OSPF extrnal network tie breaker

[Vijay Bhargav]

Hi All,

Consider I have 4 routers which have a leg towards an external network. If all have equal costs to reach this external network, then which ASBR will advertise the routes to it peers in that area

 ___ R2 -- |

 |___ R3 --| external  

R1 -- broadcast network --|___ R4 --| network

 |___R5 --|

Note : R1 cost to reach any of R2, R3, R4, & R5 is of same cost

[dmuinoorallo]

I need more information about your issue.

Same cost. Ok, same are? what area? any special configuration like nssa, etc?

An area is interface specific. A router that has all of its interfaces within the same area is called an internal router (IR). A router that has interfaces in multiple areas is called an area border router (ABR). Routers that act as gateways (redistribution)between OSPF and other routing protocols (IGRP, EIGRP, IS-IS, RIP, BGP, Static) or other instances of the OSPF routing process are called autonomous system boundary router (ASBR). Any router can be an ABR or an ASBR.

more info: http://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/7039-1.html

[Vijay Bhargav]

Hi,

Thanks for the reply. I guess attached snapshot will clarify your doubts and I hope you will help me in clarifying my issue.

[dmuinoorallo]

Hi,

OSPF algoritm use this:

1. Use the cost
2. Use the interface priority
3. Use the numerical highest Router-ID
4. Use the highest loopback interface
5. Use the highest physical interface

[Vijay Bhargav]

So you mean that the router R1 will choose R5( considering R5 has the highest loopback IP) to choose the external network ?

[Richard Burts]

The original post asks about a tie breaker for OSPF external routes which implies an assumption that OSPF needs to choose a single path as the best path. But OSPF supports equal cost load sharing. So in the example given there will be 4 ASBRs with a route to the external network. Each ASBR will advertise its route to R1 and the OSPF table in R1 will have 4 equal cost routes to the external network.

HTH

Rick

[Vijay Bhargav]

Hi Rick,

Thanks for the reply.

Sorry to bug you all .I have some doubt again.

Attached is the scenario. I have tried this however R1 ip route output is as below,

R1>sh ip route 10.0.0.0
Routing entry for 10.0.00/16
  Known via "ospf 1234", distance 110, metric 11, type extern 1
  Last update from 10.X.X.3 on Tunnel0, 00:00:31 ago
  Routing Descriptor Blocks:
  * 10.X.X.3, from 10.X.X.107, 00:00:31 ago, via Tunnel0
      Route metric is 11, traffic share count is 1

10.X.X.3 - is the p2p link between R1 and R2

10.X.X.107 is the router-id for R5

Not sure y it is preferring R5

R1>sh ip os database external 10.0.0.0

            OSPF Router with ID (10.X.X.108) (Process ID 1234)

                Type-5 AS External Link States

  Routing Bit Set on this LSA in topology Base with MTID 0
  LS age: 1121
  Options: (No TOS-capability, DC, Upward)
  LS Type: AS External Link
  Link State ID: 10.0.0.0 (External Network Number )
  Advertising Router: 10.X.X.107
  LS Seq Number: 80000159
  Checksum: 0x835D
  Length: 36
  Network Mask: /16
        Metric Type: 1 (Comparable directly to link state metric)
        MTID: 0
        Metric: 0
        Forward Address: 172.X.X.6
        External Route Tag: 0

[dmuinoorallo]

Hi,

OSPF allows every router to calculate its own routing table based on the LSAs that they receive. So they must all agree on the relationship between costs and bandwidth. Suppose you set the reference bandwidth differently on two routers, so that they advertise different link costs for their Ethernet interfaces. This could cause seriously strange routing patterns as OSPF will try to avoid using the higher cost links. For example, that a Fast Ethernet interface on one router is faster than a Gigabit Ethernet interface on the other route.

Since the OSPF cost derives from interface bandwidth, changing the interface bandwidth value should change the cost.

Router#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router(config)#interface Ethernet0
Router(config-if)#bandwidth 10000000
Router(config-if)# exit

The other way of affecting the cost of an interface is through the "ip ospf cost" command on the interface itself, this will override the calculated cost.

Router1(config)#int f1/0
Router1(config-if)#ip ospf cost 12345
Router(config-if)# exit

You can also set the cost per neighbor using the "neighbor (neighbor-ip) cost value" command. This command is valid only on OSPF network types that allow neighbor command like point-to-multipoint non broadcast.

Router(config)#router ospf 100
Router(config-router)#neighbor 10.1.1.2 cost 5000
Router(config-router)#neighbor 10.1.1.3 cost 2000
Router(config-router)#exit

[Richard Burts]

I do not understand what is involved here. The original diagram showed R1 connecting through a switch to the other 4 routers and specified equal cost connections. Now the diagram shows R1 apparently with two connections and the connections show different costs. Can you clarify what is involved here?

HTH

Rick

[Vijay Bhargav]

Hi Rick, sorry for the confusion.

Can you please look into the latest diagram and suggest y routing information from R1 is getting destined to R5 rather than R2 ?

[dmuinoorallo]

The destination is Tunel 0, i need more information, but i don't know what is tunnel0, but in your schema Router 5 is DR and all routers will send announced by this route.

[Richard Burts]

I agree that we do not have enough information to figure out what is going on here. One drawing showed what seems to be connection to 4 routers and the second drawing shows what seems to be two connections. We need a clear understanding of the topology here.

The output shows that the connection is over tunnel0. So what is tunnel0? We need config details from each of the 5 routers to be able to understand the relationships here.

HTH

Rick