Default Routes in OSPF

Answered Question
Mar 17th, 2009
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OK, stupid question time.


I have an OSPF router that is learning a default route from 2 ASBRs through the default information originate command.


In the OSPF database, you can see both LSAs with the same cost.


So what does the router use as the tie breaker when deciding whcih default route to use when they both have the same metric.


arcr01#sh ip ospf database nssa-external 0.0.0.0


OSPF Router with ID (19.211.101.154) (Process ID 521)


Type-7 AS External Link States (Area 301)


Routing Bit Set on this LSA

LS age: 220

Options: (No TOS-capability, Type 7/5 translation, DC)

LS Type: AS External Link

Link State ID: 0.0.0.0 (External Network Number )

Advertising Router: 19.213.1.130

LS Seq Number: 80000E78

Checksum: 0x1338

Length: 36

Network Mask: /0

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

TOS: 0

Metric: 1

Forward Address: 19.213.1.130

External Route Tag: 0


Routing Bit Set on this LSA

LS age: 1095

Options: (No TOS-capability, Type 7/5 translation, DC)

LS Type: AS External Link

Link State ID: 0.0.0.0 (External Network Number )

Advertising Router: 19.213.1.154

LS Seq Number: 800011BD

Checksum: 0x4092

Length: 36

Network Mask: /0

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

TOS: 0

Metric: 1

Forward Address: 19.213.1.154

External Route Tag: 0


arcr01#

arcr01#sh ip ro 0.0.0.0

Routing entry for 0.0.0.0/0, supernet

Known via "ospf 521", distance 110, metric 1, candidate default path, type NSSA extern 2, forward metric 256

Last update from 19.210.108.17 on FastEthernet3/0, 18:53:22 ago

Routing Descriptor Blocks:

* 19.210.108.17, from 19.213.1.130, 18:53:22 ago, via FastEthernet3/0

Route metric is 1, traffic share count is 1



Correct Answer by Giuseppe Larosa about 8 years 1 month ago

Hello Victor,

if you have two O E2 routes 0.0.0.0/0 with same seed metric and same metric to fowarding address the local router should install both in its IP routing table


Hope to help

Giuseppe


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Giuseppe Larosa Tue, 03/17/2009 - 09:15
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Hello Victor,

I would say the OSPF metric to the fowarding address of each LSA


verify the OSPF metrics for

19.213.1.130

and for

19.213.1.154


the first one should be 256


Known via "ospf 521", distance 110, metric 1, candidate default path, type NSSA extern 2, >> forward metric 256


Hope to help

Giuseppe



lamav Tue, 03/17/2009 - 09:18
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Yeah, I told you it was a stupid question...LOL..I just realized that one defautl route is learned through an FE interface, while the other is only a T3. I thought they were both the same interface BWs. Sorry.


Now that I opened the can of worms, lets say both interfaces were FEs, how would OSPF select the best default route? If it were any other route to another network, then it can have equal cost paths, I know...but what about the default?

Correct Answer
Giuseppe Larosa Tue, 03/17/2009 - 09:22
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Hello Victor,

if you have two O E2 routes 0.0.0.0/0 with same seed metric and same metric to fowarding address the local router should install both in its IP routing table


Hope to help

Giuseppe


lamav Tue, 03/17/2009 - 09:29
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Interesting, so you CAN indeed do some load sharing by using two distinct default routes and have them both in your routing table.


I thought the router could only select one default route.


Excellent, dude!


By the way, go back to that thread about those Asia routers (ECC and FMC) because I got an update for ya! :-)


Thanks, again


Victor

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