RIP Show UDP 520??

Unanswered Question
Aug 18th, 2010

Hi,

1)

I configure the RIP Ver 2 and No auto summ. on both Router RIP working fine no issue.

Now my query is while I do DEBUG WITH UPD on any one router and it show the RIP V2 Multicast IP (224.0.0.9) and 520 Port No. this is why?

Because I study RIP is working on Application Layer.

2)

As per RIP header format RIP v2 have next hop of IP address and as per this it goes to destination but destination how get the idea this packet is come from which source.??

Thanks,

Abhinay

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Giuseppe Larosa Wed, 08/18/2010 - 03:28

Hello Abnihay,

a RIP packet is actually an UDP packet to port 520

see

http://www.faqs.org/rfcs/rfc2453.html

so on a LAN you should see:

L2 header - EtherType = 0x0800 (IP)
L3 header - IP Protocol = 17 (UDP)
L4  header - UDP Destination Port = 520 (RIP)

>> Because I study RIP is working on Application Layer.

some basic courses try ton enforce  the point of view that RIP being over UDP is an application at the encapsulation level, in TCP/IP stack Level 4 and above have not their own headers as you could see on the OSI protocol suite

However, RIP job is done at OSI layer 3 being a routing protocol

the debug output simply reflects the reality

2) being a distance vector the next-hop of the local node will be = RIP source address unless the other device would like to advertise a different one (but in same subnet)

routing by rumors is how distance vector routing protocols work

Hope to help

Giuseppe

Peter Paluch Wed, 08/18/2010 - 03:33

Hello Giuseppe,

Thanks for answering I just posted my reply and saw you have already responded. Thanks!

Best regards,

Peter

Giuseppe Larosa Wed, 08/18/2010 - 03:58

Hello Peter,

no problem better to get two different answers then no answer

Best Regards

Giuseppe

Peter Paluch Wed, 08/18/2010 - 03:33

Hi Abhinay,

1) The debug ip rip output does not show the port number. It is expected that the RIP always runs on port 520 so having this port displayed in the output would be, in most cases, useless. If you want to see the port number try adding the command debug ip udp which displays all UDP segments processed by your router

2) The recommended next-hop IP address field in RIPv2 messages is an optional field (i.e. it does not have to be filled with actual address). For most applications, it is unused. However, to get a glimpse of what is its intended usage, have a look at the RFC 2453, Appendix A - this is the RFC that describes the entire operation of the RIPv2. The link is

http://tools.ietf.org/html/rfc2453#page-37

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