EIGRP default route question

Jackace

I have 3 routers connected in a daisy chain fashion and all 3 are running eigrp.

R1---R2---R3---INTERNET

I'm am going over the methods to inject a default route into EIGRP and all the methods worked as I expected except the default route and network command option.

On router 3 I have the following config and a route is distributed to R1 and R2.

router eigrp 1
 network 0.0.0.0
 no auto-summary
!
ip route 0.0.0.0 0.0.0.0 fastEthernet 0/1

If I change the default route to point to the next-hop ip address instead of fastEthernet 0/1 the default route is no longer distributed to R1 and R2.

ip route 0.0.0.0 0.0.0.0 1.1.1.2

Just wondering why that happens?

Monkerz

EIGRP will only advertise a static default route via the "Network 0.0.0.0" command when the route is pointing to an interface.

Don't know why this is, but I ran into the same problem a year or so ago. I think someone else brought this up in the forum back then.

Jackace

Yeah I just noticed it today when doing some testing. I was hoping someone had an explanation on why one type of default route works but not the other.

nerdydad

From my understanding of it; if you point it to the interface, EIGRP interprets that as the interface is part of the 0.0.0.0 network, if you point it to the next hop, well now the router knows the interface is not a part of 0.0.0.0, so the router doesn't know where that network actually is, so it will not advertise it unless you do a redistribute static.

txraider09

Cisco just wanted to complicate things....

Danielh22185

What if he creates default routes on all the routers pointing to their respective next hop interfaces? (<-- Seems like the long way...) However I know the easiest is to set it up as a static route and use the redistribute static to populate a "default" for the other routers.

networker050184

Think of it this way. When you activate a network command it scrapes the routing table for the connected route associated with that network and advertises it.

So when you do a 'ip route 0.0.0.0 0.0.0.0 10.1.1.2' for example the routing table looks like this.

S*   0.0.0.0/0 [1/0] via 10.1.1.2

AS you see there is no connected route to match a network command. Now if you put it as 'ip route 0.0.0.0 0.0.0.0 e0/0' the routing table looks like this.

S*   0.0.0.0/0 is directly connected, Ethernet0/0

Now you get a connected route that the newtork statement can match. It works for other static routes besides the default route as well. It also the reason you get PPP peer routes in your IGP.

Knowing how a router gets the information it uses is very important to troubleshooting.

deth1k

router treats any static route pointing to an interface as a connected route, therefore network command is able to inject it into the process, once next hop is specified this becomes an external (static AD of 1) which will have to be redistributed into EIGRP process.

Jackace

Thank you all for the explanation. It makes sense.

The quote below is so true, and the main reason I asked the question. I could have just memorized the answer and passed the test, but I like to make sure I know why something happens the way it does.

Knowing how a router gets the information it uses is very important to troubleshooting.