How do OSPF and EIGRP deal with 2 exactly same routes?

Question

If you refer this diagram,

Will ospf and eigrp share the 10 network on the right to their neighbors?

I am asking the question in two separate scenarios using a single protocol at a time and not running both protocols at once.

As per my understanding link state protocols like ospf do so while eigrp being a distance vector protocol will share only one of the routes, but this case let's say both the routes are exactly same, how will this be handled then?

Answer 1

Will ospf and eigrp share the 10 network on the right to their neighbors?

As per my understanding link state protocols like ospf do so while eigrp being a distance vector protocol will share only one of the routes, but this case let's say both the routes are exactly same, how will this be handled then?

By default, the router will install the route learned via EIGRP into the routing table. This is because EIGRP has a lower Administrative Distance than OSPF.

It should also be noted that both learned routes will remain in their respective routing processes. Thus, the router knows about both but will only select one route when routing.

Routers select routes in the following order:

1. Prefix Length - The longest-matching route is preferred first. Prefix length trumps all other route attributes.
2. Administrative Distance - In the event there are multiple routes to a destination with the same prefix length, the route learned by the protocol with the lowest administrative distance is preferred.
3. Metric - In the event there are multiple routes learned by the same protocol with same prefix length, the route with the lowest metric is preferred. (If two or more of these routes have equal metrics, load balancing across them may occur.)

Jeremy Stretch does a great job going into deep details and examples of route selection here.

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EDIT:

I am asking the question in two separate scenarios using a single protocol at a time and not running both protocols at once.

In the event of all things equal and the routing process supports it, equal-cost load balancing will occur.

Exactly how the router handles the load-balancing depends on your configuration, version of IOS and potentially your router platform. Cisco dives into the matter with the following documents.

• How Does Load Balancing Work?
• Troubleshooting Load Balancing Over Parallel Links Using Cisco Express Forwarding

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What does it look like in the routing table?

The following is an example output of EIGRP speaking routers with an equal-cost route for 10.1.1.0/24:

ROUTER2#show ip route
[...omitted text...]
      10.0.0.0/8 is variably subnetted, 5 subnets, 2 masks
D        10.1.1.0/24 [90/30720] via 10.1.3.2, 00:00:17, FastEthernet1/0
                     [90/30720] via 10.1.2.2, 00:00:17, FastEthernet0/0
C        10.1.2.0/24 is directly connected, FastEthernet0/0
L        10.1.2.1/32 is directly connected, FastEthernet0/0
C        10.1.3.0/24 is directly connected, FastEthernet1/0
L        10.1.3.1/32 is directly connected, FastEthernet1/0

Here is an example for OSPF:

ROUTER2#show ip route
[...omitted text...]
      10.0.0.0/8 is variably subnetted, 5 subnets, 2 masks
O        10.1.1.0/24 [110/2] via 10.1.3.2, 00:00:02, FastEthernet1/0
                     [110/2] via 10.1.2.2, 00:00:02, FastEthernet0/0
C        10.1.2.0/24 is directly connected, FastEthernet0/0
L        10.1.2.1/32 is directly connected, FastEthernet0/0
C        10.1.3.0/24 is directly connected, FastEthernet1/0
L        10.1.3.1/32 is directly connected, FastEthernet1/0

Answer 2

If I understand your question correctly, R2 will learn two paths to the 10 network. Assuming equal metrics, both protocols will put both routes into the routing table, and the router will load balance between them. R1, of course will learn only one path -- through R2.

@tdurden gives you good references for understanding how Cisco routers perform load balancing.

Answer 3

Cisco manages traffic through what they call a "flow". When traffic arrives, it is evaluated for the 5 tuple: * Protocol (TCP, UDP, ICMP, OSPF, EIGRP) * Source IP address * Source Port * Destination IP address * Destination Port

All subsequent traffic that is identified as part of an existing flow is sent across the same link to avoid out-of-order packet (like a voice conversation that is UDP without sequence numbers). In the case of load balancing, it's effectively assigning each flow round robin.

If you ping, while watching a youtube video, the ICMP response may alternate between links due to the fact that ICMP is stateless, while watching a youtube video is essentially pinned to a link.

Both EIGRP and OSPF support Equal Cost Load Balancing.

EIGRP supports unequal cost load balancing, but it must pass the feasibility condition which in small environments usually fails; and the alternate route ends up unused.

Here in OSPF, the Cost is 2 for both routes. Therefore traffic will flow equally through both

[110/2] via 10.1.3.2, 00:00:02, FastEthernet1/0

[110/2] via 10.1.2.2, 00:00:02, FastEthernet0/0

Here in EIGRP, the cost is 30720 for both routes. Therefore traffic will flow equally through both [90/30720] via 10.1.3.2, 00:00:17, FastEthernet1/0

[90/30720] via 10.1.2.2, 00:00:17, FastEthernet0/0