Routing and Data

[Sulaiman Ejaz]

Hi,

I want to know what is difference between routing and data transfer? Is routing means to just reach a packet from source to destination, or reply must come back? I mean if we receive a ICMP error then is this still possible that we've done routing?

regards,

Sulaiman ejaz

[daniel.dib]

I think what you are asking for is control plane vs data plane.

Control plane are all packets that are destined to the router itself. Such as ICMP packets to locally connected interfaces. Routing packets such as OSPF, EIGRP, ISIS, BGP etc. Also management traffic like Telnet, SSH and SNMP have to be processed by the device in CPU or whatever supervisor it has. Sometimes this is referred to as management plane.

The data plane is all traffic that is forwarded by the router, meaning that it goes through the router. Often called transit traffic. On some platforms this traffic is processed by hardware such as ASICs.

An ICMP packet not destined to the device itself will be sent as any other packet and be processed by hardware. If the ICMP packet is destined for the local device then it has to be processed by the CPU.

Routing is the process of receiving traffic on an interface, then looking at the destination IP and looking in the routing table and sending the packet out an interface. A packet can be routed even if there is no reply back. For example if someone pings an IP but that IP is not reachable or does not want to respond to ping.

Daniel Dib
CCIE #37149

Please rate helpful posts.

[Sulaiman Ejaz]

Thankyou daniel,

let say,

R1(se1/0)----(se1/0)R2(se1/1)------(se1/0)R3(se1/1)-------(se1/0)R4(se1/1)

now if we ping se1/0 of R4 from R1 and packet also reaches there but don't come back to R1. Is this mean we have completed our routing? or packet must come back to R1

[Reza Sharifi]

Hi,

In this case since you are pinging from R1 to R4, R1 will send an ICMP request and wait for an ICMP reply and if R1 does not get the reply, it will time out.  So, routing in not complete.

HTH

[daniel.dib]

I'm not sure what you are looking for here. Every router makes local decision, the packet was routed. If/when reply comes back then that packet is routed as well. It's not the routers job to keep state for the hosts unless it is doing something like NAT.

Daniel Dib
CCIE #37149

Please rate helpful posts.

[Jon Marshall · Accepted solution]

Sulaiman

Nowadays, as Daniel alluded to, routing is generally seen as the act of seleting the next hop for the packet to be sent to. The actual sending of the traffic is often referred to as forwarding.

But if we use the term routing to mean deriving the next hop and fowarding the packet to that next hop then again, as Daniel says, routing is usually done on a hop by hop basis ie. each hop being a L3 device. So in theory whether the packet actually gets to the destination or gets back to the source is largely irrelevant (although in practice it is obviously very important ). So as an example host1 (H1) wants to talk to web server (S1)

H1 -> R1 -> R2 -> R3 -> S1

1) R2 has no route to S1's subnet.   H1 sends the packet to R1 and R1 routes the packet to R2. R2 has no route for S1's subnet so it drops the packet. But the packet has still been routed at R1.

2) R3 has no route back to H1's subnet. The packet is sent from H1 to R1, routed to R2, routed to R3, to S1. S1 sends a return packet to R3. R3 has no route for H1's subnet so it drops the packet.  Again the packet has been routed, multiple times but the communication still failed.

3) All routers have the right routes. H1 sends a packet and it gets to S1, S1 sends it back and it gets to H1. Each router has routed the packet two times.

So you can see in all of the above routing takes place and it doesn't necessarily mean that the end to end communication was successful.

If you are having routing problems you often must look at each L3 device in the path to verify the path both ways ie. as you can see from 2) just because the routing works one way it does not necessarily means it works the other way as well.

Jon

[Sulaiman Ejaz]

Thankyou jon, that was very helpful