Virtual PortChannel (vPC) allows two links that are connected to two different physical Cisco Nexus 5000 or 7000 switches to appear to the downstream device as a single PortChannel link. That downstream device could be a server, Nexus 2000, or any Classical Ethernet switch.
vPC is useful to prevent spanning tree from blocking redundant links in the topology. After all you have spent fortune and bought those expensive 10G ports and the last thing you want is for spanning tree to block them.
Having said that they are several ways to connect the Cisco Nexus Fabric Extender (FEX) to its parent the Nexus 5000 or 7000 switch. In this post I’m going to discuss supported vPC topologies for the Nexus series. I’m going to start with the Nexus 2000/5000 now and will add a separate post for the Nexus 2000/7000 options later.
Without vPC
The picture above shows the supported non-vPC topologies. Topology A on the left shows a straight forward connectivity between Nexus 2000 and 5000 with a server connected to a server port on the Nexus 2000. There is no redundancy in this topology and failure of the Nexus 5000 or 2000 would cause the server to lose connectivity to the fabric. In this design you can have up to 24 FEX’s per Nexus 5500 in L2 mode and 16 FEX’s in L3.
Topology B on the right has also no vPC and NIC teaming in this case is used for failover. The solid blue link is the primary connection and the dotted link is the backup. It’s up to the OS on the server to detect any failure upstream and fail over to the backup link. Similar to A in this design you can have up to 24 FEX’s per Nexus 5500 in L2 mode and 16 FEX’s in L3.
With vPC
The picture above hows the supported vPC topologies for the Nexus 5000. Topology C is called straight-through vPC in which each Nexus 2000 (FEX) is connected to one parent Nexus 5000 while server is dual-homed. In this case NIC on server must support LACP so that the two FEX’s appear as a single switch. Most modern Intel and HP NIC’s support LACP today. This topology supports up to 48 FEX’s (24 per Nexus 5500) in L2 mode and 32 FEX’s (16 per Nexus 5500) in L3 mode.
In topology D on the other hand each FEX is dual-homed and so is the server. So the NIC on the server must support LACP as in C. In this topology you can have up to 24 FEX’s in L2 mode and 16 FEX’s in L3.
Topology E is similar to D where each FEX is dual-homed but the server is single-homed. In this topology you can have up to 24 FEX’s in L2 mode and 16 FEX’s in L3.
Maximum Supported Cisco FEX As of Today:
| Nexus 5000 | Nexus 5500 | |
|---|---|---|
| Without vPC (L2 Mode) | 12 | 24 |
| Without vPC (L3 Mode) | X | 16 |
| Straight-through (L2 Mode) | 24 (12 per Nexus 5000) | 48 (24 per Nexus 5500) |
| Straight-through (L3 Mode) | X | 32 (16 per Nexus 5500) |
| Dual-homed FEX (L2 Mode) | 12 | 24 |
| Dual-homed FEX (L3 Mode) | x | 16 |
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