re: classfull routing protocols

Mark

What would happen if interfaces had mismatching masks is that each interface believes that it is correct and the other one(s) is wrong. So if it receives advertisement of a subnet of the network that it is in, it will apply its mask to that subnet as it is put into the routing table. And it sends any updates for subnets within that classful network whose mask matches its own mask and drops any updates whose mask is different.

But there are a couple of things in your post that need comment. First your beginning sentence gets it backwards: RIP-1 and IGRP are classful because they do NOT send subnet mask in the routing update. Second since your serial interfaces are in class B network 172.29 and your Ethernet is in class A network 10 there is no mismatch of masks. For there to be a mismatch of masks which causes a problem the interfaces with different mask must be in the same classful network.

So in your example the router may learn various subnets of 10 through the Ethernet and learn various subnets of 172.29 through the serials. And with a classful protocol it will auto summarize and send only 10.0.0.0/8 out the serials and send only 172.19.0.0/16 out the Ethernet.

HTH

Rick

New Member

Re: classfull routing protocols

Thanx, I thought I understood more than I do obviously. More reading required.

Hall of Fame Super Silver

Re: classfull routing protocols

Mark

Thanks for using the rating system to mark your question as resolved. (and thanks for the rating) It makes the forum more useful when people can read a question and can know that an answer provided resolved the question.

The forum is a good place to learn things, and a good place to try out our understanding, and sometimes a good place to realize that more reading is required. (I have done some of those myself) I encourage you to continue your study and to continue your participation in the forum.

HTH

Rick

New Member

Re: classfull routing protocols

Not a problem with the rating,

Here is a second attempt with an example, and my understanding of your response.

Router with three interfaces again, this time all serial ( not that it really matters )

Serial 0/0 172.29.1.1/26

Serial 0/1 172.29.1.65/26

Serial 0/2 172.29.1.129/25

Serial 0/2 recieves an adverstisment with network 172.29.2.0, believes it has a /25 mask. Route will be added to local table. As serial 0/0 and 0/1 have a different mask they will not propogate this route.

Serial 0/0 recieves an advertisment with network 172.29.3.0, believes it has a /26 mask. Route added to local table as before. Propogates this route out on Serial 0/1 as it has the same mask, but not Serial 0/2 as the mask is different.

Hall of Fame Super Silver

Re: classfull routing protocols

Mark

Your example and discussion are exactly right. :)

and you are right that the type of interface has nothing to do with it.

HTH

Rick

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