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  1. #1
    Join Date
    Oct 2011
    Posts
    4

    Default layers and routing protocols!!!

    I have a serious doubt.I read in a book(Implementi ng cisco IP Routing by Diane Teare) that eigrp and ospf are transport layer routing protocol because they run diectly over ip and guarentees end to end delivery.wherea s rip and bgp are application layer protocols because they rely on their lower layer(transport layer, rip uses udp and bgp uses tcp).
    But my trainer said the routing protocols run in L3 and some protocols like Bgp uses transport layer function.But u can't say that they are upper layer protocols.Which is true.What layer u opt, when there is a question in exam?

  2. #2
    Join Date
    Mar 2008
    Posts
    2,888

    Default

    They are called routing protocols because they affect/influence the routing process not because they reside at layer 3.

    In fact RFC 871, basically says that a protocol encapsulated at the "N-th" layer typically resides at the "N+1" layer. Although it is possible to reside at the same layer or "N-th" layer, it cannot reside at the "N-1" layer.

    "Why do they need the transport layer to communicate? They are not applicattions."

    If you think about what routing protocols do for us and how they allow hosts (routers in this case) to exchange information and update each other in real time. That sounds like an application to me. In fact, BGP and RIP have been identified in several sources as being application layer protocols. Given that argument, then aren't all routing protocols application layer protocols?

    You are absolutely correct in that TCP and UDP provide services for upper layer protocols. So this enforces the notion that RIP (UDP 520) and BGP (TCP 179) are indeed application layer protocols because each of them use a transport layer protocol.

    ICMP is actually part of the IP suite and it follows RFC 871. In that it resides at the "N-th" layer and is encapsulated at the "N-th" layer.

    RIP does not need port numbers it is the transport layer that needs and uses the port numbers so it knows which upper layer application to send the user PDU. In the case of RIP it is identified by UDP port 520. So when the transport layer de-capsulate's the segment and sees the port 520 it sends the user PDU to the RIP application at the next higher layer. Same with BGP except with TCP and port 179. So all the TCP/UDP ports identify the upper layer application. Which again is IAW the RFC 871 in that when a protocol is encapsulated at the "N-th" layer it typically resides at the "N+1 layer.

    HTH.
    Maddox Thomas-Clark 14/10/2008
    Bean Thomas-Clark 18/09/2007
    Big Evils Cisco World
    Linkedin

  3. #3
    Join Date
    Oct 2011
    Posts
    4

    Default which is the layer of eigrp and ospf

    thankyou, so we can say that rip and bgp are application layer protocols.so what about eigrp and ospf.are they transport layer protocols?

  4. #4
    Join Date
    Mar 2008
    Posts
    2,888

    Default

    I think that is best brief description. Although if someone was to ask me i would still say -

    BGP - Path protocol
    EIGRP - Distance Vector/DUAL/Hybrid
    OSPF - Link State
    RIP - "Man update your damn network!" LOL
    Maddox Thomas-Clark 14/10/2008
    Bean Thomas-Clark 18/09/2007
    Big Evils Cisco World
    Linkedin

  5. #5
    Join Date
    Nov 2008
    Location
    Birmingham, UK
    Posts
    1,447

    Default

    OSPF uses it's own 'layer 4' method - LSAs. When a router sends an LSU to it's neighbour, the neighbour must send back an acknowledgement to verify it recieved the update. TCP works in just the same way, so you could say OSPF has it's own method of reliability, without using TCP.

    TCP is IP protocol number 6, UDP is protocol number 17. OSPF is number 89 and EIGRP is 88. This means that both EIGRP and OSPF could be considered layer 4 protocols.

    BGP and RIP use TCP & UDP respectively, so they could be considered application layer protocols.
    CCNP R&S, CCNA DC
    Currently studying: CCIE R&S, CCNP Data Centre
    Follow my CCIE progress with study notes on my blog: http://beyondccna.blogspot.co.uk/

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