Solved: OSPF External Route Selection

goldpartner · ‎09-29-2023

On Router-A and Router-B，configure area 0 and area 1 as follows：

Router-A：

    area 0：10.5.5.5

    area 1：10.4.4.4

Router-B：

    area 0：10.5.5.6

    area 1：10.4.4.5

On Router-A, redistribute an external route into OSPF, so Router-A is an ASBR.

Question: On Router-B, what is the next hop of the external route above，10.5.5.5 or 10.4.4.4? Why?

David Ruess · ‎09-29-2023

It will load balance on both. Since the type 5 LSA doesn't have an Area association

per-se

when you redistribute into OSPF it will send it to all areas the router is a part of. That means Router B will receive the route from both links (Area 0 and Area 1). So the answer to the question is both Next hops. Now if your topology was different as in more routers or if you modify the cost on one of the links then its no longer Equal Cost so the router will choose the lowest cost path.

Hope that helps

-David

View solution in original post

MHM Cisco World · ‎09-30-2023

as I mention before but I dont have time to run lab for my answer to be sure.
I get some time to run lab and see result
the R2 have one path to 3.3.3.3 (static route toward R3 in R1) not two path
the key here is forward address of external (static route ).

View solution in original post

MHM Cisco World · ‎09-29-2023

I think the route will use router-ID as

adv router IP

So

Show ip ospf

Check router-id and it will be use.

Not so sure but I think this is case here.

Giuseppe Larosa · ‎09-29-2023

Hello @goldpartner ,

if area 1 is a standard area there should be two equal cost paths to the OSPF RID of RA from RB

you can check this on RB with

show ip ospf border

Edit:

if the OSPF RID of RA is advertised in OSPF it will be intra area only in one area : area 0 or area 1. This should be taken in account.

Hope to help

Giuseppe

David Ruess · ‎09-29-2023

It will load balance on both. Since the type 5 LSA doesn't have an Area association

per-se

when you redistribute into OSPF it will send it to all areas the router is a part of. That means Router B will receive the route from both links (Area 0 and Area 1). So the answer to the question is both Next hops. Now if your topology was different as in more routers or if you modify the cost on one of the links then its no longer Equal Cost so the router will choose the lowest cost path.

Hope that helps

-David

goldpartner · ‎09-29-2023

Thanks guys.

I did test in a lab environment. No matter the OSPF RID of RA is advertised in area 0 or area 1, there are two equal paths of the external route as showed below.


Router-B#sh ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

1.0.0.0/32 is subnetted, 1 subnets
O 1.1.1.1 [110/11] via 10.4.4.4, 00:46:55, FastEthernet0/1
2.0.0.0/32 is subnetted, 1 subnets
C 2.2.2.2 is directly connected, Loopback0
10.0.0.0/8 is variably subnetted, 3 subnets, 2 masks
C 10.5.5.0/24 is directly connected, FastEthernet0/0
C 10.4.4.0/24 is directly connected, FastEthernet0/1
O E2 10.1.1.1/32 [110/20] via 10.5.5.5, 00:45:52, FastEthernet0/0
[110/20] via 10.4.4.4, 00:45:52, FastEthernet0/1
O E2 192.168.2.0/24 [110/20] via 10.5.5.5, 00:45:54, FastEthernet0/0
[110/20] via 10.4.4.4, 00:45:54, FastEthernet0/1

The following is configuration on Router-A and Router-B:
Router-A:

router ospf 1
router-id 1.1.1.1
log-adjacency-changes
redistribute connected subnets route-map c2o
redistribute static subnets
network 1.1.1.1 0.0.0.0 area 1
network 10.4.4.0 0.0.0.255 area 1
network 10.5.5.0 0.0.0.255 area 0

interface FastEthernet1/0
ip address 192.168.1.1 255.255.255.0

interface Loopback0
ip address 1.1.1.1 255.255.255.255

interface Loopback10
ip address 10.1.1.1 255.255.255.255

interface FastEthernet0/0
ip address 10.5.5.5 255.255.255.0

interface FastEthernet0/1
ip address 10.4.4.4 255.255.255.0

interface FastEthernet1/0
ip address 192.168.1.1 255.255.255.0

ip route 192.168.2.0 255.255.255.0 192.168.1.2

access-list 10 permit 10.1.1.1

route-map c2o permit 10
match ip address 10



Router-B:

router ospf 1
router-id 2.2.2.2
log-adjacency-changes
network 2.2.2.2 0.0.0.0 area 1
network 10.4.4.0 0.0.0.255 area 1
network 10.5.5.0 0.0.0.255 area 0

interface Loopback0
ip address 2.2.2.2 255.255.255.255
!
interface FastEthernet0/0
ip address 10.5.5.6 255.255.255.0

interface FastEthernet0/1
ip address 10.4.4.5 255.255.255.0

MHM Cisco World · ‎09-29-2023

If you can Remove the manual route-id'

And make router auto select the RID and check again.

David Ruess · ‎09-29-2023

Yes. I mentioned this in my response above and also why.

goldpartner · ‎09-29-2023

Afer removing the manual route-id, it is still the same.

Router-B#sh ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

     1.0.0.0/32 is subnetted, 1 subnets
O       1.1.1.1 [110/11] via 10.4.4.4, 00:02:00, FastEthernet0/1
     2.0.0.0/32 is subnetted, 1 subnets
C       2.2.2.2 is directly connected, Loopback0
     10.0.0.0/8 is variably subnetted, 3 subnets, 2 masks
C       10.5.5.0/24 is directly connected, FastEthernet0/0
C       10.4.4.0/24 is directly connected, FastEthernet0/1
O E2    10.1.1.1/32 [110/20] via 10.5.5.5, 00:02:00, FastEthernet0/0
                    [110/20] via 10.4.4.4, 00:02:00, FastEthernet0/1
O E2 192.168.2.0/24 [110/20] via 10.5.5.5, 00:02:02, FastEthernet0/0
                    [110/20] via 10.4.4.4, 00:02:02, FastEthernet0/1

Blue_Bird · ‎09-29-2023

It will show next hop of the

external route both 10.5.5.5 and 10.4.4.4

Why? if you do the redistribution ...by default it will take as OE2(Type-2) and internal cost is not considered....will do load-balancing with default cost 20.

If you do redistribution as OE1 (Type-1), it will consider internal cost...which path is lowest cost will be preferred....in case internal cost also same will do load-balancing....

Best regards
******* If This Helps, Please Rate *******

MHM Cisco World · ‎09-29-2023

I am out when return home I will check.

Thanks a lot for update us.

Have a nice weekend

MHM

goldpartner · ‎09-29-2023

Now I have three side questions after I removed the manual router-id on Router-A and Router-B.

1. The link

state id 1.1.1.1

(networked in area 1 on Router-A) is not showing in Router Link States (Area 1) but only in Summary Net Link States (Area 0).
2. The link

state id 2.2.2.2

is showing in both Router Link States (Area 0) and Router Link States (Area 1), but I assumed it is only showing in Router Link States (Area 1) since it is networked in area 1 on Router-B.

3. The link

state id 10.1.1.1

is showing in both Router Link States (Area 0) and Router Link States (Area 1), but I assumed it is not showing in either since it is an external route to the OSPF domain.

(Note: 10.1.1.1 is the router ID of Router-A and 2.2.2.2 is the router ID of Router-B now with auto selection.)

Can anyone explain the three questions above?

Router-B#sh ip osp database

            OSPF Router with ID (2.2.2.2) (Process ID 1)

                Router Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum Link count
2.2.2.2         2.2.2.2         478         0x8000000C 0x00FADE 1
10.1.1.1        10.1.1.1        513         0x80000002 0x008C4C 1

                Net Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum
10.5.5.6        2.2.2.2         478         0x80000001 0x000BF7

                Summary Net Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum
1.1.1.1         2.2.2.2         473         0x80000001 0x008D98
1.1.1.1         10.1.1.1        515         0x80000001 0x00F535
2.2.2.2         2.2.2.2         522         0x80000001 0x00FA31
2.2.2.2         10.1.1.1        474         0x80000001 0x002CF0
10.4.4.0        2.2.2.2         473         0x80000001 0x00D246
10.4.4.0        10.1.1.1        474         0x80000001 0x009F74

                Summary ASB Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum
10.1.1.1        2.2.2.2         473         0x80000001 0x00FF1D

                Router Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum Link count
2.2.2.2         2.2.2.2         481         0x80000009 0x001BAE 2
10.1.1.1        10.1.1.1        515         0x80000002 0x007851 2

                Net Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
10.4.4.5        2.2.2.2         481         0x80000001 0x002CD9

                Summary Net Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
10.5.5.0        2.2.2.2         476         0x80000001 0x00BB5B
10.5.5.0        10.1.1.1        477         0x80000001 0x008889

                Summary ASB Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
10.1.1.1        2.2.2.2         476         0x80000001 0x00FF1D

                Type-5 AS External Link States

Link ID         ADV Router      Age         Seq#       Checksum Tag
10.1.1.1        10.1.1.1        521         0x80000001 0x00D4B1 0
192.168.2.0     10.1.1.1        523         0x80000001 0x00B077 0

Martin L · ‎09-29-2023

OSPF database is sort of hard to read and always confusing; I will try to explain it well;

The first part/section of DB where shows "link count" (top right corner) is not network advertised by me part of table; This first section is just telling us about a number of LSAs carry by Router ID. Link ID is not network id and Adv Router is router id. For this section, Link ID and Adv Router are always the same but link count may change. Compare RA to Router B who has 2 LSAs. There is another command to see details for this section of db:

show ip os database router RouterID

-will show u detailed links which should match number of links displayed in top right corner.

Then, u have type 2 network Link type called

Net Link state

Starting here, we have actual network ID and advertising router.

Finally we have details in section

Summary Net Link States

(by Areas). Those are networks advertised by router IDs. This is LSA Type-3 section. This name "summary" comes from the fact that ABR will not forward type1 and 2 outside its own area but will "summarize" them into type3. Here, u can safely replace "summary" with word "subnets"

Then type-4 Summary ABR -host route to reach ASB/ASBR.

Regards, ML
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MHM Cisco World · ‎09-30-2023

as I mention before but I dont have time to run lab for my answer to be sure.
I get some time to run lab and see result
the R2 have one path to 3.3.3.3 (static route toward R3 in R1) not two path
the key here is forward address of external (static route ).

Martin L · ‎10-02-2023

I think your topology is different from OP (original poster); can u share R1' ouptput of

sh run | s router

and

sh ip os int br

Regards, ML
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MHM Cisco World · ‎10-02-2023

Sure I will share all config when I retrun back to home.