Misc - ip route cache cef

what is ip route cache in cisco router?

What is ip route cache cef?

How Routers takes the routing decisions?

 

Routers use the Routing Table for forwarding the packets on different routes.

Suppose a packet with destination address " 50.50.50.50" has reached the router.

Now the router has to decide in whcih interface the packet is to be sent out.

Suppose, the routing table looks like this:

ip route 0.0.0.0 0.0.0.0 s0 (which actually means, whatever be the address, simplly sent it on s1 interface.)

ip route 50.0.0.0 255.0.0.0 e1 (which means, any address of format 50.x.x.x can be sent on s2 interface)

ip route 50.50.50.0 255.255.255.0 (which means, any address of format 50.50.50.x cane be sent on s3 interface)

 

Eventhough all the three entries are applicable to the address 50.50.50.50, the router will select the route which is most appropriate ( longest match). That is , it will select s3.

 

 

 

What is ip route-cache? What is fast-switching?

 

The router will look into the routing table only for the first packet and note down the route in its cache. When further packets are coming with the same destination ip, then just by refering the cache, it decides the route. This method is faster. so it is called as fast switching.

In cisco routers, the packets are forwarded using this technque by default (since 10.x onwards).

If the router checks the routing table for each and every packet, then this method is known as PROCESS SWITCHING. By default fast-switching is enabled.

What is the difference between ip route-cache and ip route-cache cef?

While route-cache is fast-switching, you can imagine, route-cache cef is superfast switching.

 

CEF stands for Cisco Express Forwarding. In the method, not even the first packet is decided by looking into the routing table. Here, the switching is performed by hardware. No software lookup. So the speed of switching is extremely fast, that is it is equal to wire speed. The cef table will be having the layer 2 information for each lookup entry.

 

In the lastet routers, ip route-cache cef is the default method for routing.

 

If you disble ir-route cef, then ip route-cache will be running.

 

Load sharing in routers?

When two equal routes are there, then the load is shared between these routes. The sharing method can be either per-destination-based load sharing or per-packet-based load sharing.

 

The cef method uses the per-destination-based load sharing by default