Brocade TurboIron 24X Series Configuration Guide - Manual (Page 470)

436 Brocade TurboIron 24X Series Configuration Guide
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Overview
1. When the Layer 3 Switch receives an IP packet, the Layer 3 Switch checks for filters on the
receiving interface. 1 If a deny filter on the interface denies the packet, the Layer 3 Switch
discards the packet and performs no further processing, except generating a Syslog entry and
SNMP message, if logging is enabled for the filter.
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-config 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-config 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
• IP 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.
1. The filter can be an Access Control List (ACL) or an IP access policy.