Cisco Routing Ip Default Network Vs Default Static Routes

Cisco Routing Ip Default Network Vs Default Static Routes



Cisco Routing: ip default-network vs .​
Default Static Routes
One point of​ confusion for​ some CCNA and​ CCNP candidates is​ the​ difference between configuring a​ static default route and​ using the​ Cisco routing command ip default-network.
At first glance, they appear to​ do the​ same thing .​
Both configure a​ destination to​ which packets should be routed if​ there is​ no more specific route in​ the​ routing table .​
The major difference between these two options is​ that configuring a​ static default route only defines a​ default route for​ the​ router you're configuring it​ on, while ip default-network will propagate the​ route via its routing protocol .​
Let's examine the​ routing tables of​ a​ hub-and-spoke network using the​ ip default-network command .​
R1 is​ the​ hub and​ R2 and​ R3 are the​ spokes .​
They are directly connected via the​ network 172.12.123.0 /24, and​ each has a​ loopback with a​ 32-bit mask that are numbered according to​ the​ router number (1.1.1.1, etc.) RIP is​ running on all three routers and​ the​ loopbacks are advertised.
R1 has another serial interface with the​ IP address 10.1.1.1 /24, and​ this network has been flagged as​ a​ default network with the​ command ip default-network 10.0.0.0 .​
It is​ not being advertised by RIP .​
The routing protocol will then advertise this route .​
With RIP, the​ default network is​ advertised as​ 0.0.0.0 .​
(With IGRP, it​ appears as​ the​ network number, but is​ marked as​ an​ IGRP External route .​
) This route has been designated a​ candidate default route on R1, as​ we see with the​ asterisk next to​ the​ 10.0.0.0 /24 network (code table removed for​ brevity):
R1#show ip route
Gateway of​ last resort is​ not set
1.0.0.0/32 is​ subnetted, 1 subnets
C 1.1.1.1 is​ directly connected, Loopback0
R 2.0.0.0/8 [120/1] via 172.12.123.2, 00:00:11, Serial0
R 3.0.0.0/8 [120/1] via 172.12.123.3, 00:00:11, Serial0
172.12.0.0/16 is​ variably subnetted, 2 subnets, 2 masks

C 172.12.21.0/30 is​ directly connected, BRI0
C 172.12.123.0/24 is​ directly connected, Serial0
* 10.0.0.0/24 is​ subnetted, 1 subnets
C 10.1.1.0 is​ directly connected, Serial1
On R2 and​ R3, a​ default RIP route is​ now seen (code tables again deleted):

R2#show ip route
Gateway of​ last resort is​ 172.12.123.1 to​ network 0.0.0.0
R 1.0.0.0/8 [120/1] via 172.12.123.1, 00:00:00, Serial0.213
2.0.0.0/32 is​ subnetted, 1 subnets
C 2.2.2.2 is​ directly connected, Loopback0
R 3.0.0.0/8 [120/2] via 172.12.123.1, 00:00:00, Serial0.213
172.12.0.0/16 is​ variably subnetted, 2 subnets, 2 masks
C 172.12.21.0/30 is​ directly connected, BRI0
C 172.12.123.0/24 is​ directly connected, Serial0.213
R* 0.0.0.0/0 [120/1] via 172.12.123.1, 00:00:00, Serial0.213
R3#show ip route
Gateway of​ last resort is​ 172.12.123.1 to​ network 0.0.0.0
R 1.0.0.0/8 [120/1] via 172.12.123.1, 00:00:27, Serial0.31
R 2.0.0.0/8 [120/2] via 172.12.123.1, 00:00:28, Serial0.31
3.0.0.0/32 is​ subnetted, 1 subnets
C 3.3.3.3 is​ directly connected, Loopback0
172.12.0.0/24 is​ subnetted, 1 subnets
C 172.12.123.0 is​ directly connected, Serial0.31
R* 0.0.0.0/0 [120/1] via 172.12.123.1, 00:00:28, Serial0.31
And the​ default route works, since we can ping 10.1.1.1 from both R2 and​ R3 .​
Since they have no other match in​ their routing tables, they use the​ default route .​
R2#ping 10.1.1.1
Type escape sequence to​ abort .​
Sending 5, 100-byte ICMP Echos to​ 10.1.1.1, timeout is​ 2 seconds:
!!!!!
Success rate is​ 100 percent (5/5), round-trip min/avg/max = 68/68/68 ms
R3#ping 10.1.1.1
Type escape sequence to​ abort .​
Sending 5, 100-byte ICMP Echos to​ 10.1.1.1, timeout is​ 2 seconds:
!!!!!
Success rate is​ 100 percent (5/5), round-trip min/avg/max = 68/68/68 ms
When deciding whether to​ use a​ default static route or​ a​ default network, keep in​ mind that if​ you want the​ routing protocol to​ propagate the​ default route, the​ ip default-network command will do that for​ you .​
But if​ you want only the​ local router to​ have the​ default route, a​ static IP route is​ the​ way to​ go.




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