Which Cisco Ios Statement Would Deny All Traffic
The Cisco IOS statement that would deny all traffic is the implicit deny at the end of an access control list, often described as implicit deny any. In practice, this means Cisco IOS blocks any packet that does not match a preceding permit statement in the ACL. It is not usually a line you type manually, but it is always part of ACL processing unless you add an explicit permit for the traffic.
What Cisco IOS statement denies all traffic?
In Cisco IOS ACL logic, the answer is the implicit deny, sometimes summarized as deny any. Every standard and extended ACL ends with this invisible rule, which drops all unmatched traffic after Cisco evaluates the list from top to bottom.
If you are studying for a Cisco exam or troubleshooting a blocked connection, the key point is this: an ACL does not allow traffic by default. It permits only what you explicitly match, and everything else is denied automatically at the end.
How ACL evaluation works in Cisco IOS
Cisco IOS processes ACL entries in order, using first-match processing. That means the router or switch checks each line from top to bottom and stops as soon as it finds a match.
- If a packet matches a
permitstatement, Cisco forwards it according to the rest of the configuration. - If a packet matches a
denystatement, Cisco blocks it immediately. - If a packet matches no line in the ACL, Cisco applies the implicit deny at the end.
This is why ACL order matters so much. A broad deny placed too early can block traffic you intended to allow later.
Implicit deny vs explicit deny
It helps to separate two similar ideas:
| Type | What it means | Is it written in the ACL? |
|---|---|---|
| Explicit deny | A visible deny line you configure yourself | Yes |
| Implicit deny | The automatic deny applied to any unmatched traffic at the end of the ACL | No |
An explicit deny is useful when you want to block specific traffic and document that choice. The implicit deny exists regardless and catches everything you did not permit.
Example ACL that blocks unmatched traffic
Here is a simple standard ACL example:
access-list 10 permit 192.168.10.0 0.0.0.255
access-list 10 deny 192.168.20.0 0.0.0.255In this example, traffic from 192.168.10.0/24 is allowed, and traffic from 192.168.20.0/24 is blocked explicitly. Any other source address is also blocked because of the implicit deny at the end.
So even though you only typed two lines, the ACL behaves as if there is a final unseen rule that says, in effect, deny everything else.
How to permit traffic before the final deny
If an ACL is denying more than you expected, the fix is usually to add a more specific permit statement above the traffic that is being blocked.
- Identify the source, destination, protocol, and port involved.
- Check the ACL order to see whether a broader deny appears before the needed permit.
- Add a permit statement that matches the intended traffic more specifically.
- Verify that the permit line appears before the implicit deny is reached.
- Test again and confirm the packet now matches the correct rule.
For example, if an extended ACL is blocking web traffic that should be allowed, you may need a line such as permit tcp for the correct destination and port before the end of the ACL.
Common troubleshooting mistakes
When Cisco IOS appears to deny all traffic, the problem is often one of these:
- Missing permit statement, the traffic never matches any allowed rule.
- Wrong ACL order, a deny line appears before the needed permit.
- Incorrect wildcard mask, the ACL does not match the address range you intended.
- Applied to the wrong interface or direction, the ACL is filtering traffic where you did not expect it.
- Standard ACL used when an extended ACL is needed, only source addresses are matched in standard ACLs.
To troubleshoot, review the ACL with show access-lists or the platform equivalent, then check interface ACL application with show ip interface or related interface commands.
Key takeaway
If you are asking which cisco ios statement would deny all traffic, the correct answer is the implicit deny at the end of the ACL. It is not a typed command in most cases, but it is always present in Cisco IOS ACL processing. To allow traffic, place the correct permit statements before that final deny and make sure the ACL is ordered correctly.
Frequently Asked Questions About which cisco ios statement would deny all traffic
What is the implicit deny in Cisco IOS?
The implicit deny is the automatic block that Cisco applies to any traffic that does not match a previous ACL entry. It is always at the end of the ACL.
Does Cisco IOS deny all traffic by default in an ACL?
Yes, unmatched traffic is denied by default in an ACL. Only traffic that matches a permit statement is allowed through.
What is the difference between deny any and implicit deny?
deny any is a common way to describe the behavior, while implicit deny refers to the automatic end-of-ACL rule that Cisco applies even if you do not type it.
How do you allow traffic after a deny all statement in Cisco IOS?
You do not place traffic after the implicit deny. Instead, you add the needed permit statements earlier in the ACL so the traffic matches before the list ends.
Why is my Cisco IOS ACL blocking traffic?
Most often, the traffic is either missing a matching permit line, matched a deny rule earlier in the ACL, or failed to match any line and was blocked by the implicit deny.
Is the implicit deny the same in standard and extended ACLs?
Yes. Both standard and extended ACLs use implicit deny behavior at the end, although they match different traffic fields.