Backup Interface:

Lessons Learned:

Tracks line protocol of "primary" interface

-if line protocol is up, backup interface is in standby

-If lin protocol is down, backup interfce is active

Configuration.

-backip {interface | load | delay|}

-placed on the promary interface

What causes the line protocol to go down?

This concept is another variation of a primary and secondary interface like floating static routes.

Back up interface orginall y came form Dial-backup senarios, where theres a pimary frame or T1 up and the backup would be an anolog or dial-up. you'd only use dial interface if the primary was down or there was an unusual load on the link. The reason being is that the analog or dial links we're orginally pay per use.

Note: this willbe under dial technologies under Cisco's configuraitpon reference guides.

If the load option is configured or the dial link. You will need to specify the load interface under the interface. the load by default is checked in 5 minute intervals. If we want to change the sample rate, under the interface we need to specify the sample time.

EX:

# Interface s0/0/0

 #load-interval 60 (sets the sampling to 60 seconds.)

Backup interface config:

Under the primary interface - we need to tell it what's the backup interface:

EX:

#interface  serial 0/0

#backup interface serail 0/1

In this config there's normally only one interface in the up/up at one time. As long as the router has a path - route to destination off each link - it doesnt matter what the metric is.

My lab has 3 routers-

R1 and R2 are connected together vis FA/0

R3 has a serial link to both routers. It has two serail links to R1, which is being used as a border router. I've installed loopbacks on R1 for networks 1.1.1.0/24 and 2.2.2.0/24.

The primary serail link to R1 is on subnet: 13.0.0.0 /24

The backup serial link (s0/1) is on subnet 14.0.0.0/24

currently they both have reachability over each link to R1.

Now - i'll confiure the backup link on S0/0 to point to S0/1.

R3(config-if)#backup interface serial 0/1

*Mar  1 00:19:04.527: %LINK-5-CHANGED: Interface Serial0/1, changed state to standby mode
*Mar  1 00:19:05.527: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/1, changed state to down
R3#sh run interface ser
R3#sh run interface serial 0/0
Building configuration...

Current configuration : 110 bytes
!
interface Serial0/0
 backup interface Serial0/1
 ip address 13.0.0.3 255.255.255.0
 clock rate 2000000
end

R3#sh run | i ip route
ip route 0.0.0.0 0.0.0.0 Serial0/0 -
ip route 0.0.0.0 0.0.0.0 13.0.0.1 2

Serial0/1                  14.0.0.3        YES manual standby mode          down   
Serial0/2                  32.0.0.3        YES manual up                    up  

Note: that serial 0/1 is in the "standby Mode" - this means that that its line protocol is down.

 

The key point is that a route can only be installed in the orutiung table if the interface that is recurses to has it's line protocol up - up. If down - no routes can be installed.

R3#sh ip route static
S*   0.0.0.0/0 is directly connected, Serial0/0

The current route installed is over the other serial link.
This trace verifies the current route.

Type escape sequence to abort.
Tracing the route to 2.2.2.2

  1 13.0.0.1 36 msec *  40 msec
R3#

If the now prmary link goes down - the standy link will go active until the primary link retunrs to up.

Once i shut the interface on teh other side of the link for S0/0 - the backup link goes active:

R3#
*Mar  1 00:37:45.695: %LINK-3-UPDOWN: Interface Serial0/1, changed state to up
R3#
*Mar  1 00:37:46.699: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/1, changed state to up

R3#traceroute 2.2.2.2

Type escape sequence to abort.
Tracing the route to 2.2.2.2

  1 14.0.0.1 44 msec *  40 msec
R3#

Once I bring the primary backup:

*Mar  1 00:41:13.615: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/0, changed state to up
R3#
*Mar  1 00:41:15.631: %LINK-5-CHANGED: Interface Serial0/1, changed state to standby mode
*Mar  1 00:41:16.631: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/1, changed state to down

 Now we can see the orginal primary link is backup - and the backup link has gone to standby.

Serial0/0                  13.0.0.3        YES manual up                    up     
FastEthernet0/1            unassigned      YES unset  administratively down down   
Serial0/1                  14.0.0.3        YES manual standby mode          down   
Serial0/2                  32.0.0.3        YES manual up                    up  

Also that the route has be installed again in the routing table.

S*   0.0.0.0/0 is directly connected, Serial0/0