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Missing something with OSPF path selection, redundant paths

rw13
Level 1
Level 1

I have a CML topology configured with 3 routers in area 0, and another router in area 10. The router in area 10 is redistributing connected loopback routes simulating LAN subnets with metric-type 1 and I have cost configured to something very high on one side vs the other. The idea was to make OSPF prefer reaching the site networks through one path vs the other, a primary/backup situation. My problem is router ABR1 has the route as E1 with the high cost and prefers it even though ABR2 has a much lower cost. If I shutdown the link between ABR1 and leaf, it sees the path through ABR2 with lower cost. Both ABR1 and ABR2 routers have E1 routes, so isn't cost the next metric to select the correct path?

 

abr1#show ip route 10.255.255.0
Routing entry for 10.255.255.0/24
Known via "ospf 1", distance 110, metric 50020, type extern 1
Last update from 10.210.2.1 on GigabitEthernet2, 00:17:38 ago
Routing Descriptor Blocks:
* 10.210.2.1, from 10.211.2.1, 00:17:38 ago, via GigabitEthernet2
Route metric is 50020, traffic share count is 1

abr2#show ip route 10.255.255.0
Routing entry for 10.255.255.0/24
Known via "ospf 1", distance 110, metric 120, type extern 1
Last update from 10.210.2.3 on GigabitEthernet3, 00:18:51 ago
Routing Descriptor Blocks:
* 10.210.2.3, from 10.211.2.1, 00:18:51 ago, via GigabitEthernet3
Route metric is 120, traffic share count is 1
1 Accepted Solution

Accepted Solutions

I run lab and I success 
1-first make the interconnect between the ABR1 and ABR2 in area 1
2- config LO user as Router ID in R4 (leaf) and advertsie this LO with

network 

command not via redistribute,
this LO is additional to LO we will redistribute as E1

why these two steps  
first LO RID will advertise as LSA1 to ABR1 via direct and via interconnect link between the ABR1and2
the LO E1 will know learn with forward address 0.0.0.0 so the abr1 will use RID to forward to select path, the LO RID which is LSA1 have two path one via 10000 (in my lab I config cost 10000) and other lower than interconnect ABR1and2 
so the ABR1 will select path for LO E1 via short path toward LO RID 
and that we you want 
thanks 

MHM

Screenshot (135).png

 

Screenshot (133).png

View solution in original post

9 Replies 9

In leaf add LO use it as router-id then

Clear ip opsf process 

Note:- this LO must config under ospf process with

network 

command 

Then check again.

MHM

That is already in my topology as well... leaf has loopback configured in area 10 and also added to OSPF as passive interface, it's also already configured as the router-id under the OSPF process. In the route printout you see it as from 10.211.2.1.

We need abr0 to prefer path than other?

If Yes then 

Share in abr0 

Show ip ospf database external 

MHM

If we need AbR1 to prefer path toward abr2 than direct to lead 

Then we need to make link interconnect abr1 and abr2 in area1 not in area0.

Because the ospf prefer 

Lsa1/lsa2 then lsa3 

And here abr1 prefer direct becuase it lsa1 even if it high cost than prefer lsa3 toward abr2 even if it low cost 

MHM

The links between ABR1 and ABR2 to LEAF are in area 1, not area 0. Are you saying the link between ABR1 and ABR2? If I will have multiple areas going across that link the future is there any work around?

I run lab and I success 
1-first make the interconnect between the ABR1 and ABR2 in area 1
2- config LO user as Router ID in R4 (leaf) and advertsie this LO with

network 

command not via redistribute,
this LO is additional to LO we will redistribute as E1

why these two steps  
first LO RID will advertise as LSA1 to ABR1 via direct and via interconnect link between the ABR1and2
the LO E1 will know learn with forward address 0.0.0.0 so the abr1 will use RID to forward to select path, the LO RID which is LSA1 have two path one via 10000 (in my lab I config cost 10000) and other lower than interconnect ABR1and2 
so the ABR1 will select path for LO E1 via short path toward LO RID 
and that we you want 
thanks 

MHM

Screenshot (135).png

 

Screenshot (133).png

I cannot make that link area 1 only unfortunately, as it is a transit for more than just this one area (not shown). I will try to make a work around, thank you!

friend you are so so welcome 
have a nice day 

MHM

ABR1 needs to not prefer its connected path the site overall and go to ABR2. This is part of a larger plan so ignore ABR0's role in this, it's just an area 0 router.

 

abr1#show ip ospf database external

OSPF Router with ID (10.211.1.0) (Process ID 1)

Type-5 AS External Link States

LS age: 231
Options: (No TOS-capability, DC, Upward)
LS Type: AS External Link
Link State ID: 0.0.0.0 (External Network Number )
Advertising Router: 10.211.3.1
LS Seq Number: 80000006
Checksum: 0x7E3A
Length: 36
Network Mask: /0
Metric Type: 2 (Larger than any link state path)
MTID: 0
Metric: 1
Forward Address: 0.0.0.0
External Route Tag: 1

LS age: 673
Options: (No TOS-capability, DC, Upward)
LS Type: AS External Link
Link State ID: 10.255.255.0 (External Network Number )
Advertising Router: 19.211.2.1
LS Seq Number: 80000003
Checksum: 0xA18
Length: 36
Network Mask: /24
Metric Type: 1 (Comparable directly to link state metric)
MTID: 0
Metric: 20
Forward Address: 0.0.0.0
External Route Tag: 0


abr2# show ip ospf database external

OSPF Router with ID (10.211.1.1) (Process ID 1)

Type-5 AS External Link States

LS age: 266
Options: (No TOS-capability, DC, Upward)
LS Type: AS External Link
Link State ID: 0.0.0.0 (External Network Number )
Advertising Router: 10.211.3.1
LS Seq Number: 80000006
Checksum: 0x7E3A
Length: 36
Network Mask: /0
Metric Type: 2 (Larger than any link state path)
MTID: 0
Metric: 1
Forward Address: 0.0.0.0
External Route Tag: 1

LS age: 709
Options: (No TOS-capability, DC, Upward)
LS Type: AS External Link
Link State ID: 10.255.255.0 (External Network Number )
Advertising Router: 10.211.2.1
LS Seq Number: 80000003
Checksum: 0xA18
Length: 36
Network Mask: /24
Metric Type: 1 (Comparable directly to link state metric)
MTID: 0
Metric: 20
Forward Address: 0.0.0.0
External Route Tag: 0

 

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