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“Routing” without Routing Protocol?

(take into account that this isn’t a real-life case, it’s just a lab practice)

This is the case:

· I used part of the “CCNA 5 router scheme”.

· 1 Router Cisco 2514 (A) with “no ip routing” set.

· 1 Router Cisco 2501 (B) with “no ip routing” set.

· Both routers are connected through it’s serial interfaces (s0 for R2514 and s1 for R2501).

· R2514’s s0 interface is set as DCE and R2501’s s1 is set as DTE.

· Each router has its ethernet interfaces configured with different Class C IP addresses.

· R2514’s s0, IP 201.100.11.1/24.

· R2514’s e0, IP 192.5.5.1/24.

· R2501’s s1, IP 201.100.11.2/24.

· R2501’s e0, IP 219.17.100.1/24.

· R2501’s s1, IP 199.6.13.1/24.

The test: I make ping from R2514 (console) to R2501’s e0 or s1, and the ping is successful. How is this possible with “no ip routing” in both routers?

R2501 must be routing the ping in someway…WITHOUT ROUTING PROTOCOL ENABLED!

Do I understand correctly this situation? If so, how is exactly working router B with that ping from A?

Thanks a lot.

1 REPLY
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Re: “Routing” without Routing Protocol?

"A" pings IP address of Serial1 on "B":

"A" sends the ping for 201.100.11.2 out its Serial0 interface, because the destination of the ping is on the same network as one of "A"'s direct connections. "B" receives the ping on its Serial1 interface, looks at the source IP address, 201.100.11.1, and sends the reply back out Serial1 because that interface is on the same network also. "A" receives the ping reply on its Serial0; you have a successful ping.

Direct-connects work this way, whether IP routing is disabled or enabled: there's no need to route, if the source and destination are on the same IP network.

While this works for the routers themselves, however, it will NOT work for any computers on the Ethernet LANs here because IP routing on the routers is disabled. The routers will not take traffic received on one interface, and send it out on another interface. So it's not a very useful configuration.

"A" pings IP address of Ethernet0 on "B":

If IP routing is not enabled, then both "A" and "B" have no routing protocols to help them learn how to reach remote IP networks and subnets. ("Remote" here means the same thing as "not-directly-connected".) I am not 100% sure that static routes are also unavailable to help the routers find their way; I will have to check this in the lab over the next day or two to confirm. But assuming that static routes are NOT available, then the routers have to rely on the "ip default-gateway" command to process traffic for remote IP networks and subnets.

Perhaps your "A" has a default-gateway that points to the IP address on Serial1 of "B". Then, when "A" pings 219.17.100.1, "A" knows it is not directly connected to that network, so it sends the packet to its default-gateway for forwarding. "B" receives the ping on its Serial1, knows that the destination IP address of the ping is the same as an IP address assigned to one of its own interfaces. "B" decides to reply, looks at the ping's source IP address, 201.100.11.1, and sends the reply back out Serial1 because that interface is on the same network as the device which initiated the ping. "A" receives the ping reply on its Serial0; you have another successful ping.

Following this logic, a successful ping from Serial1 of "B" to 192.5.5.1 to Ethernet0 on "A" would require that the default-gateway on "B" points to the IP address on Serial0 of "A".

This is an interesting exercise in getting two routers to talk to each other without routing enabled. But it doesn't have a practical application, unless the intention is to become familiar with how routers can behave when they are not set up to route. Troubleshooting scenario: Users attached to one of the Ethernet LANs cannot reach servers or users on the other Ethernet LAN, but the routers can ping each other. First question I would ask is: "Is IP routing enabled?"

Hope this helps.

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