HSRP (Hot Standby Routing Protocol)

(Andrew P) #29

Shree,
This depends on which HSRP is being used, version 1 or version 2.

For version 1, the gateway’s MAC would appear to be: 00:00:0c:07:ac:XX (where XX is the HSRP group number you have configured).

For version 2, the gateway’s MAC would appear to be: 00:00:0c:9f:fX:XX (again, where X is the configured HSRP group)

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(SAW C) #30

Hi Rene

I find it difficult to follow the lesson because I have to scroll up the screen many times to examine the topology diagram while reading your text. It would be nice if you give us a button to make the topology diagram (or any reference diagram) pop up on a separate window, so I don’t have to scroll up and down.
Thank you.

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(Rene Molenaar) #31

Hi Saw,

You should be able to do this from your browser. If you do a right mouse click on any image, you can select “open in new tab”.

This works in Google chrome and it should be the same in Firefox.

Give it a try :slight_smile:

Rene

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(SAW C) #32

Hi Sir
yes, it works, thank you.

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(florian k) #33

Hi Rene,

could you please explain the following command with respect to HSRP:

“forwarding-threshold lower lower-value upper upper-value”

Cant really find a proper explanation for it and dont really understand what it is good for.

Thanks

Florian

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(Lazaros Agapides) #34

Hello Florian!

Let’s begin with Cisco’s explanation and we’ll go from there. Cisco says that this command:

Sets the priority level used to select the active router in an HSRP group. The level range is from 0 to 255. The default is 100. Optionally, sets the upper and lower threshold values used by vPC to determine when to fail over to the vPC trunk. The lower-value range is from 1 to 255. The default is 1. The upper-value range is from 1 to 255. The default is 255.

(See http://www.cisco.com/c/en/us/td/docs/switches/datacenter/sw/5_x/nx-os/unicast/configuration/guide/l3_cli_nxos/l3_hsrp.html)

An example of this command is the following:
switch1(config-if-hsrp)# priority 60 forwarding-threshold lower 40 upper 50

(Please note, when I refer to “switch” in the following paragraphs, I am referring to an L3 switch.)

Keep in mind that the forwarding-threshold keyword is used as part of the priority command. The priority command is used to determine which router will be the active router. The addition of the forwarding-threshold keyword is used in conjunction with vPC (Virtual Port Channel). vPC is a feature that is available on the Cisco Nexus series switches and allows the creation of a “Virtual” port channel where the physical ports of the port channel can span two switches that are functioning as an HSRP group. In such a virtual port channel, under normal conditions, vPS forwards traffic to both the active and standby switches with ports participating in the vPC.

The purpose of the forwarding-threshold keyword and its configuration parameters is to determine when a switch participating in HSRP/vPC is considered “down” so that the ports in the vPC will forward traffic only to the “good” switch. If the standby router priority falls below the lower threshold, HSRP sends all standby router traffic accross the vPC trunk to forward through the active HSRP router. HSRP maintains this scenario until the standby HSRP router priority increases above the upper threshold.

Keep in mind that the priority of an interface on a switch can dynamically change based on the Object Tracking functionality of HSRP. Take a look at this for more information: http://www.cisco.com/c/en/us/td/docs/switches/datacenter/sw/5_x/nx-os/unicast/configuration/guide/l3_cli_nxos/l3_hsrp.html#17650

I hope this has been helpful!

Laz

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(florian k) #35

Hi Laz,

thanks for your help! Ok, i thought this command is also used when HSRP is configured without vPC, but thats not the case.

Regards

Florian

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(Lazaros Agapides) #36

Hello Florian.

Glad I could be of help. Actually, during my research I learned a lot too!

Laz

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(Ronak D) #37

hi
Rene

have you got tutorial/lab for this one

thanks Ronak

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(Andrew P) #38

Ron,
Try this on the GNS3Vault sister site:

http://gns3vault.com/network-services/hot-standby-routing-protocol/

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(Shantel - Networklessons.com) split this topic #39

19 posts were merged into an existing topic: HSRP (Hot Standby Routing Protocol)

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(Abhishek D) #40

Hi,

I have 2 router and both has a port-channel and then port channel has 3 sub interface. port-channel on both routers have 3 sub-interface which is for a different vlan coming from switch.

now i want to config hsrp between 2 routers. My doubt is :

do I config port channel this way

standby 1 ip <>
standby 2 ip <>
standby 3 ip <>

OR DO I config 3 sub-interface this way

standby 1 ip <> on first sub-interface
standby 2 ip <> on first sub-interface
standby 3 ip <> on first sub-interface

I am also getting a warning " address is not within a subnet on this interface "
"
is it just a warning or hsrp wont work in this case ?

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(Lazaros Agapides) #41

Hello Abhishek.

In order to simplify the specific topology, if I have understood it correctly, we can disregard the etherchannel and subinterface configurations. Let’s assume we have three physical interfaces on each router, each on VLAN 1, VLAN 2 and VLAN 3. The configuration will essentially be the same.

So, if you want to configure HSRP, you would have to configure three instances of it, one for each pair of interfaces on the same VLAN. it would go as follows:

Router 1
Interface 1 IP Address: 10.10.10.2 standby 10.10.10.1 VLAN1
Interface 2 IP Address: 10.10.20.2 standby 10.10.20.1 VLAN2
Interface 3 IP Address: 10.10.30.2 standby 10.10.30.1 VLAN3

Router 2
Interface 1 IP Address: 10.10.10.3 standby 10.10.10.1 VLAN1
Interface 2 IP Address: 10.10.20.3 standby 10.10.20.1 VLAN2
Interface 3 IP Address: 10.10.30.3 standby 10.10.30.1 VLAN3

Now if you’re getting a warning that ”address is not within a subnet on this interface” HSRP will NOT work. For each instance of HSRP, the physical IP addresses of the associated interfaces and the virtual IP MUST be in the same subnet (as well as the same VLAN).

I hope this has been helpful!

Laz

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(Nilesh J) #42

Hi Rene,
Your HSRP networklesson is awesome.
Can you please put some light on gratituous arp process that happens during failover as well as l2 mac id of hsrp routers that sw learns

Thanks and Regards

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(Rene Molenaar) #43

Hi Nilesh,

Glad to hear you like it. When the active router disappears and the standby router takes over, a gratuitous ARP is sent so that all devices can update their MAC and/or ARP tables. Here’s what it looks like:

Wireshark capture HSRP gratuitous ARP

HSRP uses the 0000.0c07.acXX MAC address where XX is the HSRP group number.

Rene

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(Steven t) #44

Hi Rene, a bit confuse with HSRP Timers and preemption delay, what’s the difference between them two and which occurs first between them two. Thanks

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(Lazaros Agapides) #45

Hello Steven

Here’s a review of the timers:

HSRP timers consist of the hellotimer and the holdown timer. Let’s say we have Router A and Router B functioning in an HSRP group where Router A is the Active router and Router B is the standby router. Timers by default are set to the following: hello = 3 seconds, holdown = 10 seconds. Hellos are sent every 3 seconds. If Router A goes down and stops sending hello timers, Router B will wait 10 seconds (the holdown timer) before becoming Active.

So the purpose of the hello and holdown timers is to essentially define under what conditions a Standby router becomes an Active router.

In order to understand the preemption delay, it is important to understand preemption. Using our example above, Router A is Active and Router B is Standby. If Router A goes down, Router B will become Active (after the holdown timer expires). Let’s say Router A comes back up. Router B remains Active UNLESS preemption has been configured on Router A, and Router A has a higher priority than Router B. If this is the case, Router A will be forced to assume the Active state and Router B goes into passive.

Now the preemption delay is a certain amount of time that must elapse before Router A assumes the Active state once again. So, contrary to the other timers, it is the amount of time a HSRP router with a higher priority waits before assuming the Active state after it comes back up.

Now you may ask, why is that important? When is it used? Well, when a router first comes up, it does not have a complete routing table. You can set a preemption delay that allows preemption to be delayed for a configurable time period, say 60 seconds. (The default is 0 seconds). This delay period allows the router to populate its routing table before becoming the Active router.

I hope this has been helpful!

Laz

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(Nilesh J) #46

Hi Rene,
While tracing from any source to destination there is HSRP configuration in between in a trace result I observe Physical IP instead of Virtual IP.Can you please explain the reason.

Thanks
Nilesh

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(Lazaros Agapides) #47

Hello Nilesh!

Let’s say you have a HSRP configuration where the virutal IP address is 10.1.1.1, the physical address of R1 is 10.1.1.2 and that of R2 is 10.1.1.3. Let’s say that R1 is currently the active router. If you ping 10.1.1.1, the reply message you get indicates that the echo comes from 10.1.1.1. If you traceroute to an IP address beyond the HSRP pair of routers, then, yes, you will get the IP address of the active router, that is, 10.1.1.2.

According to Cisco, traceroute specifically responds using the physical address of the active router. This is how HSRP is designed. Cisco states the following:

Q. Which IP address must be seen when a reply is received for traceroute?
A. When a reply for traceroute is received from a hop that runs HSRP, the reply must contain the active physical IP adddress and not the virtual ip address.

This can be found at http://www.cisco.com/c/en/us/support/docs/ip/hot-standby-router-protocol-hsrp/9281-3.html#tr

I hope this has been helpful!

Laz

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(Shantel - Networklessons.com) split this topic #48

19 posts were merged into an existing topic: HSRP (Hot Standby Routing Protocol)

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