What is the purpose of the Advertise messages in HSRP?

[Mitrixsen]

Hello, everyone.

I was configuring HSRP and I found these messages in Wireshark

What exactly do these messages do/achieve? I haven't found much explanation online.

Thank you.

David

[Flavio Miranda]

@Mitrixsen 

This is some kind of hearbit. The router states that its status is Active or passive. 

https://networkinwireshark.wordpress.com/2018/04/29/hsrphot-standby-redundancy-protocol-in-wireshark/

[MHM Cisco World]

• Hello :-  Opcode 0   (To indicate that a router is running to become/already in active or standby state.
• Advertise:- Opcode 3 (When a router want to advertise it own state active/passive. It will be the initiating packet every time the port is configured or turns up)

Both hello and advertise seem same' I check rfc 2281 dont mention this op code' but I think there is new rev. And add this new op code.

So there is thin line I need to check what is exactly different between two op code

Thanks 

MHM

[Richard Burts]

David

I am not clear about your question. Are you asking what this message does, or are you asking about the fields in this message. So let me try to address both.

This message is sent to a multicast destination address, which will be received by devices in the local subnet, and which notifies the devices that this device is active in the HSRP protocol. It could be the beginning of forming an HSRP negotiation if there is another device running HSRP in this subnet.

As for the fields in the message, it is using UDP port 1985 which if for HSRP. A device running HSRP would be interested in this packet and would process it. A device not running HSRP would ignore this packet. The message indicates that this device is running version 0 of HSRP which would be important if there is another device in the subnet running HSRP. The message indicates that this is an advertisement which could invite negotiation of HSRP relationship if there is another device in the subnet running HSRP. The message length is 15 and the state is passive (there is not an existing HSRP relationship on this interface). Since it has not yet found an HSRP peer there is not any active HSRP group and the HSRP group configured on this interface is passive (has not negotiated an active peer).

[Mitrixsen]

Hello Richard.

Thank you for taking your time to address the message. I wasn't quite sure what the purpose of this message was because as for the fields and its purpose, it's very similar, if not identical to an HSRP hello message. If both the hello and the advertise messages are used by a router to notify the other devices that this local device is runnng HSRP, isn't their use somewhat redundant?

Thank you.

David

[Giuseppe Larosa]

Hello David,

 I think you have received an excellent answer from Rick.

the key concepts are : HSRP any version is a FHRP, there are Finite State Machine built by using UDP multicast .

Hope to help

Giuseppe

[Richard Burts]

David

You ask an interesting question which I had not considered before. And yes to some extent they are a bit redundant, but they are sent at different times in the formation of HSRP relationships. I am not authoritative on this, but I believe that the explanation is that while these messages are quite similar they represent different stages of processing of a router with HSRP. I believe that the explanation is that when a router initializes and is configured for HSRP that it sends hello messages announcing that it is present on the network and running HSRP (and is looking to see if other routers are on this network and running HSRP). After the router has determined the status of the network (whether there is already an active router, standby router) it shifts from sending hello to sending advertise in which it sends its status (in this case it is passive).