L2 Border Handoff Limitation

[techno.it]

Hello

I have Cat9300 that I want to use as a Layer 2 Border handoff for migration from traditional network to legacy network?

Are there are any limitations that need to be considered?

Can I use 2 Cat9300 as Layer 2 Border at a time ?

[Scott Hodgdon · Accepted solution]

techno.it,

You can use a stack of Catalyst 9300 as a border with L2 Handoff. In this way, you can resilience of the handoff if you assign it to a port channel interface that has ports on both 9300s in the stack.

A Catalyst 9300/9300L can support a total of 8000 endpoints when doing L2 handoff, with a maximum of 6,000 of those endpoints connected beyond the Layer 2 handoff. 

So as an example, if a subnet has 6,000 endpoints inside the fabric , then only 2,000 endpoints can be in the traditional network beyond the Layer 2 handoff. Why ? Because 6000 (fabric endpoints) + 2000 (traditional endpoints) = 8000 (total endpoint scale of 9300).

As another example, if a subnet has 1000 endpoints inside the fabric, then only 6000 endpoints can be in the traditional network beyond the Layer 2 handoff. Why ? Because 6000 is the maximum that can be connected beyond the Layer 2 handoff, even if the total in this case is below the platform's maximum capacity.

Cheers,

Scott Hodgdon

Senior Technical Marketing Engineer

Enterprise Networking and Cloud Group

[Sylvain_Che]

Thanks Scott for this explanation.

Does it exist an official documentation explaining this statement for C9300 and C9500?

Sylvain.

EDIT: found it on DNAC datasheet and clearly stated.

[jalejand]

Other than endpoint limit, there are no "hardcoded/strict" limitations but keep in mind that L2 handoff was designed for migration purposes and some caveats exists, specially for flooded traffic.

if L2 extension exists, flood arp-nd will be configured on the VLAN, ARP is flooded in the fabric using an existing underlay multicast group, make sure your multicast configuration in the underlay is working before stretching a VLAN in L2 handoff

Additionally, some DHCP deployments will include the DHCP server in one of the extended VLANs, this is possible and it will work, however, DHCP discovers must always use the DHCP relay method to reach the DHCP server (using VXLAN encap between Edges and L2 Border), in summary, you can't rely on broadcasting a DHCP discover on a VLAN to reach the DHCP server if the end hosts are in the same VLAN as the server, this is only possible by disabling DHCP snooping (or using the L2 Only Pool option), but still, this is not a suggested approach.

[techno.it]

Thank you @Scott Hodgdon & @jalejand 

I have single Cat 9300 and 9200L Can we use two seperate L2 border?

@jalejand L2 border is only temporary for migration purpose only.

Regarding DHCP, Core switch is configured as DHCP server in legacy network So how to handle and manage the DHCP during migration. Can we have another DHCP server in DC for Fabric ?

[jalejand]

DHCP can be located outside of the network, after a L3 handoff (Fusion Router), no issues if you go this path, you can have any DHCP server configured in fabric if its reachable via DHCP relay (helper address)

[techno.it]

Thanks for the quick reply.

I have devices in legacy network using multicast feature. Two systems have multicast server in DC and  other systems server connected to Edge Nodes.

Therefore, could you please let me know about multicast forwarding/routing for migration.

[Scott Hodgdon]

techno.it,

9200 / 9200L cannot be a border.

For L2 Handoff, you will need to use L2 Selective Flood in the fabric, which means multicast in the underlay of the fabric. This can be done as part of Lan Automation if you are using that.

For Multicast with SD-Access, please review the session Demystifying IP Multicast in SD-Access - BRKRST-2820 in the On-Demand Library at ciscolive.com .

Remember that for DHCP with SDA, the DHCP serve must support Option 82. 

Cheers,
Scott Hodgdon

Senior Technical Marketing Engineer

Enterprise Networking and Cloud Group

[techno.it]

Thank you so much Scott. All the points are clarified and understood.