EIGRP default route

[vishalpatil86]

Hi,

I am trying to propagate eigrp default route

Please find attached topology and R2 configuration

But, the network 200.1.1.0 is propagated as /30 network.

Shouldn't it be advertised as /24?

please Advice.

Vish

[Masoud Pourshabanian]

edited

[Masoud Pourshabanian]

Hello,

"The two steps to perform when configuring default routing with IGRP follow:
1- Define a default network. IGRP does not recognize the address of 0.0.0.0 as a default route. The IGRP actually advertises a route as an       external network. To "flag" or define a network as external,  two things must happen. First, the route must be flagged by the ip default-network a.b.c.d command. Second, for the route to be advertised as external, the interface advertising the route must not be in the same major  class boundary as the default network.
2- Ensure that IP classless is enabled on the router".

I tried to advertis default route with this method in EIGRP, but apparently, it is not supported anymore. You can simply have a default route and redistribute it by  "redistribute static"  under router EIGRP.

Hope it helps,

Masoud

[vishalpatil86]

Hi Masoud,

I have "3700 Software (C3725-ADVENTERPRISEK9-M), Version 12.4(15)T7, RELEASE SOFTWARE (fc3)"

but, still I am getting classless default candidate

[Masoud Pourshabanian]

Hello Vish

Could you please share the output of "sh ip route" on your 3750 which is supposed to get default route (output from R1 or R3)?

[vishalpatil86]

R1 -

Gateway of last resort is 12.6.25.130 to network 200.1.1.0

     200.1.1.0/30 is subnetted, 1 subnets
D*      200.1.1.0 [90/20537600] via 12.6.25.130, 00:00:14, Serial0/0
     12.0.0.0/8 is variably subnetted, 9 subnets, 6 masks
D       12.6.26.0/23 [90/40640000] via 12.6.25.134, 00:00:14, Serial1/1
                     [90/21152000] via 12.6.25.130, 00:00:14, Serial0/0
D       12.6.24.0/30 [90/41024000] via 12.6.25.134, 00:00:18, Serial1/1
                     [90/21024000] via 12.6.25.130, 00:00:18, Serial0/0
C       12.6.30.0/23 is directly connected, Loopback2
C       12.6.28.0/25 is directly connected, Loopback0
D       12.6.28.0/22 is a summary, 00:00:16, Null0
C       12.6.29.0/24 is directly connected, Loopback1
C       12.6.25.128/30 is directly connected, Serial0/0
C       12.6.28.128/26 is directly connected, FastEthernet0/0
C       12.6.25.132/30 is directly connected, Serial1/1

R3 -

     200.1.1.0/30 is subnetted, 1 subnets
D*      200.1.1.0 [90/20537600] via 12.6.24.2, 00:01:10, Serial1/0
     12.0.0.0/8 is variably subnetted, 9 subnets, 6 masks
C       12.6.26.0/26 is directly connected, Loopback0
D       12.6.26.0/23 is a summary, 00:01:10, Null0
C       12.6.27.0/24 is directly connected, Loopback2
C       12.6.24.0/30 is directly connected, Serial1/0
D       12.6.28.0/22 [90/40640000] via 12.6.25.133, 00:01:10, Serial1/1
                     [90/21152000] via 12.6.24.2, 00:01:15, Serial1/0
C       12.6.26.64/26 is directly connected, FastEthernet0/0
C       12.6.26.128/25 is directly connected, Loopback1
D       12.6.25.128/30 [90/41024000] via 12.6.25.133, 00:01:18, Serial1/1
                       [90/21024000] via 12.6.24.2, 00:01:18, Serial1/0
C       12.6.25.132/30 is directly connected, Serial1/1

[Masoud Pourshabanian]

Hello Vish,

Thanks for your input. You are right. I messed up. I forgot to activate " no auto summary" under EIGRP.

I ran the test one more time and got the same result as yours.

I edited my posts. I apologize for causing confusion. 

In IGRP , Candidate route shoud be classful. In EIGRP, it does not have to.

Do you have any more concerns? Are you trying to test "IP default-network" to learn or you want to distribute default route because there is easier way to do that.

[vishalpatil86]

No need to apologise..

Actually, getting candidate default as /24 was one of my instructor's requirements.

So, I was wondering if I am doing anything wrong....

[Masoud Pourshabanian · Accepted solution]

No, you are not doing wrong, but redistribute in this way is not common in EIGRP.

IGRP does not recognize 0.0.0.0/0 so if you redistribute static, IGRP does not redistribute default route; however, EIGRP does.

In IGRP, you need to use IP default-network to advertise default route.

Do the same thing as I did. Put auto-summary under EIGRP. Other routers will receive the Candidate route with the subnet of 24, but it causes two problems in your network.

1- EIGRP does not advertise classless subnets.

2- Receiving subnet of /24 may cause unreachable areas in your network.

Masoud

[vishalpatil86]

got it!!

thank you...

[Masoud Pourshabanian]

Glad it helped.

[paul driver]

Hello

Looking at your config if int fa0/0 had a /24 subnet then this would allow the candidate default to be advertised, as these kind of defaults requre a classful address not a subnetted one.

@Masoud

"I tried to advertis default route with this method in EIGRP, but apparently, it is not supported anymore"

This is an incorrect statement - Candidate defaults can still be used in certain IOS but i guess not in the most recent trains, However given the other options available to implement a default I guess wasnt  used.as often.

res
Paul

[Masoud Pourshabanian]

Hello Paul.

Not to disrespect. Word is easy. My sentence in my previous comment shows that I was not %100 sure, but you seem you are completely sure. Please do not be direct if you do not provide any link or proof.

Vish,

First, I tested your scenario with IOS version 15.*. I did not get the result I expected, so I searched to find the reason and came across the link below. The same problem with IOS 15.

https://supportforums.cisco.com/discussion/11472151/propagation-ip-default-network-eigrp

Then I used another IOS to make sure that having a classful subnet for propagating default route is necessary or not. The reasult was not necessarily.

I Attached the result. R1 is connected to R2 by fa0/0.  Ip default-network is configured on R2 with the ip  201.1.1.1 255.255.255.252

As you see, last resort has been set and R1 is able to ping 192.168.2.1 using default route

Hope it helps,

Masoud

[paul driver]

Hello Masoud

Apologies if you took my last post as a point to proof you worng - It was no way my intention to do such a thing- I just pointed out that your comment regards eigrp candidate defaults not being support was an incorrect statement.

As for not providing any proof - Just for clarity I did

In my last post I mentioned the reason why VISH and now I see your example for a Candidate Default route didnt work was that the interfaces related to the  Candidate route was subnetted and not classful.

Here is what I mean:

int fa0/0
ip address 201.1.1.1 255.255.255.252 ( subnetted)  -- needs to 255.255.255.0

router eigrp 1
network 201.1.1.0

ip default-network 201.1.1.0  ( classful)



res

Paul

[Masoud Pourshabanian]

Thanks for your reply.

Please check the link in my previous post. It is a discussion about newer Cisco IOS and IP default-network. It can be a proof that my statement is not completely wrong.

As for classful subnets, please look at the picture attached to my previous post. My candidate Default route is working even though the candidate route is a classless IP.

It was my expriment in EIGRP. Candidate route in IGRP might need to be classful. I was not able to test it with IGRP since I could not find an IOS supporting IGRP. I will try again and update the post.

Masoud