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Switching Methods (Process, Fast Forwarding, CEF)

I understand the differences between the three, I just wanted to get some clarification on a few things.

With Process Switching, a route lookup is done against the the destination ip address in the packet, will go out through the next hop inteface, which is in the routing table, and the L2 mac-rewrite is done via the ARP table. This is done for each and every packet, without regard to SIP/DIP pair.

With Fast Switching, a route lookup is done against the destination ip address in the packet, if its a new remote network (not in the fast cache), the first packet is process switched, and the next hop/L2 mac-rewrite information is put in a route-cache. Now, if another packet comes in for a host on this network, (seeing as if the forwarding information cache hasn't been flushed), it adds a specific host entry, and already knows the next hop intf/L2 mac-rewrite.

CEF, once the route-lookup is done, it doesn't need to look up the forwarding information, since the CEF and AT already have this information, and off it goes.

Please correct me if I'm wrong on the above statements.

My next question is, when you see the /24 and /32, what exactly determines if a /24 or /32 is put in the route-cache, FIB?

I'm pretty sure I know the answer, but I would like people smarter than me to respond first

1 ACCEPTED SOLUTION

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Beginner

Assuming you mean the /32 is in the same subnet as the /24. Both are added. Longest prefix match match wins.

For example

Ip route 192.168.1.0 255.255.255.0 s0/0

Ip route 192.168.1.0 255.255.255.128 s0/0

Ip route 192.168.1.1 255.255.255.255 s0/0

1. 192.168.1.0/24

2. 192.168.1.0/25

3. 192.168.1.1/32

All exists in CEF table.

Destination 192.168.1.1 matches #3

Destination 192.168.1.2 matches #2

Destination 192.168.1.130 matches #1

Sent from Cisco Technical Support iPad App

View solution in original post

2 REPLIES 2
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Beginner

Assuming you mean the /32 is in the same subnet as the /24. Both are added. Longest prefix match match wins.

For example

Ip route 192.168.1.0 255.255.255.0 s0/0

Ip route 192.168.1.0 255.255.255.128 s0/0

Ip route 192.168.1.1 255.255.255.255 s0/0

1. 192.168.1.0/24

2. 192.168.1.0/25

3. 192.168.1.1/32

All exists in CEF table.

Destination 192.168.1.1 matches #3

Destination 192.168.1.2 matches #2

Destination 192.168.1.130 matches #1

Sent from Cisco Technical Support iPad App

View solution in original post

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I kinda figured that's how it worked. Thanks for the clarification!

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