Autosummarization question

If you have 2 routers, Router A with a 172.16.10.0 255.255.255.0 lan and Router B with a 172.17.10.0 255.255.255.0 lan. The link joining the two together is 172.16.20.0 255.255.255.0. When you do a show ip route on the router A, shouldn't you see the 172.17.0.0 network as summarized?

If i switch the ip address from 172.17... to a 10.10.50.1, then router A shows that route as summarized.

Thanks

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Comments

fly351

jwills wrote: »

If you have 2 routers, Router A with a 172.16.10.0 255.255.255.0 lan and Router B with a 172.17.10.0 255.255.255.0 lan. The link joining the two together is 172.16.20.0 255.255.255.0. When you do a show ip route on the router A, shouldn't you see the 172.17.0.0 network as summarized?

If i switch the ip address from 172.17... to a 10.10.50.1, then router A shows that route as summarized.

Thanks

Nope. Auto summary will summarize at the classful boundaries.

So in order to make 172.17.x.x part of the current summarized routes, then you would have to change it to 172.16.x.x

btw... auto summary is pretty bad... Maybe some of the network engineers here can post their experiences for us

I threw what you had in to Visio real fast

jwills

Can you explain what you mean by "Auto summary will summarize at the classful boundaries."

Are you saying that in my previous example, that because the 10... was not in the class B like the other subnets, thats what triggered the autosummary?

Thanks

alan2308

jwills wrote: »

Can you explain what you mean by "Auto summary will summarize at the classful boundaries."

Are you saying that in my previous example, that because the 10... was not in the class B like the other subnets, thats what triggered the autosummary?

Thanks

Router A has the network 172.16.10.0/24 connected. Since this is a Class B address, autosummary will cause Router A to advertise 172.16.0.0/16 rather than 172.16.10.0/24. The classfull boundary is each /16 block.

Dilbert65

Good rule of thumb is the second command, for most routing protocols, should alwas be "no auto". If not you are asking for headaches.

tech-airman

jwills wrote: »

If you have 2 routers, Router A with a 172.16.10.0 255.255.255.0 lan and Router B with a 172.17.10.0 255.255.255.0 lan. The link joining the two together is 172.16.20.0 255.255.255.0. When you do a show ip route on the router A, shouldn't you see the 172.17.0.0 network as summarized?

If i switch the ip address from 172.17... to a 10.10.50.1, then router A shows that route as summarized.

Thanks

jwills,

Is this your network?

{172.16.10.0 255.255.255.0}(RouterA){172.16.20.0 255.255.255.0}(RouterB){172.17.10.0 255.255.255.0}

jwills

This is a network i created on a sim to experiment with autosummarization. I understand the concept of summarization. What i'm trying to understand is what "activates autosummarization in a classful ntwk". In Odoms ICND2 book p.219, he says, if a router has interfaces in networks 10.0.0.0 and 11.0.0.0 in a classful ntwk, the 10.0.0.0 interface will advertise a single route for 11.0.0.0 (autosummarize it). So wouldn't it be correct to say the same thing would happen if a router had an interface in the 172.16.0.0 ntwk and an interface in the 172.17.0.0 ntwk?

When i did a show ip route it did not show the 172.17.0.0 ntwk as summarized, which i thought it should have. (Remember its on a simulator) I just wanted to make sure i understood the autosummarization concept correctly.

Thanks

tech-airman

jwills wrote: »

This is a network i created on a sim to experiment with autosummarization. I understand the concept of summarization. What i'm trying to understand is what "activates autosummarization in a classful ntwk". In Odoms ICND2 book p.219, he says, if a router has interfaces in networks 10.0.0.0 and 11.0.0.0 in a classful ntwk, the 10.0.0.0 interface will advertise a single route for 11.0.0.0 (autosummarize it). So wouldn't it be correct to say the same thing would happen if a router had an interface in the 172.16.0.0 ntwk and an interface in the 172.17.0.0 ntwk?

What class of network address is a 172.x.x.x network?

jwills wrote: »

When i did a show ip route it did not show the 172.17.0.0 ntwk as summarized, which i thought it should have. (Remember its on a simulator) I just wanted to make sure i understood the autosummarization concept correctly.

Thanks

fly351

tech-airman wrote: »

What class of network address is a 172.x.x.x network?

I believe he is talking about the classful boundaries.

fly351

jwills wrote: »

This is a network i created on a sim to experiment with autosummarization. I understand the concept of summarization. What i'm trying to understand is what "activates autosummarization in a classful ntwk". In Odoms ICND2 book p.219, he says, if a router has interfaces in networks 10.0.0.0 and 11.0.0.0 in a classful ntwk, the 10.0.0.0 interface will advertise a single route for 11.0.0.0 (autosummarize it). So wouldn't it be correct to say the same thing would happen if a router had an interface in the 172.16.0.0 ntwk and an interface in the 172.17.0.0 ntwk?

When i did a show ip route it did not show the 172.17.0.0 ntwk as summarized, which i thought it should have. (Remember its on a simulator) I just wanted to make sure i understood the autosummarization concept correctly.

Thanks

I just pulled out that book.. pg 219's explanation with the 10.0.0.0 and 11.0.0.0 is pretty much just wrong.

btw, here is the page from the book:
http://books.google.com/books?id=jNN7kBkScm8C&pg=PA234&lpg=PA234&dq=%22in+other+words,+if+r3+has+interfaces+in+networks+10.0.0.0+and+11.0.0.0%22&source=bl&ots=2Xox0a7nsq&sig=wcoeg9xCeR6n7USf709Ni_nY_Zs&hl=en&ei=tVRATLtowqqUB4bVsPUN&sa=X&oi=book_result&ct=result&resnum=1&ved=0CBIQ6AEwAA#v=onepage&q=%22in%20other%20words%2C%20if%20r3%20has%20interfaces%20in%20networks%2010.0.0.0%20and%2011.0.0.0%22&f=false

TesseracT

The autosummarization breakdown for the 3 internal IP ranges:

10.0.0.0 /8
172.16.0.0 /16
192.168.0.0 /24

so for the 172.16.0.0 network you were using,

172.16.1.0
172.16.2.0
172.16.3.0
etc etc

would all be summarized back to the classful boundary, which is 172.16.0.0 255.255.0.0

The 172.17.0.0 is not within this classful boundary so would not be summarized

jwills

Can anyone recommend any good resources on autosummarization, since Odom's explanation that he gives is not correct?

Thanks

alan2308

fly351 wrote: »

I just pulled out that book.. pg 219's explanation with the 10.0.0.0 and 11.0.0.0 is pretty much just wrong.

btw, here is the page from the book:
CCNA ICND exam certification guide ... - Google Books

What is wrong with that explanation? I'm not seeing it.

mella060

So auto-summarization (even though it is on by default) will not occur if a router has all interfaces using the same major network with the same mask? Example: A router has S0/0/0 with 172.16.1.0/24 and S0/1/0 with 172.16.2.0/24 and Fa0/0 with 172.16.3.0/24. Auto summarization will not occur here?
That is correct, it will only happen when you cross a major network boundary regardless of your subnet mask so you have a subnet of 172.16.x.x and when it crosses another boundary that does not have 172.16 it will summarize it to 172.16.0.0/16

Auto summarization only happens when you cross a major network boundary or connect dis contiguous networks such as 172.16.x and 10.x or 192.168.1.x