becomes primary, utilizing the same gateway address. The change is transparent to the user, and
the host traffic continues to be routed.
to the hosts on the LAN. This set is known as an HSRP group, or a standby group. A single
router elected from the group is responsible for forwarding the packets that hosts send to the
virtual router. This router is known as the active router. Another router is elected as the standby
router. In the event that the active router fails, the standby assumes the packet-forwarding duties
of the active router. Although an arbitrary number of routers may run HSRP, only the active
router forwards the packets sent to the virtual router.
Protocol (IRDP) is another method of dynamically determining a gateway router.
option called proxy ARP lets you configure hosts with no default gateway so that the host
always sends an Address Resolution Protocol (ARP) request for every destination. In this
situation, the router is configured to respond to any ARP if it has a route in its routing table.
Another even less-desirable option is having the hosts listen to Routing Information Protocol
(RIP). This method requires RIP broadcasts and redistribution if other routing protocols are
used.
such as RIP may send the entire routing table out at certain intervals, whereas EIGRP sends a
partial update, conserving bandwidth. Link-state protocols such as OSPF send local link status
information to peers. The LSA propagates to every peer router in the area. The routing table is
built using an algorithm. After a router knows that its interfaces are functioning, it uses the
OSPF Hello protocol to acquire neighbors, which are routers with interfaces to a common
network. The router sends hello packets to its neighbors and receives their hello packets. In
addition to helping acquire neighbors, hello packets also act as keepalives to let routers know
that other routers are still functional. Table 6-3 shows how IGPs can be categorized by their use.