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Router On A Stick (ROAS), but with multiple routers

I've completed ROAS exercises in the past with 100% completion, PC's in different VLANs can communicate with each other, as is the goal with ROAS. When using Routers instead of PCs, the routers can ping ROAS, but not each other. My question is how come the PCs can ping each other, but the routers can't? I'll show the configuration, but it really won't provide much insight. Also I just want to know why they won't ping each other, not an explanation on how to fix it. E.G.> They can't ping each other because their routes stop, but PCs work because of their frames.

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Advocate

Re: Router On A Stick (ROAS), but with multiple routers

Hi @NicholasCastaneda7657 

 

Short answer:

Because PCs work differently than Routers.

 

In simple words:

When a PC sends a ping, it searches its ARP table and if it has a previous association between the destination IP and a MAC address, then it sends the ICMP packet (ping).
If the PC does not have that association in its ARP table, it sends ARP packets to know the MAC address of the next device.

On the other hand, the Routers, in addition to doing the same process as the PCs, look in their routing table if they have a route to the destination IP.
If they don't have a route to that destination IP, they discard the packet.

 

Regards

View solution in original post

3 REPLIES 3
VIP Advisor

Re: Router On A Stick (ROAS), but with multiple routers

how is your center switch config ?

 

Router 1 act as a Router on a stick :

 

1. R1 fa 0/0 connect to switch (configured and) act as trunk and allow VLAN 20, 30

2. Router2  face 0/0 - connect port on the switch should belong to VLAN 20

3. Router3 fas 0/0 connected port on the switch should belong to VLAN 30 to communicate with the gateway .

 

 

BB
*** Rate All Helpful Responses ***
Advocate

Re: Router On A Stick (ROAS), but with multiple routers

Hi @NicholasCastaneda7657 

 

Short answer:

Because PCs work differently than Routers.

 

In simple words:

When a PC sends a ping, it searches its ARP table and if it has a previous association between the destination IP and a MAC address, then it sends the ICMP packet (ping).
If the PC does not have that association in its ARP table, it sends ARP packets to know the MAC address of the next device.

On the other hand, the Routers, in addition to doing the same process as the PCs, look in their routing table if they have a route to the destination IP.
If they don't have a route to that destination IP, they discard the packet.

 

Regards

View solution in original post

Hall of Fame Guru

Re: Router On A Stick (ROAS), but with multiple routers

 

Just to add to Luis's explanation. 

 

On r2 and r3 you have configured a default gateway but the routers will not use this (unlike a PC) because they have routing enabled so they will consult their routing table only. 

 

If you want them to behave like a PC simply type "no ip routing" and leave in the default gateway and now you will be able to ping between them. 

 

Jon

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