EIGRP Default Route advertisement | network 0.0.0.0

[Americo Massotti]

Hello Everybody.

I would ask you regard this way to advertise a default route into EIGRP.

I've read it on some blog:

"

Option #2

The second method for injecting a default route into EIGRP is to configure a network command with 0.0.0.0. You must have the static default route configured; otherwise, with network 0.0.0.0, all existing and future directly connected interfaces will be advertised in the configured AS. The default route will be an internal EIGRP route.

R1(config-router)#do sh run | i ip route
ip route 0.0.0.0 0.0.0.0 Null0
R1(config-router)#no redistrib stat
R1(config-router)#netw 0.0.0.0
R1(config-router)#

"

http://www.ccieordie.com/eigrp-default-route-injection

I've read the same approach also on some books.

However I tried on EVE-NG and the result of "network 0.0.0.0" is to enable EIGRP on all the connected interface, not to only advertise the static default route (With Interface as next hop).

Why? It's correct? I miss something?

Have you a nice day.

Americo

[pman]

When we advertise network in eigrp with "network 0.0.0.0" It will enable EIGRP on all interfaces that have IP addresses falling into that network (0.0.0.0).

[Americo Massotti]

Pman hello.

First of all thanks your reply.

I'm sorry but my question it's different:

Suppose I configure:

router eigrp 100

network 10.1.1.0 0.0.0.255

!

ip route 10.1.1.0 255.255.255.0 ethernet0

Based on the article that I posted above:

"You must have the static default route configured; otherwise, with network 0.0.0.0, all existing and future directly connected interfaces will be advertised in the configured AS"

So, seem that the quote affirm that with a static default route configured "ip route 10.1.1.0 255.255.255.0 ethernet0" and the network 0.0.0.0 statement as above, the Router should only advertise the route without enabling the EIGRP process on all the existing directly connected interfaces.

What I mean as I stated before it's:

"However I tried on EVE-NG and the result of "network 0.0.0.0" is to enable EIGRP on all the connected interface, not to only advertise the static default route"

[pman]

if you want to advertise only the 0.0.0.0/0 as internal:

the next command causes only the default route to be advertised, all other routing updates are suppressed.

under the interface: "ip summary-address eigrp 100 0.0.0.0 0.0.0.0"

Hope I helped.

[Richard Burts]

Americo

The option suggested by @pman to use the summary-address to advertise a default route is, in fact, the option 3 suggested in the article that you reference. And it would work. But you are asking a question about something else. So let me try to address what you are asking.

Option 2 in that article is an unusual approach. What it does is to associate the default route with an interface (null0) and then to make EIGRP active on that interface. Using network 0.0.0.0 does make all the interfaces, the physical interfaces, and the virtual interfaces (both loopback and null interfaces) active in EIGRP. And it results in advertisement of the default route without needing redistribution of the default route.

Your concern is about whether all the interfaces are active. I will point out that in that article task 1 did explicitly make all of the interfaces on the router active in EIGRP. So having all interfaces active is really not an issue. I do not agree with this statement from that article:

"You must have the static default route configured; otherwise, with network 0.0.0.0, all existing and future directly connected interfaces will be advertised in the configured AS." The result of using network 0.0.0.0 is that all connected interface addresses will be advertised. And if you look in the routing table for R2 in that article it is learning all of the interface routes from R1.

The first part of that statement "You must have the static default route configured" is quite correct. But the reason it is required is not about whether all interface addresses are advertised or not. The reason it is required is that if you did not have the static default route configured then there would be no default route to advertise. 

[paul driver]

Hello

Best approach would be to use summary interface command but keep in mind this will be advertised as an eigrp internal route(admin distance 90) also a discard route will be installed on the rtr that is summarising the default

int x/x
ip summary-address eigrp x 0.0.0.0 0.0.0.0 

[Richard Burts]

Perhaps best approach is the summary command. Or perhaps it is redistributing a static default route. Certainly no one would suggest that network 0.0.0.0 was the best approach. But the article mentioned in the original post was not really about what is best. As with many things with a CCIE slant the question is not so much "what should it be" but is "what could it be". And network 0.0.0.0 is a viable way to accomplish advertising a default route. The original post is asking about implication of using this approach.

[Giuseppe Larosa]

Hello @paul driver ,

I agree that the per interface

ip summary-address eigrp <ASnumber> 0.0.0.0 0.0.0.0

can be used but there are some special cases and effects of this

EIGRP summary routes are classified as with AD 5.

The command above has an optional parameter that allows to change the AD of the route.

I had an issue several years ago where using the command without the optional parameter caused issues.

The default route was learned from a dynamic routing protocol probably IS-IS with AD 115.

The edge router was taking part in IS-IS and also in EIGRP that was used in a small block .

Using the command ip summary-address eigrp 100 0.0.0.0 0.0.0.0 made the edge routers to install the EIGRP summary route over the IS-IS default route.

The fix was to remove the command and to add it again with a final 200  ( actually a 200 AD).

In this way the IS-IS core default route was still installed and the EIGRP default route was sent to the edge block.

So in my opinion redistributing the default route in EIGRP is probably the safer way to do it.

But as noted by @Richard Burts here we are in the filed of lab options to prepare for a CCIE lab.

Hope to help

Giuseppe

[Richard Burts]

@Giuseppe Larosa thanks for your observations about possible issues in using the summary address command. And for acknowledging that the original post is more about what could be and less about what should be.