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Routing Issue

Hi,

I am using Cisco 3620 router with 2 10/100 interface. "Interface 1" is connected to a Wireless AP and "Interface 2" is 10/100 interface is connected to my uplink service provider's router via Cat5 cable.

I configure a few secondary IP addresses at "interface 1"

Headend router:

Interface Fa 1

ip address 203.1.12.1 255.255.255.240

ip address 203.1.12.61 255.255.255.252 secondary

ip address 203.1.12.65 255.255.255.252 secondary

!

At the remote site:

I have my customer's Cisco 1721 router with 2 ethernet ports. "Interface 1" to wireless CPE and "Interface 2" connected to their firewall using cross cable. So, their external interface is "203.1.12.62/255.255.255.252". Their IOS is 11.2

My client's router has no problem to "ping" 203.1.12.61, but, if I use normal "ping" from my headend router to "203.1.12.62", it failed. However, if I extended "ping" from the headend router to "203.1.12.62", I manage to get the reponse.

I don't encounter this problem if the IOS is 12.0 and above. Can I know this happen? Is it something to do with the IOS ?

1 ACCEPTED SOLUTION

Accepted Solutions

Re: Routing Issue

Depending on how the routing tables are built, you should check whether ip classless is enabled. This feature was disabled by default in older IOS versions.

ip classless enables routing to the best matching supernet route. Without it, the dest-subnet has to be in the routing table explicitly, or the packet is discarded.

This could be an issue in your case, as you are routing in 203.1.12.x with different subnets

4 REPLIES
Bronze

Re: Routing Issue

Hi,

Most probably with IOS. Try to "debug ip icmp" both ends to see what's happening to not reply with normal ping.

Regards..

Ashok.

With best regards... Ashok ----------- Pls kindly rate if helpful or answered your question.
New Member

Re: Routing Issue

When you do an extended ping, what source address do you use? When you do a 'normal' ping the router may be using 203.1.12.1 If the router on the other end does not know the route back to 203.1.12.1 it can't reply. Look in the route forwarding table for that subnet. When you do an extended ping, if you use 203.1.12.61 as the source, the router on the other end can reply because it knows where that subnet is. That is my best guess without seeing the rest of the configs.

Re: Routing Issue

Depending on how the routing tables are built, you should check whether ip classless is enabled. This feature was disabled by default in older IOS versions.

ip classless enables routing to the best matching supernet route. Without it, the dest-subnet has to be in the routing table explicitly, or the packet is discarded.

This could be an issue in your case, as you are routing in 203.1.12.x with different subnets

New Member

Re: Routing Issue

Hi sir,

Yes. You are right. Thank you very much.

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