Route Summarization Help!

Christopher Dobkowski

Hi to everyone!

I'm currently studying for my CCNA certification exam . While doing so, I came across the Route Summarization topic.
I did sat down and studied it, and Google'd and Youtube'd, but I just couldn't understand some concepts.
Well, yeah I know you do route sum. to keep your routing tables clean and to save resources. Everything's smooth and cool.
But my question is, HOW to do this? I did subnetting and it was fairly easy (very easy I might even say), but supernetting seems like black magic to me.
Can anyone explain me or give me some links to the math behind supernetting and examples?

Thanks in advance!

Zartanasaurus

If you can subnet, you can summarize. You just move in the opposite direction.
So I'll give you an easy example. Summarize 172.16.0.0/24 - 172.16.3.0/24 using the most efficient summary address.

Convert the network addresses into binary.

10101100.00010000.00000000.00000000 = 172.16.0.0
10101100.00010000.00000001.00000000 = 172.16.1.0
10101100.00010000.00000010.00000000 = 172.16.2.0
10101100.00010000.00000011.00000000 = 172.16.3.0

Starting from the left, group all bits that are the same across all of the networks. Once ANY ONE OF THEM does not match the others, stop.

10101100.00010000.00000000.00000000 = 172.16.0.0
10101100.00010000.00000001.00000000 = 172.16.1.0
10101100.00010000.00000010.00000000 = 172.16.2.0
10101100.00010000.00000011.00000000 = 172.16.3.0

After bit 22, not all of the values are the same, IOW they have 22 consecutive bits in common.
Your summary address is 172.16.0.0/22

Christopher Dobkowski

So it's that easy?!
Oh my god
And the CIDR is then summarized mask right? :

Thank You very much Zartanasaurus! I appreciate your help!

Hondabuff

The faster way with no binary math is by block Sizes. Summarize 172.16.0.0/24 - 172.16.3.0/24
Block sizes to cover the networks 2,4,8,16,32,64,128,256

172.16.0.0
172.16.3.0

Takes a block size of 4 to cover 0-3.
If you associate the block size like counting on your fingers, 2=1 finger up, 4=2 fingers up, 8=3 fingers up,,you get the point.
A block size of 4 uses two fingers or 2 bits.
Always start with 8 and subtract the number of bits you counted 8-2=6
Now 172.16 uses 16 bits.
Now add your 6 to your 16, 16+6=22 or /22
Once you get the concept you can blow thru these type of questions with very little math and you are not writing down 1's and 0's
First time I saw this was like seeing Odoms magic number method for the first time.

Christopher Dobkowski

Mhmmm, this method seems legit and easy
Nice! Thank You!!

2URGSE

Route Summarization is a character of classless routing protocols. (RIP2, EIGRP, OSPF)

Let's say you have a router with 4 different LAN subnets (10.1.1.0, 10.1.2.0, 10.1.3.0, 10.1.4.0, /24 mask is used) and you're running EIGRP. This router would normall advertise 4 different routes to other routers for them to reach these local area networks.

Now think about your routing table (SHOW IP ROUTE), after this configuration:

ROUTER EIGRP 400

NETWORK 10.1.1.0
NETWORK 10.1.2.0
NETWORK 10.1.3.0
NETWORK 10.1.4.0

A neighboring router who is part of your EIGRP 400 "club", would see 4 different routes to these 4 subnets. Imagine what happens on a large network, where you have routing tables that are really large, the more enteries, the more processing is needed and more overhead occurs on the hardware in order to process packets and send them to where they need to be.

Now if you summerize the routes, your routing tables will be smaller, and you'll have 1 route to these 4 networks containing all the IP addresses needed.

On the outbound interface (let's say S0/0), you can configure this:

ip summary-address EIGRP 400 10.1.0.0 255.255.0.0

The neighboring router will then see this in its routing table:

D 10.1.0.0/16 [90/2684416], via .......

so now it can reach any of the 4 LAN subnets on the other router via 1 route.

Christopher Dobkowski

After 3 responses in total I fully understood the concept!
Now I'm reading Link-State protocols

Thank You all!

lantech

2URGSE wrote: »

Route Summarization is a character of classless routing protocols. (RIP2, EIGRP, OSPF)

Let's say you have a router with 4 different LAN subnets (10.1.1.0, 10.1.2.0, 10.1.3.0, 10.1.4.0, /24 mask is used) and you're running EIGRP. This router would normall advertise 4 different routes to other routers for them to reach these local area networks.

Now think about your routing table (SHOW IP ROUTE), after this configuration:

ROUTER EIGRP 400

NETWORK 10.1.1.0
NETWORK 10.1.2.0
NETWORK 10.1.3.0
NETWORK 10.1.4.0

A neighboring router who is part of your EIGRP 400 "club", would see 4 different routes to these 4 subnets. Imagine what happens on a large network, where you have routing tables that are really large, the more enteries, the more processing is needed and more overhead occurs on the hardware in order to process packets and send them to where they need to be.

Now if you summerize the routes, your routing tables will be smaller, and you'll have 1 route to these 4 networks containing all the IP addresses needed.

On the outbound interface (let's say S0/0), you can configure this:

ip summary-address EIGRP 400 10.1.0.0 255.255.0.0

The neighboring router will then see this in its routing table:

D 10.1.0.0/16 [90/2684416], via .......

so now it can reach any of the 4 LAN subnets on the other router via 1 route.

.
That would be one way to summarize the route but I don't think it's the most efficient way. Plus if you have a discontigious network the routing may encounter some issues.

I would think that summarizing it to 10.1.0.0/21 would be more efficient and reduce the chance of routing issues in a discontigous network.

atorven

I have a question that Lantech may have touched on, in production networks at what point do you summarize? If you only have a small network connected to other smaller network is there any point in summarizing? Because I don’t think a few routes will be that intensive on the router if they aren’t summarized or is it worth doing it for good practice?
Another question, let’s say you have a nicely planned network where most of the subnets can easily be summarized, going with 2URGSE’s example into 10.1.0.0/21, now let’s say you get another network 10.1.8.0 is it worth summarizing to 10.1.0.0/16? What about all the other networks that you’re covering that you don’t have?

RouteMyPacket

Christopher Dobkowski wrote: »

So it's that easy?!
Oh my god
And the CIDR is then summarized mask right? :

Thank You very much Zartanasaurus! I appreciate your help!

CIDR rules!

boredgamelad

atorven wrote: »

I have a question that Lantech may have touched on, in production networks at what point do you summarize? If you only have a small network connected to other smaller network is there any point in summarizing? Because I don’t think a few routes will be that intensive on the router if they aren’t summarized or is it worth doing it for good practice?
Another question, let’s say you have a nicely planned network where most of the subnets can easily be summarized, going with 2URGSE’s example into 10.1.0.0/21, now let’s say you get another network 10.1.8.0 is it worth summarizing to 10.1.0.0/16? What about all the other networks that you’re covering that you don’t have?

You're on the right track. You're right about not wanting to summarize more than you're using (10.1.0.0/16 is far too much). But here's a question for you: there is a way to summarize just those networks, but using a /16 is not quite it. What would be better?

atorven

Hey boredgamelad, I realize now that I gave a crappy example, a /20 could cover that new address but my question is, in real networks how much is too much when trying to summarize?