Brocade Multi-Service IronWare Switching Configuration Guide (Supporting R05.6.00) - Manual (Page 735)

Multi-Service IronWare Switching Configuration Guide 705
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The IP packet flow
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IP forwarding cache
The Brocade device maintains a software cache table for fast processing of IP packets that are
forwarded or generated by the CPU. The cache also contains forwarding information that is
normally contained in the IP routing table. For example, the cache contains information on the
physical outgoing port, priority, VLAN, and the type of cache entry. Also, cache entries have
hardware information, which is useful for debugging and aging.
There are two types of IP cache entries.
1. Directly connected host entries – These entries are created when the CPU receives the first
packet destined to a directly connected host. Host entries are set to age out after a certain
period if no traffic is seen for that entry.
2. Network entries – These entries are created when a route table entry is created in software.
These entries are not subjected to aging. A route table entry is created when routes are
learned by routing protocols such as OSPF or when routes are statically configured.
Example : IP forwarding cache
Each IP forwarding cache entry contains the IP address of the destination, and the IP address and
MAC address of the next-hop router interface to the destination. If the destination is actually an
interface configured on the Brocade device itself, as shown here, then next-hop information
indicates this. The port through which the destination is reached is also listed, as well as the VLAN
and Layer 4 QoS priority associated with the destination if applicable.
To display the IP forwarding cache, refer to “Displaying the forwarding cache” .
IP packet queuing
When the user wants to send a packet to a local host, the software looks up the IP in the ARP
cache. If the address is found, it gets the MAC address, constructs an Ethernet header with the
correct source or destination MAC addresses, and sends it.
If the address is not found in the table, ARP broadcasts a packet to every host on the Ethernet,
except the one from which it received the packet. The packet contains the IP address for which an
Ethernet address is sought. If a receiving host identifies the IP address as its own, it will send its
Ethernet address back to the requesting host.
For management of IP packet queuing when a packet is received for a directly connected host
when there is no MAC address available, the ip drop-arp-pending-packets command has been
added to allow the packets in the CPU to be dropped.
To set all packets in the LP buffer to be dropped when ARP resolution is going on, enter a command
such as the following:
Brocade(config)#ip drop-arp-pending-packets
Syntax: [no] ip drop-arp-pending-packets
Use the no ip drop-arp-pending-packets command to return to the default behavior of continue with
pending IP packets while ARP resolution.
IP Address Next Hop MAC Type Port Vlan Pri
1 192.168.1.11 DIRECT 0000.0000.0000 PU n/a 0