Brocade Virtual ADX Switch and Router Guide (Supporting ADX v03.1.00) - Manual (Page 78)

64 Brocade Virtual ADX Switch and Router Guide
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Configuring IP parameters
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The Virtual ADX encapsulates IP packets in Layer 2 packets regardless of whether the ultimate
destination is locally attached or is multiple router hops away. Since the Virtual ADX’s IP route table
and IP forwarding cache contain IP address information but not MAC address information, the
Virtual ADX cannot forward IP packets based solely on the information in the route table or
forwarding cache. The Virtual ADX 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 Virtual ADX
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 Virtual ADX 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 Layer 3 Switch does the
following:
• First, the Virtual ADX 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 Virtual ADX 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 Virtual ADX 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 Virtual ADX
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 Virtual ADX, the
device sends an ARP response containing its MAC address. The response is a unicast packet
addressed directly to the Virtual ADX. The Virtual ADX 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
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 Virtual ADX knows of no route to
the destination address), the router sends an ICMP Host Unreachable message to the source.
Configuring forwarding parameters
The following configurable parameters control the forwarding behavior of a Virtual ADX running
router code:
• Time-To-Live (TTL) threshold
• Forwarding of directed broadcasts