ABRs and Summary LSAs

[dwyerr]

Hello,

I would appreciate if someone could validate my observations on how Internal routers choose summary LSAs heard from two ABRs.

It seems as though if an Internal router is directly attached to the ABRs it will use both LSAs. (also assuming metric is the same etc)

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Div-R3#show ip ospf database summary 192.168.2.4

OSPF Router with ID (9.9.9.9) (Process ID 1)

Summary Net Link States (Area 1)

Routing Bit Set on this LSA

LS age: 221

Options: (No TOS-capability, DC, Upward)

LS Type: Summary Links(Network)

Link State ID: 192.168.2.4 (summary Network Number)

Advertising Router: 1.1.1.1

LS Seq Number: 80000001

Checksum: 0xF595

Length: 28

Network Mask: /30

TOS: 0 Metric: 65

Routing Bit Set on this LSA

LS age: 1306

Options: (No TOS-capability, DC, Upward)

LS Type: Summary Links(Network)

Link State ID: 192.168.2.4 (summary Network Number)

Advertising Router: 2.2.2.2

LS Seq Number: 80000001

Checksum: 0xD7AF

Length: 28

Network Mask: /30

TOS: 0 Metric: 65

Div-R3#show ip ospf border-routers

OSPF Process 1 internal Routing Table

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

i 1.1.1.1 [1] via 10.1.1.2, Vlan100, ABR, Area 1, SPF 139

i 2.2.2.2 [1] via 10.1.1.1, Vlan100, ABR, Area 1, SPF 139

Div-R3#show ip route 192.168.2.4

Routing entry for 192.168.2.4/30

Known via "ospf 1", distance 110, metric 66, type inter area

Last update from 10.1.1.2 on Vlan100, 00:07:53 ago

Routing Descriptor Blocks:

* 10.1.1.1, from 2.2.2.2, 00:07:53 ago, via Vlan100

Route metric is 66, traffic share count is 1

10.1.1.2, from 1.1.1.1, 00:07:53 ago, via Vlan100

Route metric is 66, traffic share count is 1

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If the Internal router is not directly attached to the ABR it chooses the ABR with the highest router id.

====================

Hub-u1#show ip ospf database summary 192.168.2.4

OSPF Router with ID (6.6.6.6) (Process ID 1)

Summary Net Link States (Area 1)

Routing Bit Set on this LSA

LS age: 1864

Options: (No TOS-capability, DC, Upward)

LS Type: Summary Links(Network)

Link State ID: 192.168.2.4 (summary Network Number)

Advertising Router: 1.1.1.1

LS Seq Number: 80000002

Checksum: 0xF396

Length: 28

Network Mask: /30

TOS: 0 Metric: 65

Routing Bit Set on this LSA

LS age: 1762

Options: (No TOS-capability, DC, Upward)

LS Type: Summary Links(Network)

Link State ID: 192.168.2.4 (summary Network Number)

Advertising Router: 2.2.2.2

LS Seq Number: 80000002

Checksum: 0xD5B0

Length: 28

Network Mask: /30

TOS: 0 Metric: 65

Hub-u1#show ip ospf border-routers

OSPF Process 1 internal Routing Table

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

i 1.1.1.1 [2] via 10.2.2.253, FastEthernet0/1, ABR, Area 1, SPF 33

i 1.1.1.1 [2] via 10.2.2.249, FastEthernet0/0, ABR, Area 1, SPF 33

i 2.2.2.2 [2] via 10.2.2.253, FastEthernet0/1, ABR, Area 1, SPF 33

i 2.2.2.2 [2] via 10.2.2.249, FastEthernet0/0, ABR, Area 1, SPF 33

Hub-u1#show ip route 192.168.2.4

Routing entry for 192.168.2.4/30

Known via "ospf 1", distance 110, metric 67, type inter area

Last update from 10.2.2.249 on FastEthernet0/0, 01:05:12 ago

Routing Descriptor Blocks:

10.2.2.253, from 2.2.2.2, 01:05:12 ago, via FastEthernet0/1

Route metric is 67, traffic share count is 1

* 10.2.2.249, from 2.2.2.2, 01:05:12 ago, via FastEthernet0/0

Route metric is 67, traffic share count is 1

====================

I was under the impression that this behaivor was unique to summaries from an NSSA area when there is type7/type5 conversion.

Thanks.

-Rob

[Harold Ritter]

Or let me rephrase that. What would it buy you to use the summary LSA from both ABR A and ABR B when they lead you to the same results.

Hope this helps,

[Harold Ritter]

Rob,

This is normal behavior given the summary route is received from two ABRs that are reachable not only with the same cost but also via the same next hops (10.2.2.249 and 10.2.2.253).

Hope this helps,

[Harold Ritter]

BTW, the behavior for lsa type 7 to 5 translation is a totally different case since only the ABR with the highest RID will do the translation. In this scenario you are describing, the internal router receives from both ABRs and uses the summary from the ABR with the highest RID.

Hope this helps,

[dwyerr]

Thanks Harold, and yes in retrospect the comparison to nssa was a little flawed.

I wonder though, why ignore the second LSA in this scenario ? Doesn't seem to buy you anything.

-Rob

[Harold Ritter]

It doesn't matter if you pick the LSA from ABR A or B since the internal router has exactly the same cost and same paths to either ABRs. This doesn't prevent loadbalancing.

Hope this helps,