Foundry Networks Switch and Router Installation And Configuration Manual page 446

Foundry Switch and Router Installation and Configuration Guide
Figure 15.1 shows the following packet flow:
1. When the Layer 3 Switch receives an IP packet, the Layer 3 Switch checks for filters on the receiving
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interface. 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 port(s) 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's source address and TCP or UDP
port, the Layer 3 Switch looks in the IP forwarding cache for an entry that matches the packet's 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 port(s) 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's 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 port(s).
• If the running-config contains a Policy-Based Routing (PBR) definition or 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 a PBR definition or 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's 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-configured) entries. The software places a
dynamic entry in the ARP cache when the Layer 3 Switch learns a device's 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:
1.The filter can be an Access Control List (ACL) or an IP access policy.
15 - 4 December 2000