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How many switches are there in the path?

Hi All,

I just got this doubt. As we can see how many hops are packet is taking to reach the destination.

Do we have any command to see how many switches a frame is hopping to reach the destination ?

I'm practicing for my CCNA.

I was just playing with PT to practice some STP concepts, don't know from where i got this doubt.

My toplogy has 8 switches connected as i like and i connected two PC's to two of my switches. Now what i want to see is how the frame is travelling, i mean how many switches the frame is travelling ?

Regards,

Chandu

Regards, Chandu
1 ACCEPTED SOLUTION

Accepted Solutions
Cisco Employee

How many switches are there in the path?

Hi Chandu,

You are preparing very diligently for your CCNA - good luck!

There is no way of finding out how many switches are there in a row between a source and a destination. Unlike routers that decrease the TTL field in IP packet headers, optionally discarding the packet and sending an ICMP Unreachable message back to the sender in the process, switches are totally transparent devices. They do not send any frames on their own, they do not modify any part of a frame, and in fact, you can not even talk to them: your frames merely pass through them.

I know that Cisco switches send lots of frames on their own (STP, CDP, VTP, LOOP, Telnet/SSH/HTTP/SNMP and many more). However, these are control and management functions related to controlling the switches' operation, and they are not directly tied to the process of forwarding a frame - it can be done without them as well. There are cheap unmanaged switches that do not even have their own MAC address and are completely invisible in the network.

So the answer is - no, in general case, you do not know how many switches a frame has traversed. There can be management protocols that can find that out for you but those need to be specifically built into the switches just for this purpose (CDP and Ethernet OAM among others).

Best regards,

Peter

2 REPLIES
Cisco Employee

How many switches are there in the path?

Hi Chandu,

You are preparing very diligently for your CCNA - good luck!

There is no way of finding out how many switches are there in a row between a source and a destination. Unlike routers that decrease the TTL field in IP packet headers, optionally discarding the packet and sending an ICMP Unreachable message back to the sender in the process, switches are totally transparent devices. They do not send any frames on their own, they do not modify any part of a frame, and in fact, you can not even talk to them: your frames merely pass through them.

I know that Cisco switches send lots of frames on their own (STP, CDP, VTP, LOOP, Telnet/SSH/HTTP/SNMP and many more). However, these are control and management functions related to controlling the switches' operation, and they are not directly tied to the process of forwarding a frame - it can be done without them as well. There are cheap unmanaged switches that do not even have their own MAC address and are completely invisible in the network.

So the answer is - no, in general case, you do not know how many switches a frame has traversed. There can be management protocols that can find that out for you but those need to be specifically built into the switches just for this purpose (CDP and Ethernet OAM among others).

Best regards,

Peter

How many switches are there in the path?

Without writing a 10 page paper, here is what I would do.

Spanning Tree, basically creates a giant web, of all redundant paths in the switched network, an one path is loop free, because if all paths with available, you would have loops galore.

First you need to understand how your STP topology is (And this will be per vlan by default).

'show span vlan x'

This will show you if your local switch is the root, and you can follow the root port to the root switch for your specific vlan(s).

So if you have a host on SwitchA which needs to communicate with a host on SwitchZ, it will take the shortest path through the spanning tree topology for that specific vlan, to get to the host on SwitchZ, it won't necessarily have to go through the root switch.

Hope this helped some, feel free to ask more questions if you want.

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