OSPF Exterior LSA comprises a subject referred to as Forwarding Handle. Primarily based on the forwarding deal with the metrics of a specific route could fluctuate.
Allow us to take the under instance to check the impact of OSPF forwarding deal with on metrics:
On this topology Router R1, R2 and R3 are in OSPF Space 1 configured as NSSA. Related subnet 9.9.34.0/24 is redistributed on R3 and is acquired on R1 and R2 as a Sort-7 LLSA within the NSSA space.
CONFIGURATIONS: OSPF Ahead Handle
R1:
community 9.9.13.0 0.0.0.255 space 1
R2:
community 9.9.23.0 0.0.0.255 space 1
R3:
redistribute linked metric-type 1 subnets
community 9.9.13.0 0.0.0.255 space 1
community 9.9.23.0 0.0.0.255 space 1
Although R1 and R2 have similar connectivity to R3, we see that route for 9.9.34.0 on R1 is learnt with a metric of 22, whereas on R2 it’s 21.
Identified by way of “ospf 1”, distance 110, metric 22, sort NSSA extern 1
Final replace from 9.9.13.3 on FastEthernet0/0, 00:02:21 in the past
Routing Descriptor Blocks:
* 9.9.13.3, from 9.9.23.3, 00:02:21 in the past, by way of FastEthernet0/0
Route metric is 22, site visitors share rely is 1
Identified by way of “ospf 1”, distance 110, metric 21, sort NSSA extern 1
Final replace from 9.9.23.3 on FastEthernet0/0, 00:03:14 in the past
Routing Descriptor Blocks:
* 9.9.23.3, from 9.9.23.3, 00:03:14 in the past, by way of FastEthernet0/0
Route metric is 21, site visitors share rely is 1
Now allow us to perceive why there’s a distinction in metric on each R1 and R2 for the path to subnet 9.9.34.0/24.
We verify the ahead deal with for the exterior LSA on each R1 and R2 as highlighted under:
Sort-7 AS Exterior Hyperlink States (Space 1)
Routing Bit Set on this LSA in topology Base with MTID 0
LS age: 398
Choices: (No TOS-capability, Sort 7/5 translation, DC, Upward)
LS Sort: AS Exterior Hyperlink
Hyperlink State ID: 9.9.34.0 (Exterior Community Quantity )
Promoting Router: 9.9.23.3
LS Seq Quantity: 80000002
Checksum: 0x4D3C
Size: 36
Community Masks: /24
Metric Sort: 1 (Comparable on to hyperlink state metric)
MTID: 0
Metric: 20
Ahead Handle: 9.9.23.3
Exterior Route Tag: 0
Sort-7 AS Exterior Hyperlink States (Space 1)
Routing Bit Set on this LSA in topology Base with MTID 0
LS age: 429
Choices: (No TOS-capability, Sort 7/5 translation, DC, Upward)
LS Sort: AS Exterior Hyperlink
Hyperlink State ID: 9.9.34.0 (Exterior Community Quantity )
Promoting Router: 9.9.23.3
LS Seq Quantity: 80000002
Checksum: 0x4D3C
Size: 36
Community Masks: /24
Metric Sort: 1 (Comparable on to hyperlink state metric)
MTID: 0
Metric: 20
Ahead Handle: 9.9.23.3
Exterior Route Tag: 0
We see the ahead deal with is Sort-7 LSA for 9.9.23.3 which is straight linked on R2 however is one-hop away from R1. Therefore there was a distinction of metric on R1 & R2.
In case R3 would have chosen 9.9.13.3 because the forwarding deal with a reverse situation would have been seen i.e. metric 21 on R1 and 22 on R2.
Forwarding deal with is chosen on ASBR utilizing the next guidelines:
- If there’s a loopback configured within the space then IP deal with of loopback is chosen as forwarding deal with.
- If first situation shouldn’t be met then IP deal with of first interface on the OSPF interface record is chosen as forwarding deal with. You possibly can see OSPF interface record by utilizing “present ip ospf interface temporary” command. The interface on prime would be the final interface which was connected to OSPF.
Since there was no loopback configured on R3 the second situation will prevail, so allow us to verify the output of “sh ip ospf interface temporary” to see which IP is listed first.
Fa1/0 1 1 9.9.23.3/24 1 BDR 1/1
Fa0/0 1 1 9.9.13.3/24 1 BDR 1/1
Proceed Studying:
The right way to configure OSPF Digital Hyperlink?
Configuring OSPF Multi-Space Adjacency : Instance State of affairs
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