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I am Val Glinskiy, network engineer specializing in data center networks. TIME magazine selected me as Person of the Year in 2006.

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Monday, February 11, 2013

Port-security side effect

I discovered interesting side effect of configuring port-security which can prevent its deployment in certain circumstances. Let's have a look. Here is part of pertaining interface configuration:

 switchport port-security maximum 20
 switchport port-security
 switchport port-security aging time 1440
 switchport port-security violation restrict
 switchport port-security aging type inactivity

You can find explanations of what each command does here.

switch# show port-security interface gi 0/45
Port Security                      : Enabled
Port Status                          : Secure-up
Violation Mode                  : Restrict
Aging Time                        : 1440 mins
Aging Type                        : Inactivity
SecureStatic Address Aging : Disabled
Maximum MAC Addresses      : 20
Total MAC Addresses              : 1
Configured MAC Addresses    : 0
Sticky MAC Addresses            : 0
Last Source Address:Vlan        : abcd.ef12.3456:1234
Security Violation Count          : 0

In case you were wondering, the MAC address above is completely made up, but it associated with "floating" IP assigned to active device in cluster. Let's see what happens when active IP has to be moved to other device in cluster due to fail-over:

port_security-2-psecure_violation: security violation occurred, caused by mac address abcd.ef12.3456 on port gigabitethernet0/46.
Oops. Even though port-security configuration allows to learn up to 20 MAC addresses and there are no MAC addresses on Gi0/46, we got port-security violation. Why? Let's see debug port-security:

 PSECURE: psecure_add_addr_check: Found duplicate mac-address abcd.ef12.3456, It is already secured on Gi0/45
%PORT_SECURITY-2-PSECURE_VIOLATION: Security violation occurred, caused by MAC address abcd.ef12.3456 on port GigabitEthernet0/46.
PSECURE: Security violation, TrapCount:346
PSECURE: Read:2830, Write:2831
PSECURE: swidb = GigabitEthernet0/46 mac_addr = abcd.ef12.3456 vlanid = 1234
PSECURE: Adding abcd.ef12.3456 as dynamic on port Gi0/46 for vlan 1234
PSECURE: Violation/duplicate detected upon receiving abcd.ef12.3456 on vlan 1234: port_num_addrs 1 port_max_addrs 20 vlan_addr_ct 1: vlan_addr_max 20 total_addrs 4: max_total_addrs 6144

Port-security violation happened because MAC address has not been deleted from original port yet, hence "duplicate mac-address" message. To mitigate, but not completely alleviate the problem, we can reduce aging timer to 1 minute minimum. It still means that in case of fail-over, the floating IP address will not be accessible for another minute, which could be 1 minute too long.

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