Cisco 3750 Redundancy Test

The Cisco 3750 Catalyst Series offer high class switching functionality such as StackWise support with a 32 Gbit dual ring (16+16G). This means that you have a 32 Gigabit throughtput between Stack Switches.

During some testing with 2 stacked switches, I was able to simulate a failure by issuing the command:

reload slot 2 -> This will reload the module 2 switch.

switch# reload ?
LINE Reason for reload
at Reload at a specific time/date
cancel Cancel pending reload
in Reload after a time interval
slot Slot number card
standby-cpu Standby RP


Switch# sh switch det
Switch/Stack Mac Address : 0021.XXXX.XXXX
H/W Current
Switch# Role Mac Address Priority Version State
----------------------------------------------------------
*1 Master 0021.XXXX.XXXX 15 1 Ready
2 Member 0022.XXXX.XXXX 1 1 Ready



Stack Port Status Neighbors
Switch# Port 1 Port 2 Port 1 Port 2
--------------------------------------------------------
1 Down Down None None
2 Ok Ok 1 1

How does DHCP relay redundancy works?

Having the following configuration:

interface X
ip helper-address 1.1.1.101
ip helper-address 1.1.1.102

This will configure DHCP relay in the router. It receives a broadcast DHCP request and converts it into a unicast message directed exclusivly to both DHCP servers.
The router "sees" the DHCPDISCOVER packet and forwards it to bothaddresses simultaneously. Then, both DHCP servers will make an offer (DHCPOFFER), if they are up and received the request.
The client will receive each offer at one time, one first then the other. if it finds the offer agreeable, itwill send another broadcast, a DHCPREQUEST, specifically requestingthose particular IP parameters. Why does the client broadcast therequest instead of unicasting it to the server? A broadcast is usedbecause the first message, the DHCPDISCOVER, may have reached more thanone DHCP server. If more than one server makes an offer, thebroadcasted DHCPREQUEST allows the other servers to know which offerwas accepted. The offer accepted is usually the first offer received.
Sometimes, in other to make one server responde faster then the other, you can configure the delay of response in the DHCP server.Also, for sincronization between DCHP server, either they support this feature or you will have to have your subnet divided betweenthe 2 servers without overlapping.