Junos
Highlighted
Junos

Reason for Dual Routing engine requirement on each Juniper MX 3D member to build a Virtual Chassis.

‎10-21-2017 03:07 PM

Hello dear colleagues,

 

Juniper states that we should have  dual RE on each member routers  to build a VC. (which makes four REs and all four should be the same model).

Ref--> https://www.juniper.net/documentation/en_US/junos/information-products/pathway-pages/config-guide-hi...

 

My question is, since we will have two members and  each let's say with one RE, then why do we need two REs on each? Should not having one RE on each member be enough..? In this case one will be active and other standby and in a failover case standby will take over and control both PFEs?

 

I hope my question is clear.  (why I cant make VC with two MXs having one RE on each)

 

Thanks in advance for your help!

Cheers!

Networking or Notworking
5 REPLIES 5
Highlighted
Junos

Re: Reason for Dual Routing engine requirement on each Juniper MX 3D member to build a Virtual Chassis.

‎10-22-2017 04:13 AM

Hello,

 

In case of one routing engine and if there is a hardware or software component failure or shutdown, it will isolate
 the primary router from the Virtual chassis.
In case of dual routing engine, local swithover of routing engines can prevent virtual chassis master router switchover.
so to ensure redundancy in a two-member Virtual Chassis configuration, each of the two member routers must be configured with dual Routing Engines.

 

As per the juniper document dual routing engines are recommended:
Ensure that both MX Series routers in the Virtual Chassis have dual Routing Engines installed, and that all four Routing Engines in the Virtual Chassis are the same model.
https://www.juniper.net/documentation/en_US/junos/topics/task/configuration/virtual-chassis-mx-serie...


Regards,

Rahul

Please mark my solution as accepted if it helped.

Highlighted
Junos

Re: Reason for Dual Routing engine requirement on each Juniper MX 3D member to build a Virtual Chassis.

‎10-22-2017 08:03 AM

Hi Rahul and Thank you for your comment,

 

But in this case, do not we lose the meaning and the reason for VC?

Lets imagine I want to built a VC just becasue I have two routers each one with one RE.(that is where VC makes sence for me) I assume that if RE0 will fail in Master router, backup router's RE will be notified and will assume primary role.The router which RE failed will not be isolated from VC.  Its line cards sill will be active and controlled by backup.

 

As you mentioned, If I have two routers with dual RE let's say  R1 (re0 and re1) master and R2 (re0 and re1) secondary. 

If re0 fails in R1 then other routing engine re1 on R1 will take over. In this case R2 will never became master (only in case if both REs fail on R1 at the same time- very very unlucky!) Is not it a vaste of expensive resources?

If I have a router with dual REs in very rare cases, say just for port density or other design requirements  I would need to build a VC other than that I do not see real life scenario why anyone would do it. 

 

What I understood is, it is not recommended, it is mandatory to have dual REs on each router to build VC in MX.

 

Thanks!

Networking or Notworking
Highlighted
Junos

Re: Reason for Dual Routing engine requirement on each Juniper MX 3D member to build a Virtual Chassis.

‎10-22-2017 10:55 PM

Hi!

 

Switchovers behavior is different with virtual-chassis as compared to what you expect. In your example if R1 re0(master on R1) Master fails, it will be R2 re0(master on R2) Backup, who will take over the mastership. So the mastership between VC members also switches along with and RE switchovers. 

 

There are couple of switchover scenarios, that are very well documented and explained in the below document on page 34, you might want to have a look.

 

https://www.juniper.net/documentation/en_US/junos/information-products/pathway-pages/config-guide-hi...

 

 

--------------------------------------------------------------------------------------------------------
If this post was helpful, please mark this post as an "Accepted Solution".
Kudos are always appreciated!
--------------------------------------------------------------------------------------------------------

Thanks
Amit
Highlighted
Junos

Re: Reason for Dual Routing engine requirement on each Juniper MX 3D member to build a Virtual Chassis.

‎10-23-2017 05:26 AM

Hi,
Yes, I understand your query is right. From isolation of the primary router I mean the global swithover.
In the VC scenario, the VC master router is the protocol master and runs the chassis management process, controls protocols, manages both master and backup memeber routers, receives and processes all the traffic destined to the VC.
If it has  only one routing engine, the global siwthcover will happen in case of failure. However, if we have dual routing engines in the VC master, the local swithover happens on master VC router.

And in a very rare situation lets say we are running single routing engine in a two member VC and primary RE fails. And the backup RE on VC backup router also delays to take mastership and synchronize. This is not expected, but if we think of the worst situation.
Also if backup VC routing engine fails, we lose redundancy since there is no backup till we replace the RE or fix it.

So, considering all these factors what I understand is that by dual routing engines we reduce the probabilty of failures and achieve redundancy.

 

Regards,

Rahul

Please mark my solution as accepted if it helped.

Highlighted
Junos

Re: Reason for Dual Routing engine requirement on each Juniper MX 3D member to build a Virtual Chassis.

‎01-15-2018 08:01 AM

Thanks Rahul for you comment. 

But all these still should not make having two REs compulsory.  Still did not get the benefit behind it. As I said rarely I will use VC for redundancy if I have dual REs. I was in a case that I had two MX480s with single RE and im this case having VC would make sense for me.

 

Thanks for your thoughts!

Feedback