OSPF theory time! Can you help me figure out why I'm getting THAT default route?

anestis.iliadis · ‎08-12-2013

Hi all,

I'm having a purely theoretical problem where I have two devices both injecting an def. route in the OSPF domain and I'm trying to figure out what forces the election of one as a candidate default over the other.

So I have 3 devices all on the same LAN 10.89.228.0/24

Dev 1 (firewall):

ip: 10.89.228.40/24

router-id -same as ip

advertises 0.0.0.0.0/0.0.0.0 with metric 10

Dev 2 (router)

ip: 10.89.228.1/24

router id: 192.168.100.1

advertises 0.0.0.0.0/0.0.0.0 with metric 1

Dev 3 (router)

ip: 10.89.228.252/24

router id 172.16.100.1

receives default route of 10.89.228.40 NOT 10.89.228.1

Any ideas why this is happening? I'm scratching my head!!

CONFIGS:

Dev 2

router ospf 1

router-id 192.168.100.1

area 0 authentication message-digest

network 0.0.0.0 255.255.255.255 area 0

default-information originate metric 1

ip route 0.0.0.0 0.0.0.0 10.89.228.40

Dev 3

router ospf 1

log-adjacency-changes

network 0.0.0.0 255.255.255.255 area 0

#sh ip ospf data external 0.0.0.0

SHOW COMMANDS ON DEV3:

#sh ip ospf data external 0.0.0.0 adv-router 192.168.100.1

OSPF Router with ID (172.16.100.1) (Process ID 1)

Type-5 AS External Link States

Routing Bit Set on this LSA in topology Base with MTID 0

LS age: 274

Options: (No TOS-capability, DC)

LS Type: AS External Link

Link State ID: 0.0.0.0 (External Network Number )

Advertising Router: 192.168.100.1

LS Seq Number: 80000001

Checksum: 0xDA98

Length: 36

Network Mask: /0

Metric Type: 2 (Larger than any link state path)

MTID: 0

Metric: 1

Forward Address: 10.89.228.40

External Route Tag: 1

#sh ip ospf data external 0.0.0.0 adv-router 10.89.228.40

OSPF Router with ID (172.16.100.1) (Process ID 1)

Type-5 AS External Link States

LS age: 1664

Options: (No TOS-capability, No DC)

LS Type: AS External Link

Link State ID: 0.0.0.0 (External Network Number )

Advertising Router: 10.89.228.40

LS Seq Number: 8000001F

Checksum: 0xB287

Length: 36

Network Mask: /0

Metric Type: 2 (Larger than any link state path)

MTID: 0

Metric: 10

Forward Address: 0.0.0.0

External Route Tag: 0

#sh ip route 0.0.0.0

Routing entry for 0.0.0.0/0, supernet

Known via "ospf 1", distance 110, metric 1, candidate default path

Tag 1, type extern 2, forward metric 1

Last update from 10.89.228.40 on GigabitEthernet0/0, 00:09:26 ago

Routing Descriptor Blocks:

* 10.89.228.40, from 192.168.100.1, 00:09:26 ago, via GigabitEthernet0/0

Route metric is 1, traffic share count is 1

Route tag 1

Shouldn't the .252 (dev3) be getting the default route with the lowest metric? Why is it picking .40 as default route even though it clearly says metric 10 (vs metric 1) ?

I'm scratching my head!!

cadet alain · ‎08-13-2013

Hi,

One route has a forward address of 0.0.0.0 and not the other, see this link about forward address and path selection

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080124c7d.shtml

Regards

Alain

Don't forget to rate helpful posts.

lveloso11 · ‎08-13-2013

Hi anestis,

Looks like this is correct. As all 3 devices are in the same subnet (10.89.228.0/24), OSPF is smart enough to send the traffic directly to the firewall. It knows that Router 2 will forward the traffic to the firewall anyway, so why send it to Router 2 if it can send it straigh to the firewall?

I don't know exaclty what is the reason you are trying to do it. If you give us more info we can try to design a better solution.

Regards,

Leo