OSPF route selection

[from88]

Hello,

I have 4 OSPF external LSA's which represents: 10.33.0.0/16 network: 

show ip ospf database 10.33.0.0 detail vrf lan
 OSPF Router with ID (10.2.254.251) (Process ID 1 VRF lan)

 Type-5 AS External Link States 

 LS age: 103
 Options: 0x20 (No TOS-capability, DC)
 LS Type: Type-5 AS-External
 Link State ID: 10.33.0.0 (Network address)
 Advertising Router: 10.1.254.202
 LS Seq Number: 0x8000029f
 Checksum: 0xb4c8
 Length: 36
 Network Mask: /16
 Metric Type: 2 (Larger than any link state path)
 TOS: 0
 Metric: 50
 Forward Address: 0.0.0.0
 External Route Tag: 100

 LS age: 245
 Options: 0x20 (No TOS-capability, DC)
 LS Type: Type-5 AS-External
 Link State ID: 10.33.0.0 (Network address)
 Advertising Router: 10.2.254.2
 LS Seq Number: 0x80000007
 Checksum: 0x9847
 Length: 36
 Network Mask: /16
 Metric Type: 2 (Larger than any link state path)
 TOS: 0
 Metric: 50
 Forward Address: 0.0.0.0
 External Route Tag: 100

 LS age: 1141
 Options: 0x20 (No TOS-capability, DC)
 LS Type: Type-5 AS-External
 Link State ID: 10.33.0.0 (Network address)
 Advertising Router: 10.3.254.236
 LS Seq Number: 0x80000cc2
 Checksum: 0x73b8
 Length: 36
 Network Mask: /16
 Metric Type: 2 (Larger than any link state path)
 TOS: 0
 Metric: 50
 Forward Address: 0.0.0.0
 External Route Tag: 100

 LS age: 453
 Options: 0x2 (No TOS-capability, No DC)
 LS Type: Type-5 AS-External
 Link State ID: 10.33.0.0 (Network address)
 Advertising Router: 10.254.2.211
 LS Seq Number: 0x80000062
 Checksum: 0xbd13
 Length: 36
 Network Mask: /16
 Metric Type: 2 (Larger than any link state path)
 TOS: 0
 Metric: 50
 Forward Address: 0.0.0.0
 External Route Tag: 100

as you see these LSA's are identical. I don't understand by which OSPF number these two routes are slelected:

JAY1# sh ip route 10.33.0.0/16 vrf lan
10.33.0.0/16, ubest/mbest: 2/0
*via 10.254.2.2, Eth1/7, [110/50], 03:25:24, ospf-1, type-2, tag 100
*via 10.254.2.209, Vlan432, [110/50], 03:25:24, ospf-1, type-2, tag 100
 
JAY1# sh ip ospf route 10.33.0.0/16 vrf lan
OSPF Process ID 1 VRF lan, Routing Table for prefixes 10.33.0.0/16
(D) denotes route is directly attached (R) denotes route is in RIB
10.33.0.0/16 (type-2)(R) area 0.0.0.0, tag 100
via 10.254.2.2/Eth1/7 , cost 50 distance 110
via 10.254.2.209/Vlan432 , cost 50 distance 110

 If these 4 routes have a metric of 50. How correct ones are selected ? What is the tiebreaker ? Metric to next hop ? Thanks

[Seb Rupik · Accepted solution]

Hi there,
As these are LSA Type 5, metric type 2 routes the metric value you see listed is just the external cost to reach the destination prefix.

With OSPF route selection, if metric type 2 routes have the same external cost and destination prefix, then the path with the lowest cost to the ASBR is selected. In your case OSPF is performing ECMP which is why you have two ASBRs listed in your routing table.

https://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/7039-1.html

cheers,
Seb.