Static routes selection

[Konstantin Alekseev]

Hi all!

I have one question about selecting static routes:

If router have two routes to some destination like:

ip route 192.168.1.1 255.255.255.255 172.16.1.1 (tunnel 1)

ip route 192.168.1.1 255.255.255.255 172.16.2.1 (tunnel 2)

Status of tunnels is UP.

Both routes will appear in routing table.

The question is: what criteria does router use to choose final route? 

[syed kazim abbas]

If you configure more than one static route to the same destination, and the routes have different next-hop gateways but have the same metrics and administrative distance, the Layer 3 device load balances among the routes using basic round-robin. 

HTH

kazim

[Konstantin Alekseev]

I have to notice, that "show ip route" always shows the first route, that has been added to router, as used.

And there is no  load balancing.

May be router uses the oldest route?

[syed kazim abbas]

you are right, router perform load balancing if both routes are in routing table. if exit one then used only one, no load balancing.

you have to verify both tunnels are up and next hope are valid.

HTH

[Konstantin Alekseev]

All tunnels are up. And both routes in routing table. But if i do tracert router always uses the oldest route. When i do "sh ip route" the oldest route is always with * and router uses it. I have no load balancing only the oldest route.

[syed kazim abbas]

you try extended ping utility with record route option, it will clear to you.

make 3-4 test with different source and destination combination.

[hkkalra]

Hi,

If you put in two static Routes on a Router both for the exact same destination with both the interface up. Router will start doing load balancing for that destination prefix, however this load balancing is not exactly per packet. CEF is used to do equal cost load balancing. 

How CEF load balancing works

CEF is an advanced Layer 3 switching technology inside a router. Usually a router uses a route cache to speed up packet forwarding. The route cache is filled on demand when the first packet for a specific destination needs to be forwarded. If the destination is on a remote network reachable via a next hop router, the entry in the route cache is consisting of the destination network. If parallel paths exist this does not provide load balancing, as only one path would be used. Therefor the entry in the route cache now relates to a specific destination address, or host. If multiple hosts on the destination network are receiving traffic a route cache entry for each individual host is made, balancing the hosts over the available paths. This provides per destination load balancing. The problem that arises is that for a backbone router carrying traffic for several thousands of destination hosts a respective number of cache entries is needed. This consumes memory and makes cache maintenance a demanding task. In addition the decision about which path to use is done at the time the route-cache is filled, and it is based on the utilization of the individual links at that point in time. However the amount of traffic on individual connections can change over time, possibly leading to a situation where some links carry mostly idle connections and others are congested. CEF takes a different approach as it calculates all information necessary for the forwarding task in advance and decouples the forwarding information from the next hop adjacency, which allows for effective load balancing.

For further understanding of how CEF works please use the following link. 

http://www.cisco.com/en/US/products/hw/modules/ps2033/prod_technical_reference09186a00800afeb7.html#wp16220

Regards, 

HK