Can not route from L3 Switch to P2P router

Unanswered Question
Mar 15th, 2008

We are trying to setup L3 switching. The layer 3 switch has multiple VLANs and is routing fine between them. On the l3 switch I routing all unknown ips through an actual 1841 router that is locally on one of the vlans and connected to the l3 switch. I have attached the config. I can ping the 1841 router but when I tracert traffic dies at the l3 switch? I can ping the 1841 from the l3 switch. What am I doing wrong?

I have this problem too.
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lamav Sat, 03/15/2008 - 13:00

You can PING the 1841 from the L3 switch and your PC because they are all on vlan 220. The 1841's interface is a directly connected interface. You dont even have to route, per se. Its all L2 forwarding.

Now, what are you trying to reach when you do a traceroute?

What's the routing look like on the 1841?

We need a bit more info.




mloraditch Sat, 03/15/2008 - 13:11

I have attached the router config and the other switch. I am trying to tracert from a trunked switch on vlan 77 to the ip of another 1841 that is at the opposite end of the P2P T-1 (the p2p works fine) but the traffic dies as at the L3 switch (ip on vlan 77 and on vlan 220). Even if something is wrong with the P2P t-1, shouldn't the traffic die at the local 1841?

Richard Burts Sat, 03/15/2008 - 13:12


I have looked at the config that you posted and I do not see any obvious issues with it. When you tracert where are you doing that from? What is the source address of the tracert?

Especially if the tracert is coming from somewhere other than VLAN 220 my first guess would be that the 1841 does not have a route to that network. If the tracert is coming from within VLAN 220 then we need to look for some other explanation. Is it possible that the 1841 has an access list that is impacting traffic? Would you be able to post the config of the 1841?



Richard Burts Sat, 03/15/2008 - 13:17


Thanks for posting the router config. I believe that I see the problem and it is the one that I had guessed. If you source the tracert from VLAN 77 then its source address is 192.191.77 and it gets to the 1841. But the 1841 does not have a route for that network. The 1841 has this default route:

ip route

so it will attempt to send responses to to 192.191.77 using as the next hop. To fix this you need to configure (or configure dynamic routing so it can learn) at route to 192.191.77.

[edit] to answer your question about where the traffic appears to die, you see a response from the switch because the switch has a route to the source. You do not see a response from the 1841 because the 1841 does not have a route. But not seeing a response from the 1841 does not necessarily mean that the switch is where the traffic dies.



mloraditch Sat, 03/15/2008 - 13:18


I just posted the 1841 config in my reply to other post. As you will see the 1841 has no access lists on it. The 1841's just routes all traffic across a P2P t-1. so if that t-1 is working (and it is) should not my traceroute die somewhere on the other side? and even if the t-1 went down, shouldn't the traceroute die at the local 1841?

I may be wrong, this is the first time I am doing all this myself in the past all I have dealt with is other's configs so I certainly could have screwed up but in looking at various docs on Cisco's site, I don't think so.

Thanks so much for all the help!!

Richard Burts Sat, 03/15/2008 - 13:23


While you were posting this message I was posting a response with my explanation of what the problem is and why the traffic appears to die at the switch. But just because the switch is the last one to respond does not necessarily mean that the traffic is dieing there and does not mean that the problem is necessarily on the switch.

For your setup to work the 1841 must have a route to 192.191.77.



mloraditch Sat, 03/15/2008 - 13:47


Thanks again!! You have been very helpful. I think I know what's going and I think I know what I need to do to get everything working.

With Many Thanks,



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