HSRP Query ARP

[ray_stone]

Hello Experts.

This question is related to know the traffic flow in redundancy network where i have two cisco core switches and a VLAN 70 is configured with virtual IP address 192.168.1.1/24 as we are using HSRP protocol.

Physical IP address of Core Switch 1 is 192.168.1.2

Physical IP address of Core Switch 2 is 192.168.1.3

Both core switches are connected with seperate L2 switches and VLAN 70 is also configured and the etherchannel is configured in between Core Switches. The Core Switch is Active and Core 2 Switch is standby.

Now we have one ESX server which has two NIC cards one is connected with first L2 Switch and  second one is connected with second L2 Switch so the diagram will be like as below:

Core Switch 1-------------------Ether Channel------------------Core Switch2

          |                                                                                                      | 

          |                                                                                                      |

First  L2 Switch                                                                                  Second  L2 Switch

          |                                                                                                    |

              |                                                                                          |

                    |                                                                          |

                          |                                                        |

        ESX Server NIC Card                          ESX Server NIC Card.

As i know while configuring the HSRP it's created a virtual MAC ID which binds with Virtual IP that will be the gateway of clients and that is completely understood.

Lets take an example one VM Server is configured which IP address is 192.168.1.10 and GW is 192.168.1.1. so will the traffic will go from both connected interface, if yes then what will happen with the traffic which is going to Second L2 Switch. I just want to understand the traffic flow here. How the ARP will play a role here.

Thanks.

[bhargavdesai]

Hi,

First of all i m not too experience with VM...

I guess VM Switch will elect root port and traffic will flow through that port to reach your HSRP VIP.

H2H

Bhargav

[ray_stone]

Can anybody else share thr opinion on this...

[John Blakley]

HSRP assigns a virtual mac address for the group that's assigned with the standby command. The server that's using 192.168.1.1 as the default gateway will have already arp'd for the mac address and the virtual mac (0000.0c07.acxx) should already be in the arp table on the server. In theory, if the 1st switch goes down, the server shouldn't have to arp again for the address since it already knows about it; in other words it wouldn't get a different answer from the 2nd switch. The 2nd switch would start answering requests for that vIP on that virtual mac address.

HTH,

John

[ray_stone]

I know how HSRP works and when i run this command arp -a then it shows the gateway virtual IP bind with virtual mac as you mentioned.

My confusion is to know the traffic flow as to why server will choose the first L2 switch to send the traffic and how the server concludes here that the traffic should not go to second L2 switch until the first core switch is down.

Please explain in depth, if possible.

Thanks..

[bhargavdesai]

Look this as a switching point of view.

Core Switch 1 or Core Switch 2 one of this switch Should be your Root Bridge for VLAN 70. (Most likely to configure your Active HSRP Switch As your Root Bridge)

Lets assume CORE SWITCH 1 is Root.

Now Core Switch 2, First L2 Switch and Second L2 Switch will find best path to Core Switch 1 as it is the Root.

So in this way your traffic will flow through First L2 Switch >> Core Switch 1

H2H

Bhargav

[ray_stone]

Thanks for your response to clarify this.

Let's assume there is no switch down and traffic is going via first L2 and then move to Core Switch 1. I stated in above the ESX Server second NIC is connected with Second L2 switch and the ESX will use both NIC adapter to send the traffic (pls correct me if i m wrong here) so what will happen with that traffic hitting to second L2 Switch and if the traffic being blocked as the STP running so where it would be blocked? Is it on Core Switch (second one) or second L2 Switch port which is connected with Server second NIC card.

Or else, the server won't use the second NIC to send the traffic (if yes) then please explain about this behaviour in depth.

Thanks... 

[ray_stone]

Can somebody else response on this?

Thanks.

[bhargavdesai]

If i m not wrong VM Ware create a Switch of your NICs.

So it will use STP to block traffic.

In scenario i mentioned above one the NIC on your VM Ware will block traffic. as normal STP topology.

Basic Diagram will give you basic Idea.

Plz correct me if i m wrong.

H2H

Bhargav