3.3 IPv6 Summary Route

Hi, Could someone please explain why the SG uses a 2001:200::/22 as a summary.  The summary route will always be 2001::/22 as the '2' won't be matched by the mask.  If the mask was /23 then i'd understand why the '2' was needed.



  • Right...if your just using loops 100, 101, and 103 as the determiners for the summary I get that it should be 2001:200::/26.  The other ip addresses on R6 start with 2001:cc1e...so if we were using them that boundary would be pushed back to a /16.  I see mentions in past threads about this of a 192...maybe it changed.  Cristian...any comments?

  • The task didn't specified that the IPv6 Loopback on R6 is excluded from the summarization. So, because I took Loopback 0 into consideration the summary end up with 2001::/16



  • Same here ...


    Rack1R6#sh ipv6 int brief
    FastEthernet0/0            [up/up]
    FastEthernet0/0.16         [up/up]
    FastEthernet0/0.63         [up/up]
    FastEthernet0/1            [administratively down/down]
    Serial0/0/0                [up/up]
    Virtual-Access1            [down/down]
    Virtual-Template1          [down/down]
    Virtual-Access2            [up/up]
    Loopback0                  [up/up]
    Loopback100                [up/up]
    Loopback101                [up/up]
    Loopback103                [up/up]
    Tunnel56                   [up/up]
    Rack1R6#confi gt
    % Invalid input detected at '^' marker.

    Rack1R6#config t
    Enter configuration commands, one per line.  End with CNTL/Z.
    Rack1R6(config)#int tunnel56
    Rack1R6(config-if)#ipv6 summary eigrp 100 2001::/16 ?
      <1-255>  Administrative distance

    Rack1R6(config-if)#ipv6 summary eigrp 100 2001::/16


    For me the tasks says: R6 should send ONE  optimal prefix for all R6 prefixes ... hence I included Loop0 !


    *Jun 16
    Rack1R5#sh ipv6 route eigrp
    IPv6 Routing Table - Default - 7 entries
    Codes: C - Connected, L - Local, S - Static, U - Per-user Static route
           B - BGP, M - MIPv6, R - RIP, I1 - ISIS L1
           I2 - ISIS L2, IA - ISIS interarea, IS - ISIS summary, D - EIGRP
           EX - EIGRP external
           O - OSPF Intra, OI - OSPF Inter, OE1 - OSPF ext 1, OE2 - OSPF ext 2
           ON1 - OSPF NSSA ext 1, ON2 - OSPF NSSA ext 2
    D   2001::/16 [90/26882560]
         via FE80::9601:606, Tunnel56




  • Indeed. Task said R5 should only receive a SINGLE optimal summary. Lo0 also has an IPv6 address that's not included in the /22

  • this must be one of those 'you think it's a trick question, but it's not' . It needs to be specified that Loopback0 is excluded from the summarization. The lab is tricky enough w/o these additional complications :)

  • Since task asks to have single route on R5 to reach to all routes on R6, this makes sense to summarize all the prefix on R6 into ipv6 eigrp.

  • Since task asks to have single route on R5 to reach to all routes on R6, this makes sense to summarize all the prefix on R6 into ipv6 eigrp.

    OK I think the SG is wrong for summary in this instance.

    However here is my question.  How are we supposed to meet the last two task requirements at the same time -

    • R6 should not advertise any address space that it does not have a more specific route for.
    • R5 should receive only a single optimal summary prefix from R6.

    So if you choose a summary for example 2001:200::/29 for example, this covers networks 2001:200::/32 2001:201::/32 and so on up to 2001:207::/32.  Clearly we only have more specific routes to 2001:205:90:31::/64 but no specif routes to  2001:206:90:31::/64 for example.  The second bullet to me says that we can summarise one address space only but more specific prefixes are allowed.  What does it mean by optimal?  R5 only has one way to reach R6 via the ipv6ip tunnel so all summaries in this case would be optimal.  I accept this isn't alway the case and is topology dependent.


  • I used 2001::/16

    The problem quite clearly said one optimal route, and anything else leave out the loopback.

    Please correct me if my action was wrong as I'm confused?

  • My initial config file (dated 2011 era) had the 192 address which also is problematic with /22 being correct.


    interface FastEthernet0/0
     no shutdown
     no ip address
    no shut
     ipv6 address 2001:192:1::/64 eui-64


  • I agree with welshydragon, this question doesn't make sense..

  • Glad this was already discussed since I interpreted the question the same way.  Hopefully the question or the solution will be corrected in the future.  

  • I have also used 2001::/16 for the summary




  • Agreed on the 2001::/16 address and the SG still has the same wording and odd solution plus the 2001:192:: address for other R6 interfaces.

    Tech Edit team to sort this one out.


  • r6:

    interface FastEthernet0/0
     no shutdown
     no ip address
    no shut
     ipv6 address 2001:192:1::/64 eui-64

    Actually, we could use the /22 if you consider that this range is advertised, but it actually doesn't go anywhere. It is on the same physical interface , as where the sub-interfaces are. Besides that the switch only opened up vlan 16,63. 

    Still INE need to fix this, but they'll need to fix the initial configs. (or we had to be smart enough to remove it, and this was one of the faults the left in the configs)...



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