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Community Member

Stub area and route external to its AS or external to its area ?

Cisco says:

"stub area is an area that does not accept information about routes external to the AS (with AS my confusion), such as route from non-OSPF sources,,,,"

My query:

Should not it have said :

"stub area is an area that does not accept information about routes external to its area (the origin difinition was AS),,,,,

11 REPLIES

Re: Stub area and route external to its AS or external to its ar

No, the first statement is correct because:

Stub area does accept routes external to its area i.e IA(inter-area/type3) but it doesn't accept routes from external AS (external/type5).

HTH,

Sundar

Community Member

Re: Stub area and route external to its AS or external to its ar

Thanks for that, to clarify my confusion

Cisco Employee

Re: Stub area and route external to its AS or external to its ar

"stub area is an area that does not accept information about routes external to its area". The above definition would actually apply to a totally stubby area.

Note that although a stubby area excludes external routes, it still allows routes from other area (inter-area routes).

The proper definition for a stubby area would be as follow:

"stub area is an area that does not accept information about routes external to the AS, such as routes redistributed into ospf from any sources including routes redistributed from another ospf process."

Hope this helps,

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
Cisco Employee

Re: Stub area and route external to its AS or external to its ar

Just wanted to stress the fact that the definition of external routes was incorrectly stating "non-ospf routes". ospf routes redistributed from one ospf process to another are also seen as external routes.

Hope this helps,

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
Community Member

Re: Stub area and route external to its AS or external to its ar

According to Cisco definition:

"stub area is an area that does not accept information about routes external to the AS (with AS my confusion),such as route from non-OSPF sources,,,,"

My understanding to what you said :

1- Routes (OSP,IGRP , EIGRP,,,) redisributed from different AS is considered as external routes.

2- Routes are redistributed from different OSPF pocess id, is considered as external route as well, thought they are within same AS (not from different AS) ,,,Am i right ?

Cisco Employee

Re: Stub area and route external to its AS or external to its ar

1- correct.

2- The concept of AS usually means all routers under a common administration. If you use two different ospf processes on the same router it probably means that you are connecting two separate networks, which to me are two different ASes just as when another routing protocol is used.

Hope this helps,

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
Community Member

Re: Stub area and route external to its AS or external to its ar

"The above definition would actually apply to a totally stubby area".

External routes here mean; external to its area and external to its AS as well,,,both of them ,,,Am I right ?

"Note that although a stubby area excludes external routes, it still allows routes from other area (inter-area routes). "

You meant here by: "excludes external routes" ; excludes external routes to its AS,,,Am I right ?

Cisco Employee

Re: Stub area and route external to its AS or external to its ar

External in the context of ospf means external to its AS (ie: redistributed routes whatever the source is).

and yes on the second question.

Hope this helps,

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
Community Member

Re: Stub area and route external to its AS or external to its ar

Internetwork Expert lab says:

"A stub area blocks OSPF external routes (LSA5) from enering arae"

As you said that OSPF from one process to another process, will be external as well.

Why did he specify OSPF only,,,why did not he say :

"A stub area blaocks external routes to its AS.

And external routes can be RIP, OSPF, IGRP,,,etc

Re: Stub area and route external to its AS or external to its ar

The above statement is correct, We are talking about OSPF here and OSPF generates LSA 5 when ever it redisributes routes from other routing domain or another ospf process ID. These routes are always tagged as LSA5.Donot think here according to the different protocls, Just remember that once those routes are in OSPF they are carried as LSA5.

HTH,

-amit singh

Community Member

Re: Stub area and route external to its AS or external to its ar

Hi,

The following are considered as external routes:

1.Routes from another OSPF process

2.Routes from another Autonomous system( may be OSPF routing domain)

3.Routes from other Routing domain such as RIP,EIGRP.

4.Static routes

All the above routes are tagged as AS-external LSA in OSPF , which are blocked for Stub areas.

Thanks,

Vijay

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