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Popup Hotspot Using ISR 1000 with WiFi/LTE for Teleworkers and Micro Branchesr
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Best Way to Configure Backup T1 Interface

Hi,

I was wondering what was the best practice is for configuring a backup T1 interface? The primary interface is a DS3 to HQ.

Could it be possible with a DS3 to have an interface show as up but the DS3 not function?

I was thinking if this is the case - maybe I should configure a IP SLA pinging the other side and when that fails, I could run an EEM script to bring up the T1 and when the pings come back I could shut the T1.

Any ideas or docs on how to set up?

Also, any docs on how to set up the T1 interface?

Thank you, Pat.

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Advisor

Re: Best Way to Configure Backup T1 Interface

If you're using strictly eigrp, you won't need floating statics unless your needs call for it. Floating routes are mainly a backup route that's not put into the table unless an exact route that has a lower AD is removed from the routing table. For example:

ip route 0.0.0.0 0.0.0.0 192.168.1.1

ip route 0.0.0.0 0.0.0.0 192.168.2.1 254

The default route above pointing to 192.168.1.1 has a lower admin distance than the exact route with an AD of 254. The default route in the routing table will be the one that points to 192.168.1.1 unless the interface that's used to get to that route (usually connected) goes down. If that happens, the 192.168.2.1 route will be inserted into the table.

HTH,
John

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Re: Best Way to Configure Backup T1 Interface

Pat,

Maybe I'm not explaining myself well enough

You have a branch router and a router at HQ. My understanding, taking these 2 sites alone, is that you run eigrp between these two locations. Assuming that you're advertising every subnet between the two locations, then a default route would be needed on the branch router pointing to HQ (assuming that you are providing an egress point for internet access for the remote branch). But all of the other routes are eigrp learned, so for the failover to happen, eigrp is smart enough to know that the link it was using to get to the HQ site is now down and it will have to route over the T1. The default route that you'll install on the remote site though would point toward your ds3 circuit and then you would do a floating route for the same route pointing to the t1. If the ds3 were to go down, all of your routes that were eigrp learned and the floating default will traverse the t1 now:

R1 = branch

R2 = HQ

R1 ----- R2

R1#sh run | in ip route

ip route 0.0.0.0 0.0.0.0 172.12.0.2

ip route 0.0.0.0 0.0.0.0 192.168.12.2 254  <--- Floating default route

R1#sh run | s router eigrp

router eigrp 100

network 172.12.0.1 0.0.0.0

network 192.168.12.1 0.0.0.0

no auto-summary

Below is the topology table. Notice I have 2 paths for each route. S0/1 is set at 45Mb (DS3) and S0/0 is 1544 (T1) S0/1 is the successor at a FD of 696832 for 2.2.2.0/24 (meaning it wins the right to be in the routing table):

R1#sh ip eigrp top

IP-EIGRP Topology Table for AS(100)/ID(192.168.12.1)

Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,

       r - reply Status, s - sia Status

P 2.2.2.0/24, 1 successors, FD is 696832

        via 172.12.0.2 (696832/128256), Serial0/1

        via 192.168.12.2 (2297856/128256), Serial0/0

P 22.22.22.0/24, 1 successors, FD is 696832

        via 172.12.0.2 (696832/128256), Serial0/1

        via 192.168.12.2 (2297856/128256), Serial0/0

The routing table on R1. Notice there is only 1 route pointing to s0/1 (DS3) because of it being the successor. Also notice the static route for the default pointing to 172.12.0.2: (The 2 bolded routes are the ones that we'll focus on.)

C    192.168.12.0/24 is directly connected, Serial0/0

     2.0.0.0/24 is subnetted, 1 subnets

D       2.2.2.0 [90/696832] via 172.12.0.2, 00:07:48, Serial0/1

     172.12.0.0/24 is subnetted, 1 subnets

C       172.12.0.0 is directly connected, Serial0/1

     22.0.0.0/24 is subnetted, 1 subnets

D       22.22.22.0 [90/696832] via 172.12.0.2, 00:07:48, Serial0/1

S*   0.0.0.0/0 [1/0] via 172.12.0.2

I'll shut the DS3 interface to simulate a failure:

R1(config)#int s0/1

R1(config-if)#shut

R1(config-if)#

*Mar  1 00:15:35.979: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 100: Neighbor 172.12.0.2 (Serial0/1) is down: interface down

R1(config-if)#

*Mar  1 00:15:37.959: %LINK-5-CHANGED: Interface Serial0/1, changed state to administratively down

*Mar  1 00:15:38.959: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/1, changed state to down

R1(config-if)#

Now look at the routing table:

R1#sh ip route

Gateway of last resort is 192.168.12.2 to network 0.0.0.0

C    192.168.12.0/24 is directly connected, Serial0/0

     2.0.0.0/24 is subnetted, 1 subnets

D       2.2.2.0 [90/2297856] via 192.168.12.2, 00:00:55, Serial0/0

     22.0.0.0/24 is subnetted, 1 subnets

D       22.22.22.0 [90/2297856] via 192.168.12.2, 00:00:55, Serial0/0

S*   0.0.0.0/0 [254/0] via 192.168.12.2

So when the interface goes down, the floating default will get put in the routing table because the next hop that it was originally pointing to is no longer available due to the interface that it would use to get to it being down. Also notice that 2.2.2.0 is now pointing to the other interface s0/0 (T1). If you look at the topology table for eigrp now, you'll see that the original route is no longer in the table:

P 2.2.2.0/24, 1 successors, FD is 696832

        via 192.168.12.2 (2297856/128256), Serial0/0

P 22.22.22.0/24, 1 successors, FD is 696832

        via 192.168.12.2 (2297856/128256), Serial0/0

P 192.168.12.0/24, 1 successors, FD is 2169856

        via Connected, Serial0/0

But, when I bring that interface back up it will get reentered into the table and it's preferred again:

R1#sh ip eigrp top

IP-EIGRP Topology Table for AS(100)/ID(192.168.12.1)

Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,

       r - reply Status, s - sia Status

P 2.2.2.0/24, 1 successors, FD is 696832

        via 172.12.0.2 (696832/128256), Serial0/1

        via 192.168.12.2 (2297856/128256), Serial0/0

P 22.22.22.0/24, 1 successors, FD is 696832

        via 172.12.0.2 (696832/128256), Serial0/1

        via 192.168.12.2 (2297856/128256), Serial0/0

Oh, and just to show you the floating route concept, you'll also notice that since I brought that interface back up, the floating default that we had is now no longer the default route; it's the original one that we had at first pointing to 172.12.0.2 because of its lower AD (1 vs 254):

R1#sh ip route | inc 0.0.0.0

Gateway of last resort is 172.12.0.2 to network 0.0.0.0

S*   0.0.0.0/0 [1/0] via 172.12.0.2

HTH,

John

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Re: Best Way to Configure Backup T1 Interface

I do believe it's GNS. Your real world experience is going to be much faster when the router notices that the link is down.

HTH,
John

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Best Way to Configure Backup T1 Interface

Pat,

Can you tell us a little about how you're set up? Do the 2 interfaces connect to the same ISP? Do you run a routing protocol or static routes? You may be able to do something as simple as using the "backup interface" command on your primary interface or you can use floating statics with tracking. What type of card do you have to configure for T1?

HTH,
John

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Re: Best Way to Configure Backup T1 Interface

Thanks for the response

The interfaces connect to two different providers but, they are both dedicated links. I will be running eigrp on this branch router and the router at HQ. I'm not experienced with T1 interfaces, I believe they are PRI - it's not in front of me but, I can confirm tomorrow morning. What should I look for with the T1s as far as type?

Thank you, Pat.

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Advisor

Re: Best Way to Configure Backup T1 Interface

In my experience, sometimes you have to configure the card with a "card type t1" command in order to get your interfaces, but then there are some cards that when inserted automatically give you your serial interfaces to configure. If you do a "show run" you'll see "card type in slot not configured" or something like that if you need to configure it manually.

Are these internet links or links to an mpls vpn? If their vpn, you could keep both up and just set the metrics where your ds3 is always preferred or you can set the ds3 to be the primary and the t1 to be the backup:

DS3 interface:

backup interface s0/1

T1 interface (s0/1):

ip address 192.168.1.1 255.255.255.0

The T1 will stay down unless the router sees that the DS3 went down and then the T1 interface will be brought up. You can't shut the ds3 interface though to test it because it has to be "up/down" I believe for the failover to work. If you shut the interface it will also bring down the t1. If this is an internet link for both, you'll need to take into account all of your natting and how things will change when going over the T1 instead. It's late and I hope I'm making sense

HTH,
John

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Re: Best Way to Configure Backup T1 Interface

Thanks John - sorry if you should be sleeping.

I think I will have to do the card type t1 command as I had to do card type t3 to get the DS3 interface to show up. These are not Internet connections, they are dedicated circuits. Not sure if they are MPLS vpn - I just know that they are a layer 2 connection and I will have a /30s between the HQ and the branch on the DS3 and the T1.

My concern is this - if I use the metrics method or the backup interface method, and the DS3 is not working (meaning the brach cannot reach the HQ via the DS3) couldn't the interface be still up or am I wrong - if the branch loses a layer 3 connection with HQ it will show as down and the T1 will take over?

Also, instead of setting metrics - won't the metrics appear automatically and the metrics for the DS3 will be better so EIGRP will prefer that route (which is what I want)?

Thank you, Pat.

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Advisor

Best Way to Configure Backup T1 Interface

Pat,

That could happen. So you want to take into account the far side going down yet your connection to the ISP is still showing up. You may have to use sla and eem in order to do this like you had originally stated. Tracking will let you track routes or interfaces via glbp/hsrp, etc, but I don't believe it's going to be possible to bring up an interface based on a tracked object. Your eem script can take advantage of the tracked object though, so I'd create an icmp sla that pings the opposite end of the link and then your eem script can act on the failure of that object.

If you are using static routes only between the two sites, you could, for example, set your default route to your DS3 and track on that (same icmp sla). You could also have another default route that has a higher AD and when the tracked object fails, the tracked default static route will be removed from the table and the default that points to the T1 will be put in its place. Let me know if you'd like to do the sla with eem and I can help you with that if you need....

HTH,
John

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Re: Best Way to Configure Backup T1 Interface

Thanks John,

If the branch DS3 link loses Layer 3 connectivity to the HQ (which is what I am concerned about) but retained Layer2 connectivity with the carrier, wouldn't the T1 take over because of EIGRP? Shouldn't EIGRP sense theL3 connection is down and immediately route through the T1? Or, could it be possible, like you and I said that the router would still route packets through the DS3 even though they would be getting blackholed.

I would rather not configure an IP SLA and a EEM script if I don't have to but, if that is the only option then I will. I'm looking for immediate change over during a link fail and I don't thinka EEM script can neccessarily do that. I guess I could have a combo of both - if the link goes down EIGRP will move the traffic to the T1 and if the branch link stays up but the HQ is down, the IP SLA would trigger the EEM script.

Thank you.

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Advisor

Re: Best Way to Configure Backup T1 Interface

Pat,

If you shut down the far side interface as a test, do you see your side go down? Technically, eigrp would prefer the next path. If you do a "show ip eigrp topol" do you have multiple entries for the same route (at least 2), and if so, do you have single entries in your routing table now? You'd need to have at least 2 in your topology table for it to immediately fail over, but in working conditions you'd really only want your traffic to go out of your ds3 if you're not wanting to load balance. Load balancing would require that you have identical metrics through two different paths (your ds3 and t1). Being that the t1 is a lot slower than your ds3, I'm also going to assume that all of your routes would only go through the ds3 at this point in time.

In short, yes, if eigrp is engineered correctly, I would say that it would use the t1 as a backup but I could lab this up to just double check.

HTH,
John

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Re: Best Way to Configure Backup T1 Interface

Yes I need to test this before I head out to the branch which is probably 800 miles away. I'll have to use completey different addressing as the branch is up at this moment.

As far as the T1 interface - I did the command as you suggested and now I see the T1s under show run. the default config is:

controller T1 0/2/0

framing esf

linecode b8zs

cablelength long 0db

!

Is this the only config the interface will need other than an IP and mask?

Thank you

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Re: Best Way to Configure Backup T1 Interface

Pat,

It should be the only config. Since you see that, you should have "int s0/2/0" now and be able to configure you addressing information and everything else you need.

I'll try to lab this up today. Do you know if your two sites share the same subnet on their wan interfaces? If you look at a route to the other location on this router with the DS3, does it show the other site as the next hop for a subnet that the other site owns?

HTH,
John

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Re: Best Way to Configure Backup T1 Interface

Thanks John,

yes, the HQ WAN and the branch WAN connection share the same subnet. The DS3 link is one subnet and the T1link is another.

I don't see an option to configure and address on the T1:

WM-1002-WAN1(config-controller)#?

Controller configuration commands:

  bert            Start BERT test

  cablelength     Specify the cable length for a DS1 link

  channel-group   Specify the timeslots to channel-group mapping for an

                  interface

  clock           Specify the clock source for a DS1 link

  default         Set a command to its defaults

  description     Controller specific description

  exit            Exit from controller configuration mode

  fdl             Specify the FDL standard for a DS1 data link

  framing         Specify the type of Framing on a DS1 link

  help            Description of the interactive help system

  linecode        Specify the line encoding method for a DS1 link

  logging-events  LOG T1 Events

  loopback        Put the entire T1 line into loopback

  no              Negate a command or set its defaults

  shutdown        Shut down a DS1 link (send Blue Alarm)

  yellow          Yellow Alarm Configuration

Thank you

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Advisor

Re: Best Way to Configure Backup T1 Interface

Pat,

You'd configure your address under the standard interface now. Usually with those cards, you won't see the standard interfaces until you've done the "card type" command as you've done. Now you should be able to do a "show ip int brie" and see the interface s0/2/0.

To add the address just do the norm as in:

int s0/2/0

ip address x.x.x.x x.x.x.x

If you share the same subnet on the other end, I have a feeling that you'd see the interface go down on the HQ side if you were to shut the other side. If that's the case, you could still go with the backup interface command or just tweak eigrp.

HTH,
John

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Re: Best Way to Configure Backup T1 Interface

So I still need to tweak EIGRP? Won't the metrics decide who is the better route by default and only use the T1 when the DS3 route is down?

So, I should see both routes with the sh ip eigrp topology but only the DS3 with the sh ip route, correct?

thank you.

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Advisor

Re: Best Way to Configure Backup T1 Interface

So, I should see both routes with the sh ip eigrp topology but only the DS3 with the sh ip route, correct?

Technically, yes The metrics should be much preferred for the DS3 over the T1. If that's the case, you may not need to do anything at all, but it all depends on what the routing table says.

And as a side note, if in fact you shut down the other side and you see the HQ side go to a up/down state, then EIGRP would fail over to the T1 if it does have a route in the topology table to fail over to. Otherwise, eigrp will query for the route and the failover won't be as fast as if there were 2 routes in the topology table.

HTH,
John

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Best Way to Configure Backup T1 Interface

Thanks John,

But I still need a floating default route, correct?

Is this a fairly simple config. Do you have a good doc?

Thank you