How to set Backup delay for serial interface to ethernet interface ?

Unanswered Question
Jun 14th, 2010

Hi

I have two point 2 point links from two different ISP from  my branch office to head office. One is Serial interface and another one is Ethernet interface.

i want to config serial as primary link and ethernet as backup link  (automatic switch over).i herewidth attach the diagram also.

plz send the differnet  type of backup delay config.

Thnks

Vicky

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gatlin007 Mon, 06/14/2010 - 06:58

You may be able to address this with a ‘floating static’ route.  Is there a specific requirement to have a delay prior to shifting traffic to a surviving link?

A primary static route will send traffic out the Ethernet interface.  In the event the Ethernet circuit is not available a secondary, or floating route will forward traffic over the serial interface.

Ip route 0.0.0.0 0.0.0.0

Ip route 0.0.0.0 0.0.0.0 225

The 225 attribute above (Admin Distance) ensures that route won’t be installed in the routing table unless the Ethernet circuit is unavailable.



Chris

Mohamed Sobair Mon, 06/14/2010 - 07:38

AlKady Recalls , recalls , recalls.


The backup interface will make the backup interface (Line Protocol down) when the primary is active.

to configure Backup interface use the following command on the primary interface:

interface x

backup interface y

To configure the delay, use the following command:

interface x

backup interface y

backup delay 0 300

which means if the primary link is back up again, the secondary should wait for 5 minutes (300 sec) before switching the traffic to the primary.

HTH

Mohamed

Paolo Bevilacqua Tue, 06/15/2010 - 17:37

Both recommandations above are incorrect or incomplete, as "backup interface" commands are totaally inadequte to provide backup for ethernet terminate WAN, because in most cases, the ethernet interface will never go down, even when the service has actually failed.

Recommend you get in touch with a reputable consultant or certified cisco partner for proper configuration.

Mohamed Sobair Wed, 06/16/2010 - 00:39

Viky,

My answer should be sufficent for you as per your requirment. However, what I dont understand is you rating. since you rated it, please tell me why your rate it with 2?

Paolo,

You are telling the original poster to find a consultant to solve his problem. what makes you believe my answer is not correct?

Please try this command on ethernet segment, and see the output. Once you activate the command , immediately the Back up interface will be in Standby mode and once the primary interface is back up, it should wait for whatever seconds configured as it will preempt to check if the interface is alive and waits for the specified amount before the traffic is back to the primary interface.

HTH

Mohamed

Paolo Bevilacqua Wed, 06/16/2010 - 11:29

You are telling the original poster to find a consultant to solve his problem. what makes you believe my answer is not correct?

For the exact reason I've stated above. Instead for example, Itesh answer is correct.

Mohamed Sobair Wed, 06/16/2010 - 13:26

Paolo,

Execuse me here,  you are absolutely incorrect.

I am not saying Hitesh example is not correct, However, my example should produce the desired result as well.

Please check it out and you will notice my answer is correct.

HTH

Mohamed

Jon Marshall Wed, 06/16/2010 - 13:57

Mohamed

I'm not sure your solution will work. The OP wants to use the ethernet link as the backup interface but routers only support using serial or ISDN interfaces as backup interfaces. The only exception to this is the 7600 router but i think the OP was referring to the ISR routers.

Jon

Mohamed Sobair Wed, 06/16/2010 - 14:13

Jon,

However, I have tested it on GNS using  (3640 , 3725 , 7200) router series, and I check it also on a real 1841 router.

They all have given the same output. using either backup interface (ethernet Or serial).

HTH

Mohamed

Jon Marshall Wed, 06/16/2010 - 14:36

msobier123 wrote:

Jon,

However, I have tested it on GNS using  (3640 , 3725 , 7200) router series, and I check it also on a real 1841 router.

They all have given the same output. using either backup interface (ethernet Or serial).

HTH

Mohamed

Mohamed

Okay, good to know, thanks.

I checked the latest IOS version 15 command references and it does say it is only supported on serial/isdn interfaces other than the 7600 router. So perhaps the command references are wrong or it works but is not "officially" supported.

Thinking about it if the primary was ethernet and the backup serial then i suspect there might be a problem in that the remote end might go down but the local ethernet interface stay up/up because it's connection to the switch is still active so it would never fail over to the backup link. So if the primary link was ethernet then you would indeed need to use IP SLA.

However if the primary was serial and the backup was ethernet then i could see how it would work as the backup would know when the primary had gone down without needing to use IP SLA and would also know when it came backup. I'm assuming here that the backup command when used simply instructs the router to keep checking the status of the line protocol on the primary link which i think is how it works.

So unless i'm missing something i can't see why it wouldn't work now that you have confirmed you have made it work on an ISR.

Oops, just rated the wrong post but as it was another post by you anyway it should be fine

Jon

Paolo Bevilacqua Thu, 06/17/2010 - 10:21

You fail to understand that:

1. - OP never asked for exact clarification on the working of "backup interface".

2. - In the real world, ethernet circuit stays up whiel WAN is don, consequently "backup interface is of zero use. Actually in almost 20 years of cisco networking, I never saw anybody using it.

Jon Marshall Fri, 06/18/2010 - 04:26

p.bevilacqua wrote:

You fail to understand that:

1. - OP never asked for exact clarification on the working of "backup interface".

2. - In the real world, ethernet circuit stays up whiel WAN is don, consequently "backup interface is of zero use. Actually in almost 20 years of cisco networking, I never saw anybody using it.

1. OP asked for a solution and that is what Mohamed supplied

2. Ethernet circuit does stay up which is why if the ethernet circuit is the primary interface you would indeed need IP SLA. However if the primary circuit is serial and the backup ethernet then i can see how it would work as if the serial goes down then the ethernet takes over. If the serial comes back up then it takes over again. So unless i misunderstand how the backup interface command works it all relies on the router recognising whether the serial interface is up or down and for that you do not need IP SLA.

Jon

Hitesh Vinzoda Fri, 06/18/2010 - 04:38

Why we are using IP SLA here is for reliable failover instead of just using interface down status to rely on.. For example if you have serial interface and encapsulation is frame relay than there would not be any end to end keepalive, the router receives LMI status from the frame switch and declares interface is up while some where in service provider's network frame relay is broken... So its better to avoid such situations... and if you have luxury of tools like IP SLA why not use it ....

Hitesh Vinzoda

Jon Marshall Fri, 06/18/2010 - 05:06

Hitesh

I don't disagree with you at all and your solution using IP SLA will work fine.

But the question as to whether the backup interface command will work in this scenario is an interesting one. I personally didn't think it would work at first due to the command reference notes but Mohamed then confirmed it does indeed work on an ISR.

Paolo stated in his first post that it would simply not work and argued that it wouldn't work because ethernet always stays up which indeed it does and if ethernet was the primary interface then that would be a good reason. But in this scenario it is the serial interface that is the primary interface not ethernet.

So to state it won't work fullstop is misleading and is a misrepresentation of what Mohamed was saying.

Jon

gatlin007 Fri, 06/18/2010 - 08:17

The OP was specifically interested in a ‘type of backup delay config’.  The ‘backup interface’ feature addresses that and floating statics triggered by IP SLA come close to what the OP was inquiring about.

If we are splitting hairs as to a proper solution, I’m sure most would agree it would be a dynamic routing protocol properly tuned for fault tolerance meeting their requirements. 


Chris

Hitesh Vinzoda Tue, 06/15/2010 - 23:13

Hi,

I would like suggest have floating static default route as below with IP sla tracking

ip route 0.0.0.0 0.0.0.0 track 1

ip route 0.0.0.0 0.0.0.0 < next-hop secondary ISP> 254   >> (254 is AD, so route 1 will be always preferred, track this route with IP SLA)

track 1 rtr 1

ip sla 1
icmp-echo
frequency 5
ip sla schedule 1 life forever start-time now

Route no. 1 is only installed, if the ping response to ISP 1 is there, so line protocol is up but its not reachable the route is withdrawn and secondary route with AD 254 is installed in routing table and used.

When the ping is successfull route in installed again and preferred due to less AD.

HTH

Hitesh Vinzoda

Pls rate useful posts

gatlin007 Wed, 06/16/2010 - 06:38

Be careful when choosing the SLA target host in this scenario.  Chances are the target will also be available over the surviving link; causing the router to constantly install/remove the default route over the circuit that has lost integrity.

A creative way to force the SLA target to only take the link it’s intended to protect will have to be utilized such as a floating static to null0.



Chris

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