VPC in Mixed Chassis Mode

VPC in Mixed Chassis Mode

Posted on Jan 24, 2020 (0)

VPC in Mixed Chassis Mode

What is Mixed Chassis Mode ?, A chassis which contains both M and F Line cards is called as Mixed Chassis Mode.

M cards provides the Layer 2 and Layer 3 function and  F1 module supports Layer 2 function and the interoperability between M1 and F1 ports are provided by L3 Internal proxy routing and this is provided by M1. When traffic enters at F1 ports and are needed to be routed (Inter VLAN routing or sent to outside DC), M1 line cards provides that proxy routing function.

Layer 3 Internal Proxy Routing:

Following are the Layer 3 internal Proxy routing characteristics.

  • For Unicast traffic: Proxy Routing for all F1 module can be done by all M1 interface in the VDC.
  • For Multicast Traffic: Proxy layer 3 replication for F1 module can be done by all M1 replication engines.

As soon as M and F cards are available in particular Chassis, the internal Proxy routing is enabled by default and any user can modify the proxy routing configuration by following commands:

Hardware proxy layer-3 routing

Since NX-OS 5.1(2) release maximum number of proxy forwarders for Layer 3 traffic for F1 has increased from 16 to 128.

7K1# sh hardware proxy layer-3 detail
Global Information:
F1 Modules:        Count: 1     Slot: 8
M1 Modules:       Count: 4     Slot: 1-3,9
Replication Rebalance Mode: Manual
Number of proxy layer-3 forwarders: 16
Number of proxy layer-3 replicators: 12
Forwarder Interfaces        Status          Reason
Eth1/1-12                             up           SUCCESS
Eth1/13-24                           up           SUCCESS
Eth1/25-26                           up           SUCCESS

sh hardware proxy layer-3 counters brief gives some statistics data about the usage of L3 forwarders.

Mixed Chassis Mode vPC:

When we mix the M1 and f1 cards they provide great benefits:

  • F1 cards provide capability for Bridge traffic only
  • Any routed traffic coming from F1 card are proxies to M1 cards ports for inter VLAN routing or sending Traffic outside DC.

In Mixed Chassis mode, any card F1 or M1 cards are used for Peer-link configuration. Below is the figure which provides the complete picture of vPC system in Mixed Chassis Mode.

[pms-restrict subscription_plans="1315, 1316, 1317, 1735"]
When a F1 cards are present in Mixed Chassis Mode and is used in peer-link, it provides the following characteristics:

  • M1 ports are used for L3 uplink
  • Total 16K MAC address is supported
  • F1 ports is used for vPC member ports
  • Use peer-gateway exclude vlan < VLAN-LIST > to exclude VLAN that belong to backup routing path. (This command only applied to vPC system in mixed chassis mode with vPC peer-link on F1).

Attention: When you deploy a vPC system in a mixed chassis mode with peer-link on F1 ports, it is recommended that you disable the vPC peer gateway. If the peer gateway is configured, all the packets forwarded over the vPC peer link will be punted to the CPU, resulting in high CPU usage.

When a M1 cards are present in Mixed Chassis Mode and is used as Peer-link, it provides the following characteristics:

  • M1 port are used for L3 uplinks and vPC peer-link
  • Total MAC supported is 128K
  • F1 ports are used for vPC member ports
  • No need to configure peer-gateway exclude vlan < VLAN-LIST >

Be careful when you deploy a vPC system in mixed chassis mode with peer-link on F1 ports and you need to use peer-gateway function in addition to backup routing path.

By default, backup routing path configured over vPC peer-link are processed in software with CPU intervention when peer-gateway knob is enabled.

To force the traffic carried over backup routing path to be processed in hardware without any performance penalty, use peer-gateway exclude-vlan <VLAN list> knob.

vPC Mixed Chassis Mode with Peer-link on F1 with one M1 Line card:

With One M1 card in vPC system, and all other F1 cards, this design is not recommended and to support this design at least two M1 cards are recommended.

But with One M1 cards and all other F1 cards, vPC also works fine where M1 card is used for L3 routing and F1 card are used for Layer 2 traffic.

But in case of M1 cards failure, because system has only one M1 cards, it as following effect:

  • Inter Vlan traffic are black holed due to vPC loop avoidance rule

 So in a vPC system, with Mixes Chassis Mode, use at least two M1 line cards because doing this it will provide higher resiliency for L3 internal Proxy Routing and for Layer 3 Interface.


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