36 Brocade ICX 6650 Layer 3 Routing Configuration Guide
53-1002603-01
Configuring IP parameters – Layer 3 Switches
For example, to forward a packet whose destination is multiple router hops away, the Layer 3
Switch 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 destination. In
each case, the Layer 3 Switch must encapsulate the packet and address it to the MAC address of a
locally attached device, the next-hop router toward the IP packet destination.
To obtain the MAC address required for forwarding a datagram, the Layer 3 Switch does the
following:
•First, the Layer 3 Switch 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 Layer 3 Switch receives an ARP reply or
receives an ARP request (which contains the sender 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 Layer 3 Switch 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 Layer 3 Switch
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 Layer 3 Switch, the
device sends an ARP response containing its MAC address. The response is a unicast packet
addressed directly to the Layer 3 Switch. The Layer 3 Switch 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 Layer 3 Switch. A MAC broadcast is not routed to other
networks. However, some routers, including Brocade Layer 3 Switches, can be configured to
reply to ARP requests from one network on behalf of devices on another network. Refer to
“Enabling proxy ARP” on page38.
NOTE
If the router receives an ARP request packet that it is unable to deliver to the final destination
because of the ARP timeout and no ARP response is received (the Layer 3 Switch knows of no route
to the destination address), the router sends an ICMP Host Unreachable message to the source.
Rate limiting ARP packets
You can limit the number of ARP packets the Brocade device accepts during each second. By
default, the software does not limit the number of ARP packets the device can receive. Since the
device sends ARP packets to the CPU for processing, if a device in a busy network receives a high
number of ARP packets in a short period of time, some CPU processing might be deferred while the
CPU processes the ARP packets.