OSPF Summarization is done only on ABR and ASBR routers in your OSPF domain, and use two completely different commands, but what if a router is an ABR and an ASBR?
For example, did you know that using the command “default-information originate …” you are telling the router to create a Type 5 LSA to be propagated throughout the network, thus turning that router into an ASBR?
Another very interesting fact I did not know – OSPF will not allow you to redistribute a static default route. It cannot be done.
Being that I have never knew either of these things that seem like fairly good questions for exam day, I wanted to give them a run for their money to see if that they are true:
R1(config)#ip route 0.0.0.0 0.0.0.0 null0
R1(config)#router ospf 1
R1(config-router)#redistribute static subnets
R1(config-router)#
ASR#2
[Resuming connection 2 to r2 … ]
R2#sh ip route
Gateway of last resort is not set
1.0.0.0/32 is subnetted, 1 subnets
O IA 1.1.1.1 [110/65] via 172.12.123.1, 00:00:11, Serial0/0
2.0.0.0/32 is subnetted, 1 subnets
C 2.2.2.2 is directly connected, Loopback2
3.0.0.0/32 is subnetted, 1 subnets
O IA 3.3.3.3 [110/65] via 172.12.123.3, 00:00:11, Serial0/0
172.12.0.0/24 is subnetted, 3 subnets
O IA 172.12.15.0 [110/65] via 172.12.123.1, 00:00:11, Serial0/0
C 172.12.23.0 is directly connected, FastEthernet0/0
C 172.12.123.0 is directly connected, Serial0/0
R2#sh ip ospf data
OSPF Router with ID (2.2.2.2) (Process ID 1)
Router Link States (Area 0)
Link ID ADV Router Age Seq# Checksum Link count
1.1.1.1 1.1.1.1 45 0x80000005 0x00DC9D 1
2.2.2.2 2.2.2.2 1013 0x80000004 0x009AD9 1
3.3.3.3 3.3.3.3 132 0x80000005 0x006008 1
Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
172.12.123.1 1.1.1.1 905 0x80000004 0x0023BE
Summary Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
1.1.1.1 1.1.1.1 1416 0x80000003 0x0043EE
2.2.2.2 2.2.2.2 1013 0x80000003 0x00F633
3.3.3.3 3.3.3.3 321 0x80000001 0x00AE75
172.12.15.0 1.1.1.1 1154 0x80000005 0x0072F9
172.12.23.0 2.2.2.2 696 0x80000001 0x000460
172.12.23.0 3.3.3.3 692 0x80000009 0x00D582
Nothing! I never knew that was a behavior before, so you HAVE to use the default-information originate command to propagate a static route even though it still uses a Type 5 LSA just like redistribution would have!!!
Keep that in mind on exam day, if you see redistribution in ospf of a static default route, that is beyond a red flag.
Now. Back to this about the default-information originate command making a router an ASBR, I don’t really want to assign a default route to the logical trash bin (null0), so I’m just going to add “always” so no static default route is needed:
R1(config)#router ospf 1
R1(config-router)#default-information originate always
R1(config-router)#
ASR#2
[Resuming connection 2 to r2 … ]
R2#sh ip route
Gateway of last resort is 172.12.123.1 to network 0.0.0.0
1.0.0.0/32 is subnetted, 1 subnets
O IA 1.1.1.1 [110/65] via 172.12.123.1, 00:07:10, Serial0/0
2.0.0.0/32 is subnetted, 1 subnets
C 2.2.2.2 is directly connected, Loopback2
3.0.0.0/32 is subnetted, 1 subnets
O IA 3.3.3.3 [110/65] via 172.12.123.3, 00:07:10, Serial0/0
172.12.0.0/24 is subnetted, 3 subnets
O IA 172.12.15.0 [110/65] via 172.12.123.1, 00:07:10, Serial0/0
C 172.12.23.0 is directly connected, FastEthernet0/0
C 172.12.123.0 is directly connected, Serial0/0
O*E2 0.0.0.0/0 [110/1] via 172.12.123.1, 00:00:12, Serial0/0
R2#
There we go, now R2 has a default route, and what appears to be an External Type 5 LSA route so I am guessing when I go back to R1:
R1(config-router)#do sh ip ospf
Routing Process “ospf 1” with ID 1.1.1.1
Start time: 00:00:18.800, Time elapsed: 01:39:06.588
Supports only single TOS(TOS0) routes
Supports opaque LSA
Supports Link-local Signaling (LLS)
Supports area transit capability
Event-log enabled, Maximum number of events: 1000, Mode: cyclic
It is an area border and autonomous system boundary router
Redistributing External Routes from,
Router is not originating router-LSAs with maximum metric
The interesting thing here is that I’ve never seen any other protocol leave the “Redistributing External Routes from” field empty, and it sure is both an ABR and an ASBR now.
So can I do both types of Summarization now? Lets break some stuff and find out! To be clear on how real this is getting:
R1(config-if)#do sh ip int bri
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 unassigned YES NVRAM administratively down down
FastEthernet0/1 172.12.15.1 YES NVRAM up up
Serial0/0/0 172.12.123.1 YES NVRAM up up
Serial0/0/1 100.100.100.1 YES NVRAM administratively down down
Loopback1 1.1.1.1 YES NVRAM up up
Loopback8 172.16.8.1 YES manual up up
Loopback9 172.16.9.1 YES manual up up
Loopback10 172.16.10.1 YES manual up up
Loopback11 172.16.11.1 YES manual up up
Loopback101 100.1.0.1 YES manual up up
Loopback102 100.2.0.1 YES manual up up
Loopback103 100.3.0.1 YES manual up up
Loopback104 100.4.0.1 YES manual up up
Loopback105 100.5.0.1 YES manual up up
Loopback106 100.6.0.1 YES manual up up
Loopback107 100.7.0.1 YES manual up up
Summary Address = 172.16.8.0 /22
Summary Address = 100.0.0.0 /13
Now for the ABR, the routes need to be put in via the “network” command, being that you are specifying the Area containing the routes, so they need to be entered into OSPF in the same Area.
I was actually just cursing looking at that for some reason thinking the Loopback # dictated the Area # or something, but I got it now lets give it a go here:
R1(config-if)#router ospf 1
R1(config-router)#network 100.1.0.0 0.0.255.255 area 100
R1(config-router)#network 100.2.0.0 0.0.255.255 area 100
R1(config-router)#network 100.3.0.0 0.0.255.255 area 100
R1(config-router)#network 100.4.0.0 0.0.255.255 area 100
R1(config-router)#network 100.5.0.0 0.0.255.255 area 100
R1(config-router)#network 100.6.0.0 0.0.255.255 area 100
R1(config-router)#network 100.7.0.0 0.0.255.255 area 100
R1(config-router)#area 100 range 100.0.0.0 255.248.0.0 ?
advertise Advertise this range (default)
cost User specified metric for this range
not-advertise DoNotAdvertise this range
<cr>
R1(config-router)#area 100 range 100.0.0.0 255.248.0.0
R1(config-router)#
Cost can be defined as a modifier to the command as highlighted in red there, otherwise OSPF will use the best Prefix’s Cost value for the Summary Route which I think should be left alone unless you have a reason to change it.
So lets take a look at R2’s OSPF route table to verify we have one type of summarization at work:
R2#sh ip route ospf
1.0.0.0/32 is subnetted, 1 subnets
O IA 1.1.1.1 [110/65] via 172.12.123.1, 00:43:36, Serial0/0
100.0.0.0/13 is subnetted, 1 subnets
O IA 100.0.0.0 [110/65] via 172.12.123.1, 00:16:54, Serial0/0
3.0.0.0/32 is subnetted, 1 subnets
O IA 3.3.3.3 [110/65] via 172.12.123.3, 00:43:36, Serial0/0
172.12.0.0/24 is subnetted, 3 subnets
O IA 172.12.15.0 [110/65] via 172.12.123.1, 00:43:36, Serial0/0
O*E2 0.0.0.0/0 [110/1] via 172.12.123.1, 00:16:49, Serial0/0
R2#sh ip ospf data
OSPF Router with ID (2.2.2.2) (Process ID 1)
Router Link States (Area 0)
Link ID ADV Router Age Seq# Checksum Link count
1.1.1.1 1.1.1.1 750 0x80000006 0x00DA9E 1
2.2.2.2 2.2.2.2 1590 0x80000005 0x0098DA 1
3.3.3.3 3.3.3.3 920 0x80000006 0x005E09 1
Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
172.12.123.1 1.1.1.1 1487 0x80000005 0x0021BF
Summary Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
1.1.1.1 1.1.1.1 1971 0x80000004 0x0041EF
2.2.2.2 2.2.2.2 1590 0x80000004 0x00F434
3.3.3.3 3.3.3.3 920 0x80000002 0x00AC76
100.0.0.0 1.1.1.1 1028 0x80000001 0x00409A
172.12.15.0 1.1.1.1 1730 0x80000006 0x0070FA
172.12.23.0 2.2.2.2 1347 0x80000002 0x000261
172.12.23.0 3.3.3.3 1421 0x8000000A 0x00D383
So it is being advertised as an Inter-Area (Type 3 LSA) route as can be seen both in the IP route table, as it should because this is the ABR way to summarize routes. Ahem.
Also if you want to get granular with how you look at the LSA Database, to see this summary route for example, you can type in as follows:
R2#sh ip ospf data summ 100.0.0.0
OSPF Router with ID (2.2.2.2) (Process ID 1)
Summary Net Link States (Area 0)
Routing Bit Set on this LSA
LS age: 1347
Options: (No TOS-capability, DC, Upward)
LS Type: Summary Links(Network)
Link State ID: 100.0.0.0 (summary Network Number)
Advertising Router: 1.1.1.1
LS Seq Number: 80000001
Checksum: 0x409A
Length: 28
Network Mask: /13
TOS: 0 Metric: 1
This command will give you a ton of output, like the Database itself, except with details which makes it incredibly hard to dig through if you have a decent amount of Areas it is reporting all these details before.
However, I did want you to see, you can verify if a route is a Summary from the LSA Database – And that is a good thing to know. You can also look at sections of it with “sh ip ospf data summ” and so on but I won’t flood the page with all that output.
So all this ABR Summarization is all fine and good you say, but what about ASBR Summarization? I am glad you asked.
I am not sure if it requires the networks to be entered via the “network” command, so I’ll test out whether they need to be added, lets take a look:
R1(config-router)#summary-address 172.16.8.0 255.255.252.0
R1(config-router)#
ASR#2
[Resuming connection 2 to r2 … ]
R2#sh ip route ospf
1.0.0.0/32 is subnetted, 1 subnets
O IA 1.1.1.1 [110/65] via 172.12.123.1, 00:55:55, Serial0/0
100.0.0.0/13 is subnetted, 1 subnets
O IA 100.0.0.0 [110/65] via 172.12.123.1, 00:29:13, Serial0/0
3.0.0.0/32 is subnetted, 1 subnets
O IA 3.3.3.3 [110/65] via 172.12.123.3, 00:55:55, Serial0/0
172.12.0.0/24 is subnetted, 3 subnets
O IA 172.12.15.0 [110/65] via 172.12.123.1, 00:55:55, Serial0/0
O*E2 0.0.0.0/0 [110/1] via 172.12.123.1, 00:29:08, Serial0/0
R2#
Well that stinks. Let me add the routes via “network” on R1 and try that again:
R1(config-router)#
R1(config-router)#network 172.16.8.0 0.0.0.255 area 51
R1(config-router)#network 172.16.9.0 0.0.0.255 area 51
R1(config-router)#network 172.16.10.0 0.0.0.255 area 51
R1(config-router)#network 172.16.11.0 0.0.0.255 area 51
R1(config-router)#
ASR#2
[Resuming connection 2 to r2 … ]
R2#sh ip route ospf
1.0.0.0/32 is subnetted, 1 subnets
O IA 1.1.1.1 [110/65] via 172.12.123.1, 00:58:21, Serial0/0
100.0.0.0/13 is subnetted, 1 subnets
O IA 100.0.0.0 [110/65] via 172.12.123.1, 00:31:40, Serial0/0
3.0.0.0/32 is subnetted, 1 subnets
O IA 3.3.3.3 [110/65] via 172.12.123.3, 00:58:21, Serial0/0
172.12.0.0/24 is subnetted, 3 subnets
O IA 172.12.15.0 [110/65] via 172.12.123.1, 00:58:21, Serial0/0
172.16.0.0/32 is subnetted, 4 subnets
O IA 172.16.9.1 [110/65] via 172.12.123.1, 00:00:11, Serial0/0
O IA 172.16.8.1 [110/65] via 172.12.123.1, 00:00:21, Serial0/0
O IA 172.16.11.1 [110/65] via 172.12.123.1, 00:00:01, Serial0/0
O IA 172.16.10.1 [110/65] via 172.12.123.1, 00:00:11, Serial0/0
O*E2 0.0.0.0/0 [110/1] via 172.12.123.1, 00:00:06, Serial0/0
R2#
Now things are getting interesting, because if I remove the summarization R1 is doing as an ABR, will the summarization command as an ASBR kick into action? Lets see:
R1(config-router)#no area 100 range 100.0.0.0 255.248.0.0
R1(config-router)#
ASR#2
[Resuming connection 2 to r2 … ]
R2#sh ip route ospf
1.0.0.0/32 is subnetted, 1 subnets
O IA 1.1.1.1 [110/65] via 172.12.123.1, 01:01:04, Serial0/0
100.0.0.0/32 is subnetted, 7 subnets
O IA 100.5.0.1 [110/65] via 172.12.123.1, 00:00:12, Serial0/0
O IA 100.4.0.1 [110/65] via 172.12.123.1, 00:00:12, Serial0/0
O IA 100.7.0.1 [110/65] via 172.12.123.1, 00:00:12, Serial0/0
O IA 100.6.0.1 [110/65] via 172.12.123.1, 00:00:12, Serial0/0
O IA 100.1.0.1 [110/65] via 172.12.123.1, 00:00:12, Serial0/0
O IA 100.3.0.1 [110/65] via 172.12.123.1, 00:00:12, Serial0/0
O IA 100.2.0.1 [110/65] via 172.12.123.1, 00:00:12, Serial0/0
3.0.0.0/32 is subnetted, 1 subnets
O IA 3.3.3.3 [110/65] via 172.12.123.3, 01:01:04, Serial0/0
172.12.0.0/24 is subnetted, 3 subnets
O IA 172.12.15.0 [110/65] via 172.12.123.1, 01:01:04, Serial0/0
172.16.0.0/32 is subnetted, 4 subnets
O IA 172.16.9.1 [110/65] via 172.12.123.1, 00:02:54, Serial0/0
O IA 172.16.8.1 [110/65] via 172.12.123.1, 00:03:04, Serial0/0
O IA 172.16.11.1 [110/65] via 172.12.123.1, 00:02:45, Serial0/0
O IA 172.16.10.1 [110/65] via 172.12.123.1, 00:02:55, Serial0/0
O*E2 0.0.0.0/0 [110/1] via 172.12.123.1, 00:00:08, Serial0/0
R2#
No it did not, so I am wondering if perhaps order of commands comes into play here, as I configured the summary-address of routes that weren’t in the OSPF config yet.
So after a lot of failure with trying to redistribute an actual static route to make it an official “ASBR”, remove and re-add commands, I caved and watched the Summarization portion of my training video for summary address and I’ll be damned if this can’t ONLY be done by the ASBR because you redistribute the friggin connected routes! Gah!
R1#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#router ospf 1
R1(config-router)#no network 172.16.8.0 0.0.0.255 area 51
R1(config-router)#no network 172.16.9.0 0.0.0.255 area 51
R1(config-router)#no network 172.16.10.0 0.0.0.255 area 51
R1(config-router)#no network 172.16.11.0 0.0.0.255 area 51
R1(config-router)#redistribute connected subnets
R1(config-router)#area 100 range 100.0.0.0 255.248.0.0
R1(config-router)#summary-address 172.16.8.0 255.255.252.0
R1(config-router)#
Now for the moment of truth (I removed 172.x routes from OSPF):
R2#sh ip route ospf
1.0.0.0/32 is subnetted, 1 subnets
O IA 1.1.1.1 [110/65] via 172.12.123.1, 01:31:24, Serial0/0
100.0.0.0/13 is subnetted, 1 subnets
O IA 100.0.0.0 [110/65] via 172.12.123.1, 00:01:19, Serial0/0
3.0.0.0/32 is subnetted, 1 subnets
O IA 3.3.3.3 [110/65] via 172.12.123.3, 01:31:24, Serial0/0
172.12.0.0/24 is subnetted, 3 subnets
O IA 172.12.15.0 [110/65] via 172.12.123.1, 01:31:24, Serial0/0
172.16.0.0/22 is subnetted, 1 subnets
O E2 172.16.8.0 [110/20] via 172.12.123.1, 00:01:14, Serial0/0
O*E2 0.0.0.0/0 [110/1] via 172.12.123.1, 00:01:14, Serial0/0
R2#
FINALLY!! So that is why summary-address can only be done on the ASBR, because you need to redistribute the sequential routes to be summarized before entering the command to summarize them!
Also we now know that we can issue both commands on R1 as an ABR, and an ASBR with no problems.
HOWEVER WE ARE NOT DONE YET, AS WE HAVEN’T GONE INTO THE SECOND WAY OSPF CAN CREATE A STATIC ROUTE – AND THIS TIME IT AIN’T A TYPE 5 LSA!
The other way is to make an Area a Stub Area. By doing this, the Stub creates a default route for itself out of the network, does not allow LSA Type 5’s into the Area at all actually, so the default route created in this case is a Summary Type 3 LSA.
Lets look at Area 34 quick to wrap this one up:
R3(config-router)#area 34 stub
R3(config-router)#
ASR#4
[Resuming connection 4 to r4 … ]
R4#
R4#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R4(config)#router ospf 1
R4(config-router)#area 34 stub
That is all there is to the stub command, and the default route can be seen here, but there is still a LOT of clutter from Inter-Area routes:
R4(config-router)#do sh ip route ospf
Gateway of last resort is 172.12.34.3 to network 0.0.0.0
O*IA 0.0.0.0/0 [110/2] via 172.12.34.3, 00:00:15, FastEthernet0/1
1.0.0.0/32 is subnetted, 1 subnets
O IA 1.1.1.1 [110/66] via 172.12.34.3, 00:00:15, FastEthernet0/1
2.0.0.0/32 is subnetted, 1 subnets
O IA 2.2.2.2 [110/66] via 172.12.34.3, 00:00:15, FastEthernet0/1
3.0.0.0/32 is subnetted, 1 subnets
O IA 3.3.3.3 [110/2] via 172.12.34.3, 00:00:15, FastEthernet0/1
100.0.0.0/13 is subnetted, 1 subnets
O IA 100.0.0.0 [110/66] via 172.12.34.3, 00:00:15, FastEthernet0/1
172.12.0.0/16 is variably subnetted, 5 subnets, 2 masks
O IA 172.12.15.0/24 [110/66] via 172.12.34.3, 00:00:15, FastEthernet0/1
O IA 172.12.23.0/24 [110/2] via 172.12.34.3, 00:00:15, FastEthernet0/1
O IA 172.12.123.0/24 [110/65] via 172.12.34.3, 00:00:15, FastEthernet0/1
R4(config-router)#
In the LSDB under the Area 34 Summary Header we can see the route there as well:
Summary Net Link States (Area 34)
Link ID ADV Router Age Seq# Checksum
0.0.0.0 3.3.3.3 320 0x80000001 0x0057DA
1.1.1.1 3.3.3.3 320 0x80000001 0x00AB42
2.2.2.2 3.3.3.3 320 0x80000001 0x007D6C
3.3.3.3 3.3.3.3 320 0x80000001 0x00CC59
100.0.0.0 3.3.3.3 320 0x80000001 0x00A4EF
172.12.15.0 3.3.3.3 320 0x80000001 0x00DE4B
172.12.23.0 3.3.3.3 320 0x80000001 0x00045E
172.12.123.0 3.3.3.3 320 0x80000001 0x002C92
Now the thing that kind of amazes me, is the only verification command I could find outside of “show run” to verify this router is a stub router, was to do “sh ip ospf” and scroll all the way down under the Area 34 Header to find it:
Area 34
Number of interfaces in this area is 1
It is a stub area
Area has no authentication
SPF algorithm last executed 00:09:14.524 ago
SPF algorithm executed 4 times
Area ranges are
Number of LSA 11. Checksum Sum 0x0528C8
Number of opaque link LSA 0. Checksum Sum 0x000000
Number of DCbitless LSA 0
Number of indication LSA 0
Number of DoNotAge LSA 0
Flood list length 0
So to finish this off, lets make it a total stub, and get rid of those Inter-Area routes all together:
R3(config-router)#no area 34 stub
R3(config-router)#area 34 stub no-summary
R3(config-router)#
ASR#4
[Resuming connection 4 to r4 … ]
*May 19 00:03:42.155: %OSPF-5-ADJCHG: Process 1, Nbr 3.3.3.3 on FastEthernet0/1 from LOADING to FULL, Loading Done
R4#sh ip route ospf
Gateway of last resort is 172.12.34.3 to network 0.0.0.0
O*IA 0.0.0.0/0 [110/2] via 172.12.34.3, 00:12:49, FastEthernet0/1
R4#
So lets see if waaaay across the Topology R5 can still ping 4.4.4.4:
R5#ping 4.4.4.4
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 4.4.4.4, timeout is 2 seconds:
U.U.U
Success rate is 0 percent (0/5)
R5#traceroute 4.4.4.4
Type escape sequence to abort.
Tracing the route to 4.4.4.4
1 172.12.15.1 0 msec 0 msec 4 msec
2 172.12.15.1 !H * !H
R5#
That was interesting traceroute traffic, upon looking at R1, it does have the network 172.12.34.0 in its Summary Type 3 LSA’s, but no Area 34 or Area 4 at all in its LSDB. However I know what’s going on here, as 4.4.4.4 belong to Area 4 which to Area 34 would be blocked as an Inter-Area route, so if we do this:
R4(config)#router ospf 1
R4(config-router)#no network 4.4.4.4 0.0.0.0 area 4
R4(config-router)#network 4.4.4.4 0.0.0.0 area 34
R4(config-router)#
Then we should now be able to do this:
R5#ping 4.4.4.4
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 4.4.4.4, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 64/65/69 ms
R5#
There we go, logical thinking isn’t always easy, but it does usually work.
I have one very last thing to add to this and I am done on this topic, seriously.
It has to do with the default-information originate command, because you can actually set it to track a certain route, and if that route goes down OSPF “Poisons” the default route and removes it from route tables / LSDB’s.
Lets take a look at the configuration:
R1#conf t
R1(config)#int lo99
R1(config-if)#ip add 99.99.99.99 255.255.255.255
R1(config)#access-list 99 permit 99.99.99.99
R1(config)#route-map 99bananas permit 10
R1(config-route-map)#match ip add 99
R1(config-route-map)#route-map 99bananas permit 20
R1(config-route-map)#exit
R1(config)#router ospf 1
R1(config-router)#default-information originate always route-map 99bananas
R1(config-router)#
Adding this route-map to it will “track” that route, so if that route or interface goes bye bye, so does our default route! Lets see this in action:
R2#sh ip route
Gateway of last resort is 172.12.123.1 to network 0.0.0.0
1.0.0.0/32 is subnetted, 1 subnets
O IA 1.1.1.1 [110/65] via 172.12.123.1, 02:13:02, Serial0/0
2.0.0.0/32 is subnetted, 1 subnets
C 2.2.2.2 is directly connected, Loopback2
100.0.0.0/13 is subnetted, 1 subnets
O IA 100.0.0.0 [110/65] via 172.12.123.1, 00:42:57, Serial0/0
3.0.0.0/32 is subnetted, 1 subnets
O IA 3.3.3.3 [110/65] via 172.12.123.3, 02:13:02, Serial0/0
4.0.0.0/32 is subnetted, 1 subnets
O IA 4.4.4.4 [110/66] via 172.12.123.3, 00:09:41, Serial0/0
99.0.0.0/32 is subnetted, 1 subnets
O E2 99.99.99.99 [110/20] via 172.12.123.1, 00:05:35, Serial0/0
172.12.0.0/24 is subnetted, 4 subnets
O IA 172.12.34.0 [110/65] via 172.12.123.3, 00:18:17, Serial0/0
O IA 172.12.15.0 [110/65] via 172.12.123.1, 02:13:06, Serial0/0
C 172.12.23.0 is directly connected, FastEthernet0/0
C 172.12.123.0 is directly connected, Serial0/0
172.16.0.0/22 is subnetted, 1 subnets
O E2 172.16.8.0 [110/20] via 172.12.123.1, 00:09:38, Serial0/0
O*E2 0.0.0.0/0 [110/1] via 172.12.123.1, 00:01:02, Serial0/0
R2#
Now lets remove the loopback and see the havoc it wreaks:
R1(config)#no int lo99
R1(config)#
*May 19 01:32:13.539: %LINK-5-CHANGED: Interface Loopback99, changed state to administratively down
*May 19 01:32:14.539: %LINEPROTO-5-UPDOWN: Line protocol on Interface Loopback99, changed state to down
R1(config)#
ASR#2
[Resuming connection 2 to r2 … ]
R2#sh ip route
Gateway of last resort is not set
So that is something excellent to know for exam day and the real world, that your default routes can have dependencies or be conditional upon other routes being available.
Pretty cool stuff. Ok this post has gone on way too long, that its for these topics!
The DEVNET GRIND! 