Port Security and Filtering
When tracing the path a frame takes through LAN switches, different kinds of filters can discard frames, even when all the interfaces are up. For example, LAN switches can use filters called access control lists (ACL) that filter based on the source and destination MAC address, discarding some frames. Additionally, routers can filter IP packets using IP ACLs. (This book does not discuss ACLs for LAN switches, but it does discuss IP ACLs for routers in Chapter 22, “Basic IPv4 Access Control Lists,” and Chapter 23, “Advanced IPv4 ACLs and Device Security.”)
Additionally, port security, as discussed in Chapter 8, “Configuring Ethernet Switching,” also filters frames. In some cases, you can easily tell that port security ...
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