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LSA Type-5, not honoring metrics?

Hi, i've read http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080124c7d.shtml and it is stated

"When the metric of the redistributed route from multiple ASBRs are equal as illustrated in the document, the forwarding address changes the behavior of the type 5 LSA path selection. When a router receives two type 5 LSAs to the same destination with the forwarding addresses set on both LSAs, the router makes a comparison based on the metric to the forwarding addresses. The LSA with a forwarding address that offers the smaller metric is placed into the routing table.

If the metric of the redistributed routes are different, the routers prefer the route with the lowest metric and not the lowest metric to the forwarding address."

Well, I have similar case but different outcome.

  Routing Bit Set on this LSA
  LS age: 1049
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 10.10.10.0 (External Network Number )
  Advertising Router: 10.52.24.11
  LS Seq Number: 80001072
  Checksum: 0x8A61
  Length: 36
  Network Mask: /24
        Metric Type: 1 (Comparable directly to link state metric)
        TOS: 0 
        Metric: 10 
        Forward Address: 0.0.0.0
        External Route Tag: 16

  Routing Bit Set on this LSA
  LS age: 1174
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 10.10.10.0 (External Network Number )
  Advertising Router: 10.52.56.1
  LS Seq Number: 80001072
  Checksum: 0xC008
  Length: 36
  Network Mask: /24
        Metric Type: 1 (Comparable directly to link state metric)
        TOS: 0 
        Metric: 12 
        Forward Address: 0.0.0.0
        External Route Tag: 27

  LS age: 1986
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 10.10.10.0 (External Network Number )
  Advertising Router: 10.52.56.2
  LS Seq Number: 80000022
  Checksum: 0xDB44
  Length: 36
  Network Mask: /24
        Metric Type: 1 (Comparable directly to link state metric)
        TOS: 0 
        Metric: 20 
        Forward Address: 0.0.0.0
        External Route Tag: 27

From the above LSA Type-5, I should see the route prefers the 1st LSA as it has the lowest metric. However, it chooses 2nd LSA as it has the lower metric to ASBR.

Router#sh ip ospf border-routers 

OSPF Process 100 internal Routing Table

Codes: i - Intra-area route, I - Inter-area route

i 192.168.160.29 [1] via 192.168.160.29, Vlan750, ASBR, Area 0, SPF 92
i 192.168.160.129 [1] via 192.168.160.129, Vlan750, ASBR, Area 0, SPF 92
i 10.52.24.2 [2] via 192.168.160.33, Vlan750, ASBR, Area 0, SPF 92
i 10.52.24.1 [2] via 192.168.160.33, Vlan750, ASBR, Area 0, SPF 92
i 10.52.24.11 [2] via 192.168.160.33, Vlan750, ASBR, Area 0, SPF 92
i 10.52.24.13 [1] via 192.168.160.33, Vlan750, ASBR, Area 0, SPF 92
i 10.52.56.2 [1] via 10.52.63.246, Port-channel10, ASBR, Area 0, SPF 92
i 10.52.56.1 [1] via 10.52.63.242, Port-channel9, ASBR, Area 0, SPF 92
i 10.52.56.11 [1] via 10.52.63.130, Port-channel2, ASBR, Area 0, SPF 92

Can someone explain why?

rgds,

David Sudjiman

3 REPLIES
Hall of Fame Super Silver

Re: LSA Type-5, not honoring metrics?

Hello David,

your LSAs are external type 1 not type 2 so metric to reach ASBR is summed to seed metric in external LSA

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

This means that you need to calculate the SPF taking in account cost to reach ASBR

first LSA = 10 + 2

second LSA = 12 +1

it should choice the first, but there is one specific issue that I see:

first LSA :LS Seq Number: 80001072

second LSA: LS Seq Number: 80001072

This should not happen two LSAs generated by different OSPF routers should have different and unique sequence numbers for this reason the younger entry is not considered.

Are these real routers or you are running dynamips ?

Hope to help

Giuseppe

Community Member

Re: LSA Type-5, not honoring metrics?

Giuseppe, Thanks for the explanation.

I'm aware of the metric calculation and you're right about it. This is a live in-production network.

Based on your explanation it should choose the metric first then older LSA seq #, but it's not.

Could someone please elaborate?

thx

David Sudjiman

Community Member

Re: LSA Type-5, not honoring metrics?

It this because the next-hop

10.52.24.11 [2]
and
10.52.56.1 [1]

Therefore is chooses 10.52.56.1?

thx

David Sudjiman

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