IP packet flow through a Layer 3 device

Figure 3  IP Packet flow through a Brocade Layer 3 device
  1. When the Layer 3 device receives an IP packet, the device checks for filters on the receiving interface. The filter may be an Access Control List (ACL) or an IP access policy. If a deny filter on the interface denies the packet, the Layer 3 device discards the packet and performs no further processing, except for generating a Syslog entry and an SNMP message, if logging is enabled for the filter.
  2. If the packet is not denied at the incoming interface, the Layer 3 device 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 device 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 device 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 device 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 device forwards the packet to the IP address in the entry. The device sends the packet to a queue on the outgoing ports listed in the forwarding cache. The device 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 device checks the IP route table for a route to the packet destination. If the IP route table has a route, the device 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 device 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 device uses the new cache entry to forward subsequent packets to the same destination.
Parent topic: IP configuration overview