From: Godswill Oletu (oletu@inbox.lv)
Date: Thu Sep 08 2005 - 20:24:37 GMT-3
Chris,
Though that work for you, but your interpretation of how HSRP, tracking, etc
might still need some clearification....
In hsrp, you can influence which router should be the active by manipulating
the priority value and just like dollars, more is better. Once an active
router has been elected, just like ospf DR/BDR, no election takes place just
because a router with a higher priority showed up, with the same augument,
if the priority of the active router is reduced (eg by decreament value)
even below the other standby router's priority, the router is still going to
be the active router its now lower priority notwithstanding, your experiment
below show that, despite the fact that the priority of R3 was reduce to 95
which was less than that of R1, R3 was still the active router. This is the
default behaviour of HSRP.
Each hsrp peer send keepalives to each other on their common interface and
if the active router do not response to keepalive after a predefined period
of time, the standy router will take over and become the new active router.
This happened when you rebooted R3, R1 became the active router (you will
achieve the same result by shutting/no shutting the ethernet interface of
R3), when R3 came back online even with its higher priority vlaue, it will
become the standy router. (this has nothing to do with tracking or the
interface serial you referred to or the prempting, this is the default
behaviour of hsrp peers)
The prempt option changes this default behaviour. Prempt does not mean that,
the router with the prempt configured will always become the active router.
Prempt just gives the router the power to call for or 'prempt-election' each
time it came online and like before the priority values of each routers will
determine who becomes the active router at the end of that prempted
election. The former active router will become the active again if its
priority is still the highest, however if during the downtime of the active
router with the prempt configure, if you increase the priority of the
standby router (now active ) above that of the down active router, when the
old active router with prempt comes online, it will call for or prempt an
election but it will fail become the active router because their is another
router with a higher priority.
Configuring prempt on both routers will only make sure that there is an
election each time one of the peers goes from up to down and back to up
state.
Configuring decreament on each or one of the routers without the prempt
option will only reduce its priority by the decreament amount if the tracked
interface goes down, but the routers will still retain their various status
(active or standby) because a new election was not prempted. So tracking an
interface/decreament go hand in hand with the prempt option for it to work
effectively.
You can test these behaviour in your lab and tell us what your conclusions
are. I have not tested these on a lab but that is my understanding of how
HSRP works.
HTH
Godswill Oletu
----- Original Message -----
From: "Chris Lewis (chrlewis)" <chrlewis@cisco.com>
To: "Bob Nelson" <nelsnjr@cox.net>; <ccielab@groupstudy.com>
Sent: Thursday, September 08, 2005 11:03 AM
Subject: Clarification of respoinse; HSRP and Track - 3550
> I'd like to clarify my previous post by examining the behavior of the
> commands in the command reference;
>
> I take the following configuration directly from the documentation and
> post it in to two routers in this configuration. I have to admit that
> upon closer reading of the documentation, I have to be extremely
> generous in my interpretation of the wording for it to be viewed as
> totally correct.
>
> S3/0
> |
> R3 (E0/0)----------(E0/0) R1 (e1/0)
>
> R3
> !
> track 100 interface serial3/0 ip routing
> !
> interface Ethernet0/0
> ip address 10.1.0.21 255.255.0.0
> standby 1 ip 10.1.0.1
> standby 1 priority 105
> standby 1 track 100 decrement 10
>
> R1
> track 100 interface E1/0 ip routing
> !
> interface Ethernet0/0
> ip address 10.1.0.22 255.255.0.0
> standby 1 ip 10.1.0.1
> standby 1 priority 100
> standby 1 track 100 decrement 10
>
> If I do a show standby on R1 immediately after entering these configs I
> show this
>
> Router1#show standby
> Ethernet0/0 - Group 1
> State is Speak
> Virtual IP address is 10.1.0.1
> Active virtual MAC address is 0000.0c07.ac01
> Local virtual MAC address is 0000.0c07.ac01 (default)
> Hello time 3 sec, hold time 10 sec
> Next hello sent in 2.240 secs
> Preemption disabled
> Active router is 10.1.0.21, priority 105 (expires in 8.300 sec)
> Standby router is unknown
> Priority 100 (default 100)
> Track object 100 state Up decrement 10
> IP redundancy name is "hsrp-Et0/0-1" (default)
>
> Then after a while I see the following on R3
> R3#
> *Sep 8 14:27:49.191: %SYS-5-CONFIG_I: Configured from console by
> console
> *Sep 8 14:27:58.631: %HSRP-6-STATECHANGE: Ethernet0/0 Grp 1 state
> Standby -> Active
>
> If I check back on R1, I now see this
>
> Router1#show standby
> Ethernet0/0 - Group 1
> State is Standby
> 1 state change, last state change 00:00:23
> Virtual IP address is 10.1.0.1
> Active virtual MAC address is 0000.0c07.ac01
> Local virtual MAC address is 0000.0c07.ac01 (default)
> Hello time 3 sec, hold time 10 sec
> Next hello sent in 0.472 secs
> Preemption disabled
> Active router is 10.1.0.21, priority 105 (expires in 8.532 sec)
> Standby router is local
> Priority 100 (default 100)
> Track object 100 state Up decrement 10
> IP redundancy name is "hsrp-Et0/0-1" (default)
>
> Which shows R1 moving from Speak to Standby, R3 is now the active
> router. If I shut down S3/0 on R3, the priority is reduced to 95, but it
> remains as the active router (but note the standby router has changed to
> R1, which could be a trick wording in the exam), as shown by the output
> of the debug standby terse and show standby command
>
> R3(config)#int s3/0
> R3(config-if)#shut
> R3(config-if)#
> *Sep 8 14:36:35.195: HSRP: Et0/0 Grp 1 Track 100 object changed, state
> Up -> Down
> *Sep 8 14:36:35.195: HSRP: Et0/0 Grp 1 Priority 105 -> 95
> R3#
> *Sep 8 14:36:37.027: %LINK-5-CHANGED: Interface Serial3/0, changed
> state to administratively down
> *Sep 8 14:36:38.067: %LINEPROTO-5-UPDOWN: Line protocol on Interface
> Serial3/0, changed state to down
>
> R3#sho stand
> Ethernet0/0 - Group 1
> State is Active
> 2 state changes, last state change 00:17:56
> Virtual IP address is 10.1.0.1
> Active virtual MAC address is 0000.0c07.ac01
> Local virtual MAC address is 0000.0c07.ac01 (default)
> Hello time 3 sec, hold time 10 sec
> Next hello sent in 2.652 secs
> Preemption disabled
> Active router is local
> Standby router is 10.1.0.22, priority 100 (expires in 8.480 sec)
> Priority 95 (configured 105)
> Track object 100 state Down decrement 10
> IP redundancy name is "hsrp-Et0/0-1" (default)
>
> However if I reload R3, to simulate a failure of R3 (which is the way I
> generously interpreted the phrase "if IP routing on serial interface 1/0
> in Router A fails") this happens:
>
> Router1#
> *Sep 8 14:47:28.631: HSRP: Et0/0 Grp 1 Resign in 10.1.0.21 Active pri
> 95 vIP 10.1.0.1
> *Sep 8 14:47:28.631: HSRP: Et0/0 Grp 1 Standby: i/Resign rcvd
> (95/10.1.0.21)
> *Sep 8 14:47:28.631: HSRP: Et0/0 Grp 1 Active router is local, was
> 10.1.0.21
> *Sep 8 14:47:28.631: HSRP: Et0/0 Grp 1 Standby router is unknown, was
> local
> *Sep 8 14:47:28.631: HSRP: Et0/0 Grp 1 Standby -> Active
> *Sep 8 14:47:28.631: %HSRP-6-STATECHANGE: Ethernet0/0 Grp 1 state
> Standby -> Active
>
> I think I was too generous in my interpretation of what the
> documentation says. If it is relying on a failure of the router, there
> is no need to have the track serial command.
>
> Chris
>
> -----Original Message-----
> From: Chris Lewis (chrlewis)
> Sent: Thursday, September 08, 2005 8:29 AM
> To: 'Bob Nelson'; ccielab@groupstudy.com
> Subject: RE: HSRP and Track - 3550
>
> The quotes to be taken from the command reference and it does seem to be
> correct. It does not talk about router A re-establishing itself after it
> fails, which is what the preempt command does. I guess the example in
> the command reference is assuming the interfaces are brought up at the
> same time and they negotiate the router with higher priority as the
> active one. Regarding your second bullet, if Router A fails, Router B is
> the only choice for taking over, regardless of priority or preempt.
>
> The discussion of preempt in the configuration guide below may be of
> additional help
>
> http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/
> fipr_c/ipcprt1/1cfip.htm)
>
> Chris
>
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
> Bob Nelson
> Sent: Wednesday, September 07, 2005 9:19 PM
> To: ccielab@groupstudy.com
> Subject: HSRP and Track - 3550
>
> Hello All:
>
> I want to make sure I am clear on the preempt and track for HSRP.
> Here is an example from the DocCD. My questions follow the example
>
> In the following example, the tracking process is configured to track
> the IP routing capability of serial interface 1/0. HSRP on Ethernet
> interface 0/0 then registers with the tracking process to be informed of
> any changes to the IP routing state of serial interface 1/0. If the IP
> state on Serial interface 1/0 goes down, then the priority of the HSRP
> group is reduced by 10.
>
> If both serial interfaces are operational, then Router A will be the
> HSRP active router because it has the higher priority.
>
> However, if IP routing on serial interface 1/0 in Router A fails, then
> the HSRP group priority will be reduced and Router B will take over as
> the active router, thus maintaining a default virtual gateway service to
> hosts on the 10.1.0.0 subnet.
> Router A Configuration
> track 100 interface serial1/0 ip routing !
> interface Ethernet0/0
> ip address 10.1.0.21 255.255.0.0
> standby 1 ip 10.1.0.1
> standby 1 priority 105
> standby 1 track 100 decrement 10
>
> Router B Configuration
> track 100 interface serial1/0 ip routing !
> interface Ethernet0/0
> ip address 10.1.0.22 255.255.0.0
> standby 1 ip 10.1.0.1
> standby 1 priority 100
> standby 1 track 100 decrement 10
>
> 1. "If both serial interfaces are operational, then Router A will be the
> HSRP active router because it has the higher priority."
> Is this necessarily true. Does it not depend on which interface
> was brought up first?. Without preempt, A will not become active if it
> is brought up second, correct?
> Without the preempt command on Router A, it will not take over
> just by virtue of the higher priority.
>
> 2. "if IP routing on serial interface 1/0 in Router A fails, then the
> HSRP group priority will be reduced and Router B will take over as the
> active router"
> Again, just because the priority on A (assuming it is the active)
> drops below router B's priority, does not mean it will take over as
> active,
> without B having preempt configured. Additionally, if B is
> configured
> to preempt A when Serial 1/0 goes down on router A, so it become the
> active,
> A must also have preempt configured to resume its active role when
> the
> serial comes up.
>
> Is this correct and they just did not put the preempt statements for
> brevity(confuse me) or am I missing the concept.
>
> Thanks for the help. Sorry for the length, wanted to be complete
>
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