ACL and sequence numbers

Question

I had the first two lines in the access list and all was well, I then added the 3rd. From what I need to put the 3rd entry (deny host 10.1.30.51) after the second entry and before the permit any. Even though I created sequence numbers in order of the 3 entries (10,20,30) the sequence numbers didnt put them in order and they dont even show up in the show run. What went wrong? How is it possible to edit an acl without sequence numbers also?
Cause if I had:
10 deny x.x.x.x
20 deny x.x.x.x
30 permit any
Then I could add say 15 deny x.x.x.x, but now I cant and I dont even know what happened to the sequence numbers when I created them.
Thanks.
Standard IP access list 1
    deny host 10.1.30.50 (4 match(es))
    permit any (8 match(es))
    deny host 10.1.30.51
Router#

Answer 1

Hi Milan,
Sequence numbers are indeed not supported if you define a numbered access list. With both standard and extended numbered ACLs, however, it is possible to do a trick: if you refer to them as named ACLs (use their number as their name), you actually are able to use the sequence numbers.
For example:
R1(config)# do show run | i access-listaccess-list 1 deny   192.0.2.1access-list 1 permit anyaccess-list 100 deny   ip host 192.0.2.1 anyaccess-list 100 permit ip any anyR1(config)# do show ip access-lStandard IP access list 1    10 deny   192.0.2.1    20 permit anyExtended IP access list 100    10 deny ip host 192.0.2.1 any    20 permit ip any anyR1(config)# ip access-list standard 1R1(config-std-nacl)# 15 deny 192.0.2.15R1(config-std-nacl)# exitR1(config)# do show access-listStandard IP access list 1    10 deny   192.0.2.1    15 deny   192.0.2.15    20 permit anyExtended IP access list 100    10 deny ip host 192.0.2.1 any    20 permit ip any anyR1(config)# ip access-list extended 100R1(config-ext-nacl)# 15 deny ip host 192.0.2.15 anyR1(config-ext-nacl)# exitR1(config)# do show access-lStandard IP access list 1    10 deny   192.0.2.1    15 deny   192.0.2.15    20 permit anyExtended IP access list 100    10 deny ip host 192.0.2.1 any    15 deny ip host 192.0.2.15 any    20 permit ip any any
The router is even smart enough to disallow to refer to a named ACL whose name is a number of the opposite type than stated on the command line:
R1(config)# ip access-list standard 101% % Invalid access list name.R1(config)# ip access-list extended 2% % Invalid access list name.
What Collin may have in mind, though, is that host entries in standard ACLs are reorganized to a different order than entered:
R1(config)# ip access-list standard TestR1(config-std-nacl)# permit 10.0.0.1R1(config-std-nacl)# deny 10.0.0.2R1(config-std-nacl)# permit 10.0.0.3R1(config-std-nacl)# deny 10.0.0.4R1(config-std-nacl)# permit 10.0.0.5R1(config-std-nacl)# deny 10.0.0.6R1(config-std-nacl)# permit 10.0.0.7R1(config-std-nacl)# deny 10.0.0.8R1(config-std-nacl)# permit anyR1(config-std-nacl)#exitR1(config)# do show access-list TestStandard IP access list Test    80 deny   10.0.0.8    20 deny   10.0.0.2    30 permit 10.0.0.3    10 permit 10.0.0.1    60 deny   10.0.0.6    70 permit 10.0.0.7    40 deny   10.0.0.4    50 permit 10.0.0.5    90 permit anyR1(config)# do show run | section Testip access-list standard Test deny   10.0.0.8 deny   10.0.0.2 permit 10.0.0.3 permit 10.0.0.1 deny   10.0.0.6 permit 10.0.0.7 deny   10.0.0.4 permit 10.0.0.5 permit any
This reordering happens only with standard ACLs and is a result of indexing the host entries in the ACL into a hash table (the hash function being XOR of individual octets of the IP address in the host entry) for faster access. When printing out the ACL, first the host items are printed out in the order they are stored in the hashing table, and only then the remaining entries that use wildcards. Wildcard entries are not reordered.
The funny thing is that the ACL is actually even stored in the configuration in the reordered form, and thus evaluated in a reordered form, which can be confusing. However, you may have noticed that the router will prohibit you from entering a host ACL after entering a wildcard ACL that also matches the IP address in a wildcard entry:
R1(config)# ip access-list standard Test2R1(config-std-nacl)# permit 10.0.1.0 0.0.0.255R1(config-std-nacl)# deny 10.0.1.1% Access rule can't be configured at higher sequence num as it is part of the existing rule at sequence num 10R1(config-std-nacl)#
Why is this? Obviously, a host entry can  either select the same action for a packet that would be taken by a more  general wildcard entry (in which case it is not necessary for the  host entry to be entered at all), or  it can override the action that would be chosen by a more general  wildcard entry. In this second case, it is necessary for this host entry  to be placed in the ACL first, otherwise it would never be reached. Ordering of host entries themselves can be arbitrary, as they do not influence each other.  This leads us to a general logic in standard ACLs - it is required to put  all host entries first, and wildcard entries last. Now it is completely logical to visit all host entries first (indexed by a hash for rapid access), and then visit the wildcard entries.
Quite a long post... sorry for that. Hopefully, we've resolved some of the doubts.
Best regards,
Peter