why use AS number in eigrp config ,when not in ospf

[krishna3010]

hi,

both eigrp and ospf are IGPs. they work within a single AS. Then why is AS number used in eigrp configuration when it is not used in ospf configuration

.And how do the routers  using ospf know to which AS they belong? Can someone help me on this plz

[Giuseppe Larosa]

Hello Krishna,

in OSPF v2 that we use to route IPv4 the process-is is locally significant: this means that the process-id is never sent out in Hello packets or other OSPF packets.

Two routers decide if they can form an adjacency using the area-id as criteria, the area type (if normal. stub or NSSA) the IP subnet, subnet mask, hello and dead interval, authentication.

So we can say OSPF v2 has no concept of AS but only the OSPF areas. Routers care only of being in the same area with the same settings and they have no knowledge of AS. It is left to the network designer to decide what routers are part of a single OSPF routing domain.

You can test this by configuring two routers with different OSPF process-id, you will see that they can build an adjacency if all the above parameters match.

EIGRP requires the AS number and the k values to match to build an adjacency, but it allows for different hello intervals.

There were old designs using multiple EIGRP domains, but they had the problem that query messages were propagated across boundaries.

To be noted that in OSPFv3 developed to route IPv6 the process-id is exported in the packets sent out and this allows to run two different processes on the same L3 interface.

The differences are part of the different design of the two protocols.

Hope to help

Giuseppe

[Peter Paluch]

Hi Krishna,

Then why is AS number used in eigrp configuration when it is not used in ospf configuration

A brief answer would be: it is a design decision. EIGRP creators decided that it is useful to include the number of the autonomous system in all EIGRP messages and to explicitly check for its value, and thereby prevent two routers in different autonomous systems from talking to each other. OSPF does not have this explicit check and instead has other management tools (passive interfaces, network declarations, authentication, perhaps instance numbers in OSPFv3) to prevent one AS from talking to another, although this prevention (except the instance numbers) must be configured manually - it is not performed automatically.

The fact, however, is that the AS number in EIGRP is merely a process number, not a real AS identifier. For EIGRP purposes, the concept of AS is very limited. All that EIGRP knows about the concept of AS is that an EIGRP router in a particular AS does not talk to an EIGRP router in a different AS. And that is it. Nothing more. But the AS concept is much broader and is fully realized only in BGP (think AS_PATH, routing loop protection based on AS_PATH, route selection and filtering based on AS_PATH). Therefore, what EIGRP calls "AS" is, frankly, only a strongly trimmed-down idea of what the AS really is.

And how do the routers  using ospf know to which AS they belong?

They do not know it, and they do not need to know it. The concept of AS is important only for BGP operation, and when configuring BGP, you are configuring your AS number as the part of the BGP configuration.

Best regards,

Peter