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Ahmed Mamoon
Beginner
Beginner
‎07-27-2013 01:41 AM
‎07-27-2013 01:41 AM

If it will be called a routing loop ?

Dear Experts,

I have recently done CCNA and now preparing for CCNP Routing Exam. I came across a problem which I think is a looping issue. Below is my network

diagram.

Network Topology.jpg

R4 is sending LSA Type 3 about some loopbacks it received from R9. These loopbacks are from (172.16.101.0 to 172.16.107.0) However when I do SHOW IP OSPF DATABASE on R9, it show me below output about the Type 3 LSAs of Area 1.

                Summary Net Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
172.16.13.0     3.3.3.3         1074        0x80000001 0x001248
172.16.13.0     4.4.4.4         1068        0x80000001 0x00769F
172.16.14.0     3.3.3.3         1065        0x80000001 0x00898F
172.16.14.0     4.4.4.4         1113        0x80000001 0x000718
172.16.18.0     3.3.3.3         1080        0x80000001 0x005DB7
172.16.18.0     4.4.4.4         1052        0x80000002 0x003DD2
172.16.48.0     3.3.3.3         1080        0x80000001 0x008BDD
172.16.48.0     4.4.4.4         1113        0x80000001 0x008629
172.16.49.0     3.3.3.3         605         0x80000001 0x009B17
172.16.49.0     4.4.4.4         604         0x80000001 0x009662
172.16.80.0     3.3.3.3         1080        0x80000001 0x00CC06
172.16.80.0     4.4.4.4         1052        0x80000002 0x002AE3
172.16.81.0     3.3.3.3         1080        0x80000001 0x00C110
172.16.81.0     4.4.4.4         1052        0x80000002 0x001FED
172.16.82.0     3.3.3.3         1080        0x80000001 0x00B61A
172.16.82.0     4.4.4.4         1052        0x80000002 0x0014F7
172.16.83.0     3.3.3.3         1080        0x80000001 0x00AEA0
172.16.84.0     3.3.3.3         1080        0x80000001 0x00A3AA
172.16.85.0     3.3.3.3         1082        0x80000001 0x0098B4
172.16.101.0    3.3.3.3         540         0x80000001 0x006716
172.16.101.0    4.4.4.4         538         0x80000001 0x006261
172.16.102.0    3.3.3.3         540         0x80000001 0x005F9C
172.16.102.0    4.4.4.4         538         0x80000001 0x005AE7
172.16.103.0    3.3.3.3         540         0x80000001 0x00D5E4
172.16.103.0    4.4.4.4         538         0x80000001 0x00D030
172.16.104.0    3.3.3.3         540         0x80000001 0x008B0E
172.16.104.0    4.4.4.4         543         0x80000001 0x008659
172.16.105.0    3.3.3.3         544         0x80000001 0x00E0A7
172.16.105.0    4.4.4.4         543         0x80000001 0x00DBF2
172.16.106.0    3.3.3.3         544         0x80000001 0x000679
172.16.106.0    4.4.4.4         543         0x80000001 0x0001C4
172.16.107.0    3.3.3.3         544         0x80000001 0x001367
172.16.107.0    4.4.4.4         543         0x80000001 0x000EB2

Now it says that it have received the info of 172.16.101.0 to 172.16.107.0 both from ABR 3.3.3.3 (R3) and ABR 4.4.4.4 (R4). Whereas, as per my understanding only R4 is supposed to be the one advertising about these Routes and and not R3. So if this is a routing loop occured in the network ? Or it is normal. Can someone explain please. And if it is a routing loop, then how it can be handled ?

Regards,

Fraz

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Peter Paluch
Hall of Fame Cisco Employee Hall of Fame Cisco Employee
Hall of Fame Cisco Employee
In response to Ahmed Mamoon
‎07-27-2013 02:44 AM
‎07-27-2013 02:44 AM

Hi,

Oh I see.

In that case, the behavior is correct - and it is not a routing loop. What happens is that R4 learns about internal networks from Area 3 and because it is an ABR, it injects them as LSA-3 into other areas. In your case, those are areas 0 and 1.

R3 is an ABR between areas 0 and 1. Every ABR uses only LSA-3 received via area 0 to compute its routing table, so even if there are LSA-3 known via area 1, R3 ignores them (the same will be valid for R4). So R3 computes its routing table using internal information from area 0 and LSA-3 received via area 0, and it will inject the contents of the routing table (internal and intra-area routes) as LSA-3 to the area 1. That is the reason why R4 can see both its own LSA-3 and R3's LSA-3 about the same networks in its own LSDB. However, as mentioned earlier, an ABR uses only LSA-3 received in backbone area 0 to compute its routing table. While the LSA-3 received via area 1 are known to R4, they will be ignored during the SPF computation. Hence, no routing loop can ensue - which is also exactly the reason why OSPF does not allow you to daisy-chain areas: ABRs would not accept LSA-3 received via non-backbone areas.

Does this make more sense now? Feel welcome to ask further!

Best regards,

Peter

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