proxy-arp?

deepc0re

Hello,

Im preparing for CCNA and have gone thru CCNA Learnkey videos (for the old exam) and have now just started to read the Sybex CCNA study guide book (640-802).

In CCNA learnkey videos they say that IP (layer 3) does a ARP when its going to send a packet to another network (outside of the local LAN/Subnet mask). The router (default gateway on the LAN) will then pickup the ARP request and proxy it on its other interface and then this ARP request will continue hop to hop (form router to router) until it hits the right host who has the IP address in the request. The ARP request will then be sent back again the same path and update all the MAC tables on the routers on its way back and in the end the default gateway will answer the APR request with MAC address to the remote host.

I found this a bit strange and unnecessary (why does it need the MAC address of the remote host?)

Then when I read in the CCNA book it says that IP will look at the destination IP address and compare it with its local IP address and subnet mask to find out if it is on the same LAN, if its not it will NOT do a ARP request and look for IP address of the default gateway instead.

So I am a bit confused here, what is the correct answer ? To me it seems like the last answer seems like the smartest in terms of bandwith usage etc.

I have another question also, is Spanning Tree Protocol needed to provide redundancy over VLAN trunks ? I thought there were other protocols to provide load balancing and redundancy over specific ports/links.

marlon23

Stop reading/listening that crap, author of that material is on crack or something..

Proxy ARP: When router receives an ARP request for an IP address which is not in the subnet assigned to receiving interface, he will check his routing table if he has a route to that subnet. If yes he will send an ARP reply claiming he is that IP host. (His MAC will be inside).

STP is not to have redundancy, the sole purpose of STP is to have loop-free L2 logical topology.

PS: Get some non-fantasy study material, recycle what you can, to reduce your CO2 footprint and obtain CiscoPress CCNA Exam Certification guide from Wendell Odom - That is the book & author to get solid knowledge foundation and not rubbish.

Pash

Your CCNA book is correct. Packet delivery has one golden rule Source and destination IP's do not change end to end unless NAT is involved. If a network device has to send a packet to a device not on it's own network it will arp for it's defaults gateways IP address and then send the packet on to the routing device, the rest is then down to the routing tables and the further golden rules you will learn about.

Proxy-arp is very different:-

Read this link on how it works and why it can be an advantage in some cases.

http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note09186a0080094adb.shtml#advantages

gojericho0

deepc0re wrote:

I read in the CCNA book it says that IP will look at the destination IP address and compare it with its local IP address and subnet mask to find out if it is on the same LAN, if its not it will NOT do a ARP request and look for IP address of the default gateway instead.

If the PC is trying to connect to another PC in the same subnet in an ethernet network it will need to MAC address and IP of the remote PC in order to build the packet to send the data right?

The same logic is applied when trying to send data to another PC on the remote network. This time however the PC will need to first send a PC to its default gateway so that the router can forward the packet to the remote network. To build the packet, the PC will use the destination IP address of the remote PC, but instead of using the remote PCs MAC address this time, it must use the default gateways mac address since the remote PC is on a different network.

deepc0re wrote:

I have another question also, is Spanning Tree Protocol needed to provide redundancy over VLAN trunks ? I thought there were other protocols to provide load balancing and redundancy over specific ports/links.

STP isn't need to provide redundancy. It is needed to help prevent loops in a redundant switched network. It uses an algorithm to create a single path to all switches[/quote]

deepc0re

Thanks for clearing that out!

I know that STP is to ensure a loop free topology, let me rephrase my question instead.

Is STP something you normally run on switch ports assigned to VLAN trunks when you need redundancy (when you are using 2 or more ports for VLAN trunking) ? What about stuff like Multi-Link Trunking ?

Pash

deepc0re wrote:

Thanks for clearing that out!

I know that STP is to ensure a loop free topology, let me rephrase my question instead.

Is STP something you normally run on switch ports assigned to VLAN trunks when you need redundancy (when you are using 2 or more ports for VLAN trunking) ? What about stuff like Multi-Link Trunking ?

Read up on Etherchannels.

elegua

marlon23 wrote:

Stop reading/listening that crap, author of that material is on crack or something..

I think this is a misunderstanding because the author of learnkey is not a newbie in cisco world, check this out:

Mike Storm
Director of Network and Security Engineering

Cisco CCIE, Cisco CCSI, CCSP, CCNP, and CCDP, Cisco Security Specialist: Firewall, VPN and IPS Specialist, Cisco MARS and NAC Expert Practitioner, NSA/CNSS INFOSec Professional, Certified Information Systems Security Professional (CISSP), Microsoft MCT, Microsoft MCSE, Novell MCNE and Novell CNI, CTT+ 

Invited speaker/presenter at the Cisco Networkers Conference for 4 years 
Published Author and Video Expert for Learnkey 
Author and subject matter expert of many of the Cisco Multimedia and Online Content for Cisco Partners (PEC) for the CCNA, CCNP, and CCSP programs. 

Career Highlights:

Seventeen (17) years as an Enterprise-level consultant focused on: INFOSec Security Solutions and Compliance Architectures, Multi-Layer Internetwork Solutions Architectures, DR, Business Continuity and System Criticality assessment for Fortune 100 and 500 companies 
Experience includes extensive consulting and innovation in Network and Security Systems Architecture, Designing and Deploying Systems Management Solutions and Network Security Solutions for Companies such as Honeywell, GE, Boeing, Intel and AT&T 
Instructor specialization includes: Information Security Systems, Penetration Testing and Counter-Hacking, Cisco Active-Defense Security, Intrusion Prevention Systems, Cisco Campus and Scalable Network Design, Microsoft Systems Solutions, Management, Exchange, and Active Directory Design and Implementation 
MCSE Charter Member 1993, MCSE Early Achiever 2000 and 2003 
Thirty-Six (36) published titles in Cisco Security and Cisco Networking, Wireless LAN Design and Implementation, Windows 2000 and 2003, Microsoft SMS and Microsoft Exchange 
Founder of the "Immersion Learning System" (ILS) at Interface and lead architect of the Interface HardHat learning program for Cisco, Security and Windows Systems technologies

This guys is a great teacher, i got from him learnkey CCNA serie (old version) and the PIX Firewall serie and he is very good.

I thought that Learnkey was going to update the CCNA series but not, very good course though.