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EIGRP Default Networks V.S. Static Default Gateway

Posted on 2006-06-15
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Last Modified: 2008-01-09
I am not getting something here and I hope you can help me...

Static routing:
Default network - ROuter says if I dont know where to send you i send you to this SPECIFIC ROUTER interface, soo...
ip route 0.0.0.0 0.0.0.0 192.168.1.1  says "send it all to 192.168.1.1" if I dont know where to go.  
That makes PERFECT sense to me....

EIGRP

ip default-network 192.168.1.0  
How the heck does it know which router on this network to send it to?  Is it selecting the first network route in the list, and using the router specifed there?
i.e. if i have two routes : IP ROute 192.168.1.0 255.255.255.120 192.168.1.1
                                     IP route 192.168.1.0 255.255.255.120 192.168.1.254
So in this case  it could select either depending  on whats avilable etc??

So if I have a network righ now that is, let's say:
IP route 0.0.0.0 0.0.0.0 10.0.0.1  (class A just for fun)
Now it would be:
ip default network 10.0.0.0.0 255.255.255.0


I think what really confuses me is where to put the default network statment. In other words you put a static Default route on every router.  Do I just need to put this on one router and all the others will pick it up?


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Question by:neoponder
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by:Danny_Larouche
ID: 16916728
They may have different metrics, so the one with lowest metric will have precedence. If you have static routes mixed with dynamic routes, static ones always have lower metric value.

default=0.0.0.0
Routers within your network doesnt need a default route, but the one acting as internet gateway does.



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Expert Comment

by:nikhil_ahluwalia
ID: 16916919
the default static route are cofigured on the end routers of the network so actually you only have one gateway possible

in the dyanamic eigrp routing you don't give default route you only advertise the networks that are directly connected to the router and than it dyanamically fills the routing table with the routes to those networks which are directly attached to the other routers which also have eigrp configured

and again if their can be more than one path between two routers the path with lowest metric is given preference although you can enable path sharing if  you want in which case the traffic would flow from both the paths
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Expert Comment

by:v_karthik
ID: 16916968
Read this carefully coz this the 'official' doc -
http://www.cisco.com/warp/public/105/default.html#ipnetwork

Now,
i.e. if i have two routes : IP ROute 192.168.1.0 255.255.255.120 192.168.1.1
                                     IP route 192.168.1.0 255.255.255.120 192.168.1.254
So in this case  it could select either depending  on whats avilable etc??


Usually if u have multiple routes, the one with the least metric is used. For a connected route, the metric is considered 0, so that its always picked. Static routes have 1 as the metric (someone correct me here if I'm wrong). So they are picked next. Apart from that, depending on what protocol u use, the metric is calculated based on different factors like bandwidth etc.  If more routes have exactly the same metric, the cisco routers can load balance between them. If I remember right, by default, 4 routes are load balanced. You can raise it up to 6 thru a command. Its not that the metric needs to be EXACTLY the same for load balancing to happen. You can set a 'variance', a factor that says, "if the difference in metric between 2 routes is less than a certain percentage, load balance between them".

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Author Comment

by:neoponder
ID: 16922065
But If I don't want to use static routes anywhere, or at least in a very limtied fashion, but I want all routers to have a default route so they can surf the net, How do that in EIGRP?

Right now I put a static default route on every router that says go "upstream" to the router that eventually ships it out to the the firewall.

How Do I get rid of those statics with EIGRP?

Here is my real world problem....
I need router A to fail over between the path to b and c, including the default route so that they get internet access....


sooo

          >------|RTRB|
RouterA                  |------ROuter D------->Firewall---------->Internet
          >------|RTRC|

So do I just need to put a "default network" statment in the router D?  right now routerD points to the firewall this way:  ip route 0.0.0.0 0.0.0.0 10.10.10.254  (meaning 10.10.10.254 is the firewall)


If I put "default network 10.10.10.0 255.255.255.0" in router D and remove statics everywhere, is that all i need?




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Accepted Solution

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Danny_Larouche earned 500 total points
ID: 16922556
>If I put "default network 10.10.10.0 255.255.255.0" in router D and remove statics everywhere, is that all i need?
>

Yes, you have default route on the routerD if all routers are using dynamic routing and your DHCP server advertise the routerD as the default gateway.

Do not confuse "default route" and "default network". Default network is 0.0.0.0 with a mask of 0.0.0.0.  
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Author Comment

by:neoponder
ID: 16923102
That is what I needed to know.  ON an intersting side note, router d is acutally a L3 3550 running EIGRP. I did the default network and it's working, accept all the sudden the switch is bogged down to the point where I cant barly get past the logon via telnet.

I did not expect that much of a performence hit.  
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Author Comment

by:neoponder
ID: 16954614
I still can't get it to propogate the route to the route out.  Shows up on the rotuer D as *10.blah blah blah

Oh well. I will figure it out one of these days.

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