This post is discussing the "backup interface" command and gives an example on how it could be used.
The backup interface as its name suggests specifies an interface that is not normally active but is brought online once a primary interface's line protocol goes down. This is a router function, however for switches there is a feature (flexlinks) which is very similar (switchport backup interface) but has more layer 2 features such as keeping the flexlink peer updated with which interface should be associated with learnt MAC addresses, however that is not the focus on this posting.
On the primary interface, we specify which interface shall be our backup interface.
interface PRIMARYINT
backup interface BACKUPINTOnce this occurs, the backup interface is placed in standby mode and the interface is brought down.
When the primary interface fails (failure does not include administratively shutting down the primary interface as IOS believes you are doing this on purpose) the backup interface is brought online on the physical and then line level. Because of this only one side of a link needs to be set up this way.
In the example below we are going to have 3 Routers. R1 is connected to R2 via Frame Relay, R2 is connected to R3 via Frame Relay, and R1 is connected to R3 via HDLC. R1 will be configured with a backup interface (the HDLC will be the backup for the Frame Relay) We'll be running a dynamic routing protocol over these links as well.
Here are the configurations:
R1
hostname R1
interface Loopback0
ip address 1.1.1.1 255.255.255.255
!
interface Serial0/0
description FR Switch
no ip address
encapsulation frame-relay
clock rate 2000000
!
interface Serial0/1
description direct to R3
ip address 10.1.13.1 255.255.255.0
clock rate 64000
!
interface Serial0/0.12 point-to-point
backup interface Serial0/1
frame-relay interface-dlci 102 ppp Virtual-Template1
class FREEK
!
interface Virtual-Template1
ip address 10.1.12.1 255.255.255.0
!
router eigrp 100
network 1.1.1.1 0.0.0.0
network 10.1.12.1 0.0.0.0
network 10.1.13.1 0.0.0.0
no auto-summary
!
map-class frame-relay FREEK
frame-relay end-to-end keepalive mode bidirectional
R2
hostname R2
interface Loopback0
ip address 2.2.2.2 255.255.255.255
!
interface Serial0/0
description FR Switch
no ip address
encapsulation frame-relay
clock rate 2000000
frame-relay interface-dlci 203 ppp Virtual-Template3
!
interface Serial0/0.21 point-to-point
description PVC to R1
frame-relay interface-dlci 201 ppp Virtual-Template1
class FREEK
!
interface Virtual-Template1
ip address 10.1.12.2 255.255.255.0
!
interface Virtual-Template3
ip address 10.1.23.2 255.255.255.0
!
router eigrp 100
network 2.2.2.2 0.0.0.0
network 10.1.12.2 0.0.0.0
network 10.1.23.2 0.0.0.0
no auto-summary
map-class frame-relay FREEK
frame-relay end-to-end keepalive mode bidirectional
R3
hostname R3
interface Loopback0
ip address 3.3.3.3 255.255.255.255
!
interface Serial0/0
no ip address
encapsulation frame-relay
frame-relay interface-dlci 302 ppp Virtual-Template1
!
interface Serial0/1
ip address 10.1.13.3 255.255.255.0
ip summary-address eigrp 100 0.0.0.0 0.0.0.0 255
!
interface Virtual-Template1
ip address 10.1.23.3 255.255.255.0
!
router eigrp 100
network 3.3.3.3 0.0.0.0
network 10.1.13.3 0.0.0.0
network 10.1.23.3 0.0.0.0
no auto-summary
Lets look at some of the interfaces here
R1>sh ip int brief | ex unassigned
Interface IP-Address OK? Method Status Protocol
Serial0/1 10.1.13.1 YES NVRAM standby mode down
Virtual-Access1 10.1.12.1 YES TFTP up up
Virtual-Template1 10.1.12.1 YES NVRAM down down
Loopback0 1.1.1.1 YES NVRAM up up
Firstly we can see that Serial0/1 is in a standby mode condition and the line protocol is down. Unfortunately there appears to be no regular show command to determine which interface has S0/1 as its backup interface, so we need to examine the router configuration
R1#sh run | i ^interface|^ backup
interface Loopback0
interface FastEthernet0/0
interface Serial0/0
interface Serial0/0.12 point-to-point
backup interface Serial0/1
interface FastEthernet0/1
interface Serial0/1
interface Virtual-Template1
Here we can see that S0/0.12 has S0/1 as its backup interface
Virtual-Templates appear as down because they aren't actually active interfaces - they have a limited amount of configuration which is used as a baseline config by the Virtual-Access interfaces. Each Virtual-Access represents a dynamic instance of an individual connection.
R1#sh int virtual-template 1
Virtual-Template1 is down, line protocol is down
Hardware is Virtual Template interface
Internet address is 10.1.12.1/24
MTU 1500 bytes, BW 100000 Kbit/sec, DLY 100000 usec,
reliability 255/255, txload 1/255, rxload 1/255
....
Looking at Virtual-Access 1
R1#sh int virtual-access 1
Virtual-Access1 is up, line protocol is up
Hardware is Virtual Access interface
Internet address is 10.1.12.1/24
MTU 1500 bytes, BW 100000 Kbit/sec, DLY 100000 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation PPP, LCP Open
Open: IPCP
PPPoFR vaccess, cloned from Virtual-Template1
Vaccess status 0x44
Bound to Serial0/0.12 DLCI 102, Cloned from Virtual-Template1, loopback not set
Keepalive set (10 sec)
....
We can see that if a PPP over Frame Relay connection using the PVC on S0/0.12 DLCI 102 and taking the the base IP configuration from the virtual-template. The interface is in an up/up state because the PVC is up and there is a PPP connection established with its peer.
R1#sh frame pvc int s0/0.12 | i PVC STATUS
DLCI = 102, DLCI USAGE = LOCAL, PVC STATUS = ACTIVE (EEK UP), INTERFACE = Serial0/0.12
On S0/0.12 we can see that we are using Frame Relay End-to-End Keep alives. If EEK messages with our peer are non responded to, interface S0/0.12 will have its line protocol go down.
Click here if you would like to know more about using Frame-Relay Keep Alives
R1#sh int s0/01 | i line
Serial0/1 is standby mode, line protocol is down
Since S0/1 is the backup interface for S0/0.12, S0/1 will then be brought out of standby mode and the physical interface will go active. Once S0/0.12 responds to EEK messages again, S0/1 will re-enter standby state and the physical interface will go down.
R2#sh ip int bri | ex unassigned
Interface IP-Address OK? Method Status Protocol
Virtual-Access1 10.1.23.2 YES TFTP up up
Virtual-Template1 10.1.12.2 YES NVRAM down down
Virtual-Access2 10.1.12.2 YES TFTP up up
Virtual-Template3 10.1.23.2 YES NVRAM down down
Loopback0 2.2.2.2 YES NVRAM up up
R2 has PVCs with R1 and R3. To ensure that FR EEK operates, as expected, a subinterface is created in order for it too to go down if EEK messages are not responded to.
R3#sh ip int bri | ex unassigned
Interface IP-Address OK? Method Status Protocol
Serial0/1 10.1.13.3 YES NVRAM up down
Virtual-Access1 10.1.23.3 YES TFTP up up
Virtual-Template1 10.1.23.3 YES NVRAM down down
Loopback0 3.3.3.3 YES NVRAM up up
R3 has a PVC to R2, and a low speed link to R1. R3 will advertise a default route to R1 using a summarization. As we don't want R3 to have a default route to null0, we shall set the admin distance of the summary to 255 (which means it wont get installed in the local routing table) as a general rule if we are doing route summarization we actually desire the route to null0 as it is used to sink traffic rate than potentially have traffic bouncing around the network until the TTL expires.
R3#sh run int s0/1
Building configuration...
Current configuration : 133 bytes
!
interface Serial0/1
ip address 10.1.13.3 255.255.255.0
ip summary-address eigrp 100 0.0.0.0 0.0.0.0 255
As this interface is normally down (brought up by R1 to come up only when its PVC to R2 is down), R1 will not distribute the default to R2
We've explained what should occur, lets test things - first lets start in the normal operating state:
R1
R1#sh ip int brief | ex unassigned
Interface IP-Address OK? Method Status Protocol
Serial0/1 10.1.13.1 YES NVRAM standby mode down
Virtual-Access1 10.1.12.1 YES TFTP up up
Virtual-Template1 10.1.12.1 YES NVRAM down down
Loopback0 1.1.1.1 YES NVRAM up up
R1#sh ip route eigrp
2.0.0.0/32 is subnetted, 1 subnets
D 2.2.2.2 [90/2713600] via 10.1.12.2, 00:28:38, Virtual-Access1
3.0.0.0/32 is subnetted, 1 subnets
D 3.3.3.3 [90/5273600] via 10.1.12.2, 00:28:38, Virtual-Access1
10.0.0.0/8 is variably subnetted, 4 subnets, 2 masks
D 10.1.23.0/24 [90/5145600] via 10.1.12.2, 00:28:38, Virtual-Access1
D 10.1.23.3/32 [90/5145600] via 10.1.12.2, 00:28:38, Virtual-Access1
R1#trace 2.2.2.2 so lo0
Type escape sequence to abort.
Tracing the route to 2.2.2.2
1 10.1.12.2 8 msec * 12 msec
R1#trace 3.3.3.3 so lo0
Type escape sequence to abort.
Tracing the route to 3.3.3.3
1 10.1.12.2 0 msec 4 msec 0 msec
2 10.1.23.3 8 msec * 16 msec
R2
R2#sh ip int bri | exc unassigned
Interface IP-Address OK? Method Status Protocol
Virtual-Access1 10.1.23.2 YES TFTP up up
Virtual-Template1 10.1.12.2 YES NVRAM down down
Virtual-Access2 10.1.12.2 YES TFTP up up
Virtual-Template3 10.1.23.2 YES NVRAM down down
Loopback0 2.2.2.2 YES NVRAM up up
R2#sh ip route eigrp
1.0.0.0/32 is subnetted, 1 subnets
D 1.1.1.1 [90/2713600] via 10.1.12.1, 00:29:32, Virtual-Access2
3.0.0.0/32 is subnetted, 1 subnets
D 3.3.3.3 [90/2713600] via 10.1.23.3, 01:10:30, Virtual-Access1
R2#trace 1.1.1.1 so lo0
Type escape sequence to abort.
Tracing the route to 1.1.1.1
1 10.1.12.1 4 msec * 16 msec
R2#trace 3.3.3.3 so lo0
Type escape sequence to abort.
Tracing the route to 3.3.3.3
1 10.1.23.3 0 msec * 12 msec
R3
R3#sh ip int bri | exc unassigned
Interface IP-Address OK? Method Status Protocol
Serial0/1 10.1.13.3 YES NVRAM up down
Virtual-Access1 10.1.23.3 YES TFTP up up
Virtual-Template1 10.1.23.3 YES NVRAM down down
Loopback0 3.3.3.3 YES NVRAM up up
R3#sh ip route eigrp
1.0.0.0/32 is subnetted, 1 subnets
D 1.1.1.1 [90/5273600] via 10.1.23.2, 00:25:01, Virtual-Access1
2.0.0.0/32 is subnetted, 1 subnets
D 2.2.2.2 [90/2713600] via 10.1.23.2, 00:30:40, Virtual-Access1
10.0.0.0/8 is variably subnetted, 4 subnets, 2 masks
D 10.1.12.1/32 [90/5145600] via 10.1.23.2, 00:30:43, Virtual-Access1
D 10.1.12.0/24 [90/5145600] via 10.1.23.2, 00:25:02, Virtual-Access1
R3#trace 1.1.1.1 so lo0
Type escape sequence to abort.
Tracing the route to 1.1.1.1
1 10.1.23.2 4 msec 0 msec 4 msec
2 10.1.12.1 4 msec * 16 msec
R3#trace 2.2.2.2 so lo0
Type escape sequence to abort.
Tracing the route to 2.2.2.2
1 10.1.23.2 0 msec * 24 msec
So the Baseline is ok, we have reachability only using the Frame Relay links, now Lets cause the PVC between R1 and R2 to fail. The routers are synched to the same time source so the timestamps will have meaning across routers:
R2#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R2(config)#int s0/0.21
R2(config-subif)#shut
R2(config-subif)#end
Mar 1 00:56:16.133: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 100: Neighbor 10.1.12.1 (Virtual-Access2) is down: holding time expired
Mar 1 00:56:25.593: %FR_EEK-5-FAILED: Interface Serial0/0.21 - DLCI 201
Mar 1 00:56:25.609: %LINK-3-UPDOWN: Interface Virtual-Access2, changed state to down
Mar 1 00:56:26.605: %LINEPROTO-5-UPDOWN: Line protocol on Interface Virtual-Access2, changed state to down
EIGRP detects the failure before FREEK does and the Virtual-Access interface is brought down
On R1 we see:
Mar 1 00:56:15.779: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 100: Neighbor 10.1.12.2 (Virtual-Access1) is down: holding time expired
Mar 1 00:56:25.823: %FR_EEK-5-FAILED: Interface Serial0/0.12 - DLCI 102
Mar 1 00:56:25.835: %LINK-3-UPDOWN: Interface Virtual-Access1, changed state to down
Mar 1 00:56:26.835: %LINEPROTO-5-UPDOWN: Line protocol on Interface Virtual-Access1, changed state to down
Mar 1 00:56:28.275: %LINK-3-UPDOWN: Interface Serial0/1, changed state to up
Mar 1 00:56:29.283: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/1, changed state to up
Mar 1 00:56:41.811: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 100: Neighbor 10.1.13.3 (Serial0/1) is up: new adjacency
EIGRP detects the failure before FREEK does and the Virtual-Access interface is brought down, and the HDLC interface (S0/1) is brought up and an EIGRP adjacency is formed
On R3 we see:
Mar 1 00:56:39.061: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/1, changed state to up
Mar 1 00:56:41.772: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 100: Neighbor 10.1.13.1 (Serial0/1) is up: new adjacency
So we can definitely know that when FREEK failed R1 brought int S0/1 up
R1#sh ip int bri | exc unas
Interface IP-Address OK? Method Status Protocol
Serial0/1 10.1.13.1 YES NVRAM up up
Virtual-Access1 10.1.12.1 YES TFTP down down
Virtual-Template1 10.1.12.1 YES NVRAM down down
Loopback0 1.1.1.1 YES NVRAM up up
R1#sh ip route eigrp
D* 0.0.0.0/0 [90/2297856] via 10.1.13.3, 00:02:49, Serial0/1
We're getting the default from R3, and all traffic is traversing the HDLC link via R3
R1#trace 2.2.2.2 so lo0
Type escape sequence to abort.
Tracing the route to 2.2.2.2
1 10.1.13.3 4 msec 4 msec 0 msec
2 10.1.23.2 4 msec * 20 msec
R1#trace 3.3.3.3 so lo0
Type escape sequence to abort.
Tracing the route to 3.3.3.3
1 10.1.13.3 0 msec * 24 msec
Lets check R2
R2#sh ip int bri | exc unas
Interface IP-Address OK? Method Status Protocol
Virtual-Access1 10.1.23.2 YES TFTP up up
Virtual-Template1 10.1.12.2 YES NVRAM down down
Virtual-Access2 10.1.12.2 YES TFTP down down
Virtual-Template3 10.1.23.2 YES NVRAM down down
Loopback0 2.2.2.2 YES NVRAM up up
R2#sh ip route eigrp
1.0.0.0/32 is subnetted, 1 subnets
D 1.1.1.1 [90/4857856] via 10.1.23.3, 00:04:47, Virtual-Access1
3.0.0.0/32 is subnetted, 1 subnets
D 3.3.3.3 [90/2713600] via 10.1.23.3, 01:23:38, Virtual-Access1
10.0.0.0/8 is variably subnetted, 3 subnets, 2 masks
D 10.1.13.0/24 [90/4729856] via 10.1.23.3, 00:04:50, Virtual-Access1
R2#trace 1.1.1.1 so lo0
Type escape sequence to abort.
Tracing the route to 1.1.1.1
1 10.1.23.3 4 msec 0 msec 4 msec
2 10.1.13.1 4 msec * 4 msec
R2#trace 3.3.3.3 so lo0
Type escape sequence to abort.
Tracing the route to 3.3.3.3
1 10.1.23.3 4 msec * 4 msec
Yes, understandably with the PVC to R1 down, R2 can only go through R3, and speaking of R3:
R3#sh ip int brie | exc unas
Interface IP-Address OK? Method Status Protocol
Serial0/1 10.1.13.3 YES NVRAM up up
Virtual-Access1 10.1.23.3 YES TFTP up up
Virtual-Template1 10.1.23.3 YES NVRAM down down
Loopback0 3.3.3.3 YES NVRAM up up
R3#sh ip route eigrp
1.0.0.0/32 is subnetted, 1 subnets
D 1.1.1.1 [90/2297856] via 10.1.13.1, 00:06:20, Serial0/1
2.0.0.0/32 is subnetted, 1 subnets
D 2.2.2.2 [90/2713600] via 10.1.23.2, 00:44:12, Virtual-Access1
R3#trace 1.1.1.1 so lo0
Type escape sequence to abort.
Tracing the route to 1.1.1.1
1 10.1.13.1 0 msec * 12 msec
R3#trace 2.2.2.2 so lo0
Type escape sequence to abort.
Tracing the route to 2.2.2.2
1 10.1.23.2 0 msec * 24 msec
All good. Lets verify that things return to normal once the PVC between R1 and R2 is restored
R2#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R2(config)#int s0/0.21
R2(config-subif)#no shut
R2(config-subif)#end
Looking at R1 we see:
Mar 1 01:09:37.623: %LINK-3-UPDOWN: Interface Virtual-Access1, changed state to up
Mar 1 01:09:38.319: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 100: Neighbor 10.1.13.3 (Serial0/1) is down: interface down
Mar 1 01:09:38.623: %LINEPROTO-5-UPDOWN: Line protocol on Interface Virtual-Access1, changed state to up
Mar 1 01:09:40.279: %LINK-5-CHANGED: Interface Serial0/1, changed state to standby mode
Mar 1 01:09:40.931: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 100: Neighbor 10.1.12.2 (Virtual-Access1) is up: new adjacency
Mar 1 01:09:41.279: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/1, changed state to down
R2 shows:
Mar 1 01:09:37.595: %LINK-3-UPDOWN: Interface Virtual-Access2, changed state to up
Mar 1 01:09:38.615: %LINEPROTO-5-UPDOWN: Line protocol on Interface Virtual-Access2, changed state to up
Mar 1 01:09:40.899: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 100: Neighbor 10.1.12.1 (Virtual-Access2) is up: new adjacency
And R3:
Mar 1 01:09:52.748: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 100: Neighbor 10.1.13.1 (Serial0/1) is down: holding time expired
Mar 1 01:10:09.095: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/1, changed state to down
We can see that once the PPPoFR Link between R1 and R2 comes up quickly and that R1 S0/1 is put into standby mode and the direct link between R1 and R3 is brought back down
R1#sh ip route eigrp
2.0.0.0/32 is subnetted, 1 subnets
D 2.2.2.2 [90/2713600] via 10.1.12.2, 00:04:10, Virtual-Access1
3.0.0.0/32 is subnetted, 1 subnets
D 3.3.3.3 [90/5273600] via 10.1.12.2, 00:04:10, Virtual-Access1
10.0.0.0/8 is variably subnetted, 4 subnets, 2 masks
D 10.1.23.0/24 [90/5145600] via 10.1.12.2, 00:04:10, Virtual-Access1
D 10.1.23.3/32 [90/5145600] via 10.1.12.2, 00:04:10, Virtual-Access1
R1#ping 2.2.2.2 so lo0
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 2.2.2.2, timeout is 2 seconds:
Packet sent with a source address of 1.1.1.1
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/4 ms
R1#ping 3.3.3.3 so lo0
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 3.3.3.3, timeout is 2 seconds:
Packet sent with a source address of 1.1.1.1
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/4 ms
R2#sh ip route eigrp
1.0.0.0/32 is subnetted, 1 subnets
D 1.1.1.1 [90/2713600] via 10.1.12.1, 00:04:53, Virtual-Access2
3.0.0.0/32 is subnetted, 1 subnets
D 3.3.3.3 [90/2713600] via 10.1.23.3, 01:36:43, Virtual-Access1
R2#ping 3.3.3.3 so lo0
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 3.3.3.3, timeout is 2 seconds:
Packet sent with a source address of 2.2.2.2
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/4 ms
R3#sh ip route eigrp
1.0.0.0/32 is subnetted, 1 subnets
D 1.1.1.1 [90/5273600] via 10.1.23.2, 00:05:33, Virtual-Access1
2.0.0.0/32 is subnetted, 1 subnets
D 2.2.2.2 [90/2713600] via 10.1.23.2, 00:56:37, Virtual-Access1
10.0.0.0/8 is variably subnetted, 4 subnets, 2 masks
D 10.1.12.1/32 [90/5145600] via 10.1.23.2, 00:05:48, Virtual-Access1
D 10.1.12.0/24 [90/5145600] via 10.1.23.2, 00:05:33, Virtual-Access1
So that's backup interfaces... Of course, there are other ways to implement similar concepts.... EEM for example:
On R1 we insert the config below - note: we need to initially shutdown S0/1 because we don't want both interfaces to be active at the same time, and normally subinterface status is not logged, since we want match upon S0/0.12 going up or down, we will turn that on
interface Serial0/1
shutdown
!
interface Serial0/0.12
no backup interface Serial0/1
logging event subif-link-status
!
event manager applet FR-Down
event syslog pattern "%LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/0.12, changed state to down"
action 1.0 cli command "enable"
action 2.0 cli command "conf t"
action 3.0 cli command "int s0/1"
action 4.0 cli command "no shut"
event manager applet FR-Up
event syslog pattern "%LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/0.12, changed state to up"
action 1.0 cli command "enable"
action 2.0 cli command "conf t"
action 3.0 cli command "int s0/1"
action 4.0 cli command "shut"
!
Now let's test it:
R1#sh ip int bri | exc unas
Interface IP-Address OK? Method Status Protocol
Serial0/1 10.1.13.1 YES NVRAM administratively down down
Virtual-Access1 10.1.12.1 YES TFTP up up
Virtual-Template1 10.1.12.1 YES NVRAM down down
Loopback0 1.1.1.1 YES NVRAM up up
R2#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R2(config)#int s0/0.21
R2(config-subif)#shut
R2(config-subif)#end
On R1 we see:
Mar 1 04:51:58.346: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 100: Neighbor 10.1.12.2 (Virtual-Access1) is down: holding time expired
Mar 1 04:52:13.382: %FR_EEK-5-FAILED: Interface Serial0/0.12 - DLCI 102
Mar 1 04:52:13.390: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/0.12, changed state to down
Mar 1 04:52:13.426: %LINK-3-UPDOWN: Interface Virtual-Access1, changed state to down
Mar 1 04:52:13.790: %SYS-5-CONFIG_I: Configured from console by vty0
Mar 1 04:52:14.426: %LINEPROTO-5-UPDOWN: Line protocol on Interface Virtual-Access1, changed state to down
Mar 1 04:52:15.730: %LINK-3-UPDOWN: Interface Serial0/1, changed state to up
Mar 1 04:52:16.738: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/1, changed state to up
Mar 1 04:52:21.338: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 100: Neighbor 10.1.13.3 (Serial0/1) is up: new adjacency
It's a similar turn of events as to what happens with the backup interface, however we see that a configuration change was made (this is where S0/1 has the no shut applied)
R1#sh ip int bri | ex unas
Interface IP-Address OK? Method Status Protocol
Serial0/1 10.1.13.1 YES NVRAM up up
Virtual-Access1 10.1.12.1 YES TFTP down down
Virtual-Template1 10.1.12.1 YES NVRAM down down
Loopback0 1.1.1.1 YES NVRAM up up
R1#sh ip route eigrp
D* 0.0.0.0/0 [90/2297856] via 10.1.13.3, 00:00:57, Serial0/1
So far, so good - lets restore the PVC
R2#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R2(config)#int s0/0.21
R2(config-subif)#no shut
R2(config-subif)#end
On R1 we see:
Mar 1 04:54:07.366: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/0.12, changed state to up
Mar 1 04:54:07.410: %LINK-3-UPDOWN: Interface Virtual-Access1, changed state to up
Mar 1 04:54:07.654: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 100: Neighbor 10.1.13.3 (Serial0/1) is down: interface down
Mar 1 04:54:07.694: %SYS-5-CONFIG_I: Configured from console by vty0
Mar 1 04:54:07.926: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 100: Neighbor 10.1.12.2 (Virtual-Access1) is up: new adjacency
Mar 1 04:54:08.418: %LINEPROTO-5-UPDOWN: Line protocol on Interface Virtual-Access1, changed state to up
Mar 1 04:54:09.634: %LINK-5-CHANGED: Interface Serial0/1, changed state to administratively down
Mar 1 04:54:10.634: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/1, changed state to down
R1#sh ip route eigrp
2.0.0.0/32 is subnetted, 1 subnets
D 2.2.2.2 [90/2713600] via 10.1.12.2, 00:00:56, Virtual-Access1
3.0.0.0/32 is subnetted, 1 subnets
D 3.3.3.3 [90/5273600] via 10.1.12.2, 00:00:56, Virtual-Access1
10.0.0.0/8 is variably subnetted, 4 subnets, 2 masks
D 10.1.23.0/24 [90/5145600] via 10.1.12.2, 00:00:56, Virtual-Access1
D 10.1.23.3/32 [90/5145600] via 10.1.12.2, 00:00:56, Virtual-Access1
Advantages to using the backup interface command:
1) In a show ip interface brief, you can see that the backup interface is in standby state (in the EEM config perhaps in the description you can put something there but thats it)
2) Less configuration, less chance of error
3) No modifications to the configuration - the EEM config keeps changing the config to shut/no shut interfaces
Advantage to using EEM:
1) The only thing EEM offers over the built in functionality is potentially more flexibility - with EEM could email you to let you know the PVC is down, clear counters or other things - this may be handy if you're not sitting in front of your syslog server
