1. Cisco Set Ip Address
2. Arp Request Command

A characteristic of the active HSRP/VRRP peer device is that it is the only one to respond to ARP requests for HSRP/VRRP VIP (Virtual IP). ARP response will contain the HSRP/VRRP vMAC which is the same on both vPC peer devices. The standby HSRP/VRRP vPC peer device just relays the ARP request to active HSRP/VRRP peer device through vPC peer-link. This use case is often confused with an attempt to detect possible duplicate IP addresses, but a Gratuitous ARP is not used for this process. To detect an IP address conflict, hosts will use ARP Probes and Announcements, which is the topic for the next article in the Address Resolution Protocol Series.

See my latest video: Internet Security or DieTRANSCRIPT Address Resolution Protocol - ARP Tutorial. TCP/IP ExplainedTRANSCRIPT:Today I teach you ARP, which I've had several requests from my viewers to do. That's Address Resolution Protocol.The first thing you want to know about any protocol when you're learning it, is where do you find the protocol on the TCP/IP Stack? We see here that ARP is located on the network layer. Not on the data link layer as some folks mistakenly believeHere we see a LAN w 3 computers on it. Notice that each computer has 2 addresses, and IP address and a Mac address. The IP address is a logical address that the admin assigns to each host either manually or more likely with DHCP.

The IP address is 32 bits long and is on the network layer of the TCP/IP Stack. The Mac address Is a physical address, that is actually burned into the network interface card or NIC. The Mac address is 48 bits long and is on the data link layer of the TCP/IP Stack.Suppose we want to send a message from host A to host B. We start at the point where the Network layer of the OSI has recd a message to send from the layers above.

This is encapsulated into a packet by appending a header to it, that has the dest IP address in it. At this point it is passed down to the data link layer where a new header needs to be appended with the dest MAC address.The question is: How does Host A know the MAC address of host B? The Answer is: Host A broadcasts an ARP RequestTo help us understand the ARP request, lets look at a ARP HeaderClick on the following fields & ad lib each: Operation, Sender HW Address, Sender Protocol Address, Target HW Address, Target Protocol AddressBack to where Host A broadcasts an ARP Request.

Cisco Set Ip Address

The reason it broadcasts is that it doesn't yet know the MAC address to send it to. Essentially the request is saying: If your IP address is 10.1.1.2, then tell me your MAC address. Let's see the parameters in the the ARP Request show parameters on Hard Copy as they magically Appear.Now Host A recvs the unicast ARP reply & it becomes aware of the Host B's MAC address & it caches it. Also when Host B recd the ARP request, it became aware of the Host A's MAC address, and it cached that. Note therefore that Host B does NOT need to send an ARP request to find Host A's MAC. So now both Computer A & B are each aware of the others MAC address, they can communicate back an forth over TCP/IP.Now let's look a a different case. Suppose that Host A wants to go to the website Packethacks.com.

How does ARP work then? Host A should already have the default gw of 10.1.1.4, which is the router that leads out to the internet. So it does an ARP request to find the MAC address of the rtr. When it recvs an ARP Reply back from the rtr, it caches the MAC address. And the Rtr caches the MAC address of Host A. Now when Computer A wants to talk to website Packethacks.com it simply sends all packets to the router, and the router is responsible for sending the packets to the next hop on the way to Packethacks.comJust to drive home the point, what are we likely to see in the rtr's ARP cache? Host A's IP & Corresponding MAC Address, Host B's IP & Corresponding MAC Address, Host C's IP & Corresponding MAC Address - because if Host B or C had recently accessed any website on the internet, then these wouls be caches.

Arp Request Command

And also another entry with the IP & MAC of the next hop rtr to get to the Packethacks.com.