OSPF-ISIS Operating Layers & Some TCP/IP Basics

‎08-11-2012 03:43 AM

Hello Community


Can some one please answer the following questions.


1-why do ospf operates using a separate protocol 89 why didn't it uses tcp/udp for its operation like BGP or LDP.


2-ISIS operates at layer 2 then why do we call it a routing protocol.


3-Why inherent security does ISIS gets by operating at layer 2 of the network.If an IP packet can be crafted then a layer 2 frame can be crafted too in order to generate attacks.


4-What is the differnce between the routing and routed protocol.Does BGP fits the example.


5-What does it really mean when we write TCP/IP what does it refers when we write it like this.


Thanks & Regards



Re: OSPF-ISIS Operating Layers & Some TCP/IP Basics

‎08-17-2012 08:06 AM

I'll take a stab at a few of these.


  1. OSPF just needs the IP protocol to function.  It is given a specific IP Protocol Type for transport, in this case IP Prot 89.  Other well known protocols like GRE (47) and ICMP (1) also use their own IP Protocol number.  See RFC 1583 Section 1.3 for a history of the OSPF protocol.
  2. The term routing protocol is pretty vague, but here is a good definition - . As long as it shares routing (connectivity) information, it can be considered a routing protocol.  This OSI layer which it operates is irrelevant.
  3. Because ISIS uses CLNS as a transport (not IP) it is insusceptible to IP attacks, but that doesn;t mean you cannot cause havoc with the protocol. -
  4. I think this is trying to differentiate between two vague terms.
  5. Transmission Control Protocol / Internet Protocol - you are operating at layer 4 of the OSI reference model.



Re: OSPF-ISIS Operating Layers & Some TCP/IP Basics

‎08-17-2012 01:01 PM



I would like to add for question four that a Routing Protocol is the protocol that specifies how routers communicate with each other. Border Gateway Protocol is indeed a Routing Protocol.

A Routed Protocol is when data get's passed from one place to another, TCP/IP is an example or Appletalk.

Network and telephony support engineer