Routing in IOS

Unanswered Question
Mar 15th, 2008

Hi Seniors, During the study of CCNA, especially routing, I am stuck of a very basic(might be stupid) question,

1-Describe in very clear step(s) when router discards a packet

2-Describe the situation(s) in which router forwards a packet

Please dont refer me a CISCO technical document as at this stage of learning I am unable to understand it. Thanks for reply. Tahir

I have this problem too.
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Paolo Bevilacqua Sat, 03/15/2008 - 14:17

1. (most frequent cases)

Administratively prohibited (access list)

Exceeding configured QoS

No route to destination

Malformed / invalid packet

Input queue full

Input interface throttled

Output queue fuell

Buffer allocation failure

2. all other cases.

tahir1234 Sat, 03/15/2008 - 15:12

Very good my Senior Bevilacqua, you spilled all the CCIE knowledge in few lines whereas I am working on CCNA level. Just want to know simple routing process in which

1- classless vs classfull routing is in action, OR

2- when a major network is found in table or when it is not found and when "no ip routing" is active. Hope u will come down to my level. Thanks again.

Paolo Bevilacqua Sat, 03/15/2008 - 17:23

1 - classful routing has ended 15 years ago, there are no questions anymore about that in testing, do not ever worry about it.

2 - when no ip routing, router acts as an host - "ip default-gateways" comes into effect. Some models act in were this mode during "boot image". If bridging is configured, everything will be bridged.

CSCO10892433 Sat, 03/15/2008 - 19:02

Hi, Tahir

Let's take an example.

Suppose the router's routing table has only two entries: via e0

* via e1

and two packets with dest. ip address and respectively come into the router.

Here is what happen when classful routing is configured:(use no ip classless)

1.The router match the network number first. Because belongs to the classful network and the routing table entry is also belongs to the same classful network, the routing process consider this is a match and goto step 2.

On the other hand, belongs to the classful network and there is no routing entry belongs to the same classful network, routing process consider this is no-match and goto step 3 (that is, skip the step 2)

2.Then the router will do a longest match inside that classful network (this is the key point: it will not do a longest match outside the classfull network). For packet with dest. ip address, there is no match under this step. So the router drops the packet and ends the matching algorithm.

3.the packet will match the routing entry and be routed via e1.

In case of classless routing, the algorithm is simple: just do a longest match. Both packets will end up with matching the entry and routed via e1.




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