Cisco Router Summarization

you know in life we get really busy sometimes and we have a lot of things to keep track of if somebody walked up to me and said hey Kees how would you like to keep track of these three things and that's it I'd say what's the catch it would be a lot easier to track just three things than everything else in a normal person's life however in routing we can offer that same option to routers instead of having to memorize thousands of routes we can have them memorized just a few routes and we do that with a concept called summarization in fact a default route is the ultimate summary that says you need to get a packet to anywhere you send it to me and I'll take care of it so in a summarised environment all we're doing is we're taking the option of instead of a router having to learn hundreds or dozens of routes or thousands of routes and reducing the number of routes they have to keep track of that's the whole purpose for summarization so now that we've identified why we would want to summarize routes let's take a look at the important questions of what routes to summarize and actually how to do it let's use this wind topology as a perfect example we've got several networks we've got this network here the twenty three dot one dot two dot one 28/29 bit network we've got the third one thirty 6/13 etwork the one forty network the 144 network and the one 48 network and I've conveniently put them right here so if we had a new router come online we could feed that new router all of those specific routes or data we could summarize it so let's take an example of r9 and we'll do exactly that so here is our nine he's been added to the mix he's got EAG ARP running and he's learned a whole bunch of routes from our one we can verify that by issuing a show IP route edge here Pinel shows just the eigrp learned routes that r1 is sharing now look at all these we've got a whole bunch of loopback interfaces now these these one 1 1 is the loopback interface of r1 the two 2 2 is the loopback interface of r2 so we know what those all are and over here we have all of our when networks now it's showing us that we have six seven one two three four five six we have one two three four five the extra network that's showing up is because of the point-to-point protocol being used on the LAN link between r1 and r2 so r1 has created a 32 bit host route for its good buddy r2 and that has been incorporated into EA GRP so here's all of our wine routes plus the extra peat point-to-point route as well if we want to summarize those it's so simple we could just go to r1 and say hey mister r1 instead of telling our nine about all of these networks just go ahead and tell them about one route that represents these networks that would be a summary now the question is what is the perfect summary for those and this is where it gets dicey for many so so check this out we could on r1 we could tell our one hey listen you tell r9 that you can get to twenty-three anything and that summary route would look the fact let's just do it we'll do it and we'll say we'll take a look at maybe why it wasn't a great idea so we'll go into configuration mode interface FA 0 slash 0 which is the interface that r1 is using to talk to our nine and we'll say IP summary address or AIG IP I thought is this number one we'll do 20 3.000 and the mass could be 20 to 50 5.000 now that my friends is a gross over summarization but it still is a summary now we go back to our nine who a moment ago had all of these routes in the 23 space check this out we'll do a show IP route now it's just one one summary route so r1 suppressed or didn't send all the detailed routes does it work absolutely our nine can ping let's ping 131 where my mouse is right here so we'll ping twenty-three dot one dot two dot 131 works like a great like a champ because r1 can forward the packet there and r5 because it's also running EAG ARP has a route back to this new network this between r9 and r1 that's great so what's the problem with this the problem is if our nine because he believes that our one can get to twenty-three anything tries to send the packet to twenty-three 99.99 dot fifty well that's not a route that our one has so with this gross over summarization that we just did the unfortunate part is we've summarized way way too much so let's take it off in little control a say no and now that's removed now the question is what would be the perfect summary to specify a more accurate summary where you're not advertising hey I can reach anything that would be a default route or in this case where our one was saying I can reach twenty three anything is too broad so how do we narrow down the scope of the summary the key is to find out what all these networks that our one really does know how to reach to reduce them down to a summary that correct these describes those routes so let's take a look at the networks here they are these five networks I know we have the the one additional network for the point-to-point link which is 146 and that's okay too and pick those five networks and say well 23 is too too broad to general 23.1 slash 16 is till to general 23 dot one dot two slash 24 is still too general we need to break down this last octet from each of these networks and find out what bits are common in all of them and use those common bits as part of our summary so here we've broken down 128 as 1 0 0 0 0 0 0 I break them down into noodles as well for me it's easier to visually see it if I put a little space between the first 4 bits and the last 4 bits of a given octet so these 8 bits represent 128 the last octet of this network right here here's 136 here's 140 here's 144 and here's 148 so we look at these what how many bits going from left to right do they have in common and the answer is the first three high order bits so what would that make our summary well that part's simple we just take the bits that are on and we add them up together and be 128 if we dig 128 plus zero plus zero and then we simply specify a slash 27 to represent how many bits match between all of those addresses so the summary that correctly describes all of these networks would be 23 12.1 28 which is representing the bits that are on from the three high order bits and this last 27 says I'm matching on the first / 27 bits out of 32 so our effective range if our least significant bit is 32 our next subnet would be the 160 subnet so our range that this summary is covering is 128 all the way through 159 and that would be an accurate summary or more accurate summary and check it out even includes the dot 146 that's in that range so let's go ahead and apply exactly that summary to r1 I'll bring up the console there and we'll simply hit the up arrow key and instead of saying 23000 we'll put in our perfect summary which is twenty three dot one dot two dot when we say 128 and with a twenty seven bit mask so that's 24 bits and three more bits would be 224 and that's it and I said no dating it elastic it I had to know in front of there so let's fix that there we go so there's our summary address and we go over to our nine now and do a show IP route four just eigrp there's our summary we aren't getting the five detailed routes because the summary is suppressing the detailed routes behind it now we've removed five routes from our nine actually six routes from our nine we reduce that down to one and we still have full reach ability again for the request and have a great great rest of the day you

Channel: Keith Barker

Views: 47,045

Keywords: Cisco, CCNA, IP, Address, summary route, summarization, VLSM

Id: YOpwKwol8xk

Length: 8min 55sec (535 seconds)

Published: Thu Apr 21 2011