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> Cisco IOS IPv6 Prefix List Oddness
Posted by prox, from Charlotte, on December 28, 2013 at 15:22 local (server) time

A week or so I configured some IPv6 prefix lists in my networking lab at home on a few Cisco IOS boxes running 15.1(4)M7.  Almost a day after I finalized the configuration, I noticed that the prefix lists appeared to be reordering themselves from time to time.  Specifically, the "show running-config" command would list the entries in a slightly different order, every once and awhile.

Normally, I wouldn't care.  Prefix lists are not evaluated based on order and this was just a lab, anyway.  However, I have RANCID setup to monitor all configuration file changes in my lab environment and I started getting several e-mails per day.  Here's a sample:

Index: configs/defiant
retrieving revision 1.485
diff -u -4 -r1.485 defiant
@@ -340,11 +340,11 @@
   passive-interface Loopback0
  ipv6 prefix-list DEFAULT-ROUTE permit ::/0
- ipv6 prefix-list SPOCK-NETS permit 2001:48C8:1:104::37/128
  ipv6 prefix-list SPOCK-NETS permit 2001:48C8:1:104::38/127
  ipv6 prefix-list SPOCK-NETS permit 2001:48C8:1:13A::/63
+ ipv6 prefix-list SPOCK-NETS permit 2001:48C8:1:104::37/128
  route-map ADV-DEFAULT permit 10
   match ip address DEFAULT-ROUTE

This is happening on two routers running the same exact version of IOS with a similar prefix list.  It seems like the configuration compilation that is executed from the "show running-config" command may be doing something wrong since when I display the prefix lists manually and they are shown in the same order each time:

defiant#show ipv6 prefix-list 
ipv6 prefix-list DEFAULT-ROUTE: 1 entries
   seq 5 permit ::/0
ipv6 prefix-list SPOCK-NETS: 3 entries
   seq 5 permit 2001:48C8:1:104::37/128
   seq 10 permit 2001:48C8:1:104::38/127
   seq 15 permit 2001:48C8:1:13A::/63

I haven't tried messing with different IOS versions, yet.  I may do that later.

Oh, in other news.. among other things, I got a plush NCC-1701 for Christmas:

Plush Enterprise NCC-1701

Yes, it's from ThinkGeek and makes noise.

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