08-12-2013 08:52 PM - edited 03-07-2019 02:53 PM
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!!
08-13-2013 12:00 AM
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
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08-13-2013 07:33 AM
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
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