The At 08:48 PM 1/16/02, Cisco Nuts wrote: >Hello, >I have been trying to find out what exactly does TOS(TOS0) routes mean in >OSPF but am having no luck. Tried CCO, Doyle books, CCNP books etc. >EG: >RTA#sh ip ospf >Routing Process "ospf 1" with ID 15.15.15.1 >Supports only single TOS(TOS0) routes
It's referring to the Type of Service (TOS) bits within the Type of Service field in the IP header. These bits were originally intended to help a router select a routing path when multiple paths to a particular destination were available. That meaning for the bits is obsolete now. The field is now called the Differentiated Services field. Early OSPF RFCs said that OSPF should support routing based on the setting of the Type of Services bits. Newer OSPF RFCs don't say that anymore. It proved impractical. TOS 0 means that the router doesn't pay attention to the setting of the bits. The OSPF process supports only one type of service, which could be called type 0, and is just the ordinary default behavior, without reference to the setting of the TOS bits. Here's some background info from my up-and-coming book: The IP Type of Service field specifies both precedence and type of service. Precedence helps a router determine which packet to send when several packets are queued for transmission to the same output interface. Type of service helps a router select a routing path when multiple paths to a particular destination are available. The 4-bit Type-of-Service subfield supports four types of service. (Note that the subfield has the same name as the major field, which can be confusing.) The goal of the Type of Service subfield is to help a router select a route from a set of routes to a particular destination, especially when the routes have different characteristics. Routing protocols attempt to determine the best route to a destination, but there are several definitions of best -- cheapest, fastest, most reliable, least delay, and so on. According to RFC 1349, which clarified the use of the Type of Service byte (but is now obsolete), the Type of Service subfield has four bits that specify the routing service that an application wishes to receive from a router. An application or host can set one (and only one) of the four bits: 1 The delay bit (D) tells routers to minimize delay 2 The throughput bit (T) tells routers to maximize throughput 3 The reliability bit (R) tells routers to maximize reliability 4 The cost bit (C) tells routers to minimize monetary cost Setting the D bit is suitable for interactive applications, such as Telnet and Rlogin. When the D bit is set, a router should select a path that minimizes delay, for example, a dedicated high-speed leased line instead of a shared Frame Relay link. File transfer applications, or any applications that send bulk data, can set the T bit. Setting the T bit tells routers to select high-throughput links. Network management and mission-critical applications can set the R bit. Setting the R bit tells routers to select reliable paths. Applications for which delay, throughput, or reliability are not critical -- but a low monetary cost is important -- set the C bit. Some implementations of the Network News Transfer Protocol (NNTP), which reads UseNet news, set the C bit, presumably because reading news is not a critical activity and should not use a lot of monetary resources. In practice, routing protocols and routers never had good methods for handling these bits. Selecting a path based on the setting of these bits proved to be impractical. Although early versions of the Open Shortest Path First (OSPF) and Border Gateway Protocol (BGP) routing protocols specified support for the bits, newer versions of OSPF and BGP specifications do not require support for routing based on the setting of the bits. Differentiated Services RFC 1349 was declared obsolete with the publication of RFC 2474, "Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers," and RFC 2475, "An Architecture for Differentiated Services." Per these newer RFCs, the Type of Service field became the DS field. The DS field has two currently unused (CU) bits at the end. The first six bits are called the Differentiated Services Codepoint (DSCP). And that's probably way more than you ever wanted to know. Priscilla >Can someone help? > >Thank you. > >_________________________________________________________________ >Join the worlds largest e-mail service with MSN Hotmail. >http://www.hotmail.com ________________________ Priscilla Oppenheimer http://www.priscilla.com Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=32342&t=32242 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
