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Question about RIP and hop count

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May 12th, 2006
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Hello,

I am a part of a CISCO Academy and I am studying CCNA2. I have a question about concering the RIP protocol. When a RIP is used in an AS, how exactly the next router will determine the hop count? Is there any field in the ip packet for this purpose. Thank you

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pciaccio Fri, 05/12/2006 - 12:51
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The metric field in the RIP IP packet will tick by one for every router that it the packet hits. Once it goes to 15 hops (ticks) it is declared as unreachable....

littlegrave Fri, 05/12/2006 - 13:45
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I don't mean RIP IP packet. I mean regular IP Packet with host information in it. TTL exists, but cannot be used for this purpose.

pciaccio Fri, 05/12/2006 - 14:27
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Assuming that RIP is fine and your destination is reachable from RIP's perspective, then a packet will travel to its destination. In the IP packet the TTL (Time to live) field located in the IP datagram header is the field that you are concerned about. Every time the packet passes a gateway device (router, Layer 3 device) then the device will drop the counter field by one(15 max). If the TTL field reaches zero then the packet is discarded (IT assumes that the destination is unreachable and an ICMP messages is returend to the sender as destination unreachable.

littlegrave Fri, 05/12/2006 - 14:40
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Right, but try to ping someone in your LAN. You will see TTL is not 16 when you send echo request. It puts 128 or above for TTL. I think it depends on the tcp/ip stack which decides what TTL to put. How does next router find what was the initial TTL?

a.giorgi Fri, 05/12/2006 - 14:57
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The ttl depend of the application.

See the tracert (windows command)

It put 30 steps.


a.giorgi Fri, 05/12/2006 - 14:54
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Hi there:


Sorry but I think this is not accurate.

Don't confuse IP packet with rip packets.


Rip work sending the networks the router have (direct connected) or have learn from rip announcements from other routers.

Suposse a router only know its own networks. The cost is 0 because it has a interface directly conected.

Then the router send an rip update (broadcast) using a rip packet in each interface. The metric will be 0.

In only one packet it can send 25 networks with 25 respectives metrics.

When a neighborg router receive the packet, it learn the networks and add 1 hop to the metric. Then the second router will do exactly the same. A third router will know how to reach the first networks with in 2 hops, and again and again.

This process is called routing by rumor.


Another process is switching the IP packet. A router receive an IP packet in one interfase, it look in the routing table if know the destination router (despite it know because Rip or another protocol), destroy the L2 header, subtract 1 to the ttl, calculate a new checksum for the IP heather, make another L2 header depending the destination IF and transmit the packet.


I hope this help (please rate if it does)


Alberto Giorgi from spain.



littlegrave Fri, 05/12/2006 - 15:06
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OK. Assuming that each application uses different TTLs. Then I still don't understand how a router learns the inital TTL? For example, if the inital TTL was 273, how would the 15th router learn that this packet should be dropped? How does he know that the initail TTL was 273 and it has to substract 273-15?

a.giorgi Fri, 05/12/2006 - 15:18
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I think you have a confusion with TTL and the value infinity (16 in Rip).

The infinity value prevent the infinity count and set a maximum diameter for the RIP AS.

The TTL is a value in the IP packet that prevent the packet go into a loop a exhaust the routers resources,

Can you see the diference?


littlegrave Sat, 05/13/2006 - 01:11
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I wasn't clear of the routing updates. I was a little bit confuse, becuase of the way the curriculum is written. Sometimes I cannot understand anything. I didn't notice that this hop count limit is not for regular packets.Thanks everyone

a.giorgi Sat, 05/13/2006 - 01:14
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I give up.

I can't be more clear that my posts.

Sorry


Alberto


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