If you know of a great/free/legal site to help people learn IP networking, please post it below. One of my new favorites is "Jeremy's IT Lab" on TH-cam th-cam.com/channels/0Q7Hlz75NYhYAuq6O0fqHw.html Thank you, and let's learn together!
This is my favorite site for practicing Subnet and Wildcard masks with a variety of question "types" and if there is a problem type you want to focus on, there is a checkbox for "I want only this type of question" subnettingpractice.com/index.html
Jeremy's IT lab is grate I watched it before I found your channel, but there is a huge difference between the way you teach and him And that's the level of energy and joy you give away while teaching
Keith I'm so glad we live in a time where exceptional educators such as yourself are available for the entire world to view on the internet! You have a truly unique combination of gifts that have made preparing for the CCNA and a future in IT much more enjoyable than it would have been only studying a stuffy textbook. This information is truly interesting and exciting, it's great to have an instructor who brings it to life the way you do. Cheers!
What I love more about your content is that you are so happy to teach this to us. It's always interesting to see someone teach with a smile on their face for the whole video. Great work Keith!
I work as a software engineer in a computer networking enterprise. I have no doubt that this is one of the best channels i have come across on youtube . Thanks Keith !
I envy all the young guys just getting into the networking world, starting off with access to this kind of instruction and concept explanations, at the click of a button and at no cost! Thanks for spending hours sharing your knowledge and making networking so easy to understand for everyone, Keith!
Before this video it took me FOREVER to learn how to make my cisco equipment work with DHCP! I also learned a lil bit more from your topology! Thanks bro keep it up
Thank you for all these videos. These are very helpful. Currently doing my CCNA. I love the simplicity and enthusiasm in your delivery style. Especially, your thoughtfulness in making this resource and others available to the less privileged.
thanks Keith for your time and dedication for making those amazing videos. without these free resources it would be extremely difficult for me to get CCNA. And thanks for pointing out Jermy's IT lab too. People who create high quality free resources are amazing. 😀
Great video as always! These help me put logic to stuff I see at work that I do not fully understand at the the time. Thanks for putting these videos out there!
Hey Keith, Loved the video; however, your video contradicts a book I am using to prepare for the CCNA exam written by someone who is well established in the Cisco world - I don't want to mention names. In the book, the author talks about the DHCP discover and offer messages both being broadcast messages and how the client realizes the offer message is intended for him is by the client ID. From the book "Note that all hosts in the subnet receive the Offer message. However, the original Discover message lists a number called the client ID, which includes the host's MAC address, that identifies the original host." He goes onto to say "The rest of the hosts will receive the Offer message, but notice that the message lists another device's DHCP client ID, so the rest of hosts ignore the Offer message." He also shows in a diagram the offer message source address is from the DHCP server address and not the router interface ip address configured with the ip helper-address. I understand packets don't lie, so watching your video with the packet captures is very convincing. For preparing for the CCNA exam, should I base my studies on what you presented in this video that discover messages are always a broadcasts, abandon the information about a Client ID and the packet flow once the local router receives the offer and ack messages from the dhcp server rewrites the ip header with the ip address of the client as the destination and router ip address as the source? Thanks in advance - Paul
Thank you for the question Paul! Love the detail. For the first part: In the book, the author talks about the DHCP discover and offer messages both being broadcast Answer: depends on if the client's discover has the "broadcast" bit set on or off. If that bit is off, both the offer and the ack from the server will be sent to offered L3 address, and the current L2 address of the client. If the initial broadcast bit is off in the discover, then yes all 4 destinations addresses of the packets/frames will be set to the broadcast address. In both cases, in the DHCP payload, it also includes the client's MAC address which allows the client to recognize this offer/ack is for that client. The "client id" is the MAC address. Besides the author not mentioning or explaining the slightly different process due to the "broadcast" flag in the initial discover message, we are in agreement. 😁 (also, most of the other hosts on the segment if seeing a broadcast offer, won't be expecting an offer at that moment, which would cause them to ignore that frame anyway, once they saw what it was.) For the second point, with a DHCP relay, (ip helper), the the packets will be sourced (at L3) from the IP address of the interface where IP helper is configured, and in the responses to the client, it will also include (in the DHCP payload), the "DHCP Server Identifier" so that the client does know the real IP address of the DHCP server from that information. So, regarding preparation for the CCNA exam, they won't be asking for detail at that level (not even close). Based on the exam blueprint, they will be asking questions that test your knowledge or ability to: Explain the role of DHCP Configure and Verify DHCP Client and Relay and that is the extent for CCNA. When you get to the professional or expert level certifications, some of the additional details become more important to know. I had fun in revisiting this topic, and thank you for the question. Happy studies.
Keith Barker I feel like a fool. Sorry for my other message. I just read your explanation and thank you for taking the time to answer my question. I also subscribe to CBT Nuggets and have thoroughly enjoyed your videos and labs. They really help in understanding the material presented. Take care Paul
"let there be an undo" lol. Too funny. When Keith clicked delete I flinched. I cannot get DHCP relay working properly over subinterfaces. In packet Tracer, I have a router configured as a DHCP server, and another router with subinterfaces to vlans (ROAS config), and clients on either vlan aren't getting IP addresses. I've tried putting the helper address on the subinterfaces, I've tried putting the helper address on the interface hosting the subinterfaces. I've tried putting subinterfaces on the router that is being a DHCP server, and using VLSM and giving those subinterfaces addresses that reflect the next block of addresses in a /28 (grasping at straws). I just can't get it to work. I've gotten DHCP to work across subinterfaces and vlans (in packet Tracer) and assign appropriate addresses for respective vlans, but just can't make it work with a relay agent across subinterfaces. Any tips, anybody? Thanks.
hey, Keith always nice to see ur video. I have a question the DHCP renew/release are obviously a command, but are they DHCP messages also? are they categorized as messages also ?
Thank you B R. Feel free to join my Discord sever. Lots of people there helping each other out. Each Saturday at 10am Pacific I hold my "Office Hour" where learners can ask questions about the topics they are studying. Mostly focusing on Cisco CCNA 200-301 topics. Feel free to join us there live if you are available. Here is the link ogit.online/Join_OGIT_on_Discord Thanks again B R!
Off topic, but most sysadmins I talk to are woefully ignorant of networking. They don’t understand layers or encapsulation, so everything is just a alphabet soup of protocols. One guy was asking me how switches pass down DHCP requests to PCs.
Thank you Garegin. Everyone is at different stages of learning, and we have different histories and experiences as well. I make an effort to always leave a person a little better off than I found them, knowledge wise, if possible. Happy studies!
Hi George! We are working on the CCNP content at CBT, and I am not sure if I will be doing too much of that here on TH-cam as well. Still lots of streams related to CCNA to do here on TH-cam. I will keep your idea in mind. Glad to see you here! Cheers.
Hey Keith, in our packet capture, we were able to see ARP messages right after the Discover broadcast, from the client. Is it something normal and may I know the reason behind it? Also , why do we have source and destination port to be same (67) when we capture the packet from Relay agent to DHCP server?
Thank you for the question Surya Avudaiyappan. The DHCP server will do an ARP for the IP it is about to hand out. It does that to avoid handing out an address that some device already has in use. I wouldn't expect the client to do any ARPs until after the final offer, and the client was then using the IP. PT may or may not be playing that out correctly.
It's always great to see you and you explain everything amazingly. I want to clear my confusion about dhcp Dora process that when dhcp server receives Discover a broadcast message from the client then in this message there is clients Mac address also then why dhcp broadcasts both offer and Ack message as it clearly knows client Mac address??? Thanks.
Thank you for the question Shah. The ACK from the DHCP server doesn't really provide any additional information, but rather confirms to the client that it is allowed to use (from the DHCP server's perspective) the address that was both in the offer, and that the client requested. The client may receive a DHCPNAK, instead of the ACK, which would make the client start the DORA (or in that case DORN ;) process over. Hope that helps.
@@KeithBarker Thanks a lot for your reply.Perhaps i could not explain my problem correctly and sorry for that. Actually i want to say that whether DHCP broadcasts the Offer message( Ack message also ) or unicast this message.Because Dhcp clearly knows the Mac address of client after receiving the Discover message from client.So, it should unicast the offer and ack message??? On some internet sites it is mentioned that DHCP broadcasts at layer 3 in offer and ack message but unicast at layer 2 in these messages.
@@shhhunain It depends on the client's initial discover message. In the discover packet, if the broadcast bit is set to on (a 1), then the DHCP server will send both the offer and the ack via a layer 2 broadcast. On the other hand, if the broadcast bit in the discover packet is not set (a 0), then both the Offer and ACK are sent to the layer 2 address of the client, instead of being sent as a broadcast. Hope that is useful. Here is a link for the RFC if you have interest in reading further about that. www.ietf.org/rfc/rfc2131.txt Cheers!
@@KeithBarker Thanks a lot. God bless you long and healthy life. At last i have found someone who can answer my questions that are stuck in my head from a very log time.
This is probably a dumb question but Where can I get access to the labs shown in all your videos? It seems like they are virtualized but I could not find a link in any of the videos I've watched so far. Please halp!
Packet tracer is used in some of keiths videos, but Keith likely has access to real switches , routers and servers (likely using remote desktop) There is also gns3 which david bombal demonstrates on his channel, however gns3 appears to be unsupported by Cisco (if Ive understood that correctly)
Hello. Thank you very much for putting this out there! it really helps me out :) I have a question and it would really help me clarify somethings if you could answer it In my notes and when doing research TCP/IP layers are shown as 1. network interface layer 2. internet 3. transport 4. then application. while OSI is application presentation session transport network data link and physical. Why is it that when you described a TCP/IP layer it says application transport network data link physical? I am confused!! I did as much research as I cant but can't find the answer!!! please help me its bugging me so much!!!
Thank you for the question j h. Here is what happened, in beginning with the OSI model, there were 7 layers/names/functions. Later, when a new protocol stack was created, it had its own layers and names, (a couple similar to the OSI model) Now, in the real word we use TCP/IP, but we tweaked a few labels/names and levels. Here is the current situation, from top to bottom with TCP/IP (the only protocol stack we use) Application Layer Transport Layer (layer 4) Network Layer (layer 3) Data-Link Layer (layer 2) Physical Layer (layer 1) This is the only one, in real life, that we need to understand. Hope that helps!
What is the point of client sending Request to Server when it got the offer. Why can't client just accept the Offer and send an acknowledgement to the server instead of client sending Request to server and server sending Acknowledgment to the client.
I see you have Wireshark in your desktop.. Can you teach Wireshark? To analyze slowness, traffic unreachable cause, little hack,.. I have seen more videos in Wireshark but I believe you make it simple understandable..
Thank you for the suggestion. Currently I am focusing most of my TH-cam content at the Cisco CCNA level, but will keep your suggestion in the queue as a possibility in the future. Thanks again for the idea.
It would be helpful if you could answer the following question What dora packets are broadcast or unicast ?? (Incase we donot have a relay agent) What dora packets are broadcast or unicast ?? (Incase we have a relay agent)
Thank you for the question Gagan Bhatia. All the packets between the relay agent and the server are unicast. Between the Client and a local DHCP server (or relay), there is a bit called the "broadcast flag". If that is on in the initial discover message from the client, then all 4 DORA packets will have the L2 and L3 destination as a broadcast. If that bit is OFF in the initial discover message, then the D and R from the client will both have a destination L2 and L3 broadcast, and the O and A from the server will use L3 and L2 unicast destination addresses back to the clients MAC address, and IP (the IP that is being offered). Hope that helps!
Thank you for the question CLIn7 l33tW00d. The relay and the server are on different networks. When the server responds back to the relay, the L2 frames from the server will be addressed to the server's default gateway, who will then route the packet based on the destination L3 address, back to the dhcp relay. Let me know if that makes sense, or if you have further questions. Happy to help.
Thank you Kourosh Farrehi. Feel free to join my Discord sever. Lots of people there helping each other out. Each Saturday at 10am Pacific I hold my "Office Hour" where learners can ask questions about the topics they are studying. Mostly focusing on Cisco CCNA 200-301 topics. Feel free to join us there live if you are available. Here is the link ogit.online/Join_OGIT_on_Discord Thanks again Kourosh Farrehi!
Hi Keith I'm trying to replicate this in packet tracer. I've managed to get DHCP working for all PC's on my first vlan (eg VLAN10), but how do you configure the DHCP sever to give out addresses for clients that are in a separate vlan? (VLAN20)
Thank you for the question Sean Smith. In PT, create additional pools in the DHCP server, appropriate for the subnets you want to support. Then the dhcp relay can forward requests to the server, and the server will respond back via the relay.
Hi Keith, We can use dhcp service from cisco switch as well, should we know about this for new ccna? my vlan clients get assigned a dhcp address and subnet from switch dhcp but no gateway assigned to clients, i can ping all clients from different switches in different vlans across the switches using L2 switches in topology. I even added default-router in dhcp but no success. Thanks
Thank you for the question. Regarding DHCP, the blueprint asks you to: Explain the role of it, configure and DHCP client and relay, (but not configure a server). Based on that I wouldn't think they would be testing on the configuration of a DHCP server on s Cisco device. Happy studies!
what if, the relay agent is the loopback ip. the loopback ip (relay agent ) and the dhcp client are not in the same broadcast domain if offer comes from the dhcp server to the relay agent ( loopback ip) as uni cast ,how it handover the offer message to the client machine.
i have this issue in the evpn fabric. the client and server are in different leafs. the discover was sent to the dhcp server and the server seeing it. the offer is been send by the dhcp server to the switch loopback ip ( relay agent). but the client is not receiving the offer message.
Thank you for the question @karnadhana. Perhaps this can help: www.cisco.com/c/en/us/support/docs/switches/catalyst-9300-series-switches/217366-configure-dhcp-in-ios-xe-evpn-vxlan.html
Thank you for the question Bernd Eckenfels. Yes. This may lead to multiple Discover messages being sent to the DHCP server, but at the end of the day, the client will request only 1 offer.
Keith, something that has baffled me is, is DHCP configurable on Cisco router as does on Microsoft Windows Servers, if not why too much talk about Cisco routers?
Thank you for the question Taiwo. Usually companies have another solution for DHCP servers (other than the routers acting as DHCP servers). However, knowing that the Cisco router can be configured as a DHCP server is handy, if the need ever arises, such as in a small office/home office (SOHO). Glad you are here, and thanks for the question.
Thank you for the question @aniswlidi2012. Are you asking about a reservation for a specific IP address to go with a specified MAC address? If so, that is a function of a reservation at the DHCP server.
Thank you for the question. I don't know the ratio. Some of the factors would include how much that service will be used, and by whom. I recall that Google doesn't use a traditional server for DNS, but rather created their own based on the standards.
Depends where you're heading at and on which continent you are. If you going for enterprise "corporate" administration you're heading towards windows, but if you're going to service providers, hosting providers, data center, concentrated services you're mostly going to find Linux and open source software running as fine or better with less overhead than windows does.
Thank you for the question. There are some differences based on whether the NACK was due to a renewal request, or if it is a new client. If the client is new, and doesn't successfully get an IP address, many OS(s) will default to an APIPA address. docs.microsoft.com/en-us/previous-versions/windows/it-pro/windows-2000-server/cc958957(v=technet.10)?redirectedfrom=MSDN
Awesome! Thank you for the recommendation Fredrick! The more useful resources we add here, the easier it will be for others to reach their goals. I appreciate you taking the time to add that recommendation. I looked up his channel, looks great! Here is the link for his channel: th-cam.com/channels/K4ji45I-zxeWXAFKmu3p6Q.html Cheers!
![ไปอยู่ไสมา - ช๊อปเปอร์ โตเกียวมิวสิค [ OFFICIAL MUSIC VIDEO ]](http://i.ytimg.com/vi/sWuVNbRTJbc/mqdefault.jpg)
If you know of a great/free/legal site to help people learn IP networking, please post it below. One of my new favorites is "Jeremy's IT Lab" on TH-cam th-cam.com/channels/0Q7Hlz75NYhYAuq6O0fqHw.html
Thank you, and let's learn together!
Thank you so much for this shoutout Keith! It's an honor, you've been an inspiration since I started my networking studies.
One more that I found useful with Packet Tracer labs on TH-cam th-cam.com/play/PLAqaqJU4wzYVW-_eHijUr6PjoBCnG3ixU.html
th-cam.com/channels/bXctm6VW2ZZrksHBWAg_tw.html
hi Keith
Illimitable Internetworking - International
i think this guy is intresting as well
This is my favorite site for practicing Subnet and Wildcard masks with a variety of question "types" and if there is a problem type you want to focus on, there is a checkbox for "I want only this type of question" subnettingpractice.com/index.html
@@morganscott5588 Thank you Morgan!
Jeremy's IT lab is grate I watched it before I found your channel, but there is a huge difference between the way you teach and him
And that's the level of energy and joy you give away while teaching
Wow, thanks!
Keith I'm so glad we live in a time where exceptional educators such as yourself are available for the entire world to view on the internet! You have a truly unique combination of gifts that have made preparing for the CCNA and a future in IT much more enjoyable than it would have been only studying a stuffy textbook. This information is truly interesting and exciting, it's great to have an instructor who brings it to life the way you do. Cheers!
Happy to do it, thanks for the feedback!
What I love more about your content is that you are so happy to teach this to us. It's always interesting to see someone teach with a smile on their face for the whole video. Great work Keith!
I appreciate that!
I work as a software engineer in a computer networking enterprise.
I have no doubt that this is one of the best channels i have come across on youtube .
Thanks Keith !
Thank You! That means a lot!
I envy all the young guys just getting into the networking world, starting off with access to this kind of instruction and concept explanations, at the click of a button and at no cost! Thanks for spending hours sharing your knowledge and making networking so easy to understand for everyone, Keith!
Happy to do it, thanks for the feedback sandi dhanwada.
Before this video it took me FOREVER to learn how to make my cisco equipment work with DHCP! I also learned a lil bit more from your topology! Thanks bro keep it up
You & Jeremy Cioara are HEROES & GODS in the Cisco Game since the LATE 90's and up when I was watching!
Thank you bulcub!
DORA...heckin brilliant. I kept having trouble remembering the process but DORA... I mean. Just wow.
I'm not interested in ccna, but this video has been very helpful in using dhcp relay on a mikrotik router with dhcp on a windows server. Thank you.
Thank you Mark Miller!
Keith I like the way you teach.. you enjoy when you are teaching... learning from you is a Fun.. 😀 take care .. enjoy.
Glad you enjoy it!
Thanks for the great explanation, because I’m struggling with all of this information in school.
Thank you for all these videos. These are very helpful. Currently doing my CCNA. I love the simplicity and enthusiasm in your delivery style. Especially, your thoughtfulness in making this resource and others available to the less privileged.
Keith and Team ,
I really liked how you illustrated the relationship of the DHCP communication (DORA) with the TCP/IP protocol stack.
Awesome!!!!!
Thank you Ricardo James!
thanks Keith for your time and dedication for making those amazing videos. without these free resources it would be extremely difficult for me to get CCNA.
And thanks for pointing out Jermy's IT lab too. People who create high quality free resources are amazing. 😀
Happy to do it, thanks for the feedback Ranjana Dissanayaka.
i wait for the endings of every video. Networking is so much fun with you being a mentor :D
Thank you JAVERIA ZAFAR!
Hi Keith!!! I like all your videos, Thanks for providing these materials for us.
My pleasure!
One of the best videos I have seen. Thank you!
Wow, thanks!
Keith... you are awesome.
Thank you wally19!
Great video as always! These help me put logic to stuff I see at work that I do not fully understand at the the time.
Thanks for putting these videos out there!
Happy to do it, thanks for the feedback.
Keep going Keith, all material are very useful and well understood with body language...!! Thx
Thanks, will do!
Thanks Keith, a great tutorial - learnt a tonne 👍
Thank you George Kaimakis!
I also thought about DORA the explora when I saw the acronym and it’s funny you did too 😄 !! Thanks Keith
Happy to do it, thanks for the feedback Buenos Días gracias.
Great video as always Mr Barker.... I make conscious effort to watch at least 1 video every day and follow along with in my lab...
Wow, thanks
Thank you Keith ! Good refresh...
Happy to do it, glad you are here Karthik.
LAZRO.Diaz has a very good book on IP addressing..."All you need to know about IP addressing "
Another great informative video, thanks Keith!
The UDP demo (throw) was hilarious :D
Thank you Sarmed Rahman!
Very well explained, thank you
You're Welcome!
Hey Keith, Loved the video; however, your video contradicts a book I am using to prepare for the CCNA exam written by someone who is well established in the Cisco world - I don't want to mention names. In the book, the author talks about the DHCP discover and offer messages both being broadcast messages and how the client realizes the offer message is intended for him is by the client ID. From the book "Note that all hosts in the subnet receive the Offer message. However, the original Discover message lists a number called the client ID, which includes the host's MAC address, that identifies the original host." He goes onto to say "The rest of the hosts will receive the Offer message, but notice that the message lists another device's DHCP client ID, so the rest of hosts ignore the Offer message." He also shows in a diagram the offer message source address is from the DHCP server address and not the router interface ip address configured with the ip helper-address. I understand packets don't lie, so watching your video with the packet captures is very convincing. For preparing for the CCNA exam, should I base my studies on what you presented in this video that discover messages are always a broadcasts, abandon the information about a Client ID and the packet flow once the local router receives the offer and ack messages from the dhcp server rewrites the ip header with the ip address of the client as the destination and router ip address as the source? Thanks in advance - Paul
Thank you for the question Paul!
Love the detail.
For the first part:
In the book, the author talks about the DHCP discover and offer messages both being broadcast
Answer: depends on if the client's discover has the "broadcast" bit set on or off. If that bit is off, both the offer and the ack from the server will be sent to offered L3 address, and the current L2 address of the client. If the initial broadcast bit is off in the discover, then yes all 4 destinations addresses of the packets/frames will be set to the broadcast address. In both cases, in the DHCP payload, it also includes the client's MAC address which allows the client to recognize this offer/ack is for that client. The "client id" is the MAC address. Besides the author not mentioning or explaining the slightly different process due to the "broadcast" flag in the initial discover message, we are in agreement. 😁 (also, most of the other hosts on the segment if seeing a broadcast offer, won't be expecting an offer at that moment, which would cause them to ignore that frame anyway, once they saw what it was.)
For the second point, with a DHCP relay, (ip helper), the the packets will be sourced (at L3) from the IP address of the interface where IP helper is configured, and in the responses to the client, it will also include (in the DHCP payload), the "DHCP Server Identifier" so that the client does know the real IP address of the DHCP server from that information.
So, regarding preparation for the CCNA exam, they won't be asking for detail at that level (not even close). Based on the exam blueprint, they will be asking questions that test your knowledge or ability to:
Explain the role of DHCP
Configure and Verify DHCP Client and Relay
and that is the extent for CCNA.
When you get to the professional or expert level certifications, some of the additional details become more important to know.
I had fun in revisiting this topic, and thank you for the question.
Happy studies.
Keith Barker
I feel like a fool. Sorry for my other message. I just read your explanation and thank you for taking the time to answer my question. I also subscribe to CBT Nuggets and have thoroughly enjoyed your videos and labs. They really help in understanding the material presented.
Take care
Paul
Keith Barker
BTW - Happy Father’s Day.
You are a great teacher. I can tell.
I appreciate that!
Thank you so much for this video!
Glad it was helpful!
Thank you, I always enjoy your videos
Happy to do it, thanks for the feedback Aguogieyin.
Thanks. Great tutorial. 👍🏻
"let there be an undo" lol. Too funny. When Keith clicked delete I flinched. I cannot get DHCP relay working properly over subinterfaces. In packet Tracer, I have a router configured as a DHCP server, and another router with subinterfaces to vlans (ROAS config), and clients on either vlan aren't getting IP addresses. I've tried putting the helper address on the subinterfaces, I've tried putting the helper address on the interface hosting the subinterfaces. I've tried putting subinterfaces on the router that is being a DHCP server, and using VLSM and giving those subinterfaces addresses that reflect the next block of addresses in a /28 (grasping at straws). I just can't get it to work. I've gotten DHCP to work across subinterfaces and vlans (in packet Tracer) and assign appropriate addresses for respective vlans, but just can't make it work with a relay agent across subinterfaces. Any tips, anybody? Thanks.
you are a Perfect #Guru to teach Networking allover the World Keith, Great Kudos 😍😍🙌
Thank you Am Sudheendra !
you are a king! thank you for the interesting video !!!! i can't stop watching those videos (:
Glad you like them!
Great video, thanks !
You are welcome!
Thanks for your videos Keith, it helps me a lot
Thank you Bruno!
That was very informative. Thanks a lot!
Thanks for what you are doing.
Happy to do it!
Great stuff, man. Thank you!!!
Thank you Slavko!
U are amazing
Thanks! So are you!
Thanks Keith for the info, but curious to know how do we define scope based on VLAN's
Thanks for the video and lab capture. Will you be doing a follow on for how DHCP is configured/used on CISCO gear such as a router or L3 switch?
The channel Geek's Lesson has a free networking course from beginning to end getting introduced.
Thank you Justin!
hey, Keith always nice to see ur video. I have a question the DHCP renew/release are obviously a command, but are they DHCP messages also? are they categorized as messages also ?
Thank you B R. Feel free to join my Discord sever. Lots of people there helping each other out. Each Saturday at 10am Pacific I hold my "Office Hour" where learners can ask questions about the topics they are studying. Mostly focusing on Cisco CCNA 200-301 topics. Feel free to join us there live if you are available. Here is the link ogit.online/Join_OGIT_on_Discord
Thanks again B R!
A VM can be a host.
Keith can you please give us the name of the song in the beginning?
Ray Cox yes please share
Hi Ray. Here is that music: "Little Bit of Faith" by Malena Stark
Off topic, but most sysadmins I talk to are woefully ignorant of networking. They don’t understand layers or encapsulation, so everything is just a alphabet soup of protocols. One guy was asking me how switches pass down DHCP requests to PCs.
Thank you Garegin. Everyone is at different stages of learning, and we have different histories and experiences as well. I make an effort to always leave a person a little better off than I found them, knowledge wise, if possible.
Happy studies!
Keith Barker I always tell people not to take my word for it but read the sources.
Keith, how are you? we have met at VMWare World 2018. I was wondering if in the future there will be CCNP videos (new track). That will be great!!!
Hi George! We are working on the CCNP content at CBT, and I am not sure if I will be doing too much of that here on TH-cam as well. Still lots of streams related to CCNA to do here on TH-cam. I will keep your idea in mind. Glad to see you here! Cheers.
@@KeithBarker Likewise Keith. I'm a big fan of yours! I like to see your videos and looking forward to the new content!
Hey Keith, in our packet capture, we were able to see ARP messages right after the Discover broadcast, from the client. Is it something normal and may I know the reason behind it?
Also , why do we have source and destination port to be same (67) when we capture the packet from Relay agent to DHCP server?
Thank you for the question Surya Avudaiyappan. The DHCP server will do an ARP for the IP it is about to hand out. It does that to avoid handing out an address that some device already has in use. I wouldn't expect the client to do any ARPs until after the final offer, and the client was then using the IP. PT may or may not be playing that out correctly.
It's always great to see you and you explain everything amazingly. I want to clear my confusion about dhcp Dora process that when dhcp server receives Discover a broadcast message from the client then in this message there is clients Mac address also then why dhcp broadcasts both offer and Ack message as it clearly knows client Mac address???
Thanks.
Thank you for the question Shah. The ACK from the DHCP server doesn't really provide any additional information, but rather confirms to the client that it is allowed to use (from the DHCP server's perspective) the address that was both in the offer, and that the client requested. The client may receive a DHCPNAK, instead of the ACK, which would make the client start the DORA (or in that case DORN ;) process over. Hope that helps.
@@KeithBarker Thanks a lot for your reply.Perhaps i could not explain my problem correctly and sorry for that. Actually i want to say that whether DHCP broadcasts the Offer message( Ack message also ) or unicast this message.Because Dhcp clearly knows the Mac address of client after receiving the Discover message from client.So, it should unicast the offer and ack message???
On some internet sites it is mentioned that DHCP broadcasts at layer 3 in offer and ack message but unicast
at layer 2 in these messages.
@@shhhunain It depends on the client's initial discover message. In the discover packet, if the broadcast bit is set to on (a 1), then the DHCP server will send both the offer and the ack via a layer 2 broadcast. On the other hand, if the broadcast bit in the discover packet is not set (a 0), then both the Offer and ACK are sent to the layer 2 address of the client, instead of being sent as a broadcast.
Hope that is useful.
Here is a link for the RFC if you have interest in reading further about that. www.ietf.org/rfc/rfc2131.txt
Cheers!
@@KeithBarker Thanks a lot. God bless you long and healthy life. At last i have found someone who can answer my questions that are stuck in my head from a very log time.
this maybe a dumb question but why do I need to configure DHCP relay if IP helper-address will do?
This is probably a dumb question but Where can I get access to the labs shown in all your videos? It seems like they are virtualized but I could not find a link in any of the videos I've watched so far. Please halp!
Packet tracer is used in some of keiths videos, but Keith likely has access to real switches , routers and servers (likely using remote desktop)
There is also gns3 which david bombal demonstrates on his channel, however gns3 appears to be unsupported by Cisco (if Ive understood that correctly)
Could someone clarify- Difference Between "ip helper-address global" and "ip helper-address" ?
Hello. Thank you very much for putting this out there! it really helps me out :) I have a question and it would really help me clarify somethings if you could answer it
In my notes and when doing research TCP/IP layers are shown as 1. network interface layer 2. internet 3. transport 4. then application. while OSI is application presentation session transport network data link and physical. Why is it that when you described a TCP/IP layer it says application transport network data link physical? I am confused!! I did as much research as I cant but can't find the answer!!! please help me its bugging me so much!!!
Thank you for the question j h.
Here is what happened, in beginning with the OSI model, there were 7 layers/names/functions.
Later, when a new protocol stack was created, it had its own layers and names, (a couple similar to the OSI model)
Now, in the real word we use TCP/IP, but we tweaked a few labels/names and levels.
Here is the current situation, from top to bottom with TCP/IP (the only protocol stack we use)
Application Layer
Transport Layer (layer 4)
Network Layer (layer 3)
Data-Link Layer (layer 2)
Physical Layer (layer 1)
This is the only one, in real life, that we need to understand.
Hope that helps!
What is the point of client sending Request to Server when it got the offer. Why can't client just accept the Offer and send an acknowledgement to the server instead of client sending Request to server and server sending Acknowledgment to the client.
Thank you for the question Danial Khan. What do you think the answer to that is?
@@KeithBarker What is the answer?
I see you have Wireshark in your desktop.. Can you teach Wireshark? To analyze slowness, traffic unreachable cause, little hack,.. I have seen more videos in Wireshark but I believe you make it simple understandable..
Thank you for the suggestion. Currently I am focusing most of my TH-cam content at the Cisco CCNA level, but will keep your suggestion in the queue as a possibility in the future. Thanks again for the idea.
Hello Keith, Do we have option for DHCP smart relay in viptela , if no are there any work around for getting ip for secondary addresses
It would be helpful if you could answer the following question
What dora packets are broadcast or unicast ?? (Incase we donot have a relay agent)
What dora packets are broadcast or unicast ?? (Incase we have a relay agent)
Thank you for the question Gagan Bhatia. All the packets between the relay agent and the server are unicast.
Between the Client and a local DHCP server (or relay), there is a bit called the "broadcast flag".
If that is on in the initial discover message from the client, then all 4 DORA packets will have the L2 and L3 destination as a broadcast.
If that bit is OFF in the initial discover message, then the D and R from the client will both have a destination L2 and L3 broadcast, and the O and A from the server will use L3 and L2 unicast destination addresses back to the clients MAC address, and IP (the IP that is being offered).
Hope that helps!
46:32 why is the Layer 2 destination address that of server's default gateway? Shouldn't it be of SW2 aka the other end of the ethernet cable?
Thank you for the question CLIn7 l33tW00d.
The relay and the server are on different networks. When the server responds back to the relay, the L2 frames from the server will be addressed to the server's default gateway, who will then route the packet based on the destination L3 address, back to the dhcp relay.
Let me know if that makes sense, or if you have further questions.
Happy to help.
Do you have to use a muli-layer switch to use dhcp relay? How would the traffic look if it wasn't?
Nevermind I didnt think about utilizing the svi on a regular router.
LOL! UDP!
Hi dear Keith
I am confused a little bit. When and different between ip helper-address, ip address dhcp and ip dhcp raley
Thank you Kourosh Farrehi. Feel free to join my Discord sever. Lots of people there helping each other out. Each Saturday at 10am Pacific I hold my "Office Hour" where learners can ask questions about the topics they are studying. Mostly focusing on Cisco CCNA 200-301 topics. Feel free to join us there live if you are available. Here is the link ogit.online/Join_OGIT_on_Discord
Thanks again Kourosh Farrehi!
Hi Keith
I'm trying to replicate this in packet tracer. I've managed to get DHCP working for all PC's on my first vlan (eg VLAN10), but how do you configure the DHCP sever to give out addresses for clients that are in a separate vlan? (VLAN20)
Thank you for the question Sean Smith.
In PT, create additional pools in the DHCP server, appropriate for the subnets you want to support. Then the dhcp relay can forward requests to the server, and the server will respond back via the relay.
Hi Keith,
We can use dhcp service from
cisco switch as well, should we
know about this for new ccna?
my vlan clients get assigned a dhcp address and subnet from switch dhcp but no gateway assigned to clients, i can ping all clients from different switches in different vlans across the switches using L2 switches in topology. I even added default-router in dhcp but no success.
Thanks
Thank you for the question. Regarding DHCP, the blueprint asks you to:
Explain the role of it, configure and DHCP client and relay, (but not configure a server). Based on that I wouldn't think they would be testing on the configuration of a DHCP server on s Cisco device.
Happy studies!
what if, the relay agent is the loopback ip. the loopback ip (relay agent ) and the dhcp client are not in the same broadcast domain
if offer comes from the dhcp server to the relay agent ( loopback ip) as uni cast ,how it handover the offer message to the client machine.
i have this issue in the evpn fabric. the client and server are in different leafs.
the discover was sent to the dhcp server and the server seeing it.
the offer is been send by the dhcp server to the switch loopback ip ( relay agent). but the client is not receiving the offer message.
Thank you for the question @karnadhana.
Perhaps this can help:
www.cisco.com/c/en/us/support/docs/switches/catalyst-9300-series-switches/217366-configure-dhcp-in-ios-xe-evpn-vxlan.html
@@KeithBarker thanks for the link Keith.
Can you have the helper on multiple ML Switches at the same time active?
Thank you for the question Bernd Eckenfels. Yes. This may lead to multiple Discover messages being sent to the DHCP server, but at the end of the day, the client will request only 1 offer.
Keith, something that has baffled me is, is DHCP configurable on Cisco router as does on Microsoft Windows Servers, if not why too much talk about Cisco routers?
Thank you for the question Taiwo. Usually companies have another solution for DHCP servers (other than the routers acting as DHCP servers). However, knowing that the Cisco router can be configured as a DHCP server is handy, if the need ever arises, such as in a small office/home office (SOHO). Glad you are here, and thanks for the question.
Keith Barker Insightful. Many thanks.
Hi Keith. How the IP address decided to be PREFERRED? Is it at the client's side or Server? How to configure it? Thanks
Thank you for the question @aniswlidi2012.
Are you asking about a reservation for a specific IP address to go with a specified MAC address? If so, that is a function of a reservation at the DHCP server.
Just wondering DHCP / DNS servers ( Windows vs Linux/Unix ). How often do you see one or the other?
Thank you for the question. I don't know the ratio. Some of the factors would include how much that service will be used, and by whom. I recall that Google doesn't use a traditional server for DNS, but rather created their own based on the standards.
Depends where you're heading at and on which continent you are. If you going for enterprise "corporate" administration you're heading towards windows, but if you're going to service providers, hosting providers, data center, concentrated services you're mostly going to find Linux and open source software running as fine or better with less overhead than windows does.
Hi can anyone tell me what if dhcp server sends NACK to client then what will client do if dhcp server no acknowledge to client?
Thank you for the question. There are some differences based on whether the NACK was due to a renewal request, or if it is a new client. If the client is new, and doesn't successfully get an IP address, many OS(s) will default to an APIPA address. docs.microsoft.com/en-us/previous-versions/windows/it-pro/windows-2000-server/cc958957(v=technet.10)?redirectedfrom=MSDN
Check out on Imran rafai materials on youtube
Awesome! Thank you for the recommendation Fredrick! The more useful resources we add here, the easier it will be for others to reach their goals. I appreciate you taking the time to add that recommendation.
I looked up his channel, looks great! Here is the link for his channel: th-cam.com/channels/K4ji45I-zxeWXAFKmu3p6Q.html
Cheers!