Arp - Nortel Optical Multiservice Edge 6130 Planning Manual

10-96 Appendix A: Data communications planning
Note that circuitless IP is a Bay term; Cisco uses the term 'loopback' for these
interfaces. Having an interface that always exists within a router is very useful
for the following reasons:
1 If a tunnel is set up between two router interfaces and one of the interfaces
2 If during a Telnet session on a router the interface to which the session is
3 Other interfaces can be referenced to the circuitless IP interface. This is
4 The circuitless IP interface identifies a router for OSPF (Open Shortest

ARP

ARP (Address Resolution Protocol) is used to map IP addresses to LAN
(Local Area Network) hardware addresses. When a host wishes to send a
packet to a host on another network, it sends the packet to its gateway for
forwarding. It can also do the same for a packet destined for a host within the
same network but it leads to excessively high traffic levels, especially if a large
number of hosts are on the LAN. Therefore, in order to reduce the traffic on a
LAN, a node uses ARP with another node when it determines that the
destination address is on a directly attached network. The node can determine
if the host is local by comparing the network portion of its own IP address
(including the subnet) with the target address.
Therefore, in order to avoid using the gateway, the originating host needs to
determine the destination host's local data link layer address. It achieves this
by sending out an ARP request message containing its own IP address and data
link layer address, and the IP address of the destination host. This message is
sent via the gateway. The destination host then responds with an ARP reply
message containing its own data link layer address and uses the originating
host's data link layer address as the destination address. Thus the reply does
not need to go via the gateway. The originating host and destination host store
Optical Multiservice Edge 6130
fails, the tunnel fails. However, when the tunnel is set up between two
circuitless IP interfaces, if the normal route fails, the tunnel is re-routed if
another route exists and does not fail.
connected goes down, then the session is lost. Another connection via the
IP address of an alternative interface must be made. If Telnet sessions are
set up to connect to the router using the circuitless IP interface, then loss
of one interface is not a problem, providing the router has at least one
working IP interface.
known as an unnumbered interface. This applies only to point-to-point
interfaces (that is, not an Ethernet port). This reduces the number of IP
addresses needed.
Note: It is more difficult to find faults in a network with an unnumbered
link, and topologies from some network management systems are easier to
follow with unnumbered serial links. The use of unnumbered links is still
permissible where sufficient IP addressing space is not available.
Path First), tunnelling, and management.
NT6Q92MA Rel 1.0 Iss 1 Standard September 2006