CCIE Topic: 1.2g Route Summarization

hey everyone i'm charles judd and in this video we're going to look at the 1.2 g topic of route summarization we're going to first take a quick review of how a summary route is calculated and then we'll look at how we can perform route summarization with eigrp ospf and bgp let's quickly review how we summarize a set of routes here we're working with four networks that we're going to look at summarizing 10.10.0.0 through 10.10.3.0 and the first thing we want to do is convert each of our network addresses to a binary value you can see a binary equivalent for each of these ip addresses in a table here what we want to do next is to look for the common bits in other words how many bits do these network addresses have that are exactly the same in this case we have 22 bits in common we have the first 16 bits plus an additional 6 bits for a total of 22 meaning that we can represent this entire list of networks with the address 10.10.0.0.22 or in other words that subnet mask would be 255.255.252.0 so with this in mind let's jump into a lab and see how we can use this first we'll look at this with eigrp here's the topology that we'll use and you can see we have three routers all within eigrp autonomous system 100 and r3 has loopback interfaces configured for the four 10.10 networks as we see listed here these are the same networks that we looked at in our summarization review so that summary address is going to be valid in this case we're going to use 10.10.0.0.22 so here on r1 let's say show ip route and you can see we have all four of those loopback networks advertised here from r2 if we jump over to r3 and we say show run pipe to section eigrp this is going to show us the current eigrp configuration that's in place and it's a really simple configuration we have the network 0.0.0.0 statement meaning that all of the active interfaces on this router will participate in eigrp also notice that there is no auto summary statement in this configuration and that's because by default auto summary for eigrp is disabled in ios that's something that was implemented in ios 15 and later so if you have an older version that you're running that might not be true for you but in the most recent ios versions auto summary is disabled by default if we go under global configuration mode and go under router eigrp 100 and we say auto hyphen summary we'll see our neighbor resynchronization message here come into the console so let's go over to r1 and let's again say show iprout this time we're seeing routes summarized as the 10.0.0.0.8 subnet it's still being advertised to us from r2 at 20.1.1.2 and if we ran this command on r2 we're going to see the exact same summary route in there as well now the best practice recommendation from cisco is to actually leave auto summarization disabled and to use manual summarization instead so let's do that let's go back to r3 and to remove that we can simply say no auto hyphen summary we're going to see our neighbor resync message if we go back to r1 and again say show iprout we're going to see all of those 10.10 networks listed in there again as we originally saw now let's go back to r3 and let's configure a manual summary route and in order to do that we do that at the interface level the relevant interface here on r3 is going to of course be gig zero slash zero so let's say interface gig zero slash zero and the command we want to use is ip summary hyphen address and if we look at contextual help you can see that we are able to use this for rip or eigrp in our case of course we want to use eigrp and contextual help is going to tell us that we should indicate the autonomous system number next which is 100 and we're going to follow that with the summary ip address in this case we can use the same address that we calculated already for this range of networks which is 10.10.0.0.22 we can use a network and a subnet mask or we can do that insider notation as i've done here and we again see those resync messages in the console if we go back to r1 now and again say show iprout and now we see all of our networks represented by our configured summary address if we go back to r3 and we say show iprout we will of course see all of those networks represented along with the summary address now one thing i'll point out here is that with our summary address we see this at the top the 10.10.0.0.22 it tells us this is a summary and that is associated with a null zero interface the null zero interface is a logical interface and that gets created anytime you create a summary address this is essentially a loop prevention mechanism so that if traffic arrives for an ip address that is within this summary range of 10.10.0.0.22 but that ip address doesn't actually exist or it's not in use the null zero interface is going to simply drop that traffic rather than sending the traffic back out and potentially creating a loop let's look at this from the perspective of ospf now ospf does not have an automatic summarization option as we see in eigrp also unlike eigrp where we can perform summarization on any router ospf summarization can only be performed on either an area border router an abr or an autonomous system boundary router otherwise known as an asbr the summarization command is different for each scenario depending on that router so let's first look at this from the perspective of an abr which in this case in our topology would be r2 that is sitting at the boundary of area 0 and area 1. so first on r1 let's say show ip route and we see routes to all of our networks on r3 those are all listed with the ia status code which you can see in our status code table is an ospf enter area route or in other words routes learned from an area other than the ospf area where we currently reside in let's also say show ipospf database summary and this is specifically going to display information about our summary lsas so you'll notice we see one for 10.10.0.1 10.10.1.1 10.10.2.1 and 10.10.3.1 and finally of course we see 30.1.1.0 so all of these are type 3 or summary lsas summary lsas are created by an area border router so these were all created by router 2. now an important distinction here that people confuse quite often is that type 3 lsas do not summarize routes they summarize the topology so here we're looking at our ospf database specifically those type 3 summary lsas and we see a type 3 lsa for each route prefix within other areas so this is going to change momentarily that's why i wanted to point that out let's go over to r2 and let's summarize our routes on this abr by going under router ospf 1 and first we want to say area and we want to call out a specific area in our case we want to say area 1 and we want to say range now we indicate the area that actually contains the subnets that we are summarizing and you can see those all reside in area one so that's very important we're choosing the area where the subnets exist and then our summary will be advertised into all other areas connected to this abr so the next thing we do is we put in a network and a mask so we want to say 10.10.0.0.255.255.2 and let's look at contextual help by the way some terminology i'll point out the actual routes that we are summarizing we refer to those as subordinate routes notice that in our contextual help options we can specify a cost if we want to do that if we don't specify a cost for the summary route then what ios is going to do is it's going to look at all of our subordinate routes and it's going to use the lowest cost for all of the routes included in that summarized route range or in other words the lowest cost of those subordinate routes and whatever that is ios is going to assign that cost to our summary route also notice we have the option to not advertise if we choose to do that and the default is to advertise so i'm just going to hit enter and now our abr is going to compare the range of addresses that we've just configured here with this range command it's going to compare that range with the routes for which it is creating type 3 lsas if at least one valid subnet exists within this range then the abr is going to advertise the summary address as a type 3 lsa over to r1 in this case when it's doing that the abr will not advertise type 3 lsas for those subordinate subnets any longer so again all of this hinges on the fact that a valid subordinate subnet actually exists if we don't have any valid active subnets then the abr is not going to advertise this summary address in our case we do have valid subordinate subnets so we're all set if we go back to r1 and we now say show iprout we're going to see a single enter area ospf route now for our summarized address and we see that here if we again look at our ospf database and we look at those type 3 lsa summaries this time we're only going to see two lsas one here for the 30.1.1.0 network and we see one for our summary network 10.10.0.0 with a netmask of slash 22. so now all of those subordinate type 3 lsas are missing and we only have a type 3 lsa for our summary route which is exactly what we would want and what we would expect to see still looking at ospf let's look at this now from the perspective of an autonomous system boundary router or an asbr here we have a slightly different command that we need to use and you can see that r2 is acting as the asbr between ospf and eigrp we have the exact same address scheme and the same networks that we're summarizing so here on r1 if we say show iprout we're going to see our ospf external type 2 routes listed here which of course are all four of our loopback networks on r3 again we're working from r2 which is our asbr so let's jump over there and let's go under router ospf 1 and a very simple command we say summary hyphen address if we look at contextual help we're going to follow that with an ip address and a subnet mask for our summary network which is 10.10.0.0 with our slash 22 subnet mask and we can hit enter there so you can see the summary command is slightly different we implement that a little bit differently for asbrs but it is really simple to do let's go back to r1 and let's again say show iprout and this time we see only a single ospf external type 2 route for the 10.10 network as we would expect to see one final option that i'll look at here is bgp summarization here is our topology again the same ip address scheme and summary address that we've been using with all of our options you can see that r1 is in autonomous system 65 100 while r2 and r3 are in autonomous system 65 200. we have those same 10.10 networks on r3 that we're summarizing over to r1 here on r1 let's take a look at our bgp table let's say show ip bgp and just as we would expect to see we see all four of our prefixes advertised to us coming from r2 and these of course originate on r3 these are our loopback networks that we've assigned earlier so now let's aggregate these routes and let's advertise these to our neighboring autonomous system we want to do that from r2 and that's because r2 is the edge router for autonomous system 65 200. so let's go under router bgp 65 200 and the keyword we want to use is aggregate hyphen address and we want to follow that with the summarized address that we calculated earlier again 10.10.0.0 22 subnet mask and we can simply hit enter now if we break out of here and we say show ipbgp to take a look at our bgp table on router 2 we're going to see our aggregate address listed here so that is being advertised into bgp let's jump over to r1 and verify that we see it from there as well we'll say show ipbgp and we do see our aggregate address or our summary address but we also see all four of our original prefixes being advertised as well so we can correct that with a variation of the aggregate address command back on r2 let's go under router bgp 65 200 and i'm going to arrow up to the same command this time if we look at contextual help one of the options that we have is summary hyphen only so that's going to filter out all of the prefix advertisements except our summary address so let's use that we'll put that in and hit enter and now if we jump back over to r1 and we again run show ipbgp to take a look at our bgp table on router one now we're going to see only our summarized address being advertised to r1 that's exactly what we would want to see we see 10.10.0.0.22 the next hop address of r2 now we can also use auto summarization just as we looked at with eigrp but again it's a best practice recommendation from cisco that we do manual summarization rather than using auto summarization so that's a look at several options for route summarization i hope you found this content useful and i want to thank you sincerely for watching

Channel: Charles Judd

Views: 804

Keywords: cisco, ccie, cisco enarsi, ccie enterprise infrastructure, cisco enarsi 300-410, cisco encor 350-401, ccie lab, my ccie journey, ccie training, ccie blueprint, section 1.2, routing concepts, 1.2 routing concepts, cisco routing, routing table, route maps, eigrp, ospf, bgp, route summarization, aggregate address

Id: H_8lNtVwMdU

Length: 16min 15sec (975 seconds)

Published: Thu Sep 10 2020