ok, one example. Assume you use your router R1 to provide security to a LAN behind it. Users in the LAN will use different applications but let us restrict this to HTTP.
Which ACL would you use on R1?
In the classic case you would block all traffic except TCP port 80 from outside to the LAN. This however would already punch a permanent hole, which could be exploited by a acker. He could craft TCP packets with TCP source port 80 and send it to your LAN to spoof existing machines and the like.
Reflexive ACLs will prevent this. Basically the router keeps track of session requests, i.e. it will create a dynamic ACL entry for every LAN initiated session and clse the hole when the session is terminated. There will not be a permanent hole, which could be exploited by an attacker.
More details and some configuration examples can be found in "Configuring IP Session Filtering (Reflexive Access Lists)"
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