OSPF pop question

phoeneous · July 2013

Just a lab I put together to prep for exam.

Given the following topology and routing tables, will R1 be able to ping any of R4's loopbacks? Justify your answer

.

See attached screenshot.

R1

R1#sh ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route


Gateway of last resort is not set


     1.0.0.0/32 is subnetted, 4 subnets
C       1.1.1.1 is directly connected, Loopback1
C       1.1.3.1 is directly connected, Loopback3
C       1.1.2.1 is directly connected, Loopback2
C       1.1.4.1 is directly connected, Loopback4
     2.0.0.0/32 is subnetted, 4 subnets
O IA    2.2.1.1 [110/65] via 192.168.255.2, 00:07:23, Serial0/0
O IA    2.2.3.1 [110/65] via 192.168.255.2, 00:07:23, Serial0/0
O IA    2.2.2.1 [110/65] via 192.168.255.2, 00:07:23, Serial0/0
O IA    2.2.4.1 [110/65] via 192.168.255.2, 00:07:24, Serial0/0
     3.0.0.0/32 is subnetted, 4 subnets
O IA    3.3.1.1 [110/65] via 192.168.255.6, 00:18:31, Serial0/1
O IA    3.3.3.1 [110/65] via 192.168.255.6, 00:18:31, Serial0/1
O IA    3.3.2.1 [110/65] via 192.168.255.6, 00:18:31, Serial0/1
O IA    3.3.4.1 [110/65] via 192.168.255.6, 00:18:31, Serial0/1
     4.0.0.0/32 is subnetted, 4 subnets
O IA    4.4.1.1 [110/75] via 192.168.255.6, 00:18:31, Serial0/1
                [110/75] via 192.168.255.2, 00:18:31, Serial0/0
O IA    4.4.3.1 [110/75] via 192.168.255.6, 00:18:31, Serial0/1
                [110/75] via 192.168.255.2, 00:18:31, Serial0/0
O IA    4.4.2.1 [110/75] via 192.168.255.6, 00:18:31, Serial0/1
                [110/75] via 192.168.255.2, 00:18:31, Serial0/0
O IA    4.4.4.1 [110/75] via 192.168.255.6, 00:18:31, Serial0/1
                [110/75] via 192.168.255.2, 00:18:31, Serial0/0
     10.0.0.0/30 is subnetted, 2 subnets
O IA    10.0.100.4 [110/74] via 192.168.255.6, 00:18:31, Serial0/1
O IA    10.0.100.0 [110/74] via 192.168.255.2, 00:18:31, Serial0/0
     192.168.255.0/30 is subnetted, 2 subnets
C       192.168.255.4 is directly connected, Serial0/1
C       192.168.255.0 is directly connected, Serial0/0
R1#

R2

R2#sh ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route


Gateway of last resort is not set


     2.0.0.0/32 is subnetted, 4 subnets
C       2.2.1.1 is directly connected, Loopback1
C       2.2.3.1 is directly connected, Loopback3
C       2.2.2.1 is directly connected, Loopback2
C       2.2.4.1 is directly connected, Loopback4
     3.0.0.0/32 is subnetted, 4 subnets
O IA    3.3.1.1 [110/129] via 192.168.255.1, 00:00:31, Serial0/0
O IA    3.3.3.1 [110/129] via 192.168.255.1, 00:00:31, Serial0/0
O IA    3.3.2.1 [110/129] via 192.168.255.1, 00:00:31, Serial0/0
O IA    3.3.4.1 [110/129] via 192.168.255.1, 00:00:32, Serial0/0
     4.0.0.0/32 is subnetted, 4 subnets
O       4.4.1.1 [110/11] via 10.0.100.1, 00:00:32, FastEthernet0/0
O       4.4.3.1 [110/11] via 10.0.100.1, 00:00:32, FastEthernet0/0
O       4.4.2.1 [110/11] via 10.0.100.1, 00:00:33, FastEthernet0/0
O       4.4.4.1 [110/11] via 10.0.100.1, 00:00:33, FastEthernet0/0
     10.0.0.0/30 is subnetted, 2 subnets
O       10.0.100.4 [110/20] via 10.0.100.1, 00:00:33, FastEthernet0/0
C       10.0.100.0 is directly connected, FastEthernet0/0
     192.168.255.0/30 is subnetted, 2 subnets
O       192.168.255.4 [110/128] via 192.168.255.1, 00:00:33, Serial0/0
C       192.168.255.0 is directly connected, Serial0/0

R3

R3#sh ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route


Gateway of last resort is not set


     2.0.0.0/32 is subnetted, 4 subnets
O IA    2.2.1.1 [110/129] via 192.168.255.5, 00:00:37, Serial0/0
O IA    2.2.3.1 [110/129] via 192.168.255.5, 00:00:37, Serial0/0
O IA    2.2.2.1 [110/129] via 192.168.255.5, 00:00:37, Serial0/0
O IA    2.2.4.1 [110/129] via 192.168.255.5, 00:00:37, Serial0/0
     3.0.0.0/32 is subnetted, 4 subnets
C       3.3.1.1 is directly connected, Loopback1
C       3.3.3.1 is directly connected, Loopback3
C       3.3.2.1 is directly connected, Loopback2
C       3.3.4.1 is directly connected, Loopback4
     4.0.0.0/32 is subnetted, 4 subnets
O       4.4.1.1 [110/11] via 10.0.100.5, 00:12:32, FastEthernet0/0
O       4.4.3.1 [110/11] via 10.0.100.5, 00:12:32, FastEthernet0/0
O       4.4.2.1 [110/11] via 10.0.100.5, 00:12:36, FastEthernet0/0
O       4.4.4.1 [110/11] via 10.0.100.5, 00:12:36, FastEthernet0/0
     10.0.0.0/30 is subnetted, 2 subnets
C       10.0.100.4 is directly connected, FastEthernet0/0
O       10.0.100.0 [110/20] via 10.0.100.5, 00:12:36, FastEthernet0/0
     192.168.255.0/30 is subnetted, 2 subnets
C       192.168.255.4 is directly connected, Serial0/0
O       192.168.255.0 [110/128] via 192.168.255.5, 00:11:49, Serial0/0

R4

R4#sh ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route


Gateway of last resort is not set


     2.0.0.0/32 is subnetted, 4 subnets
O IA    2.2.1.1 [110/11] via 10.0.100.2, 00:12:07, FastEthernet0/0
O IA    2.2.3.1 [110/11] via 10.0.100.2, 00:12:07, FastEthernet0/0
O IA    2.2.2.1 [110/11] via 10.0.100.2, 00:12:07, FastEthernet0/0
O IA    2.2.4.1 [110/11] via 10.0.100.2, 00:12:07, FastEthernet0/0
     3.0.0.0/32 is subnetted, 4 subnets
O IA    3.3.1.1 [110/11] via 10.0.100.6, 00:24:01, FastEthernet0/1
O IA    3.3.3.1 [110/11] via 10.0.100.6, 00:24:01, FastEthernet0/1
O IA    3.3.2.1 [110/11] via 10.0.100.6, 00:24:03, FastEthernet0/1
O IA    3.3.4.1 [110/11] via 10.0.100.6, 00:24:03, FastEthernet0/1
     4.0.0.0/32 is subnetted, 4 subnets
C       4.4.1.1 is directly connected, Loopback1
C       4.4.3.1 is directly connected, Loopback3
C       4.4.2.1 is directly connected, Loopback2
C       4.4.4.1 is directly connected, Loopback4
     5.0.0.0/32 is subnetted, 1 subnets
C       5.5.1.1 is directly connected, Loopback5
     10.0.0.0/30 is subnetted, 2 subnets
C       10.0.100.4 is directly connected, FastEthernet0/1
C       10.0.100.0 is directly connected, FastEthernet0/0
     192.168.255.0/30 is subnetted, 2 subnets
O IA    192.168.255.4 [110/74] via 10.0.100.6, 00:24:03, FastEthernet0/1
O IA    192.168.255.0 [110/74] via 10.0.100.2, 00:24:03, FastEthernet0/0

iamme4eva · July 2013

Unless I'm missing something, then yes. A standard ping (without specifying a source address) would pick one of the two interfaces as the source address (whichever it went out of - both routes are equal cost from R1). As the return path is there in R4's routing table then it should work. However, if you used a source address of one of R1's loopbacks then it wouldn't work.

Like I said, unless I'm missing something obvious?

atorven · July 2013

I would say no, just because R4 isn’t connected to Area 0, this is only from the information provided.

mrjoshuap · July 2013

R4 has no path back to R1's loopbacks. Since R1 has routes to R4's loopback I would have to say some sort of route filtering has been put into play somewhere. Claiming area 432 is a discontigous are would be irrelevant because R2 as well as R3 has got a physical serial interface in area 0 and as I said earlier R1 has a route to R4's loopbacks.

FloOz · July 2013

If they are sourced from the serial interfaces yes. If the are sourced from the loopbacks then no. R4 does not have R1s loopbacks in its table

iamme4eva · July 2013

mrjoshuap wrote: »

Since R1 has routes to R4's loopback I would have to say some sort of route filtering has been put into play somewhere.

Or there was no network statement on R1 to cover the loopbacks, so they aren't advertised.

atorven - not every router has to be connected to area 0. A router can be an internal router in any area and only have interfaces in that area. However, each area has to directly touch area 0 (barring virtual links), meaning that every ABR should have at least one interface in area 0.

phoeneous · July 2013

Keep em coming

No route filtering is in effect.

I can post configs if you'd like. I did this exercise to remind myself that sometimes you overlook the small details.

phoeneous · July 2013

atorven wrote: »

I would say no, just because R4 isn’t connected to Area 0, this is only from the information provided.

R4 is connected to area 0 through ABR's R2 and R3. The diagram is a bit misleading but if you cover the area 2 and area 3 circles with your hand, you'll see.

mrjoshuap · July 2013

Perhaps the loopback have the shutdown command issued under the interface config or as LAMME4EVA said "there was no network statement on R1 to cover the loopbacks, so they aren't advertised."

NetworkVeteran · July 2013

R4's loopback address is 4.4.4.1

4.4.1.1 is one of R4's loopback addresses (see R4 output)
R3 has a route to 4.4.1.1/32 via R4 (10.0.100.5)
R2 has a route to 4.4.1.1/32 via R4 (10.0.100.5)
R1 has a route to 4.4.1.1/32 via R2 (192.168.255.2) and R3 (192.168.255.6)

Routing is a hop-by-hop process, and all routers have a route in their table to 4.4.1.1, so YES R1 can indeed send a ping to R4. More interestingly, will it receive a reply? R1's loopback is not in R4's routing table. However, the 192.168.255.0/30 subnets are. And those in the R2/R3 tables as well since they are directly connected to them.

Final Answer:

YES! R1 should use S0/0 or S0/1 as the ping source.

NetworkVeteran · July 2013

atorven wrote: »

I would say no, just because R4 isn’t connected to Area 0, this is only from the information provided.

While you are correct R4 and R1 are in different areas, being in different OSPF areas doesn't normally prevent two devices from talking to one another. All that happens is R1 receives an "O IA" (from LSA type 3) route instead of an "O" route (from LSA type 1).

If the different areas were an issue--say R4's area wasn't connected to the backbone and there were no virtual link or GRE tunnel to compensate for this--then you wouldn't see the route to 4.4.1.1 in R1's routing table.

NetworkVeteran · July 2013

mrjoshuap wrote:

R4 has no path back to R1's loopbacks. Since R1 has routes to R4's loopback I would have to say some sort of route filtering has been put into play somewhere.

Route filtering typically occurs on ASBRs or ABRs in OSPF, and even R2/R3 (same area) don't have routes to R1's loopbacks. While filtering is possible, it's much more likely OSPF was simply never enabled on R1's loopback interfaces.

phoeneous · July 2013

NetworkVeteran wrote: »

R4's loopback address is 4.4.4.1

4.4.1.1 is one of R4's loopback addresses (see R4 output)
R3 has a route to 4.4.1.1/32 via R4 (10.0.100.5)
R2 has a route to 4.4.1.1/32 via R4 (10.0.100.5)
R1 has a route to 4.4.1.1/32 via R2 (192.168.255.2) and R3 (192.168.255.6)

Routing is a hop-by-hop process, and all routers have a route in their table to 4.4.1.1, so YES R1 can indeed send a ping to R4. More interestingly, will it receive a reply? R1's loopback is not in R4's routing table. However, the 192.168.255.0/30 subnets are. And those in the R2/R3 tables as well since they are directly connected to them.

Final Answer:

YES! R1 should use S0/0 or S0/1 as the ping source.

There you go kiddos.

To answer anyones curiousity, R1 will be able to ping R4's loopback addresses provided that you use a standard ping command like ping 4.4.1.1. The ping will go through because R1 will use it's physical interface that learned the route by default to source the ping. And since R4 knows how to reach R1's serial interfaces then it will reply with it's source of 4.4.1.1. Now if you typed ping 4.4.1.1 source 1.1.1.1 then no, the ping would not go through because R4 doesnt have a route to R1's loopbacks simply because, as NV stated, R1 isn't advertising it's loopbacks into ospf.

iamme4eva · July 2013

Good. I didn't miss something obvious then.

FloOz · July 2013

FloOz wrote: »

If they are sourced from the serial interfaces yes. If the are sourced from the loopbacks then no. R4 does not have R1s loopbacks in its table

^ So I was right too

Danielh22185 · July 2013

Cool scenario! I was going on everything NetworkVeteran said but he answered before I came across this thread. I actually did a similar lab recently with the same situation but with BGP. It had me stumped but for a bit. My situation was dealing with the same scenario where loopbacks were not advertised by the routing protocol and trying to source a ping from the non-advertised loopbacks failed, even thought routing was present between the physical directly connected links.

This is neat though, if anybody else has free time to post up a scenario it would be beneficial to us all to discuss them. Great learning experience for sure!

NetworkVeteran · July 2013

This is neat though, if anybody else has free time to post up a scenario it would be beneficial to us all to discuss them. Great learning experience for sure!

Done.

http://www.techexams.net/forums/ccnp/91531-ospf-pop-questions-ii.html

phoeneous · July 2013

We should try to do a few of these every week. Sort of our own little practice tests. I'm cooking up an eigrp one right now.

OSPF pop question

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