Switching

We are in process to migrate our existing Enterprise from a Cisco 3-tier topology to a collapsed core using EX-8216's. We intended to implement MSTP on the EX and use dot1q trunks to the legacy Cisco distribution devices. The Cisco's are currently the root bridge for all VLANs (RPVST+) .

Can we leave the Cisco topolgy 'as is' and just add the EX as a MSTP switch - adding a VLAN to the EX and the trunk with the goal of removing the Cisco distribution devices in the future?

- Is there a Juniper Design guide (or from somewhere else) that is a good refernce on a migration strategy?

Thanks -d

Hi Netdeg,

When bringing MSTP and PVST+ together, you need to be aware of the following:

Both (I'm assuming you'll have two) your 8216s running MSTP will appear as a SINGLE logical switch to the PVST+ network.  This is because only the CIST is presented to non MSTP bridges - you will see this in action if you have dual trunks (one between each EX and a Cisco) - expect one of the trunks to go into blocking mode, rather than the link between the EXs as you would normally see.

Secondly, only the native VLAN on your trunks to the EXs will form topology within the PVST+ network - PVST+ tags BPDUs for non-native VLANs, which an MSTP instance will completely ignore.  You will see this in action if there is a topology change in the PVST+ network that affects more than one VLAN trunked to your EX - the native VLAN will fail over "Rapidly" (as per normal), but other trunked VLANs may take up to 30 seconds.  This is because Topology Change Notifications (TCNs) from the PVST+ network will be seen in the native VLAN, but sent with a tag on them for non-native VLANs and again, completely ignored by the MSTP.

My advice would be to be to give your 8200s a priority lower than the existing cores, but higher than the distribution switches (if possible), and attach them during a maintenance window (it is very likely there will be a 30-second topology change).  Once all VLANs have been trunked across, then migrate your ports over as quickly as possible (remember you are now operating with a 30-second failover time for all VLANs except the native).

Finally, break the trunk and say goodbye to PVST+ forever!  You can also promote the bridge priority on your EXs afterwards, but this is entirely optional

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Thanks Dfex -

I was following you to a point:

- CIST- does it require VLAN 1 - or can I use any defined VLAN number?

- You stated 'migrate the ports as quickly as possible' - that worries me since we were thinking our Cisco environment would be around for a year or 2 - we have 250 access switches to migrate (500 fiber links).

Should I worry if we leave this environment in place too long?

Thanks again,

Doug

Hi Doug,

CIST doesn't require any VLAN as such - BPDUs will simply be sent untagged from border ports at the edge of the CIST (regardless of which VLAN is native). For consistency though, you will need to match the VLAN with whatever is native at the edge of your PVST+ network. 

The only risk is as described in my previous post - topology changes will only be recognised on the Juniper side by the native VLAN - all others will re-converge much more slowly, but if you can live with 30 second fail-overs, then you can migrate over a long period of time without issue.

Cheers,

Ben

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