WAN Link - Possibility Scenario

[Amin Shaikh]

Hello,

Is it possible to combine Two WAN Links to use it more efficiently.

Two WAN Links is different media

(One is Wireless and another is serial)

Wireless Media : 7MB

Serial Media : 5MB

[Giuseppe Larosa]

Hello Amin,

you could think to use EIGRP with variance that supports weighted load balancing over unequal cost paths.

this means using EIGRP as your routing protocol.

see

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a008009437d.shtml

Hope to help

Giuseppe

[Amin Shaikh]

Hello Giuseppe,

Thanks for your kind reply.

final design for core network is attached.

can you help me understand what would be the difference in using OSPF and EIGRP as per my scenario.

I didnt understood how to configure the variance, looking at the attached file can you help.

**Location_A***

WAN_IP From Location_A to Location_B #7MB

interface VLAN 10

IP address 10.255.254.1 255.255.255.248

Interface fa 0/10

switchport mode access

switchport access vlan 10

WAN_IP From Location_A to Location_B #5MB

interface VLAN 11

IP address 10.255.254.9 255.255.255.248

Interface fa 0/11

switchport mode access

switchport access vlan 11

WAN_IP From Location_A to Location_C #6MB

interface VLAN 12

IP address 10.255.254.17 255.255.255.248

Interface fa 0/15

switchport mode access

switchport access vlan 12

Router eigrp 2

network 10.255.254.9

network 10.255.254.1

network 10.255.254.17

**Location_B***

WAN_IP From Location_B to Location_A #7MB

interface VLAN 10

IP address 10.255.254.2 255.255.255.248

Interface fa 0/10

switchport mode access

switchport access vlan 10

WAN_IP From Location_B to Location_A #5MB

interface VLAN 11

IP address 10.255.254.10 255.255.255.248

Interface fa 0/11

switchport mode access

switchport access vlan 11

Router eigrp 2

network 10.255.254.2

network 10.255.254.10

**Location_C***

WAN_IP From Location_C to Location_A #6MB

interface VLAN 12

IP address 10.255.254.18 255.255.255.248

Interface fa 0/15

switchport mode access

switchport access vlan 12

Router eigrp 2

network 10.255.254.18

[Giuseppe Larosa]

Hello Amin,

>> can you help me understand what would be the difference in using OSPF and EIGRP as per my scenario.

with OSPF the slower link is never used for sending/receiving user traffic until the faster one is alive.

Making the two links equal cost paths with ip OSPF cost changes like

int typex/y

ip ospf cost w

this is not recommended because the two links are not equal.

With EIGRP you could get a weighted load sharing that is:

if the two links have BW ratio 2:1 the faster link gets 2/3 of traffic and the slower link 1/3

this is the reason why EIGRP could be preferred.

The variance is a multiplier a factor used to say:

take in account feasible successors for the route if the metric as calculated from the local node is less then variance times the best metric to the route.

To find the right variance value to use you need to use

sh ip eigrp topology and to look for IP routes of site A on site B router

However, because the topology has also other possible paths between site B and site A for example via site C and site D you can also use OSPF if you make the direct links between site B and Site A backup links.

if you want to use direct links between site A and site B the best choice is EIGRP.

Hope to help

Giuseppe

[Joseph W. Doherty]

re: OSPF

and

"this is not recommended because the two links are not equal. "

Giuseppe recommendation is correct, but as a recommendation, doesn't mean you always need to adhere to it.

If links are somewhat close in bandwidth, and 5 vs. 7 might so be considered, making them look equal can do more good vs. harm. Also, the fact that their bandwidth are below typical LAN bandwidths means it's possible a single flow can fill either link. I.e., there might be little benefit to using EIGRP's variance capability because routing will direct flows to the various paths, but doesn't (normally) track actual link load. (This also means, you might just "lie" to EIGRP, and have it also consider the paths equal cost. [In other past posts, another poster was extremely concerned about EIGRP variance processing load, although I'm unaware how much an issue it is.])

[Amin Shaikh]

Thanks Giuseppe & josephdoherty

In OSPF using " IP OSPF COST" I can decide which link will be primary, if there are more than one route to the same destination. Is there any other option in OSPF rather than using OSPF COST.

Is there anything in ospf "Automatic BEST Route Selection" if there are multiple route to same destination.

[Joseph W. Doherty]

On Cisco routers, bandwidth will usually set OSPF cost. So, if you configure bandwidths to be 5 and 7 Mbps, Cisco's OSPF would prefer and just use the 7 Mbps path, but if you set both links bandwidth to 6 Mbps, Cisco OSPF would set them to equal OSPF cost and use both.

[Amin Shaikh]

Thanks

Correct me if I understood correctly if the bandwidth is set to 6mbps on both links then traffic would be send on both links.

I need to understand if the config is correct.

Assume I have following network on Site A

192.168.1.0/24

192.168.2.0/24

192.168.99.0/24

and in OSPF process I have

router ospf 5

network 192.168.0.0 0.0.255.255 area 0

what are the disadvantage of using this, assuming site B is connected to Site A

Site B

network 192.168.10.0/24

router ospf 6

network 192.168.0.0 0.0.255.255 area 0

[Joseph W. Doherty]

If your two links between A & B end have the same OSPF cost, if manually costed or configured with same bandwidths and Cisco autocosted, and they're within the same area, OSPF will use both (by default, usually true for up to 4).

The disadvantage of treating different links as the same bandwidth when they're not, flows will be directed to each not in proportion to links' actual bandwidths. (Usually, if links are close in bandwidths, usually not much of a difference vs. proportional flow directing. If pure OSPF, your choices are all or none [only equal cost routing suported], with EIGRP you do have the option to use them proportionally.)

As to your networks, don't see any in common between A & B.