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  • 1.  BGP Route Reflector best path advertisement.

    Posted 09-25-2022 07:17
    Hi Experts,

    How are you doing?

    I need a small help to understand the actual behavior for BGP best path selection and advertised when Route reflector is considered.
    My expectation was RR would advertise only the best path until add-path is enabled.
     
    VM3 is my RR and receiving updated for prefix 9.9.9.9/32 from PEs 5.5.5.5 , 6.6.6.6, 7.7.7.7 .
    VM4 is redundant RR 4.4.4.4

    regress@vmx3# run show route protocol bgp 9.9.9.9/32

    bgp.l3vpn.0: 8 destinations, 16 routes (8 active, 0 holddown, 2 hidden)
    + = Active Route, - = Last Active, * = Both

    5.5.5.5:3209:9.9.9.9/32
    *[BGP/170] 10:31:57, localpref 100, from 5.5.5.5
    AS path: 200 I, validation-state: unverified
    > to 192.168.7.1 via ge-0/0/0.0, Push 17
    [BGP/170] 10:12:58, localpref 100, from 4.4.4.4
    AS path: 200 I, validation-state: unverified
    > to 192.168.7.1 via ge-0/0/0.0, Push 17
    6.6.6.6:3209:9.9.9.9/32
    *[BGP/170] 10:39:13, localpref 200, from 6.6.6.6       >>>>>>>>> Localpref 200 is highest among others.
    AS path: 200 I, validation-state: unverified
    > to 192.168.8.1 via ge-0/0/1.0, Push 16
    [BGP/170] 10:12:58, localpref 200, from 4.4.4.4.
    AS path: 200 I, validation-state: unverified
    > to 192.168.8.1 via ge-0/0/1.0, Push 16
    7.7.7.7:3209:9.9.9.9/32
    *[BGP/170] 10:23:12, localpref 100, from 7.7.7.7
    AS path: 200 I, validation-state: unverified
    > to 192.168.9.1 via ge-0/0/2.0, Push 16
    [BGP/170] 10:12:58, localpref 100, from 4.4.4.4
    AS path: 200 I, validation-state: unverified
    > to 192.168.9.1 via ge-0/0/2.0, Push 16

    When RR advertises the prefix - 9.9.9.9/32 to another end PEs. 

    regress@vmx3# run show route advertising-protocol bgp 1.1.1.1

    bgp.l3vpn.0: 8 destinations, 16 routes (8 active, 0 holddown, 2 hidden)
    Prefix Nexthop MED Lclpref AS path
    2.2.2.2:6830:192.168.6.0/30
    * 2.2.2.2 100 I
    5.5.5.5:3209:9.9.9.9/32
    * 5.5.5.5 100 3209 200 I
    5.5.5.5:3209:192.168.14.0/30
    * 5.5.5.5 100 3209 I
    6.6.6.6:3209:9.9.9.9/32
    * 6.6.6.6 200 3209 200 I
    6.6.6.6:3209:192.168.15.0/30
    * 6.6.6.6 200 3209 I
    7.7.7.7:3209:9.9.9.9/32
    * 7.7.7.7 100 3209 200 I
    7.7.7.7:3209:192.168.16.0/30
    * 7.7.7.7 100 3209 I

    The question is why RR is adverting all the paths to other ends PE , expectation is RR should only advertise the best path until add-path is enabled.
    What am I missing here ?

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    Sujit Sahu
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  • 2.  RE: BGP Route Reflector best path advertisement.

    Posted 09-26-2022 06:23

    This is correct behavior as they are different prefixes.

    They have different route distinguishers, which makes them unique. This is the very essence of how overlapping prefixes can be carried in BGP in MPLS/L3VPN.

    5.5.5.5:3209:9.9.9.9/32 does not equal 6.6.6.6:3209:9.9.9.9/32 due to the different RDs of 5.5.5.5:3209 and 6.6.6.6:3209 therefore both routes are reflected to the clients.

    If you really wanted to, you could have the same RD across all PEs per VPN and then you would see the behavior of the RR only reflecting which it thinks is best as the uniqueness would be lost  but you usually wouldn't (and basing you RD on the PE loopback pretty much prevents this which I think is a good thing).



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    STUART RIDSDALE
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