Comments on: confused with ‘ip default-network’ command http://www.networkingblog.in/confused-with-ip-default-network-command-10670 Cisco Netpro Blog Wed, 23 Jun 2010 06:07:14 +0000 hourly 1 http://wordpress.org/?v=3.0 By: karan http://www.networkingblog.in/confused-with-ip-default-network-command-10670/comment-page-1#comment-12034 karan Wed, 23 Jun 2010 06:07:14 +0000 http://www.networkingblog.in/?p=10670#comment-12034 >> What actually happens is, this doesn't happen when I have given 'no auto-summary' command given in R1 and R2. But if I again say, 'auto-summary' in R1 and R2, then in R3 I see the gateway of last resort set to R2. the ip default-network command requires to use a major network (Class A, B,C) and works by setting a flag in the internal route data structure that tells that this IP network can be used to send packets without a route for their destination. For this reason you need to use the network command under router eigrp in R1 (you have ip default-network on R1). From this derives an interaction with auto-summary settings: because your loopback emulating the default network has actually a subnet of the major network 1.0.0.0/8 when auto-summary is disabled major network 1.0.0.0/8 is not advertised to EIGRP neighbors, and you don't see the gateway of last resort set on R2 and R3. By enabling auto-summary the net 1.0.0.0/8 is advertised to R2 and the ip default network command becomes effective. if you had used ip default network 192.168.3.0 and you had configured ip address 192.168.3.1 255.255.255.0 (mask = major network class this is a Class C) you should be able to see ip default network in action even with auto-summary disabled (no auto-summary). 2) (this router) from the point of view of EIGRP process the default network is outside the EIGRP domain and this just means that: at least one route is placed in EIGRP routing domain in local node and it is the default network. >> What actually happens is, this doesn’t happen when I have given ‘no auto-summary’ command given in R1 and R2. But if I again say, ‘auto-summary’ in R1 and R2, then in R3 I see the gateway of last resort set to R2.

the ip default-network command requires to use a major network (Class A, B,C) and works by setting a flag in the internal route data structure that tells that this IP network can be used to send packets without a route for their destination.

For this reason you need to use the network command under router eigrp in R1 (you have ip default-network on R1).

From this derives an interaction with auto-summary settings:

because your loopback emulating the default network has actually a subnet of the major network 1.0.0.0/8 when auto-summary is disabled major network 1.0.0.0/8 is not advertised to EIGRP neighbors, and you don’t see the gateway of last resort set on R2 and R3.

By enabling auto-summary the net 1.0.0.0/8 is advertised to R2 and the ip default network command becomes effective.

if you had used ip default network 192.168.3.0 and you had configured ip address 192.168.3.1 255.255.255.0 (mask = major network class this is a Class C) you should be able to see ip default network in action even with auto-summary disabled (no auto-summary).
2) (this router) from the point of view of EIGRP process the default network is outside the EIGRP domain and this just means that: at least one route is placed in EIGRP routing domain in local node and it is the default network.

]]> By: Kumar http://www.networkingblog.in/confused-with-ip-default-network-command-10670/comment-page-1#comment-12033 Kumar Wed, 23 Jun 2010 06:06:49 +0000 http://www.networkingblog.in/?p=10670#comment-12033 I am not sure where the confusion is coming from. But if you look at all three routers you see the default summary route for network 1.0.0.0/8 under the "sh ip route" command. Also, you see the gateway of last resort has now been set. R1#sh run Building configuration... router eigrp 100 network 1.0.0.0 network 10.0.0.0 auto-summary no eigrp log-neighbor-changes ! ip classless ip default-network 1.0.0.0 ip http server ! R1#sh ip route Gateway of last resort is 0.0.0.0 to network 1.0.0.0 * 1.0.0.0/8 is variably subnetted, 2 subnets, 2 masks D* 1.0.0.0/8 is a summary, 01:09:23, Null0 +++++ R2#sh run Building configuration... R2#sh ip route Gateway of last resort is 10.10.10.1 to network 1.0.0.0 D* 1.0.0.0/8 [90/40640000] via 10.10.10.1, 01:11:53, Serial1 +++++ R3#sh run Building configuration... R3#sh ip route Gateway of last resort is 20.20.20.1 to network 1.0.0.0 D* 1.0.0.0/8 [90/41152000] via 20.20.20.1, 01:13:47, Serial2 +++++ Here are few words from a Cisco "pdf" regarding the "ip default-network" command. Maybe this will help as well. ip default-network 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. I am not sure where the confusion is coming from. But if you look at all three routers you see the default summary route for network 1.0.0.0/8 under the “sh ip route” command. Also, you see the gateway of last resort has now been set.

R1#sh run
Building configuration…

router eigrp 100
network 1.0.0.0
network 10.0.0.0
auto-summary
no eigrp log-neighbor-changes
!
ip classless
ip default-network 1.0.0.0
ip http server
!

R1#sh ip route

Gateway of last resort is 0.0.0.0 to network 1.0.0.0

* 1.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
D* 1.0.0.0/8 is a summary, 01:09:23, Null0

+++++
R2#sh run
Building configuration…

R2#sh ip route

Gateway of last resort is 10.10.10.1 to network 1.0.0.0

D* 1.0.0.0/8 [90/40640000] via 10.10.10.1, 01:11:53, Serial1

+++++
R3#sh run
Building configuration…

R3#sh ip route

Gateway of last resort is 20.20.20.1 to network 1.0.0.0

D* 1.0.0.0/8 [90/41152000] via 20.20.20.1, 01:13:47, Serial2

+++++

Here are few words from a Cisco “pdf” regarding the “ip default-network” command. Maybe this will help as well.

ip default-network
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.

]]> By: karan http://www.networkingblog.in/confused-with-ip-default-network-command-10670/comment-page-1#comment-12032 karan Wed, 23 Jun 2010 06:06:30 +0000 http://www.networkingblog.in/?p=10670#comment-12032 R3#sh run Building configuration... Current configuration : 1125 bytes ! version 12.1 no service single-slot-reload-enable service timestamps debug uptime service timestamps log uptime no service password-encryption ! hostname R3 ! ! ! ! ! ! ip subnet-zero no ip domain-lookup ! ! ! ! interface Loopback0 ip address 3.3.3.3 255.255.255.0 ! interface Serial2 bandwidth 64 ip address 20.20.20.2 255.255.255.0 clockrate 64000 ! router eigrp 100 network 3.0.0.0 network 20.0.0.0 no auto-summary no eigrp log-neighbor-changes ! ip classless ip http server ! end R3#sh ip route Gateway of last resort is 20.20.20.1 to network 1.0.0.0 D* 1.0.0.0/8 [90/41152000] via 20.20.20.1, 01:13:47, Serial2 D 2.0.0.0/8 [90/40640000] via 20.20.20.1, 01:13:47, Serial2 3.0.0.0/24 is subnetted, 1 subnets C 3.3.3.0 is directly connected, Loopback0 20.0.0.0/24 is subnetted, 1 subnets C 20.20.20.0 is directly connected, Serial2 D 10.0.0.0/8 [90/41024000] via 20.20.20.1, 01:13:47, Serial2 R3#sh ip protocols Routing Protocol is "eigrp 100" Outgoing update filter list for all interfaces is not set Incoming update filter list for all interfaces is not set Default networks flagged in outgoing updates Default networks accepted from incoming updates EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0 EIGRP maximum hopcount 100 EIGRP maximum metric variance 1 Redistributing: eigrp 100 Automatic network summarization is not in effect Maximum path: 4 Routing for Networks: 3.0.0.0 20.0.0.0 Routing Information Sources: Gateway Distance Last Update 20.20.20.1 90 01:14:33 Distance: internal 90 external 170 R3#sh run

Building configuration…

Current configuration : 1125 bytes

!

version 12.1

no service single-slot-reload-enable

service timestamps debug uptime

service timestamps log uptime

no service password-encryption

!

hostname R3

!

!

!

!

!

!

ip subnet-zero

no ip domain-lookup

!

!

!

!

interface Loopback0

ip address 3.3.3.3 255.255.255.0

!

interface Serial2

bandwidth 64

ip address 20.20.20.2 255.255.255.0

clockrate 64000

!

router eigrp 100

network 3.0.0.0

network 20.0.0.0

no auto-summary

no eigrp log-neighbor-changes

!

ip classless

ip http server

!

end

R3#sh ip route

Gateway of last resort is 20.20.20.1 to network 1.0.0.0

D* 1.0.0.0/8 [90/41152000] via 20.20.20.1, 01:13:47, Serial2

D 2.0.0.0/8 [90/40640000] via 20.20.20.1, 01:13:47, Serial2

3.0.0.0/24 is subnetted, 1 subnets

C 3.3.3.0 is directly connected, Loopback0

20.0.0.0/24 is subnetted, 1 subnets

C 20.20.20.0 is directly connected, Serial2

D 10.0.0.0/8 [90/41024000] via 20.20.20.1, 01:13:47, Serial2

R3#sh ip protocols

Routing Protocol is “eigrp 100″

Outgoing update filter list for all interfaces is not set

Incoming update filter list for all interfaces is not set

Default networks flagged in outgoing updates

Default networks accepted from incoming updates

EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0

EIGRP maximum hopcount 100

EIGRP maximum metric variance 1

Redistributing: eigrp 100

Automatic network summarization is not in effect

Maximum path: 4

Routing for Networks:

3.0.0.0

20.0.0.0

Routing Information Sources:

Gateway Distance Last Update

20.20.20.1 90 01:14:33

Distance: internal 90 external 170

]]> By: Kumar http://www.networkingblog.in/confused-with-ip-default-network-command-10670/comment-page-1#comment-12031 Kumar Wed, 23 Jun 2010 06:06:07 +0000 http://www.networkingblog.in/?p=10670#comment-12031 R2#sh run Building configuration... Current configuration : 1146 bytes ! version 12.1 no service single-slot-reload-enable service timestamps debug uptime service timestamps log uptime no service password-encryption ! hostname R2 ! ! ! ! ! ! ip subnet-zero no ip domain-lookup ! ! ! ! interface Loopback0 ip address 2.2.2.2 255.255.255.0 ! ! interface Serial1 bandwidth 64 ip address 10.10.10.2 255.255.255.0 clockrate 64000 ! interface Serial2 bandwidth 64 ip address 20.20.20.1 255.255.255.0 ! router eigrp 100 network 2.0.0.0 network 10.0.0.0 network 20.0.0.0 auto-summary no eigrp log-neighbor-changes ! ip classless ip http server ! end R2#sh ip route Gateway of last resort is 10.10.10.1 to network 1.0.0.0 D* 1.0.0.0/8 [90/40640000] via 10.10.10.1, 01:11:53, Serial1 2.0.0.0/8 is variably subnetted, 2 subnets, 2 masks D 2.0.0.0/8 is a summary, 01:11:58, Null0 C 2.2.2.0/24 is directly connected, Loopback0 3.0.0.0/24 is subnetted, 1 subnets D 3.3.3.0 [90/40640000] via 20.20.20.2, 01:11:50, Serial2 20.0.0.0/8 is variably subnetted, 2 subnets, 2 masks D 20.0.0.0/8 is a summary, 01:11:58, Null0 C 20.20.20.0/24 is directly connected, Serial2 10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks D 10.0.0.0/8 is a summary, 01:11:59, Null0 C 10.10.10.0/24 is directly connected, Serial1 R2#sh ip protocols Routing Protocol is "eigrp 100" Outgoing update filter list for all interfaces is not set Incoming update filter list for all interfaces is not set Default networks flagged in outgoing updates Default networks accepted from incoming updates EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0 EIGRP maximum hopcount 100 EIGRP maximum metric variance 1 Redistributing: eigrp 100 Automatic network summarization is in effect Automatic address summarization: 20.0.0.0/8 for Loopback0, Serial1 Summarizing with metric 40512000 10.0.0.0/8 for Loopback0, Serial2 Summarizing with metric 40512000 2.0.0.0/8 for Serial1, Serial2 Summarizing with metric 128256 Maximum path: 4 Routing for Networks: 2.0.0.0 10.0.0.0 20.0.0.0 Routing Information Sources: Gateway Distance Last Update (this router) 90 01:12:35 10.10.10.1 90 01:12:26 20.20.20.2 90 01:12:25 Distance: internal 90 external 170 R2#sh run

Building configuration…

Current configuration : 1146 bytes

!

version 12.1

no service single-slot-reload-enable

service timestamps debug uptime

service timestamps log uptime

no service password-encryption

!

hostname R2

!

!

!

!

!

!

ip subnet-zero

no ip domain-lookup

!

!

!

!

interface Loopback0

ip address 2.2.2.2 255.255.255.0

!

!

interface Serial1

bandwidth 64

ip address 10.10.10.2 255.255.255.0

clockrate 64000

!

interface Serial2

bandwidth 64

ip address 20.20.20.1 255.255.255.0

!

router eigrp 100

network 2.0.0.0

network 10.0.0.0

network 20.0.0.0

auto-summary

no eigrp log-neighbor-changes

!

ip classless

ip http server

!

end

R2#sh ip route

Gateway of last resort is 10.10.10.1 to network 1.0.0.0

D* 1.0.0.0/8 [90/40640000] via 10.10.10.1, 01:11:53, Serial1

2.0.0.0/8 is variably subnetted, 2 subnets, 2 masks

D 2.0.0.0/8 is a summary, 01:11:58, Null0

C 2.2.2.0/24 is directly connected, Loopback0

3.0.0.0/24 is subnetted, 1 subnets

D 3.3.3.0 [90/40640000] via 20.20.20.2, 01:11:50, Serial2

20.0.0.0/8 is variably subnetted, 2 subnets, 2 masks

D 20.0.0.0/8 is a summary, 01:11:58, Null0

C 20.20.20.0/24 is directly connected, Serial2

10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks

D 10.0.0.0/8 is a summary, 01:11:59, Null0

C 10.10.10.0/24 is directly connected, Serial1

R2#sh ip protocols

Routing Protocol is “eigrp 100″

Outgoing update filter list for all interfaces is not set

Incoming update filter list for all interfaces is not set

Default networks flagged in outgoing updates

Default networks accepted from incoming updates

EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0

EIGRP maximum hopcount 100

EIGRP maximum metric variance 1

Redistributing: eigrp 100

Automatic network summarization is in effect

Automatic address summarization:

20.0.0.0/8 for Loopback0, Serial1

Summarizing with metric 40512000

10.0.0.0/8 for Loopback0, Serial2

Summarizing with metric 40512000

2.0.0.0/8 for Serial1, Serial2

Summarizing with metric 128256

Maximum path: 4

Routing for Networks:

2.0.0.0

10.0.0.0

20.0.0.0

Routing Information Sources:

Gateway Distance Last Update

(this router) 90 01:12:35

10.10.10.1 90 01:12:26

20.20.20.2 90 01:12:25

Distance: internal 90 external 170

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