manual and auto route summarization

I am confused as to why and when you should use manual summarization? It appears that all routing protocols except OSPF use autosummarization by default. What i got from reading the ICND2 Cisco book is that the only time you need to use "no auto-summary" is for clasful routing protocols on discontiguous networks?

Can somebody explain this to me.

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ColbyG

You never really want to use auto-summarization. Modern networks do not live on classful boundaries.

As for manual summarization. This is used all the time in larger networks. Say you have a WAN with 30 sites. You give each site a /16, from the 10/8 space, which is then chopped up into several /24s (or smaller) for various VLANs at the site. There's no reason to advertise 11ty /24s into the WAN if the site is running a single, contiguous /16.

So 30 sites with a /16 per site, each site is using 30 /24s (conservative number considering how large a /16 really is). Without summarization, your WAN has 900 prefixes. If each site advertises only its /16, you have 30 prefixes.

mark076h

ColbyG wrote: »

You never really want to use auto-summarization. Modern networks do not live on classful boundaries.

As for manual summarization. This is used all the time in larger networks. Say you have a WAN with 30 sites. You give each site a /16, from the 10/8 space, which is then chopped up into several /24s (or smaller) for various VLANs at the site. There's no reason to advertise 11ty /24s into the WAN if the site is running a single, contiguous /16.

So 30 sites with a /16 per site, each site is using 30 /24s (conservative number considering how large a /16 really is). Without summarization, your WAN has 900 prefixes. If each site advertises only its /16, you have 30 prefixes.

Ok i think i got it now.

-autosummarization ALWAYS summarizes to the class A, B, or C boundary and never summarizes a group a subnets?

-To summarize a group of subnets within a classful network you must use manual summariztion.

I was confused because i did not catch in the Cisco ICND2 book that autosummarization ALWAYS summarizes to the classful boundary. I thought it would automatically summarize the group of subnets it knows about like you would want to do with manual summarization.

How come it would not automatically summarize a known group of subnets instead of the entire classful network? I figured it would be called something different like auto-classful-summarization instead of autosummarization. The way i read it in the Cisco book is that it just does manul summarization automatically for you.

So if an ISP or company is given a block from a classful network to use it would use manual summarization for all of their subnets to let the rest of the internet know?