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Network Distribution Layer Security


Distribution layer aggregates access layer switches. This is where all packet manipulation that hasn’t already been performed in access layer should take place. I’m focusing here on infrastructure protection rather than device hardening. Everything below is of little benefit if default passwords are used! Even though, properly configured ACLs restricting access to management interfaces would in some cases effectively prevent an attacker from being able to login to a switch. Hardening is essential. This is what we call defence in depth.

-          Routing infrastructure protection

o     Authenticate routing neighbours with MD5 (prevents rouge routing info injection and routing table manipulation)

o    implement route filtering (as above)

o    use default passive interfaces (tells an interface not to listen to or distribute routing protocols, this prevents infection and interception of routing data, furthermore it makes determining what routing protocol is in use more difficult)

o    log neighbour changes

-          Implement redundancy (good, resilient design is the key)

-          Implement ACLs (Network segmentation is implemented here, I.E. preventing finance VLANS access to HR servers)

-          Implement Infrastructure ACLs (or iACLs as Cisco calls them) to restrict access to infrastructure management IP addresses (be careful not to block transit traffic but only traffic aimed at Core or distribution devices / VLANS itself)

NOTE: iACLs are only applicable in the distribution layer of a multi-tier design where the routed edge interface is on the distribution switches. In a routed access design, this is enabled in the access layer.

-          Apply uRPF to block packets with spoofed IP addresses (Unicast Reverse Path Forwarding – the router will check if the source IP is reachable before forwarding a packet, the same can be accomplished using ACL that allow traffic only from subnets that exist in particular network segment)

NOTE: uRPF is only applicable in the distribution layer of a multi-tier design where the routed edge interface is on the distribution switches. In a routed access design, this is enabled in the access layer. 

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