what's the benefit after multiple ISL link configured to a PortChannel

[guama]

Hello guys,

I would like to know after multiple ISL links between two MDS switches are configured as a port-channel, what will change regarding the traffic flow?

For example, if I have two ISL links between two MDS switch, and the two links are not configured as a PortChannel, my expectation is one of the link will be blocked to avoid the loop, after they are configured as a port-channel, both ports are available for the traffic, while I found actually they both are flowing the traffic without a port-channel configured, then if it is necessary for configured them into a Port-channel?

[balaji.bandi]

It all depends on teh requirement and ports you have : (i go with port-channel where possible and supported).

some example described here :

https://www.cisco.com/c/en/us/td/docs/switches/datacenter/mds9000/sw/5_0/configuration/guides/int/nxos/cli_interfaces/pc.html

[guama]

Hi Balaji, 

Thanks for your response, I have checked that doc, it lists the benefit of Port channel: An E PortChannel refers to the aggregation of multiple E ports into one logical interface to provide higher aggregated bandwidth, load balancing, and link redundancy.  

I know it should be better to have a PO configured. But I am still not very clear about the difference. If I have no PO configured, the two ISL links will still flow the traffic, and if one link down, the other one is alive, this is a kind of redundancy as well.  then after I bind them into a port-channel, what will change? 

[Ray D · Accepted solution]

Hi,

The main benefit that I would see is that the port-channel would make the topology more stable. It doesn't cause the FSPF to re-converge if a link is added or removed. Fabric Shortest Path First (FSPF) is the standard path selection protocol used by Fibre Channel fabrics - and thus in aggregating those links you are effectively making FSPF more efficient in only seeing changes if the entire port-channel is down as opposed to utilising individual links. 

There is a nice overview on the topic here, which also has a section on failure scenarios for individual equal cost ISL's versus Port-channels.

https://www.cisco.com/c/en/us/td/docs/switches/datacenter/mds9000/sw/6_2/configuration/guides/fabric/nx-os/nx_os_fabric/fspf.html#67440

"By bundling pairs of physical links, FSPF efficiency is considerably improved by the reduced database size and the frequency of link updates. Once physical links are aggregated, failures are not attached to a single link but to the entire PortChannel. This configuration also improves the resiliency of the network. The failure of a link in a PortChannel does not trigger a route change, thereby reducing the risks of routing loops, traffic loss, or fabric downtime for route reconfiguration."

Ray.

[guama]

Thank you, Ray, that's what exactly I want to know, the key difference is not the ability to transmit, but when a of the multiple links fails, how the FSPF protocol reacts. 

[Ray D]

Hi Guama, 

No worries, thanks for the feedback. Glad it helped.

Ray.