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weird connectivity issue between serial0 connection and a switch

Posted on 2008-10-07
Medium Priority
Last Modified: 2013-11-09
I have a Cisco 1720 router with a T1 CSU/DSU WIC in it.  The building has a T3 that it splits up and gives to the companies in the building.  They have given us an IP address that I used for the serial interface.  After I configured the router (config to follow...), the WIC was in alarm.  Couldn't figure out why.  Eventually, I transformed an RJ-45 Ethernet cable into an RJ-48C cable (spliced wires 1,4 and 2,5) and the amber alarm light went off.  Serial0 interface now shows as UP.  However, the lights on the Router are all green, but there is no light on the switch.  It's like the switch doesn't see it.  I've switched cables, ports, etc. to no avail.  The local network behind the router cannot access anything outside the router.  Any ideas what is wrong?  Here's the router config:

version 12.1
no service single-slot-reload-enable
service timestamps debug datetime localtime
service timestamps log datetime localtime
service password-encryption
hostname router_axx
no logging buffered
no logging buffered
logging rate-limit console 10 except errors
enable secret 5 $1$DNwM$rmHCiTTpun2RRBHm4kK6f.
memory-size iomem 25
clock timezone MST -7
clock summer-time MDT recurring
ip subnet-zero
no ip finger
ip name-server 12.127.xxx.71
ip dhcp excluded-address
ip dhcp excluded-address
ip dhcp excluded-address
ip dhcp excluded-address
ip dhcp pool ais
   domain-name ais_regus
   dns-server 12.127.xxx.71
interface FastEthernet0
 ip address
 ip access-group 100 in
 ip access-group 100 out
 ip nat inside
 speed auto
 no cdp enable
interface Serial0
 ip address 12.41.xxx.30
 ip nat outside
 no cdp enable
ip nat pool nat-pool 12.41.xxx.30 12.41.xxx.30 netmask
ip nat inside source list 1 pool nat-pool overload
ip classless
ip route Serial0
no ip http server
access-list 1 permit
access-list 100 permit tcp any any
access-list 100 permit udp any any
access-list 100 permit ip any any
access-list 100 permit icmp any any
no cdp run
line con 0
 transport input none
line aux 0
line vty 0 4
 password 7 14300705020D397B2A
no scheduler allocate
Question by:jamjamiss
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LVL 79

Expert Comment

ID: 22664956
What, exactly, are you connecting the T1 interface to? It cannot plug into an Ethernet switch!

Author Comment

ID: 22666005
Okay, so how can this be done then?  Once again, I've got a single public IP that I have been assigned, and am using PAT on the router.  My network of 5 or so people are behind the router.  What do I connect the router's serial interface to?
LVL 12

Accepted Solution

Pugglewuggle earned 2000 total points
ID: 22666179
You can ONLY plug a T1 CSU/DSU into a T1 line! If you're trying to plug this into a T3 then that will NEVER work. if the building provides you with internet from their switch, then you will need to get an ethernet WIC for your 1720.
I have a few extras that I ordered just for 1720s and 1721s if you need one (can I say that on here?). The part number is WIC-1ENET= They're usually about $50 USD + shipping.
This should get you online once you get one of those in and configure it... One other thing... I see you're running IOS 12.1 on your 1720. Upgrade that immediately to something like 12.3 or 12.4... 12.1 is way old. Just make sure that whatever it is it supports a WIC-1ENET card. Some of the older IOS wouldn't work with certain cards.
BTW I hope you didn't fry your T1 card... it might be gone though after plugging it into an ethernet network... best of luck to you on that.
Cheers! Let me know if you have any questions!
LVL 12

Expert Comment

ID: 22666181
BTW... you connect the serial interface to nothing. :)

Author Closing Comment

ID: 31504049
Thanks Pugglewuggle.  That makes sense.  I knew it was something easy I was just missing!  I'll go buy a new WIC for it.  Thanks again!!

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