OSPF type 7 to type 5 translator election question

[Alan_4]

Hello everyone,

This network was set up.

Router ABR1 was first automatically chosen to send the type 5 lsa's

After I changed the router ID of ABR2 to a higher router ID than the one of ABR1, the output of a show command showed on Area0router that ABR2 was the advertising router for these types of lsa's now.

But after doing a trace route on area0router to 44.44.44.44 the path to ABR1 was still chosen, so my question is, does changing the router ID of an abr just let this router do the translation of type 7 to type 5 lsa's, and the path to be chosen to for example reach the destination is still going to be the one with the beter cost? If so why would for this example area0router still chose the old path, and in general aswell why would the old path get chosen? It looks like both routes have the same cost.

Any help is appreciated.

[paul driver]

Hello

Check the route table for the forwarding address in that external LSA, I guess it will have ASBR1 as the next hop even though the external lsa is now being advertised by ABSR2 the path the forwarding address of 192.168.2.0 hasn't change

res

Paul

[Georg Pauwen]

Hello,

I labbed your setup, but my outcome is completely different. How did you configure your routers, can you post the configs ?

[paul driver]

Hello Georg

My understanding it shouldn’t matter mate - the synopsis should be the same - The external lsa has a forwarding address due to the abrs and asbr being in a nssa

As such if all is equal then it all depends which summary lsa of that forward address is the newest lsa in the ospf database determines the next hop 

By by the way georg in your lab did you clear the ospf processes?

res

paul

[Georg Pauwen]

The reason I was asking for the configs is that no matter which ABR does the advertising, I always get two equal cost routes, which I think is what it should be like, and I didn't see that in the original 'sh ip route' from the Area 0 router on the left...

The part were changing the router ID causes that router to become the advertising ABR works as described though.

But the process switching path selected changes according to the advertising router:

R1#sh ip ospf database external 5.5.5.5 <-- E2

OSPF Router with ID (1.1.1.1) (Process ID 1)

Type-5 AS External Link States

LS age: 952
Options: (No TOS-capability, DC, Upward)
LS Type: AS External Link
Link State ID: 5.5.5.5 (External Network Number )
Advertising Router: 100.100.100.100 <-- ABR1
LS Seq Number: 80000003
Checksum: 0x6792
Length: 36
Network Mask: /32
Metric Type: 2 (Larger than any link state path)
MTID: 0
Metric: 1
Forward Address: 4.4.4.4
External Route Tag: 2

Routing entry for 5.5.5.5/32
Known via "ospf 1", distance 110, metric 1
Tag 2, type extern 2, forward metric 3
Last update from 192.168.13.3 on GigabitEthernet0/1, 00:05:40 ago
Routing Descriptor Blocks:
192.168.13.3, from 100.100.100.100, 00:05:40 ago, via GigabitEthernet0/1
Route metric is 1, traffic share count is 1
Route tag 2
* 192.168.12.2, from 100.100.100.100, 00:05:40 ago, via GigabitEthernet0/0
Route metric is 1, traffic share count is 1
Route tag 2

R1#sh ip ospf database external 5.5.5.5

OSPF Router with ID (1.1.1.1) (Process ID 1)

Type-5 AS External Link States

LS age: 255
Options: (No TOS-capability, DC, Upward)
LS Type: AS External Link
Link State ID: 5.5.5.5 (External Network Number )
Advertising Router: 3.3.3.3 <-- ABR2
LS Seq Number: 80000001
Checksum: 0xD4AC
Length: 36
Network Mask: /32
Metric Type: 2 (Larger than any link state path)
MTID: 0
Metric: 1
Forward Address: 4.4.4.4
External Route Tag: 2

Routing entry for 5.5.5.5/32
Known via "ospf 1", distance 110, metric 1
Tag 2, type extern 2, forward metric 3
Last update from 192.168.12.2 on GigabitEthernet0/0, 00:00:45 ago
Routing Descriptor Blocks:
* 192.168.13.3, from 3.3.3.3, 00:01:24 ago, via GigabitEthernet0/1
Route metric is 1, traffic share count is 1
Route tag 2
192.168.12.2, from 3.3.3.3, 00:00:45 ago, via GigabitEthernet0/0
Route metric is 1, traffic share count is 1
Route tag 2

[Alan_4]

Hello George,

This was not the case for me, the area0router did show only 1 route each time and that 1 route did not change. In your output it shows as if the route did change to the ABR that was elected to convert the type 7 to type 5 lsa's and advertising the route.

So the router that is advertising the lsa's with the higher router id is also going to be the one where data gets forwarded to in your setup?

[Georg Pauwen]

Hello,

--> So the router that is advertising the lsa's with the higher router id is also going to be the one where data gets forwarded to in your setup?

Exactly. It seems strange that you only see one route, there should be two. That is why it would be useful to see you router configurations...

[paul driver]

Hello georg 

IIn you case I think you are  showing the interface to the external route 5,5,5,5  NOT the forwarding address of 4,4.4.4

thst also will have a lsa type 3 and if you do a show Ip route on that that then you will see thre next hop to reach 5,5,5,5 via 4,4,4,4

res

paul

[Alan_4 · Accepted solution]

Hello everyone,

Thanks for your help it is appreciated.

I forgot to save the running configs to the startup configs, but if I recall correctly this was the set up.

also I think that Paul is correct.

If I remember correctly the 192.168.1.0 interface was first configured and afterwards the 192.168.2.0 interface.

I found info from this article, all credit goes to the writer of this article.

https://ccieblog.co.uk/ospf/ospf-nssa-translator-election-forwarding-address

To quote a bit of his article

To work out what IP is actually used as the forward address in my scenario, it’s actually not documented anywhere by Cisco, however some of the steps are listed in the RFC 3101:

“When a router is forced to pick a forwarding address for a Type-7 LSA, preference should be given first to the router’s internal addresses (provided internal addressing is supported). If internal addresses are not available, preference should be given to the router’s active OSPF stub network addresses. These choices avoid the possible extra hop that may happen when a transit network’s address is used. When the interface whose IP address is the LSA’s forwarding address transitions to a Down state (see [OSPF] Section 9.3), the router must select a new forwarding address for the LSA and then re- originate it. If one is not available the LSA should be flushed.”

FYI, when it talks about internal addresses, it is referring to loopback addresses. Anyway, Cisco have met these requirements, but made it more specific. So this is the actual order that it is done in.

1. Newest OSPF enabled loopback interface IP address (note: On some IOS platforms it’s actually the oldest loopback IP)

2. Newest OSPF enabled non-loopback interface IP address that is connected to a transit stub network

3. Newest OSPF enabled non-loopback interface IP address that is connected to a non-transit stub network

Kind regards,

[Alan_4]

Hello @Georg Pauwen 

Are you able to post your config?

This is the config used in the setup, extra lines ommited

area0router

Spoiler

interface FastEthernet0/0
ip address 192.168.4.2 255.255.255.0
duplex auto
speed auto
!
interface FastEthernet1/0
ip address 192.168.3.2 255.255.255.0
duplex auto
speed auto
!
router ospf 4
log-adjacency-changes
network 192.168.3.0 0.0.0.255 area 0
network 192.168.4.0 0.0.0.255 area 0

router abr 2

Spoiler

interface FastEthernet0/0
ip address 192.168.1.2 255.255.255.0
duplex auto
speed auto
!
interface FastEthernet1/0
ip address 192.168.3.1 255.255.255.0
duplex auto
speed auto
!
router ospf 2
router-id 200.1.1.1
log-adjacency-changes
area 1 nssa
network 192.168.1.0 0.0.0.255 area 1
network 192.168.3.0 0.0.0.255 area 0

router abr 1

Spoiler

interface FastEthernet0/0
ip address 192.168.4.1 255.255.255.0
duplex auto
speed auto
!
interface FastEthernet1/0
ip address 192.168.2.2 255.255.255.0
duplex auto
speed auto
!
router ospf 3
log-adjacency-changes
area 1 nssa
network 192.168.2.0 0.0.0.255 area 1
network 192.168.4.0 0.0.0.255 area 0

router ASBR

Spoiler

interface Loopback0
ip address 44.44.44.44 255.255.255.0
!
interface FastEthernet0/0
ip address 192.168.1.1 255.255.255.0
duplex auto
speed auto
!
interface FastEthernet1/0
ip address 192.168.2.1 255.255.255.0
duplex auto
speed auto
!
router ospf 1
log-adjacency-changes
area 1 nssa
redistribute connected subnets
network 192.168.1.0 0.0.0.255 area 1
network 192.168.2.0 0.0.0.255 area 1

---

[paul driver]

Hello

on the asbr, you are redistributing any connected network, Ammend it to the follwoing and test again.

route-map STAN
match interface loopback0

router ospf 1
no redistribute connected subnets
redistribute connected route-map STAN subnets

res

Paul

[Alan_4]

Hello Paul,

Thanks for the help, the route map command was tried for redistributing, but it gave the same result. Traceroutes go through the same routes and ABR2 is the advertising router for the LSA's.

The routes got cost balanced when a new Loopback interface was created and added to area 1 on the ASBR. Also it was possible to select only the router ABR2 for the routes, after the command area 1 nssa translate type7 always suppress-fa at router ospf configuration mode.

All credit goes to https://ccieblog.co.uk/ospf/ospf-nssa-translator-election-forwarding-address for the explanations.

@Georg Pauwen are you able to post your configurations? It would be cool to see what the differences were in our configurations that led to your routes being cost balanced.

[paul driver]

Hello

It wasnt supposed to give a differnet result, it was just a obverstaionon on the setup you had when redistrubting a specific connected network.

FYI -The link is a good read and explantion.

res

Paul

[mohammed01701]

Hi Alan_4!

I need to understand what do you want to achieve of this topology, sorry i could not understand why do you need to use nssa Area?

/Mohammed

[Alan_4]

Hello Mohammed,

this topology was used to see and test how a border router can be elected to perform the conversion of type 7 to type 5 lsa's and also to test how the routes to reach the external networks can be changed and which routes will be chosen to reach these external networks.

regards,

Alan