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OSPF load sharing

Hello,

We have two OSPF equal cost learned routes to same subnet and we want that the traffic destined to that subnet need to be load balanced between two links. 

The output of show ip route shows that there is traffic share to the destination 10.116.4.1

show ip route 10.116.4.1
Routing entry for 10.116.4.1/32
Known via "ospf 1", distance 110, metric 101, type intra area
Last update from 10.216.2.30 on BDI300, 03:19:46 ago
Routing Descriptor Blocks:
* 10.216.4.2, from 10.116.4.1, 03:19:46 ago, via BDI100
Route metric is 101, traffic share count is 1
10.216.2.30, from 10.116.4.1, 03:19:46 ago, via BDI300
Route metric is 101, traffic share count is 1

 

CEF is enabled and per-destination load sharing is used (per-packet load sharing is not available on ASR920).

But all the traffic destined to the 10.116.4.1 is redirected through one link which is BDI100.

How can we do to load balance the traffic destined to 10.116.4.1 through the two links ?

Thanks in advance.

 

6 Replies 6

balaji.bandi
Hall of Fame
Hall of Fame

Can you post fuil confioguration or check below confiured on Global level ?

platform loadbalance max-paths 2 

 

also post the output  of below :

traceroute to 10.116.4.1 from the device ?

show  ip cef 10.116.4.1

BB

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Hello Balaji,

Thank you for your reply. The command "platform loadbalance max-paths 2" is not configured on the global level and the ASR920 can't recognize this command.

 

#traceroute 10.116.4.1
Type escape sequence to abort.
Tracing the route to 10.116.4.1
VRF info: (vrf in name/id, vrf out name/id)
1 10.216.4.2 36 msec
10.216.2.30 64 msec
10.216.4.2 24 msec

 

#show ip cef 10.116.4.1
10.116.4.1/32
nexthop 10.216.2.30 BDI300
nexthop 10.216.4.2 BDI100

You have CEF enabled in default mode (flow based). If you only have a few flows, it is possible that the two links will not be used equally. You should use the following command to see which next hop will be used for each different flow:

 

sh ip cef exact-route <source ip address> 10.116.4.1

 

You can vary the source and see how the next hop changes.

 

Regards,

Harold Ritter
Sr Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México

In addition to what Harold has already noted, although as he notes asymmetrically load balancing happens most often with few flows, it can also still happen with lots of flows if few are very heavy compared to the many and/or lots of flows between a few hosts.

Sometimes, if you can increase the number of logically routed paths, load balancing improves.

Perhaps the best load balancing is provided by something like PfR/PIRO did, which did dynamic load balancing of flows.

#traceroute 10.116.4.1
Type escape sequence to abort.
Tracing the route to 10.116.4.1
VRF info: (vrf in name/id, vrf out name/id)
1 10.216.4.2 36 msec
10.216.2.30 64 msec
10.216.4.2 24 msec

As per the output, i do see that it is LB, but if we consider only 1 source that might have to go with one direction.

 

how big is the system and traffic in the network?

 

BB

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Hi friend even if CEF pre-destination, if the source " and this case I think the source is NAT from Multi-to-One" is same then the ASR will select always the same path. 

as Friend suggest try use different source. 

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