Route Summarization

Tommy2727

Allows routing protocols to advertise many networks
as one address. THE NETWORK ADDRESS USED TO ADVERTISE
THE SUMMARY ADDRESS IS ALWAYS THE FIRST NETWORK ADDRESS IN THE BLOCK.

Ugh! I's so confused - I have an example but I'm a little lost...ok a lot confused. Can someone dummy this down for me PLS.

192.168.16.0 through network 192.168.31.0

find the network address and mask used
to summarize these networks into one advertisement
Summary address in routing ? ?????????????????????

Ok I see the block size of 240 (third octet 16) thus the mask will be 255.255.240.0. But that's it..... the rest i can not graspt the concept of the purpose of summarization..

**The mask is going to be used for THE SUMMARY ADDRESS.... the octet
where we are summarizing. **

Any help is greatly appreciated.

notgoing2fail

Ok so I think the answer to this one is:

192.168.16.0 right?


Because it's 240, your increment is 16 so up to 192.168.31.0

If the question had 192.168.32.0 (as an option) as well, then your network would end up being 192.168.32.0 instead...because the 32 is the next highest bit order....

You have to do one of those and/or bit calculation things.....

I haven't done it in awhile so hopefully someone can clear things up a bit more than me. LOL...

Tommy2727

That is correct - 192.168.16.0 is network. SO ultimately 192.168.16.0 and 255.255.240.0 will be used to advertise existing networks VIA this entry in the receiving routing table ??????

notgoing2fail

Tommy2727 wrote: »

That is correct - 192.168.16.0 is network. SO ultimately 192.168.16.0 and 255.255.240.0 will be used to advertise existing networks VIA this entry in the receiving routing table ??????

Yes anything else and the other routers don't care about. They will just send the packets your way and the receiving router takes care of the internal routing to the different subnets....

This is bad analogy, but I guess in a way, it's kinda like how BGP works but internally....

johnwest43

the purpose of the summarization is to consolidate routes in the routing table. Imagine if there were 16 subnets in that network: 192.168.16.0/24, 192.168.17.0/24 ....... all the way to 192.168.31.0/24. without summarization you would have a separate routing table entry for all 16 subnets. by summarizing you have 1 route in the table instead of 16.
I hope that helps!

fieldmonkey

Tommy2727 wrote: »

Ugh! I's so confused - I have an example but I'm a little lost...ok a lot confused. Can someone dummy this down for me PLS.

Tommy2727 wrote: »

192.168.16.0 through network 192.168.31.0

I'm going to attempt to help clarify this, but someone should probably confirm my sanity, also I think others might have given you a much shorter explanation.

I gained a better understanding of route summarization by additionally understanding the definitions of Contiguous and Discontiguous networks.

Contiguous Network -- A classful network in which packets sent between every pair of subnets can pass only through subnets of that same classful network, without having to pass through subnets of any other classful network.

Discontiguous Network -- A classful network in which packets sent between at least one pair of subnets of a different classful network.


1100 0000 1001 0000 0001 0000 0000 0000 = 192.168.16.0
1100 0000 1001 0000 000111110000 0000 = 192.168.31.0

1111 1111 1111 1111 1111 0000 0000 0000 = 255.255.240.0


Though your block size is 16 it can be further sliced up into other subnets (using longer or shorter prefixes). When the network is broken into smaller chunks, each route would need to be listed as a route in the routing table, so that a packet can find its way to a specific network.

So if you have a network of 192.168.16.0 255.255.255.0 listed in your routing table exclusively a packet destined for 192.168.16.53 would find it’s way, but a packet destined for any host from 192.168.17.1 – 192.168.31.254 it wouldn’t NOT make it home =(

Therefore you would have to list each sequential network using the network command in global configuration command to cover the each block if you used the mask 255.255.255.0

network 192.168.16.0 255.255.255.0
network 192.168.17.0 255.255.255.0
network 192.168.18.0 255.255.255.0
network 192.168.19.0 255.255.255.0

and so on … until …

network 192.168.31.0 255.255.255.0


By using the mask 255.255.240.0 (essentially VLSM) it would encompass all networks all host within that range by adding just one command network 192.168.16.0 255.255.240.0

This would also save on processing power, memory and administration.

I hope this helps and doesn’t muddy the waters… and as I started with … I will wait for someone to confirm my sanity because, I too am in the final stretch studying for ICND2.

fieldmonkey

fieldmonkey wrote: »

By using the mask 255.255.240.0 (essentially VLSM) it would encompass all networks all host within that range by adding just one command network 192.168.16.0 255.255.240.0

I just read up further on this and it appears I was not fully correct. One would need to issue the ip summary-address command in interface subcommand

Page 214 - ICND2

notgoing2fail

fieldmonkey wrote: »

I just read up further on this and it appears I was not fully correct. One would need to issue the ip summary-address command in interface subcommand

Page 214 - ICND2

if you're using RIPv2 or EIGRP, it auto-summarizes. So that can be a problem so on that note,

if you want to turn it off, you'll have to use: no auto-summary

Tommy2727

Ahhhhhhhhhhhhhhhh! Now that makes complete since. It was from the routing table aspects I wasn't grasping. Thanks a million !

Contiguous Network and Discontiguous Networks where my clarification was gray and black.... I desperately needed that guy's

thank you again.

Reibe

Tommy2727 wrote: »

Ahhh! Now that makes complete since. It was from the routing table aspects I wasn't grasping. Thanks a million !

One more question if you don't mind...... By default all routing protocols does not summarize routes thus this example. EIGRP does this I think but OSPF does not, it that correct? and can you summarize why.., ha ha I used summarize !!

If you're trying to ask if all routing protocols summarize by default, then the answer is no, some do and some don't. As for the why, it is just how the protocol is programmed. Does this help clear up that question, or did I misinterpret it?

tha_dub

fieldmonkey wrote: »

I just read up further on this and it appears I was not fully correct. One would need to issue the ip summary-address command in interface subcommand

Page 214 - ICND2

AHHHa

That was the piece I was missing there... I thought well it's fine and good to advertise the 192.168.16.0 network but how do the routers know that encompasses all subnets above the initial network.