ASBR

[Tiago Eduardo Zacarias]

Hello people,

I have a doubt regarding the definition of an ASBR.

By Cisco, an ASBR information rotedor and the one that has an interface in area 0 and another one in any area injecting routes in different routing protocols OSPF domain, but I have a senario mounted in a lab at home with area 0 only set, but when I perform the verification of oSPF with the command show ip ospf it tells me that router and an ASBR. What criterion he used in this case to set the router as ASBR?

Output  show ip ospf

Routing Process "ospf 1" with ID 172.17.0.1

Supports only single TOS(TOS0) routes

Supports opaque LSA

Supports Link-local Signaling (LLS)

Supports area transit capability                                                                                                                                                             

It is an autonomous system boundary router                                                                                                                                                   

Redistributing External Routes from,                                                                                                                                                         

    connected, includes subnets in redistribution                                                                                                                                             

    static, includes subnets in redistribution                                                                                                                                                

Initial SPF schedule delay 5000 msecs                                                                                                                                                        

Minimum hold time between two consecutive SPFs 10000 msecs                                                                                                                                   

Maximum wait time between two consecutive SPFs 10000 msecs                                                                                                                                   

Incremental-SPF disabled                                                                                                                                                                     

Minimum LSA interval 5 secs                                                                                                                                                                  

Minimum LSA arrival 1000 msecs                                                                                                                                                               

LSA group pacing timer 240 secs

Interface flood pacing timer 33 msecs

Retransmission pacing timer 66 msecs

Number of external LSA 10. Checksum Sum 0x14AC45

Number of opaque AS LSA 0. Checksum Sum 0x000000

Number of DCbitless external and opaque AS LSA 7

Number of DoNotAge external and opaque AS LSA 0

Number of areas in this router is 1. 1 normal 0 stub 0 nssa

Number of areas transit capable is 0

External flood list length 0

    Area BACKBONE(0)

        Number of interfaces in this area is 1

        Area has message digest authentication

        SPF algorithm last executed 5d06h ago

        SPF algorithm executed 76 times

        Area ranges are

        Router LSA rebuild timer due in 00:02:38

        Number of LSA 5. Checksum Sum 0x08A23C

        Number of opaque link LSA 0. Checksum Sum 0x000000

        Number of DCbitless LSA 4

        Number of indication LSA 0

        Number of DoNotAge LSA 0

        Flood list length 0

[Jon Marshall · Accepted solution]

An ASBR does not have to be connected to different areas. All it's interfaces can be in the same area as you have found out with your test. So for example if you had some static routes defined on your OSPF router and all the interfaces on the OSPF router were in area 0, if you redistributed those statics into OSPF the router would now be an ASBR. An ASBR to OSPF is simply a router that serves as redistibution point between OSPF and any other routing protocol including statics.

An ABR on the other hand does have interfaces connected to different areas. An ABR will typically have an interface connected to area 0 and an interface connected to another area.

Jon

[Richard Burts]

I agree with Jon in questioning the assertion in the original post of the idea that an ASBR needs to have an interface in area 0.  And I will go a step further than he does in discussing an ABR. Jon says that ABR "typically" has an interface in area 0. In the Cisco implementation of OSPF a router must have in interface in area 0 to function as an ABR. Or to phrase it slightly differently a router with interfaces in area 2 and in area 3 (but not in area 0) will not function as an ABR. It will not generate type 3 LSA for area 2 entries into area 3 nor generate type 3 LSA for area 3 into area 2.

HTH

Rick

[Joseph W. Doherty]

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Posting

To add a bit to the confusion, a single router might be either or both an ASBR and/or ABR.  As Jon and Rick have both noted, which (or both) a border router is depends on what the border is (between autonomous systems or between OSPF areas).

[Tiago Eduardo Zacarias]

Hello Guys,

If an ASBR and a router that has a relationship with area 0 and must receive routes from an external source other than OSPF and inject into the OSPF domain, which criterion he used to sort the router as ASBR. That is the question? Not Houe injection of external routes unless the connected and static .. Then we can consider Routes redistributed as external static? according to the following check:

Routing Process "ospf 1" with ID 172.17.0.1

Supports only single TOS(TOS0) routes

Supports opaque LSA

Supports Link-local Signaling (LLS)

Supports area transit capability

It is an autonomous system boundary router

Redistributing External Routes from,

    connected, includes subnets in redistribution

    static, includes subnets in redistribution

Initial SPF schedule delay 5000 msecs

Minimum hold time between two consecutive SPFs 10000 msecs

Maximum wait time between two consecutive SPFs 10000 msecs

Incremental-SPF disabled

Minimum LSA interval 5 secs

Minimum LSA arrival 1000 msecs

LSA group pacing timer 240 secs

Interface flood pacing timer 33 msecs

Retransmission pacing timer 66 msecs

Number of external LSA 10. Checksum Sum 0x14407B

Number of opaque AS LSA 0. Checksum Sum 0x000000

Number of DCbitless external and opaque AS LSA 7

Number of DoNotAge external and opaque AS LSA 0

Number of areas in this router is 1. 1 normal 0 stub 0 nssa

Number of areas transit capable is 0

External flood list length 0

    Area BACKBONE(0)

        Number of interfaces in this area is 1

        Area has message digest authentication

        SPF algorithm last executed 5d22h ago

        SPF algorithm executed 76 times

        Area ranges are

        Router LSA rebuild timer due in 00:01:41

        Number of LSA 5. Checksum Sum 0x0768D9

        Number of opaque link LSA 0. Checksum Sum 0x000000

        Number of DCbitless LSA 4

        Number of indication LSA 0

        Number of DoNotAge LSA 0

        Flood list length 0

[Richard Burts · Accepted solution]

If I am understanding correctly the most important part of your question is this" which criterion he used to sort the router as ASBR". The answer is simple. The criteria for ASBR is that the router redistributes some routes into OSPF.

Any router that redistributes routes into OSPF is an ASBR. That is the only criteria for ASBR. An ASBR may have a connection to area 0 or it may not. And whether it has an area 0 connection or not has nothing to do with being ASBR.

The output from the router that you posted shows that it is an ASBR. This is because it redistributes routes into OSPF. That is the only criteria.

HTH

Rick

[Richard Burts]

I am glad that you have a better understanding of criteria for ASBR and that our responses were helpful. Thank you for using the rating system to mark this question as answered.

HTH

Rick

Sent from Cisco Technical Support iPhone App

[Jose Jara]

Hello,

I would like to add that an ASBR might not "redistribute" routes into OSPF and still be an ASBR. There are two cases:

- Injecting a default route via default-information originate or via default-information originate always.

- An ABR that is doing the translation between Type 7 to Type 5 LSA in NSSA Area.

I would say that it is more accurate to say that an ASBR is the router that floods external information into the OSPF domain rather than just saying the router that is redistributing routes into OSPF.

Best Regards,

Jose.