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Does and ABR need to be connected to the Backbone Area ?

Hello

We have an internal debate with some colleagues in my company, regarding a change in our OSPF configuration, due to external restrictions, that oblige as to introduce a Router connected to 2 areas (none of them the Backbone area).

I have 2 questions, and I will use try to use an example.

In the following example we have two areas (A1 and A2) besides the backbone Area (A0).

* R1 and R2 are ABR connected to the BAckbone Area.

* R1 and R3 have interfaces connected to Area A1

* R2 and R4 have interfaces connected to Area A2

* R5 has interfaces belonging to areas A1 and A2.

* R3 and R4 aren't Cisco Routers

* R5 is really a VRF in a Cisco Router.

* R3,R4,R5 are ASBR.

* We don't want to use any of the Virtual Links.

The most important thing is that we want to avoid R5 to be a transit Router, bypassing Area 0, or generating any loop.

..........................................External Routes (OE2)

............................................................|

........................................................... |

...............|---A0-->[R1]<---A1----->[R3]<----A1------|

Rest ......|.................................................................... |

of............| ....................................................................v

Network |....................................................................[R5] --- Ext.Routes (OE2)

................|....................................................................^

................|.................................................................... |

................|----A0-->[R2]<---A1----->[R4]<-----A2----|

.............................................................. |

External Routes(OE2)

I repeat again (problem with font spacing)

A0--- [R1] -- A1 -- [R3] -- A1

| ............................................|

| ..........................................[R5]

|............................................. |

A0--- [R2] -- A2 -- [R4] -- A2

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

Q1.¿ Does an ABR need to be connected to the Backbone Area

in Cisco IOS OSPF implementation?

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

RFC2328 says in 3.1. The backbone of the Autonomous System

"...The OSPF backbone always contains all area border routers. ..."

RFC2328 says in 3.2. Inter-area routing

"...Each area border router in an area summarizes for the area its

cost to all networks external to the area. ..."

RFC2328 says in 3.3. Classification of routers

"...Area border routers. A router that attaches to multiple areas."

RFC3509 "Alternative Implementations of OSPF Area Border Routers"

"...Though the definition of the Area Border Router (ABR) in the OSPF

specification does not require a router with multiple attached areas

to have a backbone connection,..."

It seems from its point of view, that it is no necessary an ABR to be connected.

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

Q2 Does [R5] generate Summary LSA (Type 3,4)

. from area A1 to area A2 and

. from area A2 to area A1 in Cisco IOS OSPF implementation ?

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

My guess is that when an ABR is not connected to the Backbone Area,

* It will not generate nor flood any LSA-Type 3 and 4.

* The only LSA that should pass from one Area to the other one should be LSA-Type 5.

Thank you in advance.

Miguel

3 REPLIES
Green

Re: Does and ABR need to be connected to the Backbone Area ?

I believe the only exception to the rule that "an ABR must be directly connected to the backbone ..." is when the ABR is connected to Area Zero by means of a virtual link; basically a tunnel through a non-Zero Area to get to the Zero Area.

And while it may not be the absolute law, convention within the industry makes it effectively so. Any routing / loop problems you have in the future will be blamed on the non-standard implementation ... it will be a member of every troubleshooting effort.

If you must connect two (non- Zero) Areas (without virtual links), have you considered a different routing protocol (or statics) and redistribution (for those two areas)?

Finding a "standard" or conventional way of getting the job done usually lends itself to better troubleshooting in the future, and removes the suspicion from non-standard / unconventional methods.

FWIW

Scott

Gold

Re: Does and ABR need to be connected to the Backbone Area ?

So, three things:

-- I'm fairly certain the type 5's will not be passed through R5. I didn't bring it up in the lab to make certain, or look at the code, but, again, I'm fairly certain this is true.

-- R5 will not become an ABR, because it's not attached to Area 0.

-- Since R5 is not an ABR, the type 5 LSA's from one area won't do any good in the other area, even if they are flooded--or at least I don't think they will work. An ABR does more than create type 3's from type 1's and 2's. It also translate type 7's, and it also creates/generates type 4's, based on the router lsa's from ASBR's in the area it is bordering. In other words, a router in area 1 would receive a typr 5 from area 2. It will examine it's local database, and discover there is no router lsa from the router which originated the type 5, so it will look for a type 4 for that border router. Finding neither, it will not use the LSA.

Since the type 1's will not be flooded through R5, and R5 won't generate the type 4's (as far as I know), routers in the other area just won't use the routes described in the type 5's, even if they receive them.

Does this make sense? I hope I've explained it right.... :-)

Note there is a proposal to be able to use type 1's and 2's from one area in another area, based on the up/down bit set on an LSA at the "non ABR border." I think this might be termed a "back door link," so you might poke around the docs for that terminology.

But OSPF isn't my strong protocol, so I could be completely wet.

:-)

Russ W ..

New Member

Re: Does and ABR need to be connected to the Backbone Area ?

I agree with you in most of your comments, but I had the opportunity to test it partially.

With OSPF debug activated I could see that.

* Type 5 LSA area passed-flooded through R5.

* R5 is not an ABR, just an ASBR and R5 does not create any type 3 and 4 LSA.

The problem arises when R5 is not a physical router but a VRF. The reason seems to be related with the Superbackbone Area in MPLS Core. No matter if you are running MPLS, R5 is converted to an ABR and in this scenario LSA Type 5 are flooded and LSA Type 3 and 4 are generated and send to the ther area.

Miguel.

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