I have three layer 3 devices in a pretty simple topology:
PC1 -> [Fe0/0] C2621 [Fe0/1] -> [Fe0/2] Modem [WAN] -> Internet
PC1: 192.168.254.100/24
C2621: Fe0/0: 192.168.254.254/24
Fe0/1: 10.0.0.3/24
Modem: Fe0/2: 10.0.0.1/24
From PC1 I'm able to ping the modem:
Ping statistics for 10.0.0.1:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 0ms, Maximum = 1ms, Average = 0ms
The modem does not have any static routes assigned to it. How is it possible that the pings go through? I made the same topology in GNS3 with three routers with the exact same IP addresses and was not able to ping the modem:
R3#ping 10.0.0.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.0.0.1, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)
This model of modem doesn't allow me to see the ARP table. Also, a traceroute shows that the ping goes to 10.1.1.1 and then 10.0.0.1. Just as it should IF the modem actually had a route back to the 10.1.1.0/24 network which it doesn't. I even tried disconnecting the cable from R1 to the modem to see if the pings would fail. They did.
It makes absolutely no sense for the modem to know where the 10.1.1.0/24 network is. How is this possible? The modem is a SBG6580 Cable Modem
Additional information: `
C2621#show ip rou
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route
Gateway of last resort is not set
10.0.0.0/24 is subnetted, 1 subnets
C 10.0.0.0 is directly connected, FastEthernet0/1
C 192.168.254.0/24 is directly connected, FastEthernet0/0.254
C2621#show run
Building configuration...
Current configuration : 1109 bytes
!
version 12.3
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
!
hostname C2621
!
boot-start-marker
boot-end-marker
!
enable secret 5 $1$sc4H$NGsbRnN8X2zErueSVyaim/
!
no aaa new-model
ip subnet-zero
ip cef
!
!
!
no ip domain lookup
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
interface FastEthernet0/0
no ip address
duplex auto
speed auto
!
interface FastEthernet0/0.254
encapsulation dot1Q 1 native
ip address 192.168.254.254 255.255.255.0
no snmp trap link-status
!
interface Serial0/0
no ip address
shutdown
no fair-queue
!
interface FastEthernet0/1
description Test Description
ip address 10.0.0.3 255.255.255.0
no ip proxy-arp
duplex auto
speed auto
!
ip classless
ip http server
!
!
!
voice-port 1/0/0
!
voice-port 1/0/1
!
voice-port 1/1/0
!
voice-port 1/1/1
!
!
!
!
!
!
line con 0
logging synchronous
line aux 0
line vty 0 1
exec-timeout 0 0
password cisco
login
transport input telnet
line vty 2 4
exec-timeout 0 0
password cisco
login
line vty 5
exec-timeout 0 0
password cisco
login
line vty 6 15
password cisco
login
!
!
end
C2621#show ip arp
Protocol Address Age (min) Hardware Addr Type Interface
Internet 10.0.0.3 - 0001.9651.dea1 ARPA FastEthernet0/1
Internet 10.0.0.1 2 94cc.b902.7976 ARPA FastEthernet0/1
Internet 192.168.254.254 - 0001.9651.dea0 ARPA FastEthernet0/0.254
Internet 192.168.254.4 11 0017.5a57.3480 ARPA FastEthernet0/0.254
Internet 192.168.254.2 11 0011.bb6d.0300 ARPA FastEthernet0/0.254
Internet 192.168.254.3 11 0015.f9f8.0900 ARPA FastEthernet0/0.254
Internet 192.168.254.1 11 0017.59eb.3580 ARPA FastEthernet0/0.254
Internet 192.168.254.100 2 e000.0000.129c ARPA FastEthernet0/0.254
C2621#show version
Cisco Internetwork Operating System Software
IOS (tm) C2600 Software (C2600-IPVOICE-M), Version 12.3(15), RELEASE SOFTWARE (fc3)
Technical Support: http://www.cisco.com/techsupport
Copyright (c) 1986-2005 by cisco Systems, Inc.
Compiled Tue 24-May-05 14:01 by ssearch
Image text-base: 0x80008098, data-base: 0x8183E8D4
ROM: System Bootstrap, Version 11.3(2)XA4, RELEASE SOFTWARE (fc1)
ROM: C2600 Software (C2600-IPVOICE-M), Version 12.3(15), RELEASE SOFTWARE (fc3)
C2621 uptime is 3 hours, 29 minutes
System returned to ROM by power-on
System image file is "flash:c2600-ipvoice-mz.123-15.bin"
cisco 2621 (MPC860) processor (revision 0x102) with 61440K/4096K bytes of memory.
Processor board ID JAB04130AL8 (912529492)
M860 processor: part number 0, mask 49
Bridging software.
X.25 software, Version 3.0.0.
2 FastEthernet/IEEE 802.3 interface(s)
1 Serial network interface(s)
2 Voice FXO interface(s)
2 Voice FXS interface(s)
32K bytes of non-volatile configuration memory.
16384K bytes of processor board System flash (Read/Write)
Configuration register is 0x2102
no ip proxy-arp
to Fa0/1 on your Cisco2621. If the behavior changes after doing this, you have proven that the cable modem is proxy-arping for the PC's source IP. – Mike Pennington Dec 27 '13 at 3:30