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Latest post 01-23-2017 5:17 AM by FrancescoV. 5 replies.
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  • 01-12-2017 6:08 PM

    Prefix-List Help !!

    Hello

    Will prefix-list entry 192.168.0.0/18 be covered under 192.168.0.0/17 le 32  or not? 

     

    I appreciate if someone could explain.

     

    Thanks

     

    • Post Points: 35
  • 01-12-2017 6:34 PM In reply to

    • Ellie
    • Top 75 Contributor
    • Joined on 08-14-2011
    • Dallas, TX
    • Expert
    • Points 4,990

    Re: Prefix-List Help !!

    Yes, I think It is covered. That prefix-list indicates anything bigger than /17 and less than /32. The reason network part is the same but the subnet bits change which indicates /17 - /32

     

    Thanks

    • Post Points: 20
  • 01-13-2017 8:44 PM In reply to

    Re: Prefix-List Help !!

    I agree

    • Post Points: 5
  • 01-23-2017 2:14 AM In reply to

    Re: Prefix-List Help !!

    Guys, I think this is not correct.

    If we consider the prefix prefix list
    permit 192.168.0.0 /18
    we are talking about a single prefix only: 192.168.0.0/18

    If we use the prefix list "192.168.0.0/17 le 32" we are including the subnets,
    192.168.00XXXXXX.XXXXXXXX/18
    192.168.01XXXXXX.XXXXXXXX/18

    so unless I misunderstood the question, I dont think /18 is included in the /17 as /17 would allow a superset of what prefix /18 would match.

    Francesco

    • Post Points: 20
  • 01-23-2017 3:19 AM In reply to

    Re: Prefix-List Help !!

    192.168.0.0/17 le 32

     

    Read as following:

    Includes prefixes with a subnet mask less or equal to /32 up to (or in this case down to) the 192.168.0.0/17 range (192.168.0.0 - 192.168.127.255.255) and 192.168.0.0/17 itself.

     

    (  R1  ) <----->  (  R2  )

     

    R1:

    interface Loopback1

     ip address 192.168.0.1 255.255.192.0

    end

     

    R1#show run | s router bgp

    router bgp 65432

     bgp log-neighbor-changes

     network 192.168.0.0 mask 255.255.192.0

     neighbor 10.10.10.2 remote-as 65431

     

    R2:

    ip prefix-list IN seq 5 permit 192.168.0.0/17 le 32

    router bgp 65431

     bgp log-neighbor-changes

     neighbor 10.10.10.1 remote-as 65432

     neighbor 10.10.10.1 prefix-list IN in

    On R2:

    *Jan 23 10:59:14.988: BGP(0): 10.10.10.1 rcvd UPDATE w/ attr: nexthop 10.10.10.1, origin i, metric 0, merged path 65432, AS_PATH

    *Jan 23 10:59:14.988: BGP(0): 10.10.10.1 rcvd 192.168.0.0/18

    *Jan 23 10:59:14.988: BGP(0): Revise route installing 1 of 1 routes for 192.168.0.0/18 -> 10.10.10.1(global) to main IP table

    *Jan 23 10:59:14.988: BGP_Router: unhandled major event code 128, minor 0

    &
    R2#show ip bgp
    BGP table version is 4, local router ID is 10.10.10.2
    Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
                  r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
                  x best-external, a additional-path, c RIB-compressed,
    Origin codes: i - IGP, e - EGP, ? - incomplete
    RPKI validation codes: V valid, I invalid, N Not found
         Network          Next Hop            Metric LocPrf Weight Path
     *>  192.168.0.0/18   10.10.10.1               0             0 65432 i

    AND

    192.168.0.0/17 also gets accepted:

    R2#show ip bgp

    BGP table version is 6, local router ID is 10.10.10.2

    Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,

                  r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,

                  x best-external, a additional-path, c RIB-compressed,

    Origin codes: i - IGP, e - EGP, ? - incomplete

    RPKI validation codes: V valid, I invalid, N Not found

     

         Network          Next Hop            Metric LocPrf Weight Path

     *>  192.168.0.0/17   10.10.10.1               0             0 65432 i

     

    BR,

    Tim

    • Post Points: 20
  • 01-23-2017 5:17 AM In reply to

    Re: Prefix-List Help !!

    thank you Timmy,

    so you are saying that, as long as we use it to block 192.168.0.0/18, it is fine since that particular prefix will fall within /17 le 32. OK, in this context, I agree with you.


    Francesco

     

    • Post Points: 5
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