OSPF Load Balancing Across Equal Cost Path

Answered Question
Nov 21st, 2009
User Badges:

Site A (rtr1) has two T3 connected to Site B (rtr2 and rtr3).

One of the T3 is connected to rtr2 and the second connects to rtr3.


I want OSPF to load balance across the two ciruict for better utilization.

Questions:

1. Does load balancing have to be on the same routers in Site B?

2. Is this a good idea or bad idea if both links are equal cost, but on different routers?

3. Would there be a problem with packet being received out of order?

4. How can I verify that the packets are being recieved out of order?


Thanks in advance.

Correct Answer by Giuseppe Larosa about 7 years 6 months ago

Hello David,

if you are using default CEF load balancing per destination you shouldn't have any out of order issue.


being the links T3 WAN links clearly there are no firewalls on the path.


with default CEF load balancing a path is chosen for each IP flow based on IP source address and IP destination address.

if you have enabled per packet load balancing at the interface level on R1 there is a potential for out of order packets and this is not recommended as explained by Paolo.


On second site the one with two routers the capability to use both links for sending traffic to site1 depends on how the site is configured: if there are internal routers or multilayer switches they can use both links because OSPF can report two equal cost paths one via R2 and one via R3.

But if R2 and R3 serve as default gateway on client vlans you may need to use GLBP to use both links in direction site2 to site1 because HSRP has only one router active at each given time for each HSRP group.


Hope to help

Giuseppe

Correct Answer by Jon Marshall about 7 years 6 months ago

davidhuynh5 wrote:


Site A (rtr1) has two T3 connected to Site B (rtr2 and rtr3).

One of the T3 is connected to rtr2 and the second connects to rtr3.


I want OSPF to load balance across the two ciruict for better utilization.

Questions:

1. Does load balancing have to be on the same routers in Site B?

2. Is this a good idea or bad idea if both links are equal cost, but on different routers?

3. Would there be a problem with packet being received out of order?

4. How can I verify that the packets are being recieved out of order?


Thanks in advance.


David


1) No it doesn't.

2) Good idea if you are paying for both links ie. you can actually have a "shadow" circuit that is only used if the primary link goes down and you are not charged for the shadow link but if you are paying for both why not use both.

3) As previous poster, firewalls are one of the main issues with out of order packets or any device that keeps state for that matter.

4) You would need to use a sniffer on the end host to see the sequence number of the packets.


Jon

  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 4.7 (3 ratings)
Loading.
mmacdonald70 Sat, 11/21/2009 - 19:15
User Badges:

I can't see any issues with the multipath configuration.  As long as you are not running a stateful firewall, there shouldn't be any issue with packets coming in out of order

paolo bevilacqua Sun, 11/22/2009 - 02:54
User Badges:
  • Super Gold, 25000 points or more
  • Hall of Fame,

    Founding Member

there shouldn't be any issue with packets coming in out of order


No issue except TCP performance will become misarable, and certain applications may stop working misteriously.


Out of order is bad thing, do not tae is ligthly.

Correct Answer
Jon Marshall Sun, 11/22/2009 - 02:36
User Badges:
  • Super Blue, 32500 points or more
  • Hall of Fame,

    Founding Member

  • Cisco Designated VIP,

    2017 LAN, WAN

davidhuynh5 wrote:


Site A (rtr1) has two T3 connected to Site B (rtr2 and rtr3).

One of the T3 is connected to rtr2 and the second connects to rtr3.


I want OSPF to load balance across the two ciruict for better utilization.

Questions:

1. Does load balancing have to be on the same routers in Site B?

2. Is this a good idea or bad idea if both links are equal cost, but on different routers?

3. Would there be a problem with packet being received out of order?

4. How can I verify that the packets are being recieved out of order?


Thanks in advance.


David


1) No it doesn't.

2) Good idea if you are paying for both links ie. you can actually have a "shadow" circuit that is only used if the primary link goes down and you are not charged for the shadow link but if you are paying for both why not use both.

3) As previous poster, firewalls are one of the main issues with out of order packets or any device that keeps state for that matter.

4) You would need to use a sniffer on the end host to see the sequence number of the packets.


Jon

Correct Answer
Giuseppe Larosa Sun, 11/22/2009 - 05:10
User Badges:
  • Super Silver, 17500 points or more
  • Hall of Fame,

    Founding Member

Hello David,

if you are using default CEF load balancing per destination you shouldn't have any out of order issue.


being the links T3 WAN links clearly there are no firewalls on the path.


with default CEF load balancing a path is chosen for each IP flow based on IP source address and IP destination address.

if you have enabled per packet load balancing at the interface level on R1 there is a potential for out of order packets and this is not recommended as explained by Paolo.


On second site the one with two routers the capability to use both links for sending traffic to site1 depends on how the site is configured: if there are internal routers or multilayer switches they can use both links because OSPF can report two equal cost paths one via R2 and one via R3.

But if R2 and R3 serve as default gateway on client vlans you may need to use GLBP to use both links in direction site2 to site1 because HSRP has only one router active at each given time for each HSRP group.


Hope to help

Giuseppe

Actions

This Discussion