Thursday, July 15, 2010

Infastructure Learning

Hi Guys,

So today I studied CCIE Voice stuff that I would normally consider basic, Boy did I learn alot!

First of all, right off the bat, did you know that the voice-VLAN is discovered via CDP? Meaning if CDP is not running.. that's right, voice-vlan is not discovered, i confirmed this myself by turning off CDP on a switch, my phones no longer correctly discovered the voice VLAN. Turning cdp back on with cdp run enabled it again (but actually went and reset every single phone on the switch without warning so keep that in mind!)

I also learnt that the ESW modules we will be using in the exam don't support switchport access vlan with a Voice VLAN configured, you have to configure a trunk.

So your config will look something like this:

int fa0/1
switchport voice vlan 10
switchport mode trunk
switchport trunk native vlan 30 <-- this is your data vlan
switchport trunk allowed vlan 10,30 (This is optional depending on if the exam asks you not to trunk unnecessarily)
spanning-tree portfast

I also learnt about DHCP in CUCM. You might not know this but CUCM actually assigns from the top of the addresses in the available pool downwards unlike an IOS DHCP.

I also discovered that a very common bug in CUCM 7.0 is that the DHCP server does not work with CIsco Security Agent running, to disable the cisco security agent, login to your CUCM using SSH and use your OS administration username and password when prompted, then issue:

#utils csa disable

This will require a restart of CUCM.

in TFTP Option 66 is used to use DNS as a TFTP configuration option instead of Option 150, keep that in mind guys!

Finally I learnt about NTP, its important to know that phone NTP references are only used for SIP handsets, so bare this in mind, other handsets (SCCP) get there time from CUCM which in turn can be pointed to an NTP reference. A subscriber ALWAYS gets its time from the publisher so if you point the NTP reference of the publisher to a router you will be fine.

I hope these little infrastructure hints help someone out there!

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