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MEC on Catalyst9400 with StackWise Virtual

Hi,

 

Can someone please explain what this means?

"Etherchannel in StackWise Virtual is capable of implementing Multi-chassis EtherChannel (MEC) across the
stack members. When access layer and aggregation layer are collapsed into a single StackWise Virtual system,
MEC across the different access layer domain members and across distribution and access layer switches will
not be supported. MEC is designed to forward the traffic over the local link irrespective of the hash result."

 

I have 2 x ISR4461s connected to 2 x 9410 via Layer3. Then I have another 2 x 9410 connected to the previous 2 via Layer 2 (attached diagram). Both switch pairs have a collapsed access layer. 

 

If my understanding is right, I should be able to configure MECs between the aggregation virtual chassis and the routers, but not between the two virtual chassis. Is that right?

 

Thank you!

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Re: MEC on Catalyst9400 with StackWise Virtual

Hello Simona,

can you provide a link to the document where you have taken the following sentence:

 

>> "Etherchannel in StackWise Virtual is capable of implementing Multi-chassis EtherChannel (MEC) across the
stack members. When access layer and aggregation layer are collapsed into a single StackWise Virtual system,
MEC across the different access layer domain members and across distribution and access layer switches will
not be supported. MEC is designed to forward the traffic over the local link irrespective of the hash result."

 

the C9500 Stackwise whitepaper explains that there is an " optimization" in MEC that makes local member links preferred over links on the other chassis as explained in the sentence you have reported.

 

https://www.cisco.com/c/dam/en/us/products/collateral/switches/catalyst-9000/nb-06-cat-9k-stack-wp-cte-en.pdf?dtid=osscdc000283

 

pag. 13 explanation of MEC different load balancing strategies

 

One unique difference between multichassis EtherChannel and regular EtherChannel interfaces is the way traffic is load-balanced across the channel group members. A regular EtherChannel link selects the appropriate channel group member to exit based purely on the hashing algorithm of choice. A multichassis EtherChannel link, however, has some extra intelligence to reduce the amount of traffic that requires transmission across the StackWise Virtual link. This optimization is accomplished by populating the index port only with the ports local to the physical switch. This allows the switch to favor the local ports of the multichassis EtherChannel link over those on the remote switch

 

I think your network design is supported only the MECs will try to avoid to send traffic over the SVL bundle in each SVL pair.

 

However, given your physical topology this does not limit  having at least two possible links avaliable on each SVL pair for the presence of the crossed links.

 

Hope to help

Giuseppe

 

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