Proxy Address Resolution Protocol
Some IP hosts use proxy Address Resolution Protocol (ARP) to select
a router. When a host runs proxy ARP, it sends an ARP request for
the IP address of the remote host it wants to contact. A router,
Router A, on the network replies on behalf of the remote host and
provides its own MAC address. With proxy ARP, the host behaves as if
it were connected to the same segment of the network as the remote
host.
If Router A fails, the host continues to send packets destined for
the remote host to the MAC address of Router A, even though those
packets have nowhere to go and are lost. You can either wait for ARP
to acquire the MAC address of another router, Router B, on the local
segment by sending another ARP request, or reboot the host to force
it to send an ARP request. In either case, for a significant and noticeable
period
of time, the host can't communicate with the remote host, even
though the routing protocol has converged, and Router B is prepared
to transfer packets that would otherwise go through Router A.
Default Gateways
It is a very common practice to specify a default gateway on a host.
If a workstation attempts to reach a remote subnet, it simply sends
the packet toward the default gateway. There may only be one default
gateway specified in some operating systems. Thus, if the
router being used as the default gateway fails, the workstation
cannot reach remote subnets. Although the network infrastructure can
recover quickly and reconverge, the host cannot and will not know
about a new default gateway. This is illustrated in the Figure.
With operating systems that support multiple default gateway entries
such as Microsoft Windows, failover between the gateway entries is
not dynamic. The operating system will send an echo to each gateway
entry in order on bootup; the first gateway to reply will be used as
the default gateway. Therefore, to use another gateway in the event
of a router failure requires a restart of the host.
Dynamic Routing Protocol
Some IP hosts run a dynamic routing protocol such as the
Routing Information Protocol (RIP) or Open Shortest Path First (OSPF)
to discover routes. The drawback of using RIP is that it is slow to
converge. Running a dynamic routing protocol
on every host may not be feasible for numerous reasons, including
administrative overhead, processing overhead, security issues, or
lack of a protocol implementation for some platforms.
ICMP Router Discovery Protocol
Some newer IP hosts use ICMP Router Discovery Protocol (IRDP) (RFC
1256) to find a new router when a route becomes unavailable. A
host that runs IRDP listens for hello multicast messages from its
configured router and uses an alternate router when it no longer
receives those hello messages. The default timer values of IRDP mean
that it is not suitable for detection of failure of the first hop.
The default advertisement rate is once every 7 to 10 minutes, and
the default lifetime is 30 minutes.
Dynamic Host Configuration
Protocol
Dynamic Host Configuration Protocol (DHCP) (RFC 1531)
provides a mechanism for passing configuration information to hosts
on a TCP/IP network. A host that runs a DHCP client requests
configuration information from a DHCP server when it boots onto the
network. This configuration information typically comprises an IP
address and a default gateway. There is no mechanism for switching
to an alternative router if the default gateway fails.
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