Level 1 & Parent routes

Been trying to search for the definition for each and not having much luck. Anyone out there able to explain?

I know a Parent route is a Level 1 but Parents don't have exit interfaces and I've read that a Level 1 has an exit interface (Ultimate route). Kind of confusing.

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Comments

miller811

Whenever a route enters the routing table that contains a subnetted network address, a child route, a parent network route is automatically created. If there is already a parent network route in the routing table, the child route is placed below the parent. All subnetted routing entries will be installed within the parent (classful) route to guarantee proper route lookup

Parent Route: 172.16.0.0/24 is subnetted, 1 subnets
Child Route: C 172.16.3.0 is directly connected, FastEthernet0

cisco_trooper

Interesting...I don't recall coming across this terminology before...

APA

yeah I find this intriguing as well as the only time I've heard level-1 used in routing terms is when talking about IS-IS.

level-1
level-2
level-1-2

level-3 for Inter Domain Routing via IS-IS

Met44

It is mentioned in one lesson of the Network Academy's newest CCNA coursework. Since reading that one lesson around two years ago, I haven't heard this terminology used inside or outside of Academy materials.

To the OP, like you said, ultimate routes are routes with an exit interface. When you see a routing table with parent and child routes like this:

172.16.0.0/24 is subnetted, 2 subnets
C  172.16.1.0 is directly connected, fa0/0
C  172.16.2.0 is directly connected, fa0/1

The Level 1 parent is not an ultimate route, but the two Level 2 children are (since they have exit interfaces).

As far as I recall, a level 1 route with no children could be a Level 1 ultimate route, but would not be considered a parent since it does not have children routes. A Level 1 route with no children would not necessarily be an ultimate route, however, since you can set a static route to not have a specific exit interface, only a next hop IP addres.

networker050184

I was wondering what an ISIS question was doing in the CCNA forums too. Maybe I missed this, but this has to be some kind of newish terminology (or really old)?

billscott92787

Camit:

Welcome to the forums

Must be a newer terminology, especially if you guys haven't seen it before. I have and it is referred to outside of IS-IS because I learned about it when first learning about ip subnet-zero and no ip subnet-zero. I haven't reached IS-IS yet and I know what it is. What text did you see this in?

It is used to determine how the routing table is used depending on whether we are using classful (no ip subnet-zero) or classless routing (ip subnet-zero). Depending on which is in use depends on how the routing decisions are made.

MET44 posted the following, which I am going to use to explain this terminology:

172.16.0.0/24 is subnetted, 2 subnets
C 172.16.1.0 is directly connected, fa0/0
C 172.16.2.0 is directly connected, fa0/1

As in the example above when talking about the parent and child routes if classful networking is in use (no ip subnet-zero) when the routing table is inspecting, lets say a packet is destined for 172.16.3.0, the packet matches the entry in the routing table for the parent route 172.16.0.0 /24 so it goes and inspects the "child routes" it doesn't find a match from 172.16.1.0 or 172.16.2.0 so the packet would then be dropped, the rest of the routing table is not looked at during classful routing.

Now, if IP subnet-zero (Classless routing) is in use, then the packet would be compared to the routing table (Destination 172.16.3.0/24) it would match the parent route 172.16.0.0 /24 but it wouldn't find a match, so then it would go and inspect the rest of the routing table for any other matches (such as a default route) if it doesn't find any then the packets are dropped.

Remember that the CHILD ROUTES (in this case: 172.16.1.0, 172.16.2.0) are only going to be inspected if there is a match of the PARENT ROUTE (172.16.0.0/24). Don't think of level 1 as being a physical layer either because it isn't. I'm not sure why they use that terminology, I can see where it can be confusing, but what you need to understand it is in this post. This is what they are trying to get across to you. At the child routes, this is where you would find the exit interface to that particular destination.

I hope this helps.

mikej412

camit34 wrote: »

I've read that a Level 1 has an exit interface (Ultimate route).

How about mentioning your source so we have a clue what you're studying and the context of the statement.

camit34

Thanks for all the replies!

mikej412 wrote: »

How about mentioning your source so we have a clue what you're studying and the context of the statement.

Sorry about that, I am taking a CCNA course through the local community college where we are using the Cisco Netacad for curriculum (http://cisco.netacad.net). We are working through chapter 8 "Routing Tables" when a huge "but if this then this..." discussion occurred about this topic and it never really was resolved.

Here a few screen captures -