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Configure T1 and passthrough IP?



I have a T1 handed off to me from my local ISP.  I have a Cisco 2620XM w/ 12.3(1a) installed.  Normally, at my other locations, I would have the T1 hit an ISP provided device and hand off to me as an Ethernet, but this was legacy before my time when it was ordered.

In short, I want to pass through this circuit to the firewall we use (Watchguard) via ethernet.  I've configured the router, and the T1 is up.  Not exactly sure what to do on the ethernet side, I did configure a network (took a private block, 192.168.0.x) but is this the best (or only) way to do this?  Here is my config:

Current configuration : 1752 bytes


version 12.3

service timestamps debug datetime msec

service timestamps log datetime msec

no service password-encryption


hostname Warehouse-Cisco


boot system flash c2600-is-mz.123-1a.bin

logging buffered 4096 debugging

enable secret 5 $A-BunchOfRandomStuff

enable password 15 NotTheRealPassword


clock timezone CST -6

clock summer-time CDT recurring

ip subnet-zero

no ip routing



no ip domain lookup

ip domain name

ip name-server

ip name-server

no voice hpi capture buffer

no voice hpi capture destination

buffers verybig permanent 0


interface FastEthernet0/0

description Ethernet Side

ip address

no ip route-cache

speed auto



interface Serial0/0

description MYCiruitInfo ABunchOfNumberAndLettersHere

ip address

encapsulation ppp

no ip route-cache


service-module t1 timeslots 1-24


interface Serial0/1

description Not in Use

mtu 2104

no ip address

no ip route-cache


clockrate 56000


ip default-gateway

ip http server

ip classless



no logging trap

Obviously my relavant information I changed to protect the innocent ;)



6 Replies 6

paolo bevilacqua
Hall of Fame
Hall of Fame

Ask you ISP what is your LAN address assigned and configure it on LAN interface. No NAT is needed.

Note: you will need to re-enable 'ip cache'

Thanks a lot!

I did have the LAN addresses (with mask), I have configured the ethernet side with that.  I will configure the other end to an address within that above mentioned range.  You mentioned turning on IP route cache.  I did that for the FastEthernet but do I need to enable it for the Serial side?

also, I have IP routing turned off, just using a default gateway.  Should that be sufficient?

Thanks again,


No, being this a router, you need to have ip routing - the default. Then configure a default route.

And have (the default) of ip route-cache enabled everywhere.

Ok, so I enabled:

removed default-gateway

IP routing

IP route (the distant endpoint of the T1).

Not pingable, but may not be a configuration issue.  Need to open ticket with Carrier.  Will update tomorrow.

THanks for all the help!


Check "show interface se0/0" to begin with.

Thank you for the nice rating and good luck!

The show Interface command gives me the following:

Serial0/0 is up, line protocol is up

  Hardware is PQUICC with Fractional T1 CSU/DSU

  Description: XO: 14/HCRT/242639..SW Loop: 03.HCRU.008646..ALPH

  Internet address is

  MTU 1500 bytes, BW 1536 Kbit, DLY 20000 usec,

     reliability 255/255, txload 1/255, rxload 1/255

  Encapsulation PPP, LCP Open

  Open: CDPCP, IPCP, loopback not set

  Last input 00:00:50, output 00:00:02, output hang never

  Last clearing of "show interface" counters 1d17h

  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0

  Queueing strategy: weighted fair

  Output queue: 0/1000/64/0 (size/max total/threshold/drops)

     Conversations  0/2/256 (active/max active/max total)

     Reserved Conversations 0/0 (allocated/max allocated)

     Available Bandwidth 1152 kilobits/sec

  5 minute input rate 0 bits/sec, 0 packets/sec

  5 minute output rate 0 bits/sec, 0 packets/sec

     34766 packets input, 2233917 bytes, 0 no buffer

     Received 0 broadcasts, 0 runts, 0 giants, 0 throttles

     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort

     35408 packets output, 2261304 bytes, 0 underruns

     0 output errors, 0 collisions, 2 interface resets

     0 output buffer failures, 0 output buffers swapped out

     1 carrier transitions

     DCD=up  DSR=up  DTR=up  RTS=up  CTS=up

The circuit is indeed up, pingable from a remote location, so I know the circuit is good.  Still waiting on carrier information.


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