Re: [mpls] BFD Packet Flow

[Lavanya Srivatsa <[e-mail]>]

Dave,

Thanks for the earlier response about the so-called 4-way handshake. I got what you are trying to say...

I had a question on the below though:
>> In some sense the question is meaningless;  the active and passive roles are really a way of describing the way the protocol behaves in a larger context.  In the >> LSP context, LSR-Z is by definition in the active role because it must send the first BFD packet.

If so, does this mean that BFD-MPLS is applicable only in the Active-Active scenario i.e. both LSR-A and LSR-Z in Active roles? The reason I am saying this is one would expect LSR-A to be playing the Active role because LSR-A is the **head-end of the forward unidirectional path** that is being monitored in case of an unidirectional MPLS path. Now if LSR-Z is also required to be in Active state because it sends the first BFD packet, then this would imply that only Active-Active scenario is applicable for BFD-MPLS, right?

The BFD-MPLS RFC also goes on to say that the node receiving a LSP Ping request MUST send a BFD control packet and MAY send an LSP Ping response.
Does this imply the Active-Passive scenario for BFD-MPLS (i.e. LSR-A in Active role and LSR-Z in Passive role)?

In the sense, if LSR-Z is Active, upon receiving an LSP Ping request, LSR-Z will respond with a BFD control packet.
If LSR-Z is Passive, upon receiving an LSP Ping request, LSR-Z will *not* send a BFD control packet but will respond with an LSP Ping response (carrying LSR-Z's discriminator) instead.

Is my understanding correct? If so, the text in the RFC seems to slightly mislead with the "MAY" clause for the LSP Ping response.

- Lavanya


Date: Sat, 21 Aug 2010 12:45:09 -0700
From: Dave Katz <[e-mail]>
Subject: Re: [mpls] BFD Packet Flow
To: "Weingarten, Yaacov (NSN - IL/Hod HaSharon)"
        <[e-mail]>
Cc: [e-mail], [e-mail], RAKESH GUPTA
        <[e-mail]>
Message-ID: <[e-mail]>
Content-Type: text/plain; charset="windows-1252"


On Aug 19, 2010, at 2:08 AM, Weingarten, Yaacov (NSN - IL/Hod HaSharon) wrote:

> Hi,
>
> Just to verify that I understand the flow of messages and the state of the BFD Session ? is the following flow correct ?
>
> Assume a LSP between LSR-A and LSR-Z ?
> 1. Both LSRs initialize the BFD Session and set the state to Down
> 2. LSR-A (the ingress LSR) sends a LSP-Ping (with the Discriminator TLV) to LSR-Z ? both sessions still in Down state.
> 3. LSR-Z receives the LSP-Ping, and (assuming that LSR-Z is in Active mode) sends a BFD Control Packet with its own Discriminator, the Discriminator received from the LSP-Ping, and State=Down to LSR-A, both session still in Down State.
> 4. LSR-A receives the BFD Control and switches to Init state (as per RFC5880), and sends a BFD Control Packet with State=Init to LSR-Z
> 5. LSR-Z receives the BFD Control and switches to Up state, and sends a BFD Control Packet with State=Up to LSR-A
> 6. LSR-A receives the BFD Control and switches to UP state
>
> Additional question ? regarding step 3 above ? must LSR-Z be in Active mode? What happens if LSR-Z is in Passive mode and LSR-A is in Active mode?

In some sense the question is meaningless;  the active and passive roles are really a way of describing the way the protocol behaves in a larger context.  In the LSP context, LSR-Z is by definition in the active role because it must send the first BFD packet.

--Dave

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