HSRP state machine

Raptien

Understanding and Troubleshooting HSRP Problems in Catalyst Switch Networks - Cisco Systems




On this diagram there is a transition from listen directly to active. The code references an event where a router in the listen state receives a lower priority hello from the active router, and subsequently enters active state itself by sending a coup message etc.

But the router in the standby state also does the same thing, so how would this work without a disagreement on which router is active?

stuh84

Raptien wrote: »

On this diagram there is a transition from listen directly to active. The code references an event where a router in the listen state receives a lower priority hello from the active router, and subsequently enters active state itself by sending a coup message etc.

But the router in the standby state also does the same thing, so how would this work without a disagreement on which router is active?

You've kinda answered your own question. Given that you should be setting priorities, one that comes in with a lower priority will be usurped, whereas one with a higher priority won't be.

So if you had say 3 routers, one with a priority 200, one with a priority 150, and another with priority 120, and the one with a priority 200 has just dropped to 100 (say, a tracking statement or something), then the one with priority 150 picks up as it hears a lower priority hello. The one with a priority 120 doesn't, as it hears the lower priority hello from the first router, but a higher priority from the second router.

That diagram seems a bit....like its missing something, as it only really says the states that something goes through, it doesn't seem complete in the messages that are exchanged. Put it this way, I've never seen a standby router take over from something of higher priority from itself in normal operation.

Raptien

I'm not sure i follow what you're saying, but let me rephrase my question...

Why would a router in the listen state ever jump straight to active after hearing a lower priority hello from the active router, when the listening router could assume that a standby exists with a higher priority than the listening router itself, and therefore the standby should be the router to become active.

It seems slightly flawed, the listening router will have a lower priority than the standby but will go to active, which presumably would cause another transition if the standby then preempts. Maybe that is the only way to do it, since the listening router will not know if preempt is enabled on the standby router, therefore the only option is to go to active, and if the standby router then wants to take over it can.

The diagram is the best thing i could find for explaining state transitions. I want to know the details but can't find any information that goes into that much depth. The RFC is hopeless.

stuh84

If you notice, the arrow from the Listen state to itself (the circular one), its acting on

Receipt of a hello message of higher priority from the active router
Receipt of a hello message of higher priority from the standby router
Receipt of a hello message of lower priority from the active router

If it ONLY receives " Receipt of a hello message of lower priority from the active router", then it would go straight to active. However if it receives one, or both of the other two messages, I think its going to move to speak.

In this case, I do not believe it would even attempt to go active. It would just go into the Speak state and carry on from there.

Raptien

stuh84 wrote: »

If you notice, the arrow from the Listen state to itself (the circular one), its acting on

Receipt of a hello message of higher priority from the active router
Receipt of a hello message of higher priority from the standby router
Receipt of a hello message of lower priority from the active router

If it ONLY receives " Receipt of a hello message of lower priority from the active router", then it would go straight to active. However if it receives one, or both of the other two messages, I think its going to move to speak.

In this case, I do not believe it would even attempt to go active. It would just go into the Speak state and carry on from there.

If it receives only a lower priority hello from the active, and nothing else, then the standby must have just failed. I can't think of any other circumstances where it would only receive that and nothing from the standby.

stuh84

Raptien wrote: »

If it receives only a lower priority hello from the active, and nothing else, then the standby must have just failed. I can't think of any other circumstances where it would only receive that and nothing from the standby.

Having no standby router in the network at all would follow the same process. I guess they are missing out the Standby with a lower priority angle too, but that I'd guess, as long as preempt is around, that'd mean going direct to active.