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Cisco 2600 Routing

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by ninet4

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My company is about to move from one location to another. We are not doing it all at once however. All of our administration is moving in the first phase, and the rest are to follow in 2-4 weeks. What this means is that I need to support both locations for near a month. I plan on doing this by creating a site-to-site VPN. The VPN is not currently my problem. At Site2 (the new location) I have a Cisco 2620 with a WIC-1DSU-T1 card, connected to a full T1. The Serial0/0 Interface is completely up, and I can ping out anywhere, including to my current network at Site1. From Site1, I can ping both the external Serial0/0 interface and the internet FastEthernet0/0 interface of the Cisco 2620 router at Site2. However, I am unable to ping anything on the internal network, from Site1. There is currently onle 1 device on the internal network at Site2, and its IP address is one that was supplied by my ISP. If I telnet from Site1 to the router at Site2, I can ping the IP address of the host in the inside of the network, so I know that it is online. The question is, why can't I ping from Site1 to the inside of Site2? Is it a routing problem? If I do a tracert, it makes it all the way to the outside interface Serial0/0 of the Cisco2620.

8 replies

by joseph moore

Ok, so at Site2, the host "its IP address is one that was supplied by my ISP."
What is the IP address of your FE0/0 interface? Is it on the same subnet as the IP of the host you are trying to reach? I sounds like it isn't.

Do a SH IP ROUTE on the Site2 router. You are gonna get something like this:

1605R#sh 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, * - candidate default
U - per-user static route, o - ODR

Gateway oflast resort is not set

172.16.0.0/24 is subnetted, 2 subnets
C 172.16.30.0 is directly connected, Ethernet0
C 172.16.20.0 is directly connected, Ethernet1
1605R#

Now, your routes should show the IP segment(s) to Site1, aswell as whatever range you use at Site2. Now, what I think is happening is the host at Site2 has an IP that is not in the same segment as FE0/0 uses.
For example, say FE0/0 is 192.168.2.1/24
Say also that the host behind this interface is 192.168.3.1/24
Well, both addresses are 24-bit subnetted addresses, so they are on different networks. You need to route between them.

by joseph moore

But (and I have done this on accident) you could PING the host from the FE0/0 interface because switches and hubs share the same broadcast domain, meaning eventhough you have no routing between the 2 different subnetted networks, you can still send out a BROADCAST to find the host.
So, what I think is happening is when you Telnet into the router and PING out FE0/0 to find the host, the routing tables on the router don't know where to send the packets (since there is no route to 192.168.3.0/24 network in my example). So, the router sends out a /32 bit broadcast to the switch the router is plugged into.
And switches do NOT break up broadcasts, they forward the broadcast to all ports on them (I don't even think VLANS break up broadcasts, but I am not totally sure right now).
So, your router floods every port in the switch fabric it is connected to, looking for the host.
And the host gets the broadcast and replies back.
But you STILL don't have a route to it!

by joseph moore

So, where does this leave you? If the IP your ISP have the host is a static IP for Internet access, then I would contact them and explain what you are doing. If you got a 2nd IP for the FE0/0 interface that was on the same IP network as the IP on the host, then you could route that way easy. At that point, the router would have a valid route to the host.
If that is not an option, then I think that you could try and change the IP address on FE0/0 to be a smaller network address that would encompass the IP address given to the host (if the host is a /24 Class C address, turn your FE0/0 interface to /16). But again, not sure if the ISP will like it if you did that.
YOu could contact the ISP and tell them the deal, and they might have a recommendation.

But basically, yes, this is a routing problem. You need a route to the network the host is a member of, and currently you don't have one.

hope this helps

by ninet4

The host that is attached to FE0/0 "IS" on the same subnet. Example (using actual mask) FE0/0 x.x.x.193 255.255.255.224, host attached to FE0/0 x.x.x.194 255.255.255.224. Network address x.x.x.192 255.255.255.224. I have done some more poking around, which has left me even more perturbed. When I telnet to my router at Site1, I can ping all the way into Site2. From inside my network at Site1, I can reach the router at Site2, but not inside the network. The same is true coming from Site2 toSite1.

by bcbvale

It sounds like you may not have the default gateway on your hosts set correctly. Try double checking the settings at site2. Send your configs, minus the password entries, for both routers to iznot@msn.com and I can take a closer look.

ChrisV

by ninet4

Default gateways are set correctly. The hosts inside the networks can reach out to every other nook and cranny of the internet. Just not to each other. If I try to access the hosts from off site (i.e. my home) I can reach them with no problem. Iam thinking that it is a problem with a route on one of my ISPs routers?

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