09-03-2016 01:23 PM - edited 03-05-2019 04:38 AM
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
09-03-2016 01:56 PM
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
09-03-2016 09:21 PM
09-03-2016 10:38 PM
Hi,
OSPF algoritm use this:
09-03-2016 10:45 PM
So you mean that the router R1 will choose R5( considering R5 has the highest loopback IP) to choose the external network ?
09-04-2016 05:53 AM
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
09-06-2016 04:26 AM
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
09-06-2016 10:11 AM
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
09-06-2016 10:31 AM
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
09-06-2016 11:32 AM
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 ?
09-06-2016 12:02 PM
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.
09-06-2016 12:43 PM
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
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