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OSPF Question

lamav
Level 8
Level 8

Hi:

Roueters RIB shows 2 default routes learned from two OSPF neighbors.

Why do I only see one LSA for th default route if two neighbors are sending LSAs?

router 4#sh ip ro 0.0.0.0

Routing entry for 0.0.0.0/0, supernet

Known via "ospf 16586", distance 110, metric 1, candidate default path

Tag 16586, type extern 2, forward metric 3

Last update from 71.22.7.157 on TenGigabitEthernet9/2, 6d18h ago

Routing Descriptor Blocks:

* 71.22.7.165, from 64.13.44.104, 6d18h ago, via TenGigabitEthernet9/1

Route metric is 1, traffic share count is 1

Route tag 16586

71.22.7.157, from 64.13.44.104, 6d18h ago, via TenGigabitEthernet9/2

Route metric is 1, traffic share count is 1

Route tag 16586

router 4#

router 4#

router 4#

router 4#

router 4#

router 4#

router 4#sh ip ospf data ext 0.0.0.0

OSPF Router with ID (64.13.44.112) (Process ID 16586)

Type-5 AS External Link States

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

LS age: 25

Options: (No TOS-capability, DC)

LS Type: AS External Link

Link State ID: 0.0.0.0 (External Network Number )

Advertising Router: 64.13.44.104

LS Seq Number: 8000032A

Checksum: 0x1B3D

Length: 36

Network Mask: /0

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

MTID: 0

Metric: 1

Forward Address: 66.192.62.1

External Route Tag: 16586

router 4#

router 4#

router 4#

router 4#sh ip ospf nei

Neighbor ID Pri State Dead Time Address Interface

64.13.44.111 1 FULL/BDR 808 msec 71.22.7.157 TenGigabitEthernet9/2

64.13.44.105 1 FULL/BDR 836 msec 71.22.7.165 TenGigabitEthernet9/1

router 4#

4 Replies 4

paul.matthews
Level 5
Level 5

The obvious thought is that the default is generated at one point in your network (64.13.44.104) but you have two equakl cost paths to get to it.

Victor,

Router4 has received this information from 64.13.44.104(Advertising Router) who generated Type5LSA. Because this router can reach 66.192.62.1(Forward Address) via 2 paths(equal cost) that's why you see 2 default routes in RIB.

However,This should be posted in the routing forum. J/K (grin)

Toshi

Yes, I see...stupid question, really..

One router originated the default and it learned it from two equal cost paths....

Why there are 2 routes in the RIB is easy to understand, I knew that. I just couldnt figure out why there was only one LSA....I guess the only LSA that should appear is from the router that orginated it....right?

Hello Victor,

>> .I guess the only LSA that should appear is from the router that orginated it....right?

yes, an LSA is a data structure with a single owner the node that originated it other nodes can just flood it.

Best Regards

Giuseppe

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