Auto-summarizations, discontiguos networks

lildeezul

10.10.10.0

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172.16.10.0

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192.168.10.0

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10.1.1.0
[A] [C]

I believe i get the concept of this summarization, can someone tell me if i am on the correct path.
Thanks

Ok, i think i get it.
So, using contigious networks, like classfull networks, router A only send the 10.0.0.0 update to router B, and router B tells router C about the 10.0.0.0 network, but router C says i already have this network and its directly connected,
but in reality, router C has the 10.1.1.0 network, and Router A has a 10.10.10.0 network, but since a classfull protocol was used Router A, and Router C will only send to classfull network such as 10.0.0.0, right???

if were using OSPF that supports discontigious networks by defualt. then Router A running OSPF will send updates to router B sayin " i know about network 10.10.10.0"
as oposed to 10.0.0.0

Am i correct.??? i belive i understand this.
Also for use of support of discontigous network in EIGRP, then i would have to issue the no summary command. Correct.?????

contigious network dont work in class C address do they??

only in class B and A am i correct??

mikearama

The autosummarization in your scenerio happens on router B, and using a classful routing protocol, will indeed give you problems.

Router B will summarize the 10.10.10.0 network from A to 10.0.0.0 (sending it in updates to Router C), and will also summarize the 10.1.1.0 network from C to 10.0.0.0 (sending that update to Router A).

Now picture Router B's routing table... it will have two paths to the 10.0.0.0/8 network, and assuming they're both 100Mb links, will load balace using both, getting it wrong on average 50% of the time.

So you're right, you'd want to run no auto-summary with any auto-summarizing protocol to prevent B from doing the above.

Oh, and if you're subnetting a class C address into smaller chunks (ie, /25, or /30, or /32), these can be summarized to a class C summary address as well.

Mike

darkuser

nice answer

now can you fix my discontigous brain ?

mikearama

Hey, I get that too.

In my case, my mouth (or typing fingers) are often discontiguous with my brain. Brilliant post ideas become mindless drivel.

Course, the mushrooms don't help! (See? Why the hell did I type that!?!?)

ITdude

darkuser wrote:

now can you fix my discontigous brain ?

I know that feeling!