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Bronze

asbr in ospf

hi every body.

My book says an asbr is the router that connects one AS to another As.

The other book says asbr is the router that redistributes other routing protocols into ospf'AS.

which one is correct?

thanks and have a nice day.

6 ACCEPTED SOLUTIONS

Accepted Solutions
Hall of Fame Super Bronze

Re: asbr in ospf

When you are redistributing, you are bringing routes from another autonomous system into OSPF so both definitions are correct - and - that's a way of connecting one AS to another AS.

__

Edison.

Re: asbr in ospf

Hi,

they both mean the same, hence both are correct.

Typical example is an ASBR will have more than 1 routing protocols, means one routing protocol runs in 1 domain and the second protocol in the other domain. Hence the router which understands both protocols to establish communications between both domains is called ASBR.

Hope this helps....

Shaheen

Hope this helps

Hall of Fame Super Bronze

Re: asbr in ospf

If you are running 2 protocols, your router is running 2 autonomous systems.

how about if we have one AS running two routing protocols, rip and ospf, will the router that redistributing rip routes into ospf be termed as asbr?

Yes.

How about if we have two ASes both running ospf, in this case ospf routes from one AS say As 1 will be advertised into other AS say AS 2 and vice versa without any restribute commands? what would that router be called that is connecting two Ases ?

How do you plan to exchange routes between OSPF processes without redistribution?

__

Edison.

Hall of Fame Super Blue

Re: asbr in ospf

Sarah

An ASBR is router that injects routes learnt from another protocol eg. EIGRP/BGP into the OSPF domain.

So an ASBR can be an internal, backbone or ABR router.

The books definition of an AS as under the same administrative authority is misleading in this case because an ASBR can be a router that is under the same administrative authority but that is still redistributing routes into the OSPF domain.

"i think we don't need anything to restribute routes between different ospf processes."

Edison is correct, you do need to redistribute between OSPF processes on the same router otherwise the routes will not be exchanged between the OSPF processes.

Jon

Hall of Fame Super Gold

Re: asbr in ospf

Just to add a slightly different perspective to this discussion: the definition of AS as "An autonomous system is one network or sets of networks under a single administrative control." is a somewhat older view of AS and is especially oriented to BGP operations. The usage of the term is slightly different when talking about AS in the context of OSPF. In the context of OSPF an AS is a group of routers that share OSPF routes with each other and the boundary of the AS is the boundary of how far OSPF updates are propagated.

The example of a situation where one router is running OSPF 1 and another router is running OSPF 2 is an example of how the OSPF process ID is only locally significant and is not an example of 2 different AS in OSPF.

If the same router is configured with OSPF 1 and with OSPF 2 then this is an example of multiple AS. The router with both OSPF 1 and OSPF 2 will have all the routes from both ASs in its routing table. But it will maintain a separate link state database for OSPF 1 and for OSPF 2. Without redistribution no LSAs from OSPF 1 will be advertised to OSPF 2. And no other router connected in OSPF 2 will learn routes from OSPF 1.

HTH

Rick

Hall of Fame Super Bronze

Re: asbr in ospf

My book says

"An autonomous system is one network or sets of networks under a single administrative control."

It's a very misleading term which has caused confusion to many students so don't feel bad :)

When you have OSPF enabled on a group of routers and routes are exchanged that's called an AS. All routes follow a single administrative control. Policies made in one router may affect other routers.

If you decide to run EIGRP on that same set of routers, policies within EIGRP won't affect OSPF hence that's considered another AS.

When bringing routes from one AS into another AS, a redistribution process takes place and when performed under OSPF, the router is considered an ASBR.

I think we don't need anything to restribute routes between different ospf processes.

I remember a lab with set up something like that:

R1-----------R2

R1

loopback 1

10.0.0.1/8

loopback 2

11.0.0.100/8

router ospf 1

network 10.0.0.0 0.255.255.255 area 0

network 11.0.0.0 0.255.255.255 area 0

R2:

loopback 3

3.0.0.1/8

router ospf 2

network 3.0.0.0 0.255.255.255 area 0

I found routes are exchanged though both routers are running ospf processes id.

Process-ID is locally significant and the routers are able to exchange routes because - they are still considered to be part of the same AS.

If you were to do that under EIGRP, no neighbors will be formed as the AS number is used on the adjacency.

HTH,

__

Edison.

9 REPLIES
Hall of Fame Super Bronze

Re: asbr in ospf

When you are redistributing, you are bringing routes from another autonomous system into OSPF so both definitions are correct - and - that's a way of connecting one AS to another AS.

__

Edison.

Bronze

Re: asbr in ospf

Hi Edison.

how about if we have one AS running two routing protocols, rip and ospf, will the router that redistributing rip routes into ospf be termed as asbr?

How about if we have two ASes both running ospf, in this case ospf routes from one AS say As 1 will be advertised into other AS say AS 2 and vice versa without any restribute commands? what would that router be called that is connecting two Ases ?

thanks and have a nice day.

Hall of Fame Super Bronze

Re: asbr in ospf

If you are running 2 protocols, your router is running 2 autonomous systems.

how about if we have one AS running two routing protocols, rip and ospf, will the router that redistributing rip routes into ospf be termed as asbr?

Yes.

How about if we have two ASes both running ospf, in this case ospf routes from one AS say As 1 will be advertised into other AS say AS 2 and vice versa without any restribute commands? what would that router be called that is connecting two Ases ?

How do you plan to exchange routes between OSPF processes without redistribution?

__

Edison.

Bronze

Re: asbr in ospf

Thanks Edison.

I will get back to you in few hours

Bronze

Re: asbr in ospf

Thanks Edison and Shaheen

"If you are running 2 protocols, your router is running 2 autonomous systems."

My book says

"An autonomous system is one network or sets of networks under a single administrative control."

According to book, possible reasons why an AS might two or more igp are :

1) Migrating from old igp to a new igp.

2)You want to use another protocol but need to keep old routing protocol because of the host system's needs. For example,Unix host-based system routers might run only rip.

3)Different departments might not want to upgrade thier routers to suport a new routing protocol.

4) If you have a mixed-router vendor enviroment, you can use a Cisco-specific routing protocol such as eigrp in cisco portion of the network and then use a common standard-based routing protocol such as ospf to communicate with non-cisc- devices.

===========================================

"How do you plan to exchange routes between OSPF processes without redistribution? "

I think we don't need anything to restribute routes between different ospf processes.

I remember a lab with set up something like that:

R1-----------R2

R1

loopback 1

10.0.0.1/8

loopback 2

11.0.0.100/8

router ospf 1

network 10.0.0.0 0.255.255.255 area 0

network 11.0.0.0 0.255.255.255 area 0

R2:

loopback 3

3.0.0.1/8

router ospf 2

network 3.0.0.0 0.255.255.255 area 0

I found routes are exchanged though both routers are running ospf processes id.

If I am wrong, please correct me.

thanks and you have a good day.

Hall of Fame Super Blue

Re: asbr in ospf

Sarah

An ASBR is router that injects routes learnt from another protocol eg. EIGRP/BGP into the OSPF domain.

So an ASBR can be an internal, backbone or ABR router.

The books definition of an AS as under the same administrative authority is misleading in this case because an ASBR can be a router that is under the same administrative authority but that is still redistributing routes into the OSPF domain.

"i think we don't need anything to restribute routes between different ospf processes."

Edison is correct, you do need to redistribute between OSPF processes on the same router otherwise the routes will not be exchanged between the OSPF processes.

Jon

Hall of Fame Super Bronze

Re: asbr in ospf

My book says

"An autonomous system is one network or sets of networks under a single administrative control."

It's a very misleading term which has caused confusion to many students so don't feel bad :)

When you have OSPF enabled on a group of routers and routes are exchanged that's called an AS. All routes follow a single administrative control. Policies made in one router may affect other routers.

If you decide to run EIGRP on that same set of routers, policies within EIGRP won't affect OSPF hence that's considered another AS.

When bringing routes from one AS into another AS, a redistribution process takes place and when performed under OSPF, the router is considered an ASBR.

I think we don't need anything to restribute routes between different ospf processes.

I remember a lab with set up something like that:

R1-----------R2

R1

loopback 1

10.0.0.1/8

loopback 2

11.0.0.100/8

router ospf 1

network 10.0.0.0 0.255.255.255 area 0

network 11.0.0.0 0.255.255.255 area 0

R2:

loopback 3

3.0.0.1/8

router ospf 2

network 3.0.0.0 0.255.255.255 area 0

I found routes are exchanged though both routers are running ospf processes id.

Process-ID is locally significant and the routers are able to exchange routes because - they are still considered to be part of the same AS.

If you were to do that under EIGRP, no neighbors will be formed as the AS number is used on the adjacency.

HTH,

__

Edison.

Re: asbr in ospf

Hi,

they both mean the same, hence both are correct.

Typical example is an ASBR will have more than 1 routing protocols, means one routing protocol runs in 1 domain and the second protocol in the other domain. Hence the router which understands both protocols to establish communications between both domains is called ASBR.

Hope this helps....

Shaheen

Hope this helps

Hall of Fame Super Gold

Re: asbr in ospf

Just to add a slightly different perspective to this discussion: the definition of AS as "An autonomous system is one network or sets of networks under a single administrative control." is a somewhat older view of AS and is especially oriented to BGP operations. The usage of the term is slightly different when talking about AS in the context of OSPF. In the context of OSPF an AS is a group of routers that share OSPF routes with each other and the boundary of the AS is the boundary of how far OSPF updates are propagated.

The example of a situation where one router is running OSPF 1 and another router is running OSPF 2 is an example of how the OSPF process ID is only locally significant and is not an example of 2 different AS in OSPF.

If the same router is configured with OSPF 1 and with OSPF 2 then this is an example of multiple AS. The router with both OSPF 1 and OSPF 2 will have all the routes from both ASs in its routing table. But it will maintain a separate link state database for OSPF 1 and for OSPF 2. Without redistribution no LSAs from OSPF 1 will be advertised to OSPF 2. And no other router connected in OSPF 2 will learn routes from OSPF 1.

HTH

Rick

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