UDP Routing

Unanswered Question
Apr 23rd, 2007

We have linux based modular messaging system running behind the core router 3745 which has a point to point T1 connection to 3825. Unfortunately, when we try to do the traceroute to the other linux host sitting behind 3825 the packets drop at the router. How should I turn the UDP on for that wan interface connecting to us on 3745?

I have this problem too.
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Harold Ritter Mon, 04/23/2007 - 06:56

This could be caused by the router not knowing the route to destination or by an ACL on the router that prevents UDP traffic from traversing the router.

Hope this helps,

saurabh1969 Mon, 04/23/2007 - 07:17

Thanks but it does know the routes to the network on which linux host resides as from my Windows machine, I could do the traceroute successfully to the linux host. We are talking about two core routers so no acl in place in-between. How should I make sure that 3825 is allowing the UDP packets to pass-thru?

Harold Ritter Mon, 04/23/2007 - 07:32

UDP messages are routed inside of IP, therefore nothing needs to be done specifically to enable the routing of these UDP messages. It should just work as long as you have IP connectivity between the endpoints and no ACL in the path to block UDP traffic.

Can you provide us with the output of your unsuccessful traceroute.

A way to get more information on why the traceroute stops at the 3745 would be to do a debug ip icmp" on this router.

Hope this helps,

mohmmad.imran Mon, 04/23/2007 - 15:22

Hi Saurabh,

As you have said the routing information is there in routing table, the next step is to see what exactly the router is doing with packets when you do the trace.

To see whether the router is dropping the packet enable the ip route-cache flow on the incoming and outgoing interface on the router and try to see the ip cache flow

# int se0/0

ip route-cache flow

# ip cache flow | in as per your convinence.

if the packet goes via the router it will show the incoming and outgoing interface with protocol.

and if the packet dropping on the router debug ip packet to see why it is dropping on the router

for ex:-

#access-lists 101 permit ip host host

#debug ip packet 101




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