OSPF load-balancing across areas

Unanswered Question
Aug 17th, 2010

Hi group,

I have an interesting scenario: i want to announce the same subnet/prefix in two different area's in OSPF.

In my topology, the cost to these two different area's is exactly the same. so i thought i would get two equal cost

routes as a result. However, i am only seeing one route being used, it looks like the one with the lowest LSA age.

Can someone explain ? What will be the tie-breaker in this scenario ?

regards,

Geert

Topology:

SUBNET A ---- AREA 1 --------------------------- CORE (AREA 0)

SUBNET A ---- AREA 2 ---------------------------------|

On the core switch, i see two summary routes in the backbone area:


>sh ip ospf database summary 10.104.244.0

            OSPF Router with ID (10.96.32.3) (Process ID 1000)

                Summary Net Link States (Area 0.0.0.0)

  LS age: 935
  Options: (No TOS-capability, DC, Upward)
  LS Type: Summary Links(Network)
Link State ID: 10.104.244.0 (summary Network Number)
  Advertising Router: 10.96.32.3
  LS Seq Number: 80000001
  Checksum: 0x5FE6
  Length: 28
  Network Mask: /24
        TOS: 0  Metric: 2

  LS age: 936
  Options: (No TOS-capability, DC, Upward)
  LS Type: Summary Links(Network)
  Link State ID: 10.104.244.0 (summary Network Number)
  Advertising Router: 10.102.90.7
  LS Seq Number: 80000001
  Checksum: 0x7F82
  Length: 28
  Network Mask: /24
        TOS: 0  Metric: 2

This is the advertisement in area 4.1.0.20 on the core:

>sh ip ospf database router 10.102.88.1

            OSPF Router with ID (10.96.32.3) (Process ID 1000)

                Router Link States (Area 4.1.0.20)

  LS age: 1834
  Options: (No TOS-capability, DC)
  LS Type: Router Links
  Link State ID: 10.102.88.1
  Advertising Router: 10.102.88.1
  LS Seq Number: 80004F51
  Checksum: 0x134F
  Length: 84
  Number of Links: 5

    Link connected to: a Stub Network
     (Link ID) Network/subnet number: 10.104.244.0
     (Link Data) Network Mask: 255.255.255.0
      Number of TOS metrics: 0
       TOS 0 Metrics: 1

<surpressed>

This is the advertisement in area 4.1.0.21 on the core:


>sh ip ospf database router 10.102.89.1

            OSPF Router with ID (10.96.32.3) (Process ID 1000)

                Router Link States (Area 4.1.0.21)

  LS age: 203
  Options: (No TOS-capability, DC)
  LS Type: Router Links
  Link State ID: 10.102.89.1
  Advertising Router: 10.102.89.1
  LS Seq Number: 80000FD1
  Checksum: 0x72A6
  Length: 84
  Number of Links: 5

    Link connected to: a Stub Network
     (Link ID) Network/subnet number: 10.104.244.0
     (Link Data) Network Mask: 255.255.255.0
      Number of TOS metrics: 0
       TOS 0 Metrics: 1

<surpressed>

However, in the routing table, i only see one route:

>sh ip route 10.104.244.0
Routing entry for 10.104.244.0/24
   Known via "ospf 1000", distance 110, metric 2, type intra area
   Last update from 10.102.89.14 on Port-channel32, 00:11:09 ago
   Routing Descriptor Blocks:
   * 10.102.89.14, from 10.102.89.1, 00:11:09 ago, via Port-channel32
       Route metric is 2, traffic share count is 1

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Giuseppe Larosa Tue, 08/17/2010 - 11:36

Hello Geert,

the last node sees an intra area route

sh ip route 10.104.244.0
Routing entry for 10.104.244.0/24
   Known via "ospf 1000", distance 110, metric 2, type intra area  <<<<
   Last update from 10.102.89.14 on Port-channel32, 00:11:09 ago
   Routing Descriptor Blocks:
   * 10.102.89.14, from 10.102.89.1, 00:11:09 ago, via Port-channel32
       Route metric is 2, traffic share count is 1

intra area route is preferred over an inter area route, this should explain why this node selects this path only: it looks like to have a link in area 4.1.0.21 (the port-channel 32).

Edit:

likely you have just post the wrong output taken from the wrong device.

Sorry if  I have pointed out something so evident.

I don't want to deny the issue you are seeing

Hope to help

Giuseppe

gnijs Fri, 08/20/2010 - 14:53

Hello Guiseppe,

Maybe i explained wrong: the last node (the one with the output above) sees the same route in two areas

Area 4.1.0.21 & Area 4.1.0.20. Cost is exactly the same in each area.

Question is then: which route will be prefered  and be put in the backbone area ?

It seems only one of the two is selected and not both routes (equal cost balancing ??) but on what base ?

regards,

Geert

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