[j-nsp] juniper cisco switch interconnection

[Joe Horton]

Here is a quick summary of the JUNOS RSTP/VSTP interop behavior.

If you enable RSTP on the Juniper side, and do NOT set any native-vlans on any of the cisco ports connected to juniper, then JUNOS will interop with the cisco RSTP frames sent untagged, and VLAN1 will block correctly.  The cisco devices will see each other via PVST on all other VLANs and block those accordingly.

If you only enable VSTP on the Juniper side and you define VLAN 1 on the Juniper side with a vlan tag of 1 specified, then bad things happen prior to 12.1r2/12.2 software.  This is due to the fact that JUNOS transmits/expects tagged packets for VLAN1 and drops all untagged STP frames.  While the cisco device will transmit untagged STP frames to both the STP MAC and the PVST MAC, but both are untagged.  So basically the Juniper and the Cisco will not see each other on the wire.
With 12.1r2 and 12.2 software, JUNOS will behave similarly to the cisco side, in that it will transmit the VSTP(PVST) frames untagged.

If you enable both RSTP and VSTP on the Juniper side and you define VLAN 1 on the Juniper side with a VLAN tag of 1 specified, the behavior is the same as above, as RSTP won't be transmitted out the interface because all VLANs have VLAN-IDs associated with them.  Again 12.1r2/12.2 and beyond address this.

If you enable RSTP and VSTP on the Juniper side and you define VLAN 1 WITHOUT a VLAN id defined, then all is well, as the Juniper will interoperate with Cisco on all VLANs other than VLAN1 using VSTP/PVST, and it will interoperate with the Cisco on VLAN1 using RSTP.  This behavior can be used on all releases prior to 12.1r2/12.2 based on what I've seen in the past.  There is a minimum release in which to run RSTP+VSTP, but I don't remember that off the top of my head, probably a low 10.x release.

Joe



-----Original Message-----
From: juniper-nsp-bounces at puck.nether.net [mailto:juniper-nsp-bounces at puck.nether.net] On Behalf Of Mike Azevedo
Sent: Monday, December 10, 2012 3:28 PM
To: Benny Amorsen
Cc: Juniper List
Subject: Re: [j-nsp] juniper cisco switch interconnection

you can bcast storm your network to a halt if you are not careful. Use VSTP on juniper side to match the cisco PVST+. I believe Juniper's RSTP uses one instance of spanning tree over the physical network not an instance per vlan like cisco's implementation. I would also watch out for vlan 1. 



----- Original Message -----

From: "Benny Amorsen" <benny+usenet at amorsen.dk>
To: "harbor235" <harbor235 at gmail.com>
Cc: "Juniper List" <juniper-nsp at puck.nether.net>
Sent: Monday, December 10, 2012 3:16:10 PM
Subject: Re: [j-nsp] juniper cisco switch interconnection 

harbor235 <harbor235 at gmail.com> writes: 

> Has anyone connected a Juniper EX series switch with a Cisco switch (I 
> have a 3550)?

Yes 

> Do you use a standard crossover cable? MDIX? 

I have only attempted 1Gbps, that just worked with a straight cable. 

> Any Layer 2 issues with RSTP and PVST+? 

It seems to work so far... 

> Any specific configuration required to make it work? 

Avoid VLAN 1. You can probably make VLAN 1 work if you try, but for me it was easier to simply not use it. 


/Benny 

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