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IP Routing Features
Configuring IP Parameters for Routing Switches
The routing switch encapsulates IP packets in Layer 2 packets regardless of
whether the ultimate destination is locally attached or is multiple router hops
away. Since the routing switch’s IP route table and IP forwarding cache
contain IP address information but not MAC address information, the routing
switch cannot forward IP packets based solely on the information in the route
table or forwarding cache. The routing switch 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 routing 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’s destination. In each case, the
routing 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’s
destination.
To obtain the MAC address required for forwarding a datagram, the routing
switch does the following:
■ First, the routing 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 routing switch 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 routing 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 routing 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 routing switch, the device sends
an ARP response containing its MAC address. The response is a unicast
packet addressed directly to the routing switch. The routing switch places
the information from the ARP response into the ARP cache.
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