Router swap - can I spoof old MAC address?

[mcreilly]

I will shortly be replacing a 2851 with a 3845 in an office that has outgrown the number of phones the 2851 will support for SRST.

Various things configured in CUCM refer to the MAC address of Gi0/0 on the current router (MTP, xcode, CFB, etc). We're using it as an H323 gateway rather than MGCP. Voice interfaces are a PRI and four FXS ports. IOS is 12.4(24)T3, CUCM is 6.1.3.

I'd prefer to make as few changes as possible, not least to keep the downtime involved in swapping in the new router as short as I can.

I'm hoping I can just copy the config of the router (editing for the obvious, e.g. my PRI will be controller 0/2/1 rather than 0/1 or whatever) and add "mac-address aaaa.bbbb.cccc" to the Gi0/0 interface config, where aaaa.bbbb.cccc is the BIA of Gi0/0 on the existing router.

Unfortunately I don't have enough lab kit to actually try this out before I have to do it for real. I'm guessing this is something other people have probably tried, so what I'd like to ask is - have you done this? Did it work? Any "gotchas" or is it a really bad idea for some reason I haven't thought of?

(I realise the biggest "gotcha" would be the fun and games if the old router was to be connected on the same network with the duplicated MAC address, but the old hardware won't stay in that office or be redeployed anywhere on the same CUCM cluster later).

Thanks all.

[paolo bevilacqua]

Yes, that can be done.

If buying new hardware, a better chocie would an ISR G2, that can hold a larger number of devices in SRTS compared to the directly equivalent ISR G1 model.

[mcreilly]

Thanks for the reply.

I already have the 3845 so not buying new for this application. Also I believe the ISR G2 would need a later version of CUCM to allow use of the newer DSPs and we're not quite ready for that upgrade yet.

[clileikis]

You can try keeping the 2851 in production, add the 3845 (using a different IP address) to Call Manager, making sure your MTP's, Xcodes, etc all register fine and everything looks good.  When your ready to do the change over add the new H323 gateway to your route plan and move the T1's over for testing.  If it doesn't work as expected you can move the T1's back to the 2851.