User`s guide

Packet Walk-Throughs
9-2
8000-A2-GB21-10
June 1997
97-15474-01
Router
155.1.2.1
155.1.3.1
ISP1
155.1.2.2
DSL Card
155.1.3.2
RTU
135.1.3.3
155.1.3.4
Unnumbered
DSL
Interface
ES1
Host/Net/Subnet
1) 155.1.3.4
2) 155.1.3.4
Subnet Mask
255.255.255.255
255.255.255.255
Next-Hop Address
155.1.3.1
135.1.3.3
S/D (Source/Destination)
src (source)
dst (destination)
Partial DSL Routing Table
ES1 pings ISP1
ISP1 issues reply to ping
1234567
1 2 3 4 5 6 7 8
When ES1 pings ISP1:
1. ES originates a packet addressed to 155.1.2.2. Because they are both on the
155.1 network, ES1 ARPs to map ISP1’s IP address into a MAC address.
2. The RTU receives the broadcast ARP request from ES1.
3. The RTU replies to the ARP request with its own MAC address (proxy ARP).
4. After ES1 receives the ARP reply, it sends the packet to the MAC address of
the RTU.
5. Upon receiving this packet, the RTU forwards it to the DSL card over its DSL
interface.
6. When the DSL card receives this packet, the DSL card consults its routing
table to determine how to route the packet. Since a source route is defined
for ES1 (route #1), the DSL card forwards the packet to the router
(151.1.3.1), which is the next-hop.
7. The router then forwards the packet to ISP1.
ISP1 then issues a reply to the ping.
1. The ISP sends the ping reply packet addressed to 155.1.3.4.
2. By normal means, the packet arrives at the router.
3. Because the router has an interface with an address 155.1.3.1 (on 155.1.3
subnet) it ARPs for 155.1.3.4.
4. Because the DSL card has a host route (marked PA=y) for 155.1.3.2, it
responds to the ARP request with its own MAC address (proxy ARP).
5. Then, the ping reply is sent directly to the DSL card.
6. The DSL card then consults its routing table to identify the next hop to
forward the packet. Since a host route is defined for ES1 (route #2), the RTU
135.1.3.3 is used as the next hop.