User`s guide

Packet Walk-Throughs
9-4
8000-A2-GB21-10
June 1997
When WS1 pings a HotWire 5446 RTU:
1. The packet addressed to 135.1.3.4 is routed to the router by normal means.
2. The router then does an ARP request for the RTU because the router’s IP
address of 135.1.3.254 is on the same subnetwork as the RTU (with an IP
address of 135.1.3.4).
Note that the router’s interface to the MCC is multihomed (i.e., it has two IP
addresses (135.1.2.1 and 135.1.3.254) assigned to the one interface).
3. The MCC does an ARP reply with its own MAC address (proxy ARP).
4. The router then forwards the ping packet to the MCC card.
5. Upon receiving the ping, the MCC card consults its routing table to identify to
which DSL card to forward the ping.
In this case, route #1 contains a host route for 135.1.3.4 with a next hop of
DSL 135.1.3.2.
6. The ping request is then forwarded to the DSL card from the MCC card’s
s1b
interface to the DSL card’s s1b interface (which is over the DSLAM system
backplane).
7. The DSL card knows that 135.1.3.4 is directly connected over s1c (one of the
DSL card’s DSL ports).
8. The DSL card then forwards the ping to the RTU over s1c.
The HotWire 5446 RTU then issues a ping reply to IP address 135.1.1.1.
1. The RTU forwards the ping reply to the DSL card.
2. The DSL card consults its routing table to identify how to forward the reply.
Route #2 is used because the destination address (135.1.1.1) is the
135.1.1 subnet. Therefore, the next-hop address is the MCC card’s
s1b
interface (135.1.3.1).
3. Similarly, upon receiving the packet, the MCC card consults its routing table
to identify how to forward the packet. Since the destination IP address of the
ping is WS1 (135.1.1.1) and this does not match any entry in the route table,
the next-hop IP address (135.1.2.1) of the default route is used.
4. The MCC card then forwards the packet to its 10BaseT interface to the
router.
5. The router forwards the packet toward WS1 by normal means.