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Routing Protocols

Aneek
Beginner
Beginner

Host L1 is connected to switch sw1, Switch S1 is connected t Switch S2, Switch S2 is connected to Router R1, Router R1 is connected Router R2, Router R2 is connected to Host L2. How will communication happen from Host H1 to Host H2?

Tell me the entire procedure from packet level. 

Pls, find the network diagram doc below. 

5 ACCEPTED SOLUTIONS

Accepted Solutions

Joseph W. Doherty
Hall of Fame Master Hall of Fame Master
Hall of Fame Master
"Entire" depends on how devices are configured and besides the (L3) packet level, there's also the (L2) frame level, but since you've asked only about the packet level, I'll exclude L2 considerations.

In brief, assuming host L1 has R1's, same network, IP as its gateway, it sends a packet to that gateway with host's L2 as the packet's destination address (it has its own IP as the source IP - that's how host L2 will know both the sender, and where to send any reply).

When R1 receives the packet, it (via some form of routing information) knows to send the packet to R2.

When R2 receives the packet, it knows host L2 is on a directly connected network, so it sends the packet to host L2.

Since you asked about "communication", as that normally implies two way data transfer, host L2 to host L1 is exactly alike, except in the reverse direction.

Again, there are L2 actions too. Also, I'm not addressing how L1 "knows" L2's address or how R1 and R2 "know" about not directly connected networks. Nor other higher levels protocols, such as perhaps using TCP (L4) to insure delivery of each packet and FTP or HTTP (L7) etc.

The switches, being L2 devices, don't deal with packets, beyond delivering them as part of a frame.

View solution in original post

"Entirely" explanations would run into "book" length, or at least a chapter or two. Highly recommend you do some Internet searches on these subjects - you should find much material. Perhaps qualify your searches with words like introduction or primer, etc. You can also often find free on-line courses, for basic networking information, too.

Cisco also has a "Learning Network", https://learningnetwork.cisco.com, and their "Academy", https://www.netacad.com, both are more oriented toward learning basic information.

That said, although I believe you can post questions on Cisco's Learning Network site, this community will usually answer specific questions, but often you'll probably find this community won't provide basic network "how it works" information, especially when it would require a very long answer. Or course, you can ask, but your query may go unanswered, or only be briefly answered. Again, your best bet is likely other sources for this information.

View solution in original post

Ah, I see you've two postings, different titles, asking much the same. I.e.: https://community.cisco.com/t5/routing/routing-protocols/m-p/4131063

At the packet level, switches are "invisible" to the host and router. As far a they are concerned, the host and router might be directly connected on the same physical wire. I.e. switch S2 does not send any message to R1 (or the converse).

View solution in original post

At the packet level, logically nothing. Again, L2 switches are "invisible" to packets (and packets are "invisible" to "true" L2 switches, beyond being "data" within the frame).

 

PS:

I also might add, between the host and router, even at L2, the switches are still logically "invisible".  However, L2 switches do "operate upon" frames entering them.  A "true" L2 (only) switch wouldn't "change" the frame, although it might drop/discard it for various reasons.  Smart/enhanced/VLAN switches, might do other "things" to the frame.

View solution in original post

Martin L
VIP Advisor VIP Advisor
VIP Advisor


In addition to above excellent information, I would like to add need for ARP frames; Before any host can communicate over the Ethernet, ARP will be sent to neighboring device. ARP does IP to MAC resolution and builds table of such info. This is true for all types of communication from simple ping to web browsing (from L3 thru L7). In case of HTTP, DNS resolution maybe needed as well. So, you will see DNS packets send over.
ARP is Layer 2 protocol while most of other communication will use L3 - L7.
In IP communication, Layer 3 IP address will stay the same during one-way journey from source to destination. While L2 ARP will be re-written by L3 device - such as router- at least twice.

 

Regards, ML
**Please Rate All Helpful Responses **

View solution in original post

11 REPLIES 11

Georg Pauwen
VIP Master VIP Master
VIP Master

Hello,

 

what exactly are you after ? There is no diagram attached. What do you mean by packet level, something like a protocol analyzer (e.g. Wireshark) output ?

I want to know how a packet travels from L1 to L2. 

I want to the address learning that happens in Switches and routers. 

What message does the switch S2 sent to Router R1? 

Ah, I see you've two postings, different titles, asking much the same. I.e.: https://community.cisco.com/t5/routing/routing-protocols/m-p/4131063

At the packet level, switches are "invisible" to the host and router. As far a they are concerned, the host and router might be directly connected on the same physical wire. I.e. switch S2 does not send any message to R1 (or the converse).

Ok. What happens in the link between Host, L1 and Switch S1?

At the packet level, logically nothing. Again, L2 switches are "invisible" to packets (and packets are "invisible" to "true" L2 switches, beyond being "data" within the frame).

 

PS:

I also might add, between the host and router, even at L2, the switches are still logically "invisible".  However, L2 switches do "operate upon" frames entering them.  A "true" L2 (only) switch wouldn't "change" the frame, although it might drop/discard it for various reasons.  Smart/enhanced/VLAN switches, might do other "things" to the frame.

Thank you. 

Joseph W. Doherty
Hall of Fame Master Hall of Fame Master
Hall of Fame Master
"Entire" depends on how devices are configured and besides the (L3) packet level, there's also the (L2) frame level, but since you've asked only about the packet level, I'll exclude L2 considerations.

In brief, assuming host L1 has R1's, same network, IP as its gateway, it sends a packet to that gateway with host's L2 as the packet's destination address (it has its own IP as the source IP - that's how host L2 will know both the sender, and where to send any reply).

When R1 receives the packet, it (via some form of routing information) knows to send the packet to R2.

When R2 receives the packet, it knows host L2 is on a directly connected network, so it sends the packet to host L2.

Since you asked about "communication", as that normally implies two way data transfer, host L2 to host L1 is exactly alike, except in the reverse direction.

Again, there are L2 actions too. Also, I'm not addressing how L1 "knows" L2's address or how R1 and R2 "know" about not directly connected networks. Nor other higher levels protocols, such as perhaps using TCP (L4) to insure delivery of each packet and FTP or HTTP (L7) etc.

The switches, being L2 devices, don't deal with packets, beyond delivering them as part of a frame.

Thanks. But pls let me know the L2 and L1 actions too. 

I want to know the operation entirely. 

"Entirely" explanations would run into "book" length, or at least a chapter or two. Highly recommend you do some Internet searches on these subjects - you should find much material. Perhaps qualify your searches with words like introduction or primer, etc. You can also often find free on-line courses, for basic networking information, too.

Cisco also has a "Learning Network", https://learningnetwork.cisco.com, and their "Academy", https://www.netacad.com, both are more oriented toward learning basic information.

That said, although I believe you can post questions on Cisco's Learning Network site, this community will usually answer specific questions, but often you'll probably find this community won't provide basic network "how it works" information, especially when it would require a very long answer. Or course, you can ask, but your query may go unanswered, or only be briefly answered. Again, your best bet is likely other sources for this information.

Martin L
VIP Advisor VIP Advisor
VIP Advisor


In addition to above excellent information, I would like to add need for ARP frames; Before any host can communicate over the Ethernet, ARP will be sent to neighboring device. ARP does IP to MAC resolution and builds table of such info. This is true for all types of communication from simple ping to web browsing (from L3 thru L7). In case of HTTP, DNS resolution maybe needed as well. So, you will see DNS packets send over.
ARP is Layer 2 protocol while most of other communication will use L3 - L7.
In IP communication, Layer 3 IP address will stay the same during one-way journey from source to destination. While L2 ARP will be re-written by L3 device - such as router- at least twice.

 

Regards, ML
**Please Rate All Helpful Responses **

Wonderful explanation. Thank you. 

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