Address resolution protocol, Figure 329 t, Table 183 – Brocade Communications Systems Brocate Ethernet Access Switch 6910 User Manual

Page 1085: Ddress resolution protocol

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Brocade 6910 Ethernet Access Switch Configuration Guide

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Address Resolution Protocol

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FIGURE 329

Tracing the Route to a Network Device

Address Resolution Protocol

The switch uses its routing tables (for static routes and directly connected subnets) to make routing
decisions, and uses Address Resolution Protocol (ARP) to forward traffic from one hop to the next.
ARP is used to map an IP address to a physical layer (i.e., MAC) address. When an IP frame is
received by this switch (or any standards-based switch/router), it first looks up the MAC address
corresponding to the destination IP address in the ARP cache. If the address is found, the switch
writes the MAC address into the appropriate field in the frame header, and forwards the frame on
to the next hop. IP traffic passes along the path to its final destination in this way, with each routing
device mapping the destination IP address to the MAC address of the next hop toward the
recipient, until the packet is delivered to the final destination.

If there is no entry for an IP address in the ARP cache, the switch will broadcast an ARP request
packet to all devices on the network. The ARP request contains the following fields similar to that
shown in this example:

When devices receive this request, they discard it if their address does not match the destination IP
address in the message. However, if it does match, they write their own hardware address into the
destination MAC address field and send the message back to the source hardware address. When
the source device receives a reply, it writes the destination IP address and corresponding MAC
address into its cache, and forwards the IP traffic on to the next hop. As long as this entry has not
timed out, the switch will be able forward traffic directly to the next hop for this destination without
having to broadcast another ARP request.

TABLE 183

Address Resolution Protocol

destination IP address

10.1.0.19

destination MAC address

?

source IP address

10.1.0.253

source MAC address

00-00-ab-cd-00-00

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