IP Source Guard

By daxm (June 3rd, 2013)

IP Source Guard requires DHCP snooping (and applied to a VLAN).

Source Guard checks inbound IP traffic on an interface against the DHCP snooping database and compares the IP (and possibly MAC) against the database to ensure that the source IP (and possibly MAC) against the associated database entry.

To match only on source IP:

int f0/0
ip verify source
To match on source IP and MAC:

int f0/0
ip verify source port-security
switchport port-security

To manually bind a source-guard entry:

(router-config)# ip source bind <mac> vlan <num> <ip> int <interface>


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Frame Relay

By daxm (February 14th, 2010)

… What is Frame Relay …

Devices that “speak” Frame Relay fall into 1 of 2 categories:  data terminal equipment (DTE) or data circuit-terminating equipment (DCE).

  • DTE equipment is at the customer premise and are usually the “user” end of the connection.  Think of terminals, PCs, routers.
  • DCE equipment is the service provider end of the connection and provide the clocking and switching services in the FR network.

Frame Relay is a connection-oriented protocol. This means that before data is sent across the FR network an end-to-end (DTE to DTE) path must be established.  This end-to-end path is known as a virtual circuit (VC). NOTE:  Multiple VCs can travel down the same physical wire(s).  Through the use of TDM (time division multiplexing) each VC can get its share of the wire’s available bandwidth.

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Switchport Protected Command

By daxm (February 8th, 2010)

Similar to Private VLAN’s concept of an isolated VLAN is a command called Switchport Protected.  It is similar in that any interface that is in the same VLAN and is in “switchport protected” mode cannot see each other but can see other ports NOT in switchport protected mode that are in the same VLAN.  This feature ONLY works on a per switch basis.  So protected interfaces on different switches can communicate with each other as if the protected command wasn’t there.

Here is my graphic to display this:

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Private VLANs

By daxm (February 1st, 2010)

To help myself better understand private VLANs I created the following picture:

Private VLAN Diagram

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Dynamic ARP Inspection (DAI)

By daxm (January 31st, 2010)

As mentioned in my previous post, here are my notes on DAI.

To enable DAI all is needed is 1 command:  (config)#ip arp inspection vlan <num> HOWEVER there is so much behind this simple command it is scary.

Unless additional parameters are configured DAI uses the DHCP snooping bindings table to know what IP-to-MAC-to-Interface mappings are permitted.  Here is a copy of my DHCP snooping table on my home switch:

daxm-home-switch#show ip dhcp snooping binding
MacAddress          IpAddress        Lease(sec)  Type           VLAN  Interface
——————  —————  ———-  ————-  —-  ——————–
00:1E:90:EC:61:7A     2747        dhcp-snooping   1     FastEthernet1/0/1
00:22:68:15:74:1E     3528        dhcp-snooping   1     FastEthernet1/0/4
00:50:56:00:00:01     3003        dhcp-snooping   1     FastEthernet1/0/1
Total number of bindings: 3


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DHCP Snooping

By daxm (January 31st, 2010)

DHCP snooping seems to me to be a strange security enhancement. Though I do agree that securing against a rogue DHCP server is important the way Cisco implements it is odd.

In order for DHCP snooping to work at least 3 configurations are  needed:

1) Enable DHCP snooping on the switch — (config)#ip dhcp snooping

2) All ports are “untrusted” by default.  Specify which port are “trusted”.  A trusted interfaces consist of interfaces along the path to the DHCP server.  So, trunk ports and the actual port connecting to the DHCP server need to be configured as trusted ports.  — (config-if)#ip dhcp snooping trust

3) Identify which VLANs DHCP snooping will monitor. — (config)#ip dhcp snooping vlan <num>

4) (Optional?)  After configuring the above commands in my local (home) network I noticed that my DHCP requests were not being answered.  Thanks to a buddy of mine (Thanks Walt!) he showed me that the DHCP snooping process was adding Option 82 to my DHCP requests by default.  Apparently my DHCP server didn’t like that option because as soon as I disabled Option 82 from being sent my DHCP requests were being answered. — (config)#no ip dhcp snooping information option

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Spanning-tree Timers

By daxm (December 8th, 2009)

Quick note about STP timers:
— STP timers are inherited from the root bridge. Therefore you only need to configure the timers on the root bridge and backup root bridge.
— STP timers are configured per VLAN. Therefore if you want to configure the timers for all the vlans you’ll need to use the ‘vlan 1-4094’ option.

So, the timers associated to the path to the root bridge are the ones that are honored not the timers configured on the current bridge.

SW04(config)#do sh spann vlan 1

Spanning tree enabled protocol ieee
Root ID    Priority    4097
Address     001e.bd01.5400
Cost        8
Port        21 (GigabitEthernet1/0/21)
Hello Time   3 sec  Max Age 10 sec  Forward Delay 10 sec

Bridge ID  Priority    8193   (priority 8192 sys-id-ext 1)
Address     001d.e665.f200
Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
Aging Time  300 sec

Check out http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a0080094954.shtml for more information.

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VLANs Traversing a Network

By daxm (December 2nd, 2009)

A quick note on VLANs traversing trunk links between switches:

Even though VLANs 1-4094 are allowed, by default, across a trunk link the remote switch will NOT process the encapsulated frame if that switch is not made aware of the incoming VLAN tag.

Therefore, ALL switches within a broadcast domain need to be made aware of ALL VLANs that will/can traverse their trunked ports.

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