Default network

Hi,
Just finishing off my notes on eigrp for CCNP route, and something simple is bothering me, but I can get it too click in my head!

If a R1 has ip default network set on it , to let's say 10.0.0.0, this will be advertised to its neighbours as the gateway of last resort right?

So if R2 receives packets where it doesn't know the destination , in my mind it has a route to this default network on R1.

So by logical reasoning, R2 would never query any neighbours if a route is lost, because it has this default network to route too? I know it doesn't do this, if a route is lost, it checks its feasible successor route and if that isn't available sends out a query... Can someone help me Make this click please..

Thank you

Find more posts tagged with

Comments

networker050184

You are confusing two different processes, the forwarding table and the EIGPR topology table. The EIGRP topology table is not trying to forward packets. It treats, for example, 10.0.0.0/8 as a different object than 10.0.0.0/24 even though both routes will match for the forwarding of a packets sake.

websponge

Thanks, but that's confused me even more. The default network will get advertised to R2 and it will put it in its topology table as a candidate default. So when a route becomes unavailble it will send it there?.

networker050184

Think of things in the topology table as A, B, C rather than routes. For instance if A is gone it looks for another A. It doesn't say well A fits into B so lets look at that to take As place.

The routing/forwarding table on the other hand does not behave this way. If your route to A is gone it will send it to B if it is a valid destination.

You need to understand the difference between control and forwarding planes.

websponge

I don't think I've asked the question correctly.. Apologies.
I understand that you can configure an ip route on R1, so if there are no routes on R1 it used the static default route, but with the default network command, this gets advertised and put into neighbouring routers topology table. The routing table will take the best routes from here, if a more specific route is not found it can use this advertised default network. Correct?

networker050184

So are you asking if the router will use the default route if a more specific routes is not in the routing table? The answer is yes.

websponge wrote: »

Thanks, but that's confused me even more. The default network will get advertised to R2 and it will put it in its topology table as a candidate default. So when a route becomes unavailble it will send it there?.

No, it will be put in the routing table as a candidate default.

websponge

Right...&#55357;&#56832;
I was getting confused, you have cleared that up for me. If a more specific route is not found it will use the known default. I was thinking the router looks at the candidate default before it actually looked at its known routes. I was also thinking if a route was lost it would just use the default. I assume the router would query its neighbours for a route, and if none are found, then it uses the default right?

Thankyou

networker050184

No, again you are confusing the control and forwarding planes. The EIGRP topology table (control plane) and queries are a separate process than the forwarding of packets (forwarding plane). The router is going to continue to forward packets using the forwarding table regardless of EIGRP and it's topology table.

websponge

I understand control plane and forwarding plane.

The default network is put in the routing table, I can see it when I type show ip route.... So explain when is this route ever used then please?

Dieg0M

websponge wrote: »

I understand control plane and forwarding plane.

The default network is put in the routing table, I can see it when I type show ip route.... So explain when is this route ever used then please?

The default network will be used (forwarding plane) while queries are done by EIGRP (control plane). If a more specific route is found, the route will be added in the EIGRP topology table (control plane). The control plane will feed the forwarding plane with what it needs to add to its forwarding table.

websponge

Dieg0M wrote: »

The default network will be used (forwarding plane) while queries are done by EIGRP (control plane). If a more specific route is found, the route will be added in the EIGRP topology table (control plane). The control plane will feed the forwarding plane with what it needs to add to its forwarding table.

This is what I meant when I asked the original question,
If a route enters the router and there is not a route for it, DUAL will run its queries (Control Plane) to find the route (Forwarding Plane) , if no routes are found, it uses the default network. This is what the Cisco website has told me.

Dieg0M

No, if a route enters the router and there is not route for it, DUAL will add it to its topology table. Queries are sent only when a route goes down and no Feasible Successor exist in its topology table. The routing table is part of the forwarding plane and EIGRP's topology table is part of the control plane, two different things. The forwarding plane carries out the commands of the control plane.

websponge

Dieg0M wrote: »

No, if a route enters the router and there is not route for it, DUAL will add it to its topology table. Queries are sent only when a route goes down and no Feasible Successor exist in its topology table. The routing table is part of the forwarding plane and EIGRP's topology table is part of the control plane, two different things. The forwarding plane carries out the commands of the control plane.

if a route enters the router, it does not add it to the topology table, the toplogy table is built from gathering the routes from its known neighbours.
I am blaming the way I have asked the question here, so apolgies for all your fustrations..

I understand totally about the control and forwarding plane, I understand what the topology and routing table are and what they do..

I built this on GNS3 last night, I pinged a destination address on a router that was advertising a default network, it was a succesful ping even though no routes were available in the routing table, so it answered my question, if a route is not in the routing table it will use the default network if one is configured.

networker050184

So what would be the point of having the default route if it didn't use it?

Dieg0M

websponge wrote: »

if a route enters the router, it does not add it to the topology table, the toplogy table is built from gathering the routes from its known neighbours.

Sorry I should of specified, if a route enters the router in a form of an update advertised by a neighbour that has a formed adjacency, it adds it to its topology table.

websponge

networker050184 wrote: »

So what would be the point of having the default route if it didn't use it?

Eh? It does use it, I just said that above... If a more specific route is not found, it will exit out the default network.. Are we speaking different languages lol

websponge

networker050184 wrote: »

So what would be the point of having the default route if it didn't use it?

It's a default network, not a default route. A default route only applies to the router that has that network. You are getting confused with the control plane and the forwarding plane

networker050184

The default network is used to generate a default route. When you look at your routing table you see the gateway of last resort is set towards this network you designate as the default network.

websponge

networker050184 wrote: »

The default network is used to generate a default route. When you look at your routing table you see the gateway of last resort is set towards this network you designate as the default network.

Thanks for your replies, makes sense now, appreciate your patience.

realdreams

IMO the default-network command should really be depreciated... It serves no purpose in today's network (the command is classful), only causes confusing and misconfiguration... The exam tests the arbitrary behavior of this command under different conditions.

fredrikjj

realdreams wrote: »

IMO the default-network command should really be depreciated... It serves no purpose in today's network (the command is classful), only causes confusing and misconfiguration... The exam tests the arbitrary behavior of this command under different conditions.

In the INE CCNP Route videos, you can see that the instructor is struggling with this command. It just doesn't behave the way you expect it to and I really don't get why EIGRP doesn't have a proper default route command like OSPF.