ip default-network command not advertising 0.0.0.0 into network

clifford.gormley · ‎03-11-2012

Hello,

Here is the Cisco link I am using to test this: http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094374.shtml

NOTE: My subnets vary a little from this link.

Here is my ISP1 router.
interface Loopback0

ip address 198.10.1.1 255.255.255.0

!

interface Serial0/0

description to Corp

ip address 172.16.2.2 255.255.255.252

sho ip proto <-You can see that I am not running any routing protocols here

ISP1#sho ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

     172.16.0.0/30 is subnetted, 1 subnets
C       172.16.2.0 is directly connected, Serial0/0
C    198.10.1.0/24 is directly connected, Loopback0

Here is my ISP2 Router:

interface Loopback0

ip address 171.70.24.1 255.255.255.0

!

interface Serial0/0

ip address 172.17.2.2 255.255.255.252

sho ip proto <-You can see that I am not running any routing protocols here

ISP2#sho ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

     171.70.0.0/24 is subnetted, 1 subnets
C       171.70.24.0 is directly connected, Loopback0
     172.17.0.0/30 is subnetted, 1 subnets
C       172.17.2.0 is directly connected, Serial0/0

HERE IS MY CORP ROUTER:

Corp1#

sho ip route:

Gateway of last resort is 171.70.24.0 to network 171.70.0.0

* 171.70.0.0/16 is variably subnetted, 2 subnets, 2 masks <-ip default-network
S* 171.70.0.0/16 [1/0] via 171.70.24.0 <-ip default-network

S       171.70.24.0/24 [1/0] via 172.17.2.2
     172.17.0.0/30 is subnetted, 1 subnets
C       172.17.2.0 is directly connected, Serial2/1
     172.16.0.0/30 is subnetted, 1 subnets
C       172.16.2.0 is directly connected, Serial2/0
     10.0.0.0/24 is subnetted, 12 subnets
R       10.1.11.0 [120/1] via 10.1.5.2, 00:00:11, Serial0/3
R       10.1.10.0 [120/1] via 10.1.5.2, 00:00:12, Serial0/3
R       10.1.9.0 [120/1] via 10.1.4.2, 00:00:10, Serial0/2
R       10.1.8.0 [120/1] via 10.1.4.2, 00:00:10, Serial0/2
R       10.1.12.0 [120/2] via 10.1.5.2, 00:00:12, Serial0/3
C       10.1.3.0 is directly connected, Serial0/1
C       10.1.2.0 is directly connected, Serial0/0
C       10.1.1.0 is directly connected, FastEthernet1/0
R       10.1.7.0 [120/1] via 10.1.3.2, 00:00:04, Serial0/1
                 [120/1] via 10.1.2.2, 00:00:17, Serial0/0
R       10.1.6.0 [120/1] via 10.1.3.2, 00:00:04, Serial0/1
                 [120/1] via 10.1.2.2, 00:00:18, Serial0/0
C       10.1.5.0 is directly connected, Serial0/3
C       10.1.4.0 is directly connected, Serial0/2
S*   198.10.1.0/24 [1/0] via 172.16.2.2 <-ip default-network

router rip

redistribute static <-I redistributed the static routes, that point to the networks used in the ip default-network commands, into RIP to have them propogate through the network, which works.

network 10.0.0.0 <-the rest of my network uses 10.x.x.x subnets.

Unfortunately, I AM PROPOGATING THE ACTUAL NETWORKS of 198.10.1.0 and 171.70.0.0, AS OPPOSED TO 0.0.0.0. 0.0.0.0. ISN'T THE 0.0.0.0 0.0.0.0 NETWORK SUPPOSED TO BE PROPOGATED TO THE DOWNSTREAM RIP ROUTERS? As you can see in a downstream rip router, the actual subnet is being received and is entered into the routing table:

ip default-network 198.10.1.0

ip default-network 171.70.0.0

ip forward-protocol nd

ip route 10.1.6.0 255.255.255.0 10.1.2.2 150

ip route 10.1.6.0 255.255.255.0 10.1.3.2 151

ip route 10.1.7.0 255.255.255.0 10.1.3.2 150

ip route 10.1.7.0 255.255.255.0 10.1.2.2 151

ip route 10.1.8.0 255.255.255.0 10.1.4.2 150

ip route 10.1.9.0 255.255.255.0 10.1.4.2 150

ip route 10.1.10.0 255.255.255.0 10.1.5.2 150

ip route 10.1.11.0 255.255.255.0 10.1.5.2 150

ip route 171.70.0.0 255.255.0.0 171.70.24.0

ip route 171.70.24.0 255.255.255.0 172.17.2.2

ip route 198.10.1.0 255.255.255.0 172.16.2.2

!

DOWNSTREAM RIP ROUTER:
Gateway of last resort is not set

R    171.70.0.0/16 [120/2] via 10.1.11.1, 00:00:08, Ethernet1/0 <-Shouldnt I have 0.0.0.0 0.0.0.0 instead?
     10.0.0.0/24 is subnetted, 12 subnets
C       10.1.11.0 is directly connected, Ethernet1/0
R       10.1.10.0 [120/1] via 10.1.11.1, 00:00:08, Ethernet1/0
R       10.1.9.0 [120/3] via 10.1.11.1, 00:00:08, Ethernet1/0
R       10.1.8.0 [120/3] via 10.1.11.1, 00:00:08, Ethernet1/0
C       10.1.12.0 is directly connected, FastEthernet0/0
R       10.1.3.0 [120/2] via 10.1.11.1, 00:00:08, Ethernet1/0
R       10.1.2.0 [120/2] via 10.1.11.1, 00:00:09, Ethernet1/0
R       10.1.1.0 [120/2] via 10.1.11.1, 00:00:09, Ethernet1/0
R       10.1.7.0 [120/3] via 10.1.11.1, 00:00:09, Ethernet1/0
R       10.1.6.0 [120/3] via 10.1.11.1, 00:00:09, Ethernet1/0
R       10.1.5.0 [120/1] via 10.1.11.1, 00:00:09, Ethernet1/0
R       10.1.4.0 [120/2] via 10.1.11.1, 00:00:12, Ethernet1/0
R    198.10.1.0/24 [120/2] via 10.1.11.1, 00:00:13, Ethernet1/0<-Shouldnt I have 0.0.0.0 0.0.0.0 instead?

Thanks

milan.kulik · ‎03-12-2012

Hi,

"If the router has a directly connected interface onto the specified network, the dynamic routing protocols running on that router will generate (or source) a default route. For RIP, this is flagged as the pseudonetwork 0.0.0.0; for IGRP, it is the network itself, flagged as an exterior route."

Read

http://www.cisco.com/en/US/docs/ios/12_1/iproute/command/reference/1rdindep.html#wp1017818

You might also read the article you are referring to for details.

HTH,

Milan

Peter Paluch · ‎03-12-2012

Hi Milan,

"If the router has a directly connected interface onto the specified network, the dynamic routing protocols running on that router will generate (or source) a default route"

Hmmm, I know this is taken from the Command Reference guide to the ip default-network command - but it does not make sense. It does not make sense to refer to a directly connected network using the ip default-network command because it does not really say via which neighbor the packets should be routed (similar to a static network entry pointing out a multiaccess interface). The point of ip default-network was to specify a remote network that is already known and reachable via some particular neighbor - and to use the very same neighbor towards that network as a default route neighbor as well.

Even the document Clifford mentioned clearly shows that the RIP in particular should inject a default route as soon as there is a route flagged as candidate default in the router's routing table - and this is how I have known the RIP to behave. Interesting enough, on my 12.4T IOS, I can not force it to work. It is strongly possible that the support for ip default-network is slowly fading out.

Best regards,

Peter

milan.kulik · ‎03-12-2012

Hi Peter,

I remember some years ago I met ip default-network command in some config and it was not generating any 0.0.0.0/0 advertisements.

Looking to the command reference today I though I found the way how to generate them - even a strange one.

Running a quick test in my lab after your note, I agree:

I'm also not able to get my router to generate any 0.0.0.0/0 advertisement using the ip default-network command.

Looking to my IGP "bible" book (Cisco IP Routing: Packet Forwarding and Intra-domain Routing Protocols by Alex Zinin), I found under the "ip default-network netAddr" command explanantion:

"RIP, in turn, sends the default route (0.0.0.0/0) to its neighbors whenever a candidate-marked route is in the routing table."

So it should!!

I'm afraid you are correct and the ip default-network command is not working anymore.

Personally, I always preferred the ip route 0.0.0.0 0.0.0.0 .. command.

It looks much easier to understand to me.

BR,

Milan

Peter Paluch · ‎03-12-2012

Hi Milan,

Yeah, the entire ip default-network concept is overly obscure. To my best knowledge, its main purpose was to work around a basic inability of IGRP to advertise the network 0.0.0.0/0 because IGRP was unable to advertise networks with a prefix length of 0. I have never used it in a production network, nor have I ever recommended using it. It's an ugly quirk and should be avoided if possible.

I agree absolutely - the proper way to do default routing is via advertising the 0.0.0.0/0 prefix. In case of RIP, this network can be either redistributed or injected via the default-information originate command (the most proper way to do it).

Milan, thank you for testing this out!

Best regards,

Peter

clifford.gormley · ‎03-12-2012

Hello, Thankyoiu for the replies.

I am using ver 12.4(25c). C3640-IK9S-M throughout my network.

clifford.gormley · ‎03-12-2012

Hi Guys,

If I have a Corp Edge router connected to two ISPs, and the router image does not support BGP.

Can I load balance across two Static Routes (SR) 0.0.0.0 0.0.0.0 routes to the two ISPs? I think not.

If I cannot load balance the two SRs, then...

Can I create two static routes in the Corp router, and assign a different Administrative Distance (AD) to each one?
That way my thinking is that the SR 0.0.0.0 0.0.0.0 with the lower AD will appear in the routing table, and will use the ISP that one connects to.

If the the lower AD SR connection drops, then I am thinking that the higher AD SR will then be placed in the routing table of the Corp, thus pointing to the 2nd ISP.

milan.kulik · ‎03-13-2012

Hi,

IMHO, it should be possible to configure load balance across two Static Routes (SR) 0.0.0.0 0.0.0.0 to the two ISPs.

But you should also configure some kind of tracking and decrease the AD of the particular SR in a case the IPS connection would not be available.

And don't forget you would load-balance only the outgoing traffic this way.

The incoming traffic load-balancing would be a challenge in this case!

HTH,

Milan

clifford.gormley · ‎03-13-2012

Hi Milan,

Thank you for your answer.

It is my understanding that an SR 0 0 will be removed from the routing table if it loses connectivity to the ISP (unless you use the PERMANENT command).

Is this what you mean by 'tracking"; or is it something else?
Thank you!

Cliff