Help with Routing on Cisco 2600

[theironcrosser]

I am having trouble routing LAN requests from a Cisco 2600 router. The WAN link is a serial T1 line and is set up on the router. I am able to ping the DNS resolver, the T1 serial line, and the ethernet line from the router.

The client laptop inside the LAN can ping the router but cannot ping the DNS. The same occurs when the laptop is connected directly to the Ethernet port of the router. I have set up IP routing amongst other things, but my concept seem to be blurred.

Will someone please explain to me the correct procedures to set up the Ethernet0/0 to route through Serial0/0? Thanks in advance.

[spremkumar]

Hi

Hav u enabled NAT in u r router ?if yes try to do a ext ping to DNS using the ethernet as the source address also post u r config.

regds

prem

[theironcrosser]

Thanks for the reply Prem. I don't have private IP addresses so I don't think enabling NAT will do it, or will it?

I have an assigned block of external IP addresses, 38.1.1.0/24 for example. My Serial0/0 is set to 39.1.1.1 and Ethernet0/0 is set to 38.1.1.254. I will post my config if still needed. Thanks!

[spremkumar · Accepted solution]

hi

Assuming u r connected with ur isp ,try pinging the DNS server ip with source as ur ethernet (38.1.1.254 in ur case)and do find with ur ISP whether they hve done the reverse routing for your local lan there in their router towards u r serial interface.

U can also chek up reverse tracing your ethernet ip from reverse traceroute sites and ensure u r reachable from outside world.

reverse sites : route-server.exodus.net , route-server.cerf.net Telnet to these route servers and chek tracing u r eth ip.

regds

prem

[theironcrosser]

Thanks Prem. I will try that.

[thisisshanky]

Please paste your relevant configs and we can add what configuration is required..

[theironcrosser]

Here is the running-config:

-----------------------

service timestamps debug uptime

service timestamps log uptime

service password-encryption

!

hostname Router

!

enable secret xxxxxx

enable password xxxxx

!

!

!

!

!

ip subnet-zero

ip name-server 66.28.0.45

ip name-server 66.28.0.61

!

!

!

!

interface Ethernet0/0

ip address x.x.x.x.x.x.x

no ip redirects

no ip proxy-arp

load-interval 30

no cdp enable

!

interface Serial0/0

ip address 38.112.x.x.x.x.252

no ip redirects

no ip proxy-arp

encapsulation ppp

load-interval 30

service-module t1 timeslots 1-24

service-module t1 remote-alarm-enable

no cdp enable

!

ip classless

ip route 0.0.0.0 0.0.0.0 38.112.17.165

ip http server

!

no cdp run

snmp-server community n RO

!

line con 0

exec-timeout 0 0

transport input none

line aux 0

line vty 0 4

password xxxx

login

!

no scheduler

--------------------

The following was assigned by the ISP:

Serial IP: 38.112.17.166 255.255.255.252

IP route: 0.0.0.0 0.0.0.0 38.112.17.165 (<---this is also your neighbor if doing BGP)

Your assigned block of IP addresses is: 38.112.77.0/24

CSU needs to be set for timeslots 1-24

this will be a PPP connection (so set the interface to

encapsulation PPP)

You must allow pings from 66.28.3.0/24 and 66.250.250.0/23

DNS Resolvers

res1.dns.cogentco.com (66.28.0.45)

res2.dns.cogentco.com (66.28.0.61)

[theironcrosser]

Here is the output of the ARP and IP Route:

------------

LS-Router#show arp

Protocol Address Age (min) Hardware Addr Type Interface

Internet 38.112.77.2 21 0006.2990.9fe1 ARPA Ethernet0/0

Internet 38.112.77.1 - 00d0.ba89.eb00 ARPA Ethernet0/0

LS-Router#show ip route

Codes: C - connected, S - static, I - IGRP, 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, E - EGP

i - IS-IS, 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 38.112.17.165 to network 0.0.0.0

38.0.0.0/8 is variably subnetted, 3 subnets, 3 masks

C 38.112.77.0/24 is directly connected, Ethernet0/0

C 38.112.17.165/32 is directly connected, Serial0/0

C 38.112.17.164/30 is directly connected, Serial0/0

S* 0.0.0.0/0 [1/0] via 38.112.17.165

------------

[thisisshanky]

Are you able to ping an IP in the internet from the router ? Also do an extended ping (type ping and hit enter and follow instructions). In the source address field, change it to the 38.112.77.1 ip address (the inside ethernet interface and see if you are able to ping ? Most probably your ISP has not yet setup routing for that network in their cloud.

[theironcrosser]

I am able to ping an IP in the internet from the router. I am also able to ping Serial0/0 on the router from the internet. I have verified with my ISP that routing has been set up.

I will have to try the extended ping later. What would the extended ping tell me if it fails?

There must be something I am missing because I am almost sure of the configuration. Anyone see something I don't? Thanks!

[spremkumar]

Hi

When ever you give a normal ping from u r router it takes the source as u r serial interface ip and goes out.Since the other end is connected to isp and has /30 ip address you would be able to reach the same withot any sort of routing (they would be running ibgp or ospf in their core and ebgp with their upstream).

But when u do a extended with source as ur ethernet it will take the source as u r fastethernet,if the isp hasnt prvodied the route for your local lan block in their PE(Peer Edge connecting to CE Customer End)router pointing towards ur link u wont be able to get into the internet world or wont be able to ping from u r local lan.

if the ext ping fails u can make sure of urself tht the reverse route isnt ther in the ISP end .

regds

prem

[theironcrosser]

Using the Extended Ping with the DNS address as my destination and Ethernet0/0 address as my source I am unable to ping the host.

Using the reverse traceroute site you mentioned earlier I can completely trace to the Serial0/0 address. However, tracing to the Ethernet0/0 address, I find a separate route taken and then times out.

For example, the path tracing to Serial0/0 looks like this: 1.1.1.1 -> 1.1.1.2 -> 1.1.1.3 -> Serial0/0 address. The path tracing to Ethernet0/0 the path looks like this: 1.1.1.1 -> 1.1.1.10 -> * -> *

To my understanding both trace routes to Serial0/0 and Ethernet0/0 should be the same path, except there should be one additional path -> Serial0/0 -> Ethernet0/0. Does this mean the ISP is not setting up their routing correctly? Thanks!

[theironcrosser]

Problem solved! Apparantly the ISP did not set up the routing correctly. Using the reverse traceroute helped prove the misconfiguration.

Thanks for all the responses and help!