KB.15.15

Figure 21 Example of show ip ospf command with router ID displayed

Configuring ARP parameters

ARP is a standard IP protocol that enables an IP routing switch to obtain the MAC address of

another device's interface when the routing switch knows the IP address of the interface. ARP is

enabled by default and cannot be disabled.

How ARP works

A routing switch needs to know a destination's MAC address when forwarding traffic, because

the routing switch encapsulates the IP packet in a Layer 2 packet (MAC layer packet) and sends

the Layer 2 packet to a MAC interface on a device directly attached to the routing switch. The

device can be the packet's final destination or the next-hop router toward the destination.

The routing switch encapsulates IP packets in Layer 2 packets regardless of whether the ultimate

destination is locally attached or is multiple router hops away. Since the routing switch's IP route

table and IP forwarding cache contain IP address information but not MAC address information,

the routing switch cannot forward IP packets based solely on the information in the route table or

forwarding cache. The routing switch needs to know the MAC address that corresponds with the

IP address of either the packet's locally attached destination or the next-hop router that leads to

the destination.

For example, to forward a packet whose destination is multiple router hops away, the routing

switch must send the packet to the next-hop router toward its destination, or to a default route or

default network route if the IP route table does not contain a route to the packet's destination. In

each case, the routing switch must encapsulate the packet and address it to the MAC address of

a locally attached device, the next-hop router toward the IP packet's destination.

To obtain the MAC address required for forwarding a datagram, the routing switch does the

following:

• First, the routing switch looks in the ARP cache (not the static ARP table) for an entry that lists

the MAC address for the IP address. The ARP cache maps IP addresses to MAC addresses.

The cache also lists the port attached to the device and, if the entry is dynamic, the age of

the entry. A dynamic ARP entry enters the cache when the routing switch receives an ARP

reply or receives an ARP request (which contains the sender's IP address and MAC address.)

A static entry enters the ARP cache from the static ARP table (which is a separate table) when

the interface for the entry comes up.

To ensure the accuracy of the ARP cache, each dynamic entry has its own age timer. The timer

is reset to zero each time the routing switch receives an ARP reply or ARP request containing

the IP address and MAC address of the entry. If a dynamic entry reaches its maximum allowable

118 Routing Basics