Nexus 7K vPC - Peer-link for Routing between chassis

[yhameed81]

Hi All,

Could you please tell me what are the disadvantage of running routing protocols e.g. iBGP between SVIs of two chassis over vPC peer-link? I heard cisco recommend to use a separate link for layer-3.

Thanks in advance.

Regards

YH

[Reza Sharifi]

Hi,

For the 7ks, a separate layer-3 link is recommnded and supported.  See table-1 in this link.

I am not sure why you need to run BGP over the link, as OSPF or EIGRP will work just fine.

http://www.cisco.com/c/en/us/support/docs/ip/ip-routing/118997-technote-nexus-00.html

HTH

[yhameed81]

Hi Reza,

The layer 3 link is supported but i curious to know why should i have a separate layer-3 link, what is the disadvantage of using peer-link for layer-3?

vPC peer-link is generally bundle with multiple physical link so should be consider more reliable. Why should i dedicate an other link for layer-3?

nexus switches have eBGP to upstream devices so it make more sense to have iBGP between switches.

Regards

YH

[Reza Sharifi]

Hi,

That is just recommendation in case you have a lot of traffic going over the Peer link, if not you can use an SVI on the existing link and use that for your L3 peering. Some platforms (5000&6000 series) don't even support a separate layer-3 link.  It has to be an SVI over the existing link.

HTH

[Masoud Pourshabanian]

Hello,

VPC has a hardware mechanism to prevent loop on VPC peer link so it may drop your routing protocol traffic.

As Reza mentioned, It is recommended using a seperate portchannel between two nexus switches for routing protocols.

Hope it helps,

Masoud

[Ganesh Hariharan]

Hello YH,

Its bit debatable stuff, With my exp. what i have implemented in my network is to have separate L3 link for routing protocol like OSPF or BGP between N7k over vPC.

After doing bit research and testing  i cant say i got the exact answer but understand the vPC behaivor.

When Packet arrives at lower end standalone switch port ( Lets Say AC1), AC1 does lookup in routing table and sees two equal paths going to (N7k1 & N7k2) and assume it chooses N7k1 based on ECMP decision , L2 lookup happens and outgoing interface is Portchannel 1 ( Between ACI and two N7Ks) and Hashing determines portchanne members as N7k2.

Packet is sent to N7k2 and N7k2 sees that it needs to send it over vPC peer link to N7k1 based on Mac address and perfoms lookup and send it to uplink switch which is connected over portchannel 2.

N7k1 perfoms check that if frame came over vPC peer link and is going out on a vPC, 

Note :- Frame will only be forwarded if outgoing interface is NOT a vPC or if outgoing vPC is not having active interface on the other vPC peer ( N7k2)

I may be bit confusing :) considering the last mechanism i make it pratice to have separte L3 link for routing protocol.

Hope it Helps..

-GI

Rate if it Helps..