Routing

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  • 1.  OSPF Router with interfaces in area 1 and area 2 become ABR

    Posted 08-22-2021 13:06
    Hi everyone.

    I have the following topology:
    My configuration:

    MX1:
    set system host-name MX1
    set interfaces ge-0/0/0 unit 0 family inet address 10.0.0.0/31
    set interfaces ge-0/0/1 unit 0 family inet address 11.0.0.0/31
    set interfaces lo0 unit 0 family inet address 1.1.1.1/32
    set protocols ospf area 0.0.0.0 interface ge-0/0/0.0 interface-type p2p
    set protocols ospf area 0.0.0.0 interface ge-0/0/1.0 interface-type p2p

    MX2:
    set system host-name MX2
    set interfaces ge-0/0/0 unit 0 family inet address 10.0.0.1/31
    set interfaces ge-0/0/1 unit 0 family inet address 12.0.0.0/31
    set interfaces ge-0/0/2 unit 0 family inet address 20.0.0.0/31
    set interfaces lo0 unit 0 family inet address 2.2.2.2/32
    set protocols ospf area 0.0.0.0 interface ge-0/0/0.0 interface-type p2p
    set protocols ospf area 0.0.0.0 interface ge-0/0/2.0 interface-type p2p
    set protocols ospf area 0.0.0.1 nssa default-lsa default-metric 1
    set protocols ospf area 0.0.0.1 nssa default-lsa metric-type 1
    set protocols ospf area 0.0.0.1 nssa no-summaries
    set protocols ospf area 0.0.0.1 interface ge-0/0/1.0 interface-type p2p

    MX3:
    set system host-name MX3
    set interfaces ge-0/0/0 unit 0 family inet address 13.0.0.0/31
    set interfaces ge-0/0/1 unit 0 family inet address 11.0.0.1/31
    set interfaces ge-0/0/2 unit 0 family inet address 20.0.0.1/31
    set protocols ospf area 0.0.0.0 interface ge-0/0/1.0 interface-type p2p
    set protocols ospf area 0.0.0.0 interface ge-0/0/2.0 interface-type p2p
    set protocols ospf area 0.0.0.2 nssa default-lsa default-metric 1
    set protocols ospf area 0.0.0.2 nssa default-lsa metric-type 1
    set protocols ospf area 0.0.0.2 nssa no-summaries
    set protocols ospf area 0.0.0.2 interface ge-0/0/0.0 interface-type p2p

    MX4:
    set system host-name MX4
    set interfaces ge-0/0/0 unit 0 family inet address 13.0.0.1/31
    set interfaces ge-0/0/1 unit 0 family inet address 12.0.0.1/31
    set interfaces lo0 unit 0 family inet address 100.100.100.100/32
    set protocols ospf export external-route-to-lo0
    set protocols ospf area 0.0.0.1 nssa
    set protocols ospf area 0.0.0.1 interface ge-0/0/1.0 interface-type p2p
    set protocols ospf area 0.0.0.2 nssa
    set protocols ospf area 0.0.0.2 interface ge-0/0/0.0 interface-type p2p
    set policy-options policy-statement external-route-to-lo0 from protocol direct
    set policy-options policy-statement external-route-to-lo0 from interface lo0.0
    set policy-options policy-statement external-route-to-lo0 then external type 1
    set policy-options policy-statement external-route-to-lo0 then accept

    So, my question:

    I have no 100.100.100.100/32 route on MX1. This happens due to external route not advertised by MX2 and MX3, because they don't tanslate LSA7 to LSA5. This role was taken by MX4 due to highest Router ID (rfc1587 - Section 4.1). But, MX4 shouldn't be ABR (rfc2328 - Section 3.1).

    Why
    MX4 becomes ABR and how to disable this feature?

    ------------------------------
    ANTON GOLOVIN
    ------------------------------

    Attachment(s)

    txt
    Configuration.txt   2 KB 1 version


  • 2.  RE: OSPF Router with interfaces in area 1 and area 2 become ABR

    Posted 08-23-2021 08:39

    Hi Anton, 

    pretty sure MX4 is expected to be an ABR, it has interfaces in more than one area.

    I ignore why you would need a design like this, but in a normal scenario when you are working with a router that is an ABR and is not attached to area 0 you would use a virtual link, but since this is an NSSA the device wont support, i bet that even if you remove the NSSA the behavior will remain the same, MX 1 wont be able to receive the external until you either connect disable one interface in MX4, or merged area 1 and 2 into a single area. 



    ------------------------------
    GABRIEL FLORES
    ------------------------------



  • 3.  RE: OSPF Router with interfaces in area 1 and area 2 become ABR

    Posted 08-23-2021 13:24
    Hi Gabriel,

    Thank you for your reply.

    As i see, the MX4 shouldn't be an ABR, as written in Juniper Documentation (Area Border Routers):
    Routing devices that belong to more than one area and connect one or more OSPF areas to the backbone area are called 
    area border routers
     (ABRs). At least one interface is within the backbone while another interface is in another area. ABRs also maintain a separate topological database for each area to which they are connected.
    Same information are written in RFC2328 - Section 3.1:
    The OSPF backbone always contains all area border routers. The backbone is responsible for distributing routing
    information between non-backbone areas.

    And in Section 3.2:
    When routing a packet between two non-backbone areas the backbone is used.


    ------------------------------
    ANTON GOLOVIN
    ------------------------------



  • 4.  RE: OSPF Router with interfaces in area 1 and area 2 become ABR

    Posted 08-23-2021 17:29

    we call area 0 the backbone, but this is nothing but a best a practice, the problem i see there is that MX4 sees one of his two areas as his "backbone" breaking the leak of routes. this isnt because of the NSSA, but rather because he is not attached to area 0. 

    from section 3.1:

            Virtual links can be configured between any two backbone routers
            that have an interface to a common non-backbone area.  Virtual
            links belong to the backbone.  The protocol treats two routers
            joined by a virtual link as if they were connected by an
            unnumbered point-to-point backbone network.  On the graph of the
            backbone, two such routers are joined by arcs whose costs are
            the intra-area distances between the two routers.  The routing
            protocol traffic that flows along the virtual link uses intra-
            area routing only.

    concept of an ABR

       3.3.  Classification of routers
    
             Area border routers
                A router that attaches to multiple areas.  Area border
                routers run multiple copies of the basic algorithm, one copy
                for each attached area. Area border routers condense the
                topological information of their attached areas for
                distribution to the backbone.  The backbone in turn
                distributes the information to the other areas.
    


    ------------------------------
    GABRIEL FLORES
    ------------------------------



  • 5.  RE: OSPF Router with interfaces in area 1 and area 2 become ABR
    Best Answer

    Posted 08-23-2021 20:13
    Hi Anton, hi Gabriel,

    This is one of those things where the answer is up to the implementation. Setting the Border bit in OSPF (0x1 in the Router LSA header) is what is introducing the lack of reachability from the backbone towards the NSSA (more on this later).

    Here is a couple of items to take into account:
    1. Cisco implementation of OSPF/RFC2328 considers an area border router (ABR) by definition a router that is between backbone (0) and a non-backbone area (!0).
    2. Juniper implementation of OSPF/RFC2328 considers an area border router (ABR) by definition a router that is between two areas.
    If you check the Border bit on MX4, it should be set on the Router LSA (in this case it should be set to 0x3 since it is an ABR and ASBR) per Router LSA in each respective area (see the show ospf database router extensive command). Since in this case, MX4 would result in the translator since it has the highest router-ID, disallowing the translation of the NSSA External LSA on MX2 and MX3 respectively.

    How to fix this? Set the router-ID of MX4 to be lower than the rest of the routers.

    This is one of those things where something that seems a minor implementation quirk, changes everything. ; )

    HTH,

    Elvin