aclrouter
Let's say I have a access-list 1 with 5 permits.
And I would like to delete the third permit listing.
I know that I can simply do "no 30"
However, can I confirm that I can
no access-list 1 permit host 192.168.1.1 command in global configuration mode. Or no access-list 1 global configuration command, and then re-enter the other 4 access-list?
R1(config)#access-list 1 R1(config)#Deny 20.1.1.1 R1(config)#Permit any R1(config)# int s0/0 R1(config-if)#ip access-group 1 inWhen i try to ping 10.1.1.1 it returns
U.U.U-----> Which means destination host unreachable.
The only thing you can do is add no ip unreachables to Serial0/0. This would make pings simply timeout instead of receiving an ICMP admin prohibited message when packets are denied on the serial interface.
Examples:
The following examples illustrate what happens:
ip unreachables is configured on Gi0/0no ip unreachables is configured on Gi0/0ip unreachables (which is the default) on the interfaceOn the router with the ACL...
ROUTER2#sh runn | i access-list 166
access-list 166 deny ip host 192.0.2.111 any
access-list 166 permit ip any any
ROUTER2#sh runn int gi0/0
!
interface GigabitEthernet0/0
ip address 192.0.2.29 255.255.255.0
ip access-group 166 in
no ip redirects
no ip proxy-arp
And on the host being blocked...
ROUTER1#debug ip icmp
ROUTER1#ping 192.0.2.29 source lo0
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.0.2.29, timeout is 2 seconds:
Packet sent with a source address of 192.0.2.111
U.U.U
Success rate is 0 percent (0/5)
ROUTER1#sh log | i administrat
Jan 16 11:02:29.251 CST: ICMP: dst (192.0.2.111) administratively
prohibited unreachable rcv from 192.0.2.29
Jan 16 11:02:31.255 CST: ICMP: dst (192.0.2.111) administratively
prohibited unreachable rcv from 192.0.2.29
Jan 16 11:02:33.263 CST: ICMP: dst (192.0.2.111) administratively
prohibited unreachable rcv from 192.0.2.29
no ip unreachablesAdding no ip unreachables on ROUTER2...
ROUTER2#conf t
ROUTER2(config)#int gi0/0
ROUTER2(config-if)#no ip unreach
Now the pings fail silently...
ROUTER1#ping 192.0.2.29 source lo0
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.0.2.29, timeout is 2 seconds:
Packet sent with a source address of 192.0.2.111
.....
Success rate is 0 percent (0/5)
ROUTER1#
The reason your router hangs is because you're editing the ACL through the same interface that it's applied to. Usually what happens is you wind up blocking yourself accidentally.
Note that I make a suggestion for reorganizing your ACLs at the bottom of this answer, although most people will be interested in this first section (the easiest way to change ACLs).
Newer Cisco IOS images (such as the image on your Cisco 1941) support IP access-list line numbering, which means you can modify your ACL without removing it...
I will use the example of your original ACL 110, although as I mentioned you should really consider reorganizing your ACLs into two different ACLs.
Step 1, ACL 110 already on the router:
LAB_RTR#sh access-list 110
Extended IP access list 110
10 permit ip 192.168.1.0 0.0.0.255 host 192.168.2.44
20 permit ip host 192.168.2.44 192.168.1.0 0.0.0.255
30 permit ip 192.168.2.0 0.0.0.255 host 192.168.1.18
40 permit ip host 192.168.1.18 192.168.2.0 0.0.0.255
50 deny ip 192.168.1.0 0.0.0.255 192.168.2.0 0.0.0.255
60 permit udp any any
70 permit ip any any
Take note of the line numbers that Cisco automatically added to ACL 110. Remember that you need to add your ACL entries before line 50, due to the deny statement.
Step 2, Modify ACL 110 in place with new entries, starting after line 40:
LAB_RTR#conf t
Enter configuration commands, one per line. End with CNTL/Z.
LAB_RTR(config)#ip access-list extended 110
LAB_RTR(config-ext-nacl)#41 permit ip 192.168.2.0 0.0.0.255 host 192.168.1.120
LAB_RTR(config-ext-nacl)#42 permit ip host 192.168.1.120 192.168.2.0 0.0.0.255
LAB_RTR(config-ext-nacl)#43 permit ip 192.168.2.0 0.0.0.255 host 192.168.1.222
LAB_RTR(config-ext-nacl)#44 permit ip host 192.168.1.222 192.168.2.0 0.0.0.255
LAB_RTR(config-ext-nacl)#end
LAB_RTR#
LAB_RTR#
LAB_RTR#
LAB_RTR#sh access-list 110
Extended IP access list 110
10 permit ip 192.168.1.0 0.0.0.255 host 192.168.2.44
20 permit ip host 192.168.2.44 192.168.1.0 0.0.0.255
30 permit ip 192.168.2.0 0.0.0.255 host 192.168.1.18
40 permit ip host 192.168.1.18 192.168.2.0 0.0.0.255
41 permit ip 192.168.2.0 0.0.0.255 host 192.168.1.120
42 permit ip host 192.168.1.120 192.168.2.0 0.0.0.255
43 permit ip 192.168.2.0 0.0.0.255 host 192.168.1.222
44 permit ip host 192.168.1.222 192.168.2.0 0.0.0.255
50 deny ip 192.168.1.0 0.0.0.255 192.168.2.0 0.0.0.255
60 permit udp any any
70 permit ip any any
LAB_RTR#
Step 3, Renumber the ACL, automatically:
The automatic line numbers are nice, because Cisco also gives you a way to renumber the config...
LAB_RTR#conf t
LAB_RTR(config)#! Renumber ACL 110, from line 10... increment in steps of 10
LAB_RTR(config)#ip access-list resequence 110 10 10
LAB_RTR(config)#end
LAB_RTR#
LAB_RTR#sh access-list 110
Extended IP access list 110
10 permit ip 192.168.1.0 0.0.0.255 host 192.168.2.44
20 permit ip host 192.168.2.44 192.168.1.0 0.0.0.255
30 permit ip 192.168.2.0 0.0.0.255 host 192.168.1.18
40 permit ip host 192.168.1.18 192.168.2.0 0.0.0.255
50 permit ip 192.168.2.0 0.0.0.255 host 192.168.1.120
60 permit ip host 192.168.1.120 192.168.2.0 0.0.0.255
70 permit ip 192.168.2.0 0.0.0.255 host 192.168.1.222
80 permit ip host 192.168.1.222 192.168.2.0 0.0.0.255
90 deny ip 192.168.1.0 0.0.0.255 192.168.2.0 0.0.0.255
100 permit udp any any
110 permit ip any any
LAB_RTR#
Originally, the safest way to change your ACL was by removing it from the interfaces in question (this prevented problems if you were changing the ACL via the same interface it was applied to).
Use this technique if your IOS doesn't support IP access-list line numbering:
interface GigabitEthernet0/1.1
no ip access-group 110 in
!
interface GigabitEthernet0/1.2
no ip access-group 110 in
Now you can add the new ACL lines, but do it before the other permit any entries...
! Reset the ACL
no access-list 110
!
access-list 110 permit ip 192.168.1.0 0.0.0.255 host 192.168.2.44
access-list 110 permit ip host 192.168.2.44 192.168.1.0 0.0.0.255
access-list 110 permit ip 192.168.2.0 0.0.0.255 host 192.168.1.18
access-list 110 permit ip host 192.168.1.18 192.168.2.0 0.0.0.255
!! New ACL lines before the deny statement
access-list 110 permit ip 192.168.2.0 0.0.0.255 host 192.168.1.120
access-list 110 permit ip host 192.168.1.120 192.168.2.0 0.0.0.255
access-list 110 permit ip 192.168.2.0 0.0.0.255 host 192.168.1.222
access-list 110 permit ip host 192.168.1.222 192.168.2.0 0.0.0.255
!! Deny statement
access-list 110 deny ip 192.168.1.0 0.0.0.255 192.168.2.0 0.0.0.255
access-list 110 permit udp any any
access-list 110 permit ip any any
Finally, reapply the ACL to the switch...
interface GigabitEthernet0/1.1
ip access-group 110 in
!
interface GigabitEthernet0/1.2
ip access-group 110 in
You should really consider using two different ACLs for Gigabit0/1.1 and GigabitEthernet0/1.2... this is a guess at what you're trying to do, but it's unclear that I'm interpreting things correctly...
access-list 111 permit ip 192.168.1.0 0.0.0.255 host 192.168.2.44
access-list 111 permit ip host 192.168.1.18 192.168.2.0 0.0.0.255
access-list 111 permit ip host 192.168.1.120 192.168.2.0 0.0.0.255
access-list 111 permit ip host 192.168.1.222 192.168.2.0 0.0.0.255
access-list 111 deny ip 192.168.1.0 0.0.0.255 192.168.2.0 0.0.0.255
access-list 111 permit udp any any
access-list 111 permit ip any any
!
interface GigabitEthernet0/1.1
no ip access-group 110 in
ip access-group 111 in
!
interface GigabitEthernet0/1.2
no ip access-group 110 in
Best Answer
As you know how to remove a single line from the ACL (using
no sequence_numberform), I would focus on your two commands:no access-list 1 permit host 192.168.1.1command indeed deletes your ENTIRE ACL, thus NEVER use this kind of command.no access-list 1command obviously deletes your ENTIRE ACL, you then re-apply other 4 lines, thus it is technically correct, but remember to remove the ACL from an interface before removing or adding the ACL.And always remember to remove the ACL from an interface before removing or adding the ACL.