152 BigIron RX Series Configuration Guide53-1001986-01The IP packet flow71. When the device receives an IP packet, the device checks for IP ACL filters on the receivinginterface. If a deny filter on the interface denies the packet, the device discards the packet andperforms no further processing. If logging is enabled for the filter, then the device generates aSyslog entry and SNMP trap message.2. If the packet is not denied, the device checks for Policy Based Routing (PBR). If the packetmatches a PBR policy applied on the incoming port, the PBR processing is performed andeither drops the packet or forwards it to a port, based on the route map rules.3. If the incoming packet does not match PBR rules, the device looks in the hardware IP routingtable to perform IP routing. The hardware routing table is pre-loaded with the complete routingtable, except for the directly connected host entries. Default and statically defined routes arealso pre-loaded in the hardware routing table. If the incoming packet matches a route entry,the packet is routed according to the information provided in the route entry. The ECMP andtrunk load balancing is done by the hardware, if needed, to select the outgoing port.4. If there is no match in the IP routing table and a default route is not configured, the packet isdropped. For an IP packet whose destination IP address is to a directly connected host, the firstpacket is forwarded to the CPU. If the ARP is resolved and the host is reachable, the CPUcreates a route entry in the hardware to route subsequent packets in hardware.The software enables you to display the ARP cache and static ARP table, the IP route table, the IPforwarding cache.ARP cache tableThe Address Resolution Protocol (ARP) is supported on the device. Refer to “IP fragmentationprotection” on page 184.The ARP cache contains entries that map IP addresses to MAC addresses. Generally, the entriesare for devices that are directly attached to the device.An exception is an ARP entry for an interface-based static IP route that goes to a destination that isone or more router hops away. For this type of entry, the MAC address is either the destinationdevice’s MAC address or the MAC address of the router interface that answered an ARP request onbehalf of the device, using proxy ARP.The ARP cache can contain dynamic (learned) entries and static (user-configured) entries. Thesoftware places a dynamic entry in the ARP cache when the device learns a device’s MAC addressfrom an ARP request or ARP reply from the device.The software can learn an entry when the device receives an ARP request from another IPforwarding device or an ARP reply. Here is an example of a dynamic entry.Each entry contains the destination device’s IP address and MAC address.Static ARP tableIn addition to the ARP cache, the device has a static ARP table.Entries in the static ARP table are user-configured. You can add entries to the static ARP tableregardless of whether the device the entry is for is connected to the device.IP Address MAC Address Type Age Port1 207.95.6.102 0800.5afc.ea21 Dynamic 0 6
