From: Bit Gossip (bit.gossip@chello.nl)
Date: Wed Sep 05 2007 - 05:14:14 ART
Prasad,
from RFC1583:
"Type 2 external paths are compared by looking at the advertised type 2
metrics, and then if necessary, the distance to the forwarding addresses. If
the new path is shorter, it replaces the present paths in the routing table
entry. If the new path is the same cost, it is added to the routing table
entry's list of paths."
So, probably SW2 is metrically closer to R4 than R3 (when the forwarding
address is set to 0.0.0.0, this is the distance to the ASBR itself). In my
case this is really the case:
R4#show ip ospf border-routers
<...>
i 150.1.3.3 [781] via 164.1.34.3, Serial0/0, ASBR, Area 0, SPF 7
i 150.1.8.8 [2] via 164.1.47.7, FastEthernet0/0, ASBR, Area 0, SPF 7
It is interesting to note that if I tweak the ospf metric so that R3 and SW2
are same distance from R4, than both routes are installed:
R4#show ip ospf border-routers
<...>
i 150.1.3.3 [781] via 164.1.34.3, Serial0/0, ASBR, Area 0, SPF 8
i 150.1.8.8 [781] via 164.1.47.7, FastEthernet0/0, ASBR, Area 0, SPF 8
R4#show ip route ospf
<...>
O E2 150.1.3.0/24 [110/20] via 164.1.47.7, 00:00:24, FastEthernet0/0
[110/20] via 164.1.34.3, 00:00:24, Serial0/0
BTW: I have no idea what is the "routing bit set"! In my lab both the
external LSA had it even if only one was installed in the RT. Any help from
the experts out there?
HTH,
Bit.
----- Original Message -----
From: "Prasad Shemrudkar (pshemrud)" <pshemrud@cisco.com>
To: "Cisco certification" <ccielab@groupstudy.com>
Sent: Monday, August 27, 2007 8:38 PM
Subject: ip ospf database
> Hi Guys,
>
> I needed a little clarification on the ospf database. Scenario and my
> question as follows:
>
> R4-------R3--------R1
> | |
> | |
> SW1 -------------------SW2
>
> SW2 and R3 are ASBR's and redistributing eigrp into ospf. Loopback of R3
> is 150.1.3.3 is advertised in EIGRP.
>
> And in R4's routing table 150.1.3.3 is received from SW2, even though R3
> is directly connected to it.
>
> Routers running ospf only: SW1,R4
> Routers running eigrp only : R1
> ASBRs: R3 and SW2
>
> Rack1R4(config-if)#do sh ip route 150.1.3.3
> Routing entry for 150.1.3.0/24
> Known via "ospf 1", distance 110, metric 20
> Tag 90, type extern 2, forward metric 30
> Last update from 164.1.47.7 on Ethernet0/0, 00:11:41 ago
> Routing Descriptor Blocks:
> * 164.1.47.7, from 150.1.8.8, 00:11:41 ago, via Ethernet0/0
> Route metric is 20, traffic share count is 1
> Route tag 90
>
> The ospf database shows that it has been indeed learnt from both SW2
> (150.1.8.8) and R3 (150.1.3.3) albeit installs only the one learnt from
> SW2. I could not find much difference between the 2 LSAs except "Routing
> Bit Set on this LSA".
>
> Rack1R4#sh ip ospf data external 150.1.3.0
>
> OSPF Router with ID (150.1.4.4) (Process ID 1)
>
> Type-5 AS External Link States
>
> LS age: 1473
> Options: (No TOS-capability, DC)
> LS Type: AS External Link
> Link State ID: 150.1.3.0 (External Network Number )
> Advertising Router: 150.1.3.3
> LS Seq Number: 80000001
> Checksum: 0xF419
> Length: 36
> Network Mask: /24
> Metric Type: 2 (Larger than any link state path)
> TOS: 0
> Metric: 20
> Forward Address: 0.0.0.0
> External Route Tag: 90
>
> Routing Bit Set on this LSA
> LS age: 1320
> Options: (No TOS-capability, DC)
> LS Type: AS External Link
> Link State ID: 150.1.3.0 (External Network Number )
> Advertising Router: 150.1.8.8
> LS Seq Number: 80000001
> Checksum: 0xB350
> Length: 36
> Network Mask: /24
> Metric Type: 2 (Larger than any link state path)
> TOS: 0
> Metric: 20
> Forward Address: 0.0.0.0
> External Route Tag: 90
>
> Is this expected or am I missing something here? The metric for both,
> being ospf external is 20, so that's ruled out.
>
> TIA,
> Prasad
>
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