Brocade FastIron Ethernet Switch Layer 3 Routing Configuration Guide - Manual (Page 25)

IP packet flow through a Layer 3 switch
FIGURE 1 IP Packet flow through a Brocade Layer 3 switch
1. When the Layer 3 switch receives an IP packet, the Layer 3 switch checks for filters on the receiving
interface. 3 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
3 The filter can be an Access Control List (ACL) or an IP access policy.
IP packet flow through a Layer 3 switch
FastIron Ethernet Switch Layer 3 Routing Configuration Guide 25
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