2. If the packet is not denied at the incoming interface, the Layer 3 switch looks in the session table for an entry that has the same
source IP address and TCP or UDP port as the packet. If the session table contains a matching entry, the Layer 3 switch
immediately forwards the packet, by addressing it to the destination IP address and TCP or UDP port listed in the session table
entry and sending the packet to a queue on the outgoing ports listed in the session table. The Layer 3 switch selects the queue
based on the Quality of Service (QoS) level associated with the session table entry.
3. If the session table does not contain an entry that matches the packet source address and TCP or UDP port, the Layer 3 switch
looks in the IP forwarding cache for an entry that matches the packet destination IP address. If the forwarding cache contains a
matching entry, the Layer 3 switch forwards the packet to the IP address in the entry. The Layer 3 switch sends the packet to a
queue on the outgoing ports listed in the forwarding cache. The Layer 3 switch selects the queue based on the Quality of
Service (QoS) level associated with the forwarding cache entry.
4. If the IP forwarding cache does not have an entry for the packet, the Layer 3 switch checks the IP route table for a route to the
packet destination. If the IP route table has a route, the Layer 3 switch makes an entry in the session table or the forwarding
cache, and sends the route to a queue on the outgoing ports:
• – If the running-cong contains an IP access policy for the packet, the software makes an entry in the session table. The
Layer 3 switch uses the new session table entry to forward subsequent packets from the same source to the same
destination.
– If the running-cong does not contain an IP access policy for the packet, the software creates a new entry in the
forwarding cache. The Layer 3 switch uses the new cache entry to forward subsequent packets to the same
destination.
The following sections describe the IP tables and caches:
• ARP cache and static ARP table
• IP route table
• IP forwarding cache
• Layer 4 session table
The software enables you to display these tables. You also can change the capacity of the tables on an individual basis if
needed by changing the memory allocation for the table.
ARP cache and static ARP table
The ARP cache contains entries that map IP addresses to MAC addresses. Generally, the entries are for devices that are directly attached
to the Layer 3 switch.
An exception is an ARP entry for an interface-based static IP route that goes to a destination that is one or more router hops away. For
this type of entry, the MAC address is either the destination device MAC address or the MAC address of the router interface that
answered an ARP request on behalf of the device, using proxy ARP.
ARP cache
The ARP cache can contain dynamic (learned) entries and static
(user-congured) entries. The software places a dynamic entry in the
ARP cache when the Layer 3 switch learns a device MAC address from an ARP request or ARP reply from the device.
The software can learn an entry when the Layer 2 switch or Layer 3 switch receives an ARP request from another IP forwarding device or
an ARP reply. Here is an example of a dynamic entry:
IP Address MAC Address Type Age Port
1 10.95.6.102 0000.00fc.ea21 Dynamic 0 6
Each entry contains the destination device IP address and MAC address.
IP conguration overview
FastIron Ethernet Switch Layer 3 Routing
26 53-1003627-04
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