AS number in RIP and OSPF Help?

[imran_mcse]

How RIP and OSPF find there AS number. As there is no configuration where we can define these numbers, as we define in IGRP and EIGRP.

Like we did

router rip and router ospf 0 (no mention of AS number)

but in eigrp and igrp

router igrp 100 and router eigrp 100 (AS mentioned)

Pls help me in detail.

[Georg Pauwen]

Hello Imran,

RIP does not have a concept of autonomous systems (AS) or areas, like OSPF or (E)IGRP, so with RIP, there is no option to define a number that specifies an AS or an area. When you configure OSPF on a router, e.g. ' router ospf 1', the ' 1' is an arbitrary number that is local to the router and only describes an OSPF instance. On the same router, you could e.g. run another OSPF process, e.g. ' router ospf 2'. Furthermore, OSPF has the concept of areas, which are configured for networks, e.g. ' network 135.13.1.0 0.0.0.255 area 0' would put network 135.13.1.0/24 into area 0. The process number and the area number are in no way related.

Does that make sense ?

Regards,

GP

www.solutionfinders.nl

[imran_mcse]

Thans for your kind reply as usual.

Sir so we call that RIP and OSPF are IGP (operate with the autonomous) as they have no any information regarding autonomous.

But one thing more that in OSPF when we configure ASBR then how it think about autonomous no.

I m confused pls explain.

[Richard Burts]

Georg was correct in his answer. It is probably most important to understand the first thing that he said in his answer: RIP and OSPF do not have any concept of an autonomous system identity. RIP and OSPF (as well as IGRP and EIGRP) are in the category of IGP or Interior Gateway Protocol. The Interior Gateway Protocols are designed to operate within a single autonomous system. The IGPs learn routes and exchange routing information with other routers within their own autonomous system. They can learn information from outside their autonomous system via redistribution. But there is no direct exchange of routing information between routers in different autonomous systems in an IGP.

And it is mostly a coincidence that IGRP and EIGRP have a thing in the configuration command called autonomous system number. If there was a router configured with autonomous system 100 and on the same Ethernet there were a router configured with autonomous system 200, they would recognize that they had different autonomous system numbers and there would be no direct exchange of routing information between them.

You asked about OSPF and ASBR. When you configure an OSPF router as an ASBR it recognizes that it is learning route information from some outside source but it does not have any concept of an autonomous system number as the source of that outside rout information.

The other major category of routing protocol in addition to IGP is EGP or Exterior Gateway Protocol. The most common EGP is BGP or Border Gateway Protocol. When you configure BGP you include its autonomous system number in the router BGP command. And when you configure its neighbors you identify the neighbor's autonomous system number. In BGP if you have a router configured in autonomous system 100 and it has a neighbor in autonomous system 200 then there will be a direct exchange of routing information between the autonomous systems. And the routers in AS 100 will recognize information that is learned from AS 200 and they know what AS it came from.

The biggest difference between the Interior routing protocols and the Exterior routing protocols is whether there is any direct exchange of routing protocols between routers of different ASes.

HTH

Rick