Getting Cisco 871 to work with cable modem (example: SB5100)

by Cyrus Lok on Wednesday, December 9, 2009 at 5:38pm

Here’s how it should go:
label VLAN1 as ip nat inside.
label interface fastethernet 4 as ip nat outside.
there is a default named standard access list created by cisco, that is useful if you… do not plan to change the subnet/
Default subnet used is 10.10.10.0/29, if you did not change this subnet, then use the default access list.
Here’s how it should go in global config mode:
ip nat inside source list 23 interface fa4 overload.SDM seems great, it seems to make configuration less intimidating….but….it sucks big time in negotiating ip address…the sdm simply…STALLS when in the “Internet(WAN)” section!… lol… it seems to be great, easy to use, less intimidating, or even idiot proof.. blah blah blah.. but still not as good as configuring in cli 😉

871 has been configured a default network 10.10.10.0/29 to work with the sdm.
FastEthernet 0 – 3 switch ports.
FastEthernet4 (WAN) belongs to router…, after I got this I felt I should get a model with two FastEthernet ports instead of one.. 😦

One thing I find extremely stupid is, fa4 was administratively shut down by default! Cisco, what are you thinking? The so call one-CD setup (SDM) does not have option to no shut that interface! (@#$%^&*)
When I first connect the straight-through cable from fa4 to SB5100, SB5100 did not have activity light (o_O), I telnet into my router and did “show ip interface brief”, and showed fa4 was adminstratively down, I did a “no shutdown” hoping that the router send a dhcpdiscover broadcast on its own… ALAS! No ip address in fa4!!! (#$%^&*)

the command to use for router to negotiate an IP address through DHCP through fa4:
cyrus(config)#int fa4
cyrus(config-if)#ip address dhcp client-id fastEthernet 4

Sweet! After I issued this command, fastethernet 4 got the public ip from dear Starhub!! 🙂
But! I could not surf the net!!! I ping my fa4 interface, there’s echo, so I believe should be the dns-server…(#$%^&), I have used Linksys router for too long making me think Cisco router is an automatic router (gosh, what was I thinking?)

I tried these commands:
cyrus(config)#ip ddns update method testdns
cyrus(DDNS-update-method)#

cyrus(DDNS-update-method)#?
Dynamic DNS update method configuration commands:
DDNS IETF standardized Dynamic DNS update
HTTP Dynamic DNS update via HTTP based protocols
default Set a command to its defaults
exit Exit from DNS dynamic update method configuration mode
internal Update internal IOS name cache
interval Specify interval between DNS updates
no Negate a command or set its defaults

cyrus(DDNS-update-method)#ddns ?
both Update both DNS A and PTR records

I used ddns both, my running-config shows this:
ip ddns update method testdns
DDNS

I did a copy run start, and reload my router, kept my fingers and toes crossed…. It works! I did ipconfig/all, here’s the results:
Ethernet adapter Local Area Connection:

Connection-specific DNS Suffix . :
Description . . . . . . . . . . . : Broadcom NetLink (TM) Gigabit Ethernet
Physical Address. . . . . . . . . : 00-0A-E4-FC-04-FA
Dhcp Enabled. . . . . . . . . . . : Yes
Autoconfiguration Enabled . . . . : Yes
IP Address. . . . . . . . . . . . : 10.10.10.2
Subnet Mask . . . . . . . . . . . : 255.255.255.248
Default Gateway . . . . . . . . . :
DHCP Server . . . . . . . . . . . : 10.10.10.1
DNS Servers . . . . . . . . . . . : 218.186.1.38
202.156.1.58
Lease Obtained. . . . . . . . . . : Wednesday, December 09, 2009 6:09:16 PM
Lease Expires . . . . . . . . . . : Thursday, December 10, 2009 6:08:16 PM

Add-on: Do remember to configure NAT overload at fa4.
Here’s how it should go:
label VLAN1 as ip nat inside.
label interface fastethernet 4 as ip nat outside.
there is a default named standard access list created by cisco, that is useful if you… do not plan to change the subnet/
Default subnet used is 10.10.10.0/29, if you did not change this subnet, then use the default access list.
Here’s how it should go in global config mode:
ip nat inside source list 23 interface fa4 overload.

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