NOTE:
RIP does not use the router ID.
If no router ID is configured, then, by default, the router ID on a routing switch is the first IP address that becomes
physically active at reboot. This is usually the lowest numbered IP interface configured on the device. However, if
no router ID is configured, and one or more user-configured loopback interfaces are detected at reboot, the
lowest-numbered (user-configured) loopback interface becomes the router ID. If the lowest-numbered loopback
interface has multiple IP addresses, the lowest of these addresses will be selected as the router ID. Once a router
ID is selected, it does not automatically change unless a higher-priority interface is configured on the routing
switch
and
OSPF is restarted with a reboot. (User-configured loopback interfaces are always higher priority than
other configured interfaces.) However, you can explicitly set the router ID to any valid IP address, as long as the
IP address is not in use on another device in the network.
NOTE:
To display the router ID, enter the
show ip ospf
CLI command at any Manager EXEC CLI
level.
Figure 23: 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
Chapter 5 Routing Basics
151
