How arp works – Brocade Multi-Service IronWare Switching Configuration Guide (Supporting R05.6.00) User Manual
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Multi-Service IronWare Switching Configuration Guide
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Configuring ARP parameters
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How ARP works
The Brocade device needs to know a destination’s MAC address when forwarding traffic, because
the Brocade device encapsulates the IP packet in a Layer 2 packet (MAC layer packet) and sends
the Layer 2 packet to a MAC interface on a device directly attached to the Brocade device. The
device can be the packet’s final destination or the next-hop router toward the destination.
The Brocade device encapsulates IP packets in Layer 2 packets regardless of whether the ultimate
destination is locally attached or is multiple router hops away. Since the Brocade device’s IP route
table and IP forwarding cache contain IP address information but not MAC address information, the
Brocade device cannot forward IP packets based solely on the information in the route table or
forwarding cache. The Brocade device needs to know the MAC address that corresponds with the
IP address of either the packet’s locally attached destination or the next-hop router that leads to
the destination.
For example, to forward a packet whose destination is multiple router hops away, the Brocade
device must send the packet to the next-hop router toward its destination, or to a default route or
default network route if the IP route table does not contain a route to the packet’s destination. In
each case, the Brocade device must encapsulate the packet and address it to the MAC address of
a locally attached device, the next-hop router toward the IP packet’s destination.
To obtain the MAC address required for forwarding a datagram, the Brocade device does the
following:
•
First, the Brocade device looks in the ARP cache (not the static ARP table) for an entry that lists
the MAC address for the IP address. The ARP cache maps IP addresses to MAC addresses. The
cache also lists the port attached to the device and, if the entry is dynamic, the age of the
entry. A dynamic ARP entry enters the cache when the Brocade device receives an ARP reply or
receives an ARP request (which contains the sender’s IP address and MAC address). A static
entry enters the ARP cache from the static ARP table (which is a separate table) when the
interface for the entry comes up.
To ensure the accuracy of the ARP cache, each dynamic entry has its own age timer. The timer
is reset to zero each time the Brocade device receives an ARP reply or ARP request containing
the IP address and MAC address of the entry. If a dynamic entry reaches its maximum
allowable age, the entry times out and the software removes the entry from the table. Static
entries do not age out and can be removed only by you.
•
If the ARP cache does not contain an entry for the destination IP address, the Brocade device
broadcasts an ARP request out all its IP interfaces. The ARP request contains the IP address of
the destination. If the device with the IP address is directly attached to the Brocade device, the
device sends an ARP response containing its MAC address. The response is a unicast packet
addressed directly to the Brocade device. The Brocade device places the information from the
ARP response into the ARP cache.
ARP requests contain the IP address and MAC address of the sender, so all devices that
receive the request learn the MAC address and IP address of the sender and can update their
own ARP caches accordingly.
NOTE
The ARP request broadcast is a MAC broadcast, which means the broadcast goes only to
devices that are directly attached to the Brocade device. A MAC broadcast is not routed to
other networks. However, some routers, including the Brocade device, can be configured to
reply to ARP requests from one network on behalf of devices on another network. Refer to