Re: Use of extended prefix lengths

[bill fumerola <billf@mu.org>]

On Thu, Jun 05, 2008 at 06:05:57PM -0400, Chuck Anderson wrote:
> On Thu, Jun 05, 2008 at 02:28:20PM -0700, bill fumerola wrote:
> > for internal point-to-point links, using ospfv3 + link-local addresses
> > + /128s on loopbacks has worked well for me. for point-to-point on the
> > edge, i've used /64s down to /120s. depending on your IGP, using global
> > addresses on point-to-point may have its benefits.
> 
> How does traceroute work when using link-local addressing on 
> point-to-point links?

rtr1.ash>sh ipv6 route 2001:500:2F::F
B   2001:500:2F::/48 [200/0]
     via 2620:0:CCC:1::1    
rtr1.ash>sh ipv6 route 2620:0:CCC:1::1
O   2620:0:CCC:1::1/128 [110/1]
    via FE80::217:FFF:FE1C:BC1B, Tunnel3800 
rtr1.ash>sh ipv6 int tun3800
Tunnel3800 is up, line protocol is up
  IPv6 is enabled, link-local address is FE80::219:7FF:FE23:7C1B 
  Description: NYC/ASH
  No global unicast address is configured
[...]
rtr1.ash>traceroute 2001:500:2F::F

Type escape sequence to abort.
Tracing the route to f.root-servers.net (2001:500:2F::F)

  1 2620:0:CCC:0:217:FFF:FE1C:BC1B 8 msec 8 msec 8 msec
  2 2001:504:1::A502:7319:1 8 msec 8 msec 8 msec
  3 f.root-servers.net (2001:500:2F::F) 8 msec 4 msec 80 msec

and...

rtr1.nyc#sh ipv6 int br
GigabitEthernet0/0         [up/up]
    unassigned
GigabitEthernet0/0.1       [up/up]
    FE80::217:FFF:FE1C:BC1B
    2620:0:CCC:0:217:FFF:FE1C:BC1B  <-- used in traceroute
GigabitEthernet0/2         [up/up]
    FE80::217:FFF:FE1C:BC19
    2001:504:1::A503:6692:1
Loopback0                  [up/up]
    FE80::217:FFF:FE1C:BC1B
    2620:0:CCC:1::1
[....]
Tunnel3800                 [up/up]
    FE80::217:FFF:FE1C:BC1B
Tunnel3802                 [up/up]
    FE80::217:FFF:FE1C:BC1B

in this case it uses an available global address. probably the lowest
number. (on this box, 2620:0:CCC:0:: < 2620:0:CCC:1::)

rtr1.nyc#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
rtr1.nyc(config)#int tun3800
rtr1.nyc(config-if)#ipv6 unnumbered lo0
rtr1.nyc(config-if)#^Z
rtr1.nyc#

Tunnel3800                 [up/up]
    FE80::217:FFF:FE1C:BC1B
    unnumbered (Loopback0)

and now...

rtr1.ash#traceroute 2001:500:2F::F         

Type escape sequence to abort.
Tracing the route to f.root-servers.net (2001:500:2F::F)

  1 2620:0:CCC:1::1 8 msec 8 msec 8 msec
  2 2001:504:1::A502:7319:1 4 msec 8 msec 12 msec
  3 f.root-servers.net (2001:500:2F::F) 8 msec 8 msec 8 msec
rtr1.ash#

... and now it uses the loopback.

-- bill