EIGRP default network advertisement

Tom Looyer · ‎08-17-2013

Hi All,

I am having trouble with EIGRP default network advertisement.

I have entered the ip default-network 172.16.0.0 command on R1.

The route is marked as a cadidate default on R2 but gateway of last resort is not set. Why?

Also, the static route on R1 was added with the ip default-network 172.16.0.0 command. Why?

Complete running configs attached.

Thanks to all who help.

Reza Sharifi · ‎08-17-2013

Hi,

That is the correct behavior

Have a look:

Unlike the ip default-gateway command, you can use ip default-network when ip routing is enabled on the Cisco router. When you configure ip default-network the router considers routes to that network for installation as the gateway of last resort on the router.

For every network configured with ip default-network, if a router has a route to that network, that route is flagged as a candidate default route. This network diagram displays the routing table taken from router 2513:

Link to doc:

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094374.shtml#ipnetwork

HTH

Tom Looyer · ‎08-18-2013

Hi Reza,

Thanks for you reply, however I think you have misunderstood my question. I am more concerned about the behavior on R2.

R2 has marked 172.16.0.0/16 as a candidate default but it has NOT set it as the gateway of last resort. Why not?

The document you linked does not explain the expected behavior on routers receiving the route.

Thanks again,

Tom

Reza Sharifi · ‎08-18-2013

Hi Tom,

I replicated your config on a couple of switches and I can see the default is set correctly on r2.

maybe during copy and paste here to the post it was missed

can you post the entire output from sh ip router just like mine below?

have a look:

r2#sh ip rou
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 192.168.100.1 to network 172.16.0.0

D*   172.16.0.0/16
           [90/130816] via 192.168.100.1, 00:02:40, GigabitEthernet1/0/1
C    192.168.100.0/24 is directly connected, GigabitEthernet1/0/1
r2#

r1#sh ip ro
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 0.0.0.0 to network 172.16.0.0

*   172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks
C*      172.16.0.0/24 is directly connected, Loopback0
D*      172.16.0.0/16 is a summary, 00:04:23, Null0
C    192.168.1.0/24 is directly connected, Vlan1
C    192.168.100.0/24 is directly connected, GigabitEthernet1/0/2
r1#

HTH

Reza

Tom Looyer · ‎08-19-2013

Hi Reza,

Below is the complete route tables from R1 and R2.

Note the static route to 0.0.0.0/0 appeared when I entered the ip default-network command.

R1#show ip route

Codes: L - local, 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, H - NHRP, l - LISP

+ - replicated route, % - next hop override

Gateway of last resort is 0.0.0.0 to network 172.16.0.0

S* 0.0.0.0/0 [5/128256] via 0.0.0.0, Null0

* 172.16.0.0/16 is variably subnetted, 3 subnets, 3 masks

D* 172.16.0.0/16 is a summary, 00:51:12, Null0

C 172.16.0.0/24 is directly connected, Loopback1

L 172.16.0.1/32 is directly connected, Loopback1

192.168.100.0/24 is variably subnetted, 2 subnets, 2 masks

C 192.168.100.0/24 is directly connected, GigabitEthernet0/0

L 192.168.100.1/32 is directly connected, GigabitEthernet0/0

R2#show ip route

Codes: L - local, 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, H - NHRP, l - LISP

+ - replicated route, % - next hop override

Gateway of last resort is not set

D* 172.16.0.0/16

[90/130816] via 192.168.100.1, 00:51:36, GigabitEthernet0/0

192.168.100.0/24 is variably subnetted, 2 subnets, 2 masks

C 192.168.100.0/24 is directly connected, GigabitEthernet0/0

L 192.168.100.2/32 is directly connected, GigabitEthernet0/0

Peter Paluch · ‎08-19-2013

Reza, Tom,

I have also observed exactly the same behavior as Tom described in our labs during routine exercises with EIGRP. Networks were flagged as candidate default but they have not actually been used as default networks. Either there has been a regression in newer IOS versions that causes this behavior, or the support for default networks is silently being phased out. I have never found out exactly.

To be a little personal here, I will not miss the ip default-network at all - it is a mess, completely unsuitable for today's needs. But if the functionality is indeed being removed, it should have been documented, and I do not recall seeing information about this command and its funcionality being discontinued. It is most probably a bug - but a bug I personally like seeing.

Best regards,

Peter