Packet Walk-Throughs
9-2
8000-A2-GB21-20
November 1997
97-15474-02
Router
155.1.2.1
155.1.3.1
NSP1
155.1.2.2
8540 DSL
Card
155.1.3.2
RTU*
155.1.3.4
Unnumbered
DSL
Interface
ES1
Host/Net
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
s1c
S/D (Source/Destination)
src (source)
dst (destination)
Partial DSL Routing Table
ES1 pings NSP1
NSP1 issues reply to ping
1
2
3
4
5
6
7
1
2
3
4
5
6
7
8
* The RTU can be a 5170, 5171, 5216, or 5246 RTU.
s1c
When ES1 pings NSP1:
1. ES originates a packet addressed to 155.1.2.2. Because they are both on the
155.1 network, ES1 ARPs to map NSP1’s IP address into a MAC address.
2. The RTU forwards the ARP to the 8540 DSL card over its DSL interface
(e.g., s1c).
3. The 8540 DSL card 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 8540 DSL card.
5. Upon receiving this packet, the RTU forwards it to the 8540 DSL card over its
DSL interface.
6. When the 8540 DSL card receives this packet, the 8540 DSL card consults
its routing table to determine how to route the packet. Since a source route is
defined for ES1 (route #1), the 8540 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 NSP1.
NSP1 then issues a reply to the ping.
1. The NSP 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 8540 DSL card has a host route (marked PA=y) for 155.1.3.4, it
responds to the ARP request with its own MAC address (proxy ARP).
5. Then, the ping reply is sent directly to the 8540 DSL card.
