Primary and backup link setup for point to point circuits

[PankajPatel1758]

Hello,

I have 2 point to point leased data circuits between two offices. These 2 links are connected to Cisco 3845 and Cisco 2811 routers WIC-2T card serial ports in both the offices. Static routing is used between these two sites and Multilink with load sharing is configured to utilize these 2 links. Now there is a problem in this setup when one of the two links is relatively less stable sometimes. Whenever the unstable link goes down then traffic switches to the 2nd link (stable link), but there is a gap during this transition every time. So basically by using Multilink my overall availability is going down even though I have a stable link which is up for the whole day.

My question is whether I can configure Primary secondary link in this setup by keeping Stable link as primary and relatively less stable link as secondary?  I have enough bandwidth available in both the links so load sharing is not the primary objective, but the overall availability of link between the sites is primary objective.

Configuration:

interface Multilink 2

ip address 10.10.100.253 255.255.255.0

ip pim-sparse mode

ppp multilink

ppp multilink group 2

interface se 0/0/0

no ip address

encapsulation ppp

ppp multilink

ppp multilink group 2

interface se 0/0/1

no ip address

encapsulation ppp

ppp multilink

ppp multilink group 2

ip route 10.10.100.0 255.255.255.0 10.10.100.254

Router-2 configuration:

Interface Multilink 2

ip address 10.10.100.254 255.255.255.0

ip pim-sparse mode

ppp multilink

ppp multilink group 2

interface se 0/0/0

no ip address

encapsulation ppp

ppp multilink

ppp multilink group 2

interface se 0/0/1

no ip address

encapsulation ppp

ppp multilink

ppp multilink group 2

ip route 0.0.0.0  0.0.0.0 10.10.100.253

Thanks

[Georg Pauwen]

Hello,

it all depends on how the routers are connected to each other. I assume you are using some sort of multichassis PPP ?

Post the running configuration of the routers...

[Richard Burts]

I do not understand this statement "Multilink with load sharing is configured to utilize these 2 links". Multi link with load sharing might make sense in the context of some dynamic routing protocol, but you tell us this is using static routes. Perhaps the suggestion to post the configuration of both routers would be helpful in trying to understand this.

But if you are not concerned about load sharing and are wiling to implement an approach with a primary (more stable) and a backup (less stable) then adding an administrative distance greater than zero to the unstable route might accomplish this.

I might also ask whether you would consider using a dynamic routing protocol. A dynamic routing protocol would allow you to use both circuits but to stop using one if the neighbor on that one is no longer reachable.

[PankajPatel1758]

Hi,

I have posted the concerned part of the configuration file.

For implementing primary backup setup with administrative distance I believe I need to configure two serial ports with the same IP address in my case (which is not possible). The only way I could find putting same IP on two serial interfaces was by using Multilink, but the Multilink solution is not suiting due to gap during the transition when one link goes down.

[PankajPatel1758]

Hi,

Router-1 configuration:

interface Multilink 2

ip address 10.10.100.253 255.255.255.0

ip pim-sparse mode

ppp multilink

ppp multilink group 2

interface se 0/0/0

no ip address

encapsulation ppp

ppp multilink

ppp multilink group 2

interface se 0/0/1

no ip address

encapsulation ppp

ppp multilink

ppp multilink group 2

ip route 10.10.100.0 255.255.255.0 10.10.100.254

Router-2 configuration:

Interface Multilink 2

ip address 10.10.100.254 255.255.255.0

ip pim-sparse mode

ppp multilink

ppp multilink group 2

interface se 0/0/0

no ip address

encapsulation ppp

ppp multilink

ppp multilink group 2

interface se 0/0/1

no ip address

encapsulation ppp

ppp multilink

ppp multilink group 2

ip route 0.0.0.0  0.0.0.0 10.10.100.253

[Georg Pauwen]

Hello,

what log message do you see when the 'unstable' member goes down ? A possible solution would be to implement a series of simple EEM scripts that immediately shut down a member interface that goes down. The scripts are below (you need to configure this on both routers):

event manager applet SERIAL0/0/0_DOWN
event syslog pattern "*Serial0/0/0, changed state to down"
action 1.0 cli command "enable"
action 2.0 cli command "conf t"
action 3.0 cli command "interface Serial0/0/0"
action 4.0 cli command "shut"
action 5.0 cli command "end"
!
event manager applet SERIAL0/0/0_UP
event syslog pattern "*Serial0/0/0, changed state to up"
action 1.0 cli command "enable"
action 2.0 cli command "conf t"
action 3.0 cli command "interface Serial0/0/0"
action 4.0 cli command "no shut"
action 5.0 cli command "end"
!
event manager applet SERIAL0/0/1_DOWN
event syslog pattern "*Serial0/0/1, changed state to down"
action 1.0 cli command "enable"
action 2.0 cli command "conf t"
action 3.0 cli command "interface Serial0/0/1"
action 4.0 cli command "shut"
action 5.0 cli command "end"
!
event manager applet SERIAL0/0/1_UP
event syslog pattern "*Serial0/0/1, changed state to up"
action 1.0 cli command "enable"
action 2.0 cli command "conf t"
action 3.0 cli command "interface Serial0/0/1"
action 4.0 cli command "no shut"
action 5.0 cli command "end"

[PankajPatel1758]

Hi,

What I mean to say by saying unstable link is: It goes down for few minutes 4-5 times a day. So during those 4-5 transitions I am observing some data gaps. I didn't understand how making the port down by EEM script will help here.

[balaji.bandi]

My question is whether I can configure Primary secondary link in this setup by keeping Stable link as primary and relatively less stable link as secondary?  I have enough bandwidth available in both the links so load sharing is not the primary objective, but the overall availability of link between the sites is primary objective.

Both cases are durable, only if we can know how these are configured here. Do you have any network Diagram of the network?

Static Routing is not the best, But you can achieve different methods - Make the stable link as Primary and another one as secondary.

Once that is Primary Link Failes and Fail over to Secondary.  can you post-show run config all the devices?

[PankajPatel1758]

Hi,

For some reason I have to stick to static routing only, I don't have the liberty to change it to dynamic routing.

I am unable to configure primary secondary setup because I can't put the same IP address to 2 different interfaces (I believe I need to do that to use administrative distance for ip route) Is there any other way to implement that ?

The main part of the configuration I have already posted. Attaching network diagram also.