OSPF genral

Unanswered Question
Nov 27th, 2008

im stuck with OSPF again :( :(

im receiving default routes from two neighbours.

Both are having

same metric

same forwarding address

same cost [ sh ip ospf border]

but one path is selected - on what basis its selecting ?

I have this problem too.
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Giuseppe Larosa Thu, 11/27/2008 - 05:34

Hello Rajveev,

may you post the output of

sh ip ospf database external 0.0.0.0

sh ip route 0.0.0.0

and

sh ip protocol summary

how many OSPF processes are involved, multiple OSPF processes are competitors to install routes in the routing table and don't share OSPF informations

Hope to help

Giuseppe

Richard Burts Thu, 11/27/2008 - 07:14

Rajveev

Is it possible that one of the default routes is an external router while the other is an intra-area (or inter-area) route? There is an order of preference in OSPF in which intra-area is preferred over inter-area, which is preferred over external routes.

HTH

Rick

rajivrajan1 Thu, 11/27/2008 - 20:29

sh ip ospf database external 0.0.0.0

OSPF Router with ID (26.63.47.1) (Process ID 10)

Type-5 AS External Link States

Routing Bit Set on this LSA

LS age: 1101

Options: (No TOS-capability, DC)

LS Type: AS External Link

Link State ID: 0.0.0.0 (External Network Number )

Advertising Router: 10.10.0.1

LS Seq Number: 8000208E

Checksum: 0x43DB

Length: 36

Network Mask: /0

Metric Type: 1 (Comparable directly to link state metric)

TOS: 0

Metric: 20

Forward Address: 0.0.0.0

External Route Tag: 10

Routing Bit Set on this LSA

LS age: 737

Options: (No TOS-capability, DC)

LS Type: AS External Link

Link State ID: 0.0.0.0 (External Network Number )

Advertising Router: 10.10.0.2

LS Seq Number: 800017EF

Checksum: 0x9530

Length: 36

Network Mask: /0

Metric Type: 1 (Comparable directly to link state metric)

TOS: 0

Metric: 20

Forward Address: 0.0.0.0

External Route Tag: 10

LS age: 1167

Options: (No TOS-capability, DC)

LS Type: AS External Link

Link State ID: 0.0.0.0 (External Network Number )

Advertising Router: 10.63.63.1

LS Seq Number: 800047B0

Checksum: 0x9395

Length: 36

Network Mask: /0

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

TOS: 0

Metric: 1

Forward Address: 0.0.0.0

External Route Tag: 10

sh ip route 0.0.0.0

Routing entry for 0.0.0.0/0, supernet

Known via "ospf 10", distance 252, metric 69, candidate default path

Tag 10, type extern 1

Last update from 10.62.253.58 on Serial0/1/0, 00:00:27 ago

Routing Descriptor Blocks:

* 10.62.253.58, from 10.10.0.1, 00:00:27 ago, via Serial0/1/0

Route metric is 69, traffic share count is 1

Route tag 10

sh ip ospf border-routers | i 10.10.0.1

i 10.10.0.1 [49] via 10.62.253.58, Serial0/1/0, ASBR, Area 0, SPF 2469

sh ip ospf border-routers | i 10.10.0.2

i 10.10.0.2 [49] via 10.62.253.58, Serial0/1/0, ASBR, Area 0, SPF 2469

1.10.10.0.1 and 10.10.0.2 both are sending default routes

2. both are having same metric in routing table & border router table also

3.on what basis 10.10.0.1 is selected - i tried clearing the process still it's selecting 10.10.0.1 only.

4 . y not 10.10.0.2 ?

Giuseppe Larosa Thu, 11/27/2008 - 23:46

Hello Rajeev,

the physical outgoing link is only one to reach the two ASBR and it is a serial interface:

sh ip ospf border-routers | i 10.10.0.1

i 10.10.0.1 [49] via 10.62.253.58, Serial0/1/0, ASBR, Area 0, SPF 2469

sh ip ospf border-routers | i 10.10.0.2

i 10.10.0.2 [49] via 10.62.253.58, Serial0/1/0, ASBR, Area 0, SPF 2469

these paths are intraarea ( leftmost i)

From a forwarding point of view being both ASBR reachable out a single point-to-point link this behavior is correct: even if both routes were installed the forwarding path is only one.

let's see paragraph 16.4 of RFC2328 external routes calculation:

http://ip-doc.com/rfc/rfc2328#s100

10.10.0.1's LSA is older and entered first the OSPF DB.

To verify this do on router 10.10.0.1

clear ip ospf process 2429 (or whatover process-id you use on that node) the other LSA that via 10.10.0.2 should be installed or just wait over time when the LSA will be renewed by 10.10.0.1 (within 30 minutes half max LSA age)

Hope to help

Giuseppe

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