Introduction:
With the introduction of rfc2328, or OSPFv2, the path selection criteria for external or summary routes changed.
Specifically, the new changes call for preferring a non-backbone path over a backbone(area 0) path for the prefix.
Previously, we relied on cost to determine path selection, without regard to whether it was a backbone or non-backbone path.
IOS has always treated rfc1583 as the default behavior for path selection, and as support for rfc2328 was added, IOS added a toggle to prefer this new method of path selection, but continued to keep rfc1583 behavior set by default.
NX-OS, however, was launched with rfc2328 only behavior by default, and then to conform with RFC specs, added a command to make it optionally rf1583 compliant.
Problem Statement:
This difference in default behavior can lead to ospf loops, where you have some devices pointing to one path as the shortest path, and other devices pointing to a different path as the shortest path.
The scenario below illustrates this problem:
ASBR1----area0-----(e3/7)ABR(e2/10)----area100----(G0/0)rtrB-----area100---ASBR2
ABR is an n7k, and rtrB is an IOS router (2800 in this case).
Both ASBR1 and ASBR2 redistribute a route, same prefix (192.168.0.0/16) into OSPF.
With their default configurations, the following takes place in the routing tables:
ABR# show ip route 192.168.0.0
192.168.0.0/16, ubest/mbest: 1/0
*via 10.0.0.2, Eth2/10, [110/160], 00:03:00, ospf-loop, type-1
------------------------------------------------------------------------------------------------------
rtrB#show ip route 192.168.0.0
Routing entry for 192.168.0.0/16, supernet
Known via "ospf 1", distance 110, metric 25, type extern 1
Last update from 10.0.0.1 on GigabitEthernet0/0, 00:20:40 ago
Routing Descriptor Blocks:
* 10.0.0.1, from 2.2.2.2, 00:20:40 ago, via GigabitEthernet0/0
Route metric is 25, traffic share count is 1
As you can see, the IOS router points out g0/0 to the n7k, and the n7k(ABR) points out e2/10 to rtrB. This is a loop!
Now, we will turn on the rfc1583compatibility command on the n7k(ABR) and it will allow us to use the metric for our decision, like the IOS router is doing.
ABR(config)# router ospf loop
ABR(config-router)# rfc1583compatibility
ABR# show ip route 192.168.0.0
192.168.0.0/16, ubest/mbest: 1/0
*via 10.10.10.1, Eth3/7, [110/24], 00:00:05, ospf-loop, type-1
ABR#
---------------------------------------------------------------
rtrB#show ip route 192.168.0.0
Routing entry for 192.168.0.0/16, supernet
Known via "ospf 1", distance 110, metric 25, type extern 1
Last update from 10.0.0.1 on GigabitEthernet0/0, 00:30:03 ago
Routing Descriptor Blocks:
* 10.0.0.1, from 2.2.2.2, 00:30:03 ago, via GigabitEthernet0/0
Route metric is 25, traffic share count is 1
As you can see, if all the ospf routers in this domain are configured to run according to the same rfc mode, then we do not experience these same loops, as the complete domain is consistent in what it considers to be the shortest path.
Configuration:
On NX-OS, you can use the "rfc1583compatibility" command to make the device align with default IOS behavior.
On IOS, you can use the "no compatible rfc1583" command to make the device align with default Nexus behavior.
Which ever of the two options you use is your choice, but I will not go into the major design differences here in this post.
