IP Summarization

[czarg]

Hi,
please who has a layman knowledge on IP summarization on Routers? Kindly post a simple and helpful text on the subject. Thanks.

[PrntRhd]

No layman understands that stuff, it is a CCNA subject!

Here is an Examcram article on the subject:
http://www.examcram2.com/articles/article.asp?p=102351&seqNum=4

[Erik]

Layman summeraization?

You can tell a router all of these routes are over there, so if you see one of them don't even look at the exact address. :D

[czarg]

Thanks. I knew it but just needed a different opinion on the subject.
[PrntRhd]: I understand yr position. I am CCNA too. Layman here meant, 'Network guys lay-man'.

Thanks all the same guys.

[juniper]

Unfortunately, engineers find they have to describe their work to corporate and need a laymans term to describe why something needs to be done.

So in laymans terms summarization of routes (also referred to as aggregation) is combining common lower bit network routes into one summarized route advertisement to be sent to other routers apposed to advertising all the routes. This is used to lower the resources needed on the routers.

Since the CCNA shows you have basic networking skills we can talk network guy lay-mans terms and gets a little more technical ..

Lets look at RIPv2 for instance it sends a full routing table update every 30 seconds to its neighbors this might be OK for your network but what if we had 150 routes that needed advertised? Well RIP can only hold 25 routes in an advertisement and would have to send 6 update packets every 30 seconds to send its full route table. Now if we had 2 routers on the link each with 150 routes each you will consume alot of your links bandwidth just to push the routing information. you will also need enough memory to hold 300 routes per router as well it would have to sort though them so it can make its routing decisions. With summarization each router will only advertise the common higher order bits of the networks it wants to advertise so essentialy you could break this down to each router only sending one update. for example router A is the gateway router to reach networks 172.16.0.0/24 thru 172.16.150.0/24 and router B is the gateway to reach 192.168.0.0/24 thru 192.168.150.0/24 instead of router A sending all 150 routes to router B it can summarize the routes into one common high order bit route of 172.16.0.0/16 and router B can just send 192.168.0.0/16.

(A little more detailed you would actualy in reality only advertise 172.16.0.0/17 and 172.16.128/20 etc then individuals after to encompass only the 150 routes you have incase of /24 discontiguous networks from 151 thru 255 range)

Some other benifits to this involves BGP peering lets look at why this is just the greatest thing since sliced bread LOL! lets say ISP 1 has 1000 routes and ISP 2 has 1000 and ISP 3 has 1000 routes. now if you think about this, this means there are theoreticly 3000 links in these three networks which makes the odds of link failure happening quite often very good. Now if ISP 1 has a link failure and no summarization it would need to update not only its routers but ISP 2 and ISP 3 would need to update theirs as well causing a huge resync between all 3 ISPs at the same time. now think of a normal teir 1 ISP that has hundreds of peering sessions and thousands of routes and my goodness the internet would be in a constant recalculation.

Now if you ever setup summarization (sometimes on by default depending on protocol) you would see that it may create automagicly a route to null0 for your summarized route, what does this do? it keeps the route up even if a link goes down that is part of the summarization, as Null0 is a logical interface in the router also known as the black hole that is always up, it keeps the summarized route advertised all the time, If a link goes down again in ISP 1 the other ISPs will not update their routing tables since they only hear the always up summarized route and will still send to the summarizing router, once the summarizing router gets the packet it looks in its routing table for a route if there isnt a more specific route (the link is down) it sends the packet to null0 which gets dropped, if there is a more specific route (the link is up) it sends it on its merry way. basicly we just contained the route resync within the single ISP and we did not make our BGP peers mad LOL! The CIDR RFC's is a good read if you really want to get technical.. rfc's 1519 and 1817

This was just a round about answer and we can get more explicit if you like, the real big reason for this is CIDR which allows us to route at any given bit apposed to the no longer internet supported classful system.

Caveats - The routing protocol must be able to pass subnet info and Summarization hates discontiguous networks you will need policy routing or manipulate the administrative distance for routes that are discontiguous yet part of a summarization.