BSR - Multiple RP Candidates

Hi,

I was just doing this LAB from Vol I and I noticed that the solution guide points out that you should use a mask value of 31 bits on the BSR Router!?! First off, can anyone explain exactly why a 31 bit mask was selected as (The longest possible mask value)??? Also if someone could explain how this works I would appreciate it, as I didn't fully understand the explanation for it!?!?

Two cRPs were used in this solution (Sw 2 and Sw 4)

I configured the BSR with a 32 bit mask value (As I wasnt sure) and I got the same results as the solution guide did with a 31 bit mask???? (See Below):

Rack1R1#sh ip pim rp-hash 224.0.0.1
  RP 150.1.8.8 (?), v2
    Info source: 150.1.5.5 (?), via bootstrap, priority 0, holdtime 210
         Uptime: 00:16:09, expires: 00:02:50
  PIMv2 Hash Value (mask 255.255.255.255)
    RP 150.1.10.10, via bootstrap, priority 0, hash value 1853965685
    RP 150.1.8.8, via bootstrap, priority 0, hash value 2125118543


Rack1R1#sh ip pim rp-hash 224.0.0.2
  RP 150.1.10.10 (?), v2
    Info source: 150.1.5.5 (?), via bootstrap, priority 0, holdtime 210
         Uptime: 00:15:54, expires: 00:02:49
  PIMv2 Hash Value (mask 255.255.255.255)
    RP 150.1.10.10, via bootstrap, priority 0, hash value 375231454
    RP 150.1.8.8, via bootstrap, priority 0, hash value 223172280


Any help would be appreciatied!!

Thanks,

Ian.

Comments

  • The hash size is most like the "accuracy" of the database. An MA/BSR's
    job is to map an RP to a group or set of groups. Each group can have
    just ONE active RP. So it's a balancing act, and the hash helps
    determine how much balancing you want. As an example, 8-bits would
    have a separate group for larger chunks (for simplicity, let's just say
    one group for 224/8, one for 225/8, etc.) 31-bits would be an RP per
    group of two multicast addresses.

    HTH,




    *Scott Morris*, CCIE/x4/ (R&S/ISP-Dial/Security/Service Provider) #4713,

    JNCIE-M #153, JNCIS-ER, CISSP, et al.

    JNCI-M, JNCI-ER

    [email protected]


    Internetwork Expert, Inc.

    http://www.InternetworkExpert.com

    Toll Free: 877-224-8987

    Outside US: 775-826-4344


    Knowledge is power.

    Power corrupts.

    Study hard and be Eeeeviiiil......



    Ianjf wrote:
    >
    > Hi,
    >
    > I was just doing this LAB from Vol I and I noticed that the solution
    > guide points out that you should use a mask value of 31 bits on the
    > BSR Router!?! First off, can anyone explain exactly why a 31 bit mask
    > was selected as (The longest possible mask value)??? Also if someone
    > could explain how this works I would appreciate it, as I didn't fully
    > understand the explanation for it!?!?
    >
    > Two cRPs were used in this solution (Sw 2 and Sw 4)
    >
    > I configured the BSR with a 32 bit mask value (As I wasnt sure) and I
    > got the same results as the solution guide did with a 31 bit mask????
    > (See Below):
    >
    > Rack1R1#sh ip pim rp-hash 224.0.0.1
    > RP 150.1.8.8 (?), v2
    > Info source: 150.1.5.5 (?), via bootstrap, priority 0, holdtime 210
    > Uptime: 00:16:09, expires: 00:02:50
    > PIMv2 Hash Value (mask 255.255.255.255)
    > RP 150.1.10.10, via bootstrap, priority 0, hash value 1853965685
    > RP 150.1.8.8, via bootstrap, priority 0, hash value 2125118543
    >
    >
    > Rack1R1#sh ip pim rp-hash 224.0.0.2
    > RP 150.1.10.10 (?), v2
    > Info source: 150.1.5.5 (?), via bootstrap, priority 0, holdtime 210
    > Uptime: 00:15:54, expires: 00:02:49
    > PIMv2 Hash Value (mask 255.255.255.255)
    > RP 150.1.10.10, via bootstrap, priority 0, hash value 375231454
    > RP 150.1.8.8, via bootstrap, priority 0, hash value 223172280
    >
    >
    > Any help would be appreciatied!!
    >
    > Thanks,
    >
    > Ian.
    >
    >
    >
    >
    > Internetwork Expert - The Industry Leader in CCIE Preparation
    > http://www.internetworkexpert.com
    >
    > Subscription information may be found at:
    > http://www.ieoc.com/forums/ForumSubscriptions.aspx
  • Thanks Scott, that helped a lot!!

    So just to check I have it right, if we had 224.1.1.1 group and 224.1.1.2 group and we used a mask of 255.255.0.0, both of these groups may end up (or definitely would?) being assigned to the same RP. But if we had a new group of say 224.2.1.1 this should be assigned to a different RP??

    The only thing I am still a tad confused about then is why the answer in the workbook solution was /31 and not /32???? Surely a /32 mask means every different group is balanced between the RPs.

    In this instance there was 2 RPs which could be used for All Groups. The Task asked:

    "Use the maximum possible hash mask length to evenly distribute the load across the RPs"

    Thanks,

    Ian.

  • Yes. My only guess on the solution is that some IOS's may have maxed
    out at /31? Otherwise I'd use /32. :)




    *Scott Morris*, CCIE/x4/ (R&S/ISP-Dial/Security/Service Provider) #4713,

    JNCIE-M #153, JNCIS-ER, CISSP, et al.

    JNCI-M, JNCI-ER

    [email protected]


    Internetwork Expert, Inc.

    http://www.InternetworkExpert.com

    Toll Free: 877-224-8987

    Outside US: 775-826-4344


    Knowledge is power.

    Power corrupts.

    Study hard and be Eeeeviiiil......



    Ianjf wrote:
    >
    > Thanks Scott, that helped a lot!!
    >
    > So just to check I have it right, if we had 224.1.1.1 group and
    > 224.1.1.2 group and we used a mask of 255.255.0.0, both of these
    > groups may end up (or definitely would?) being assigned to the same
    > RP. But if we had a new group of say 224.2.1.1 this should be assigned
    > to a different RP??
    >
    > The only thing I am still a tad confused about then is why the answer
    > in the workbook solution was /31 and not /32???? Surely a /32 mask
    > means every different group is balanced between the RPs.
    >
    > In this instance there was 2 RPs which could be used for All Groups.
    > The Task asked:
    >
    > "Use the maximum possible hash mask length to evenly distribute the
    > load across the RPs"
    >
    > Thanks,
    >
    > Ian.
    >
    >
    >
    >
    > Internetwork Expert - The Industry Leader in CCIE Preparation
    > http://www.internetworkexpert.com
    >
    > Subscription information may be found at:
    > http://www.ieoc.com/forums/ForumSubscriptions.aspx
  • That's great, thanks for the help Scott in clearing this issue up!!

    As always it's much appreciated!

    Have a good weekend!!!

    Ian.

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