CEF= Cisco Express Forwarding is the current and best switching method for routers and multilayer switches.
It is a form or routecaching.
CEF works using the routing table (topology driven) to build a FIB Forwarding Information Table.
By using more memory the device tries to prepare complete forwarding rules to be used to forward traffic.
So for example if a packet with destination net A has to be sent out a LAN interface the source and destination MAC addresses to be used are placed in the table together with the outgoing interface ID, L2 header fields to be used.
the destination MAC address is that of the next hop.
A neighbors table is kept for convenience.
More powerful devices that can perform CEF in a distributed manner have a copy of CEF tables on each linecard and the main cpu work is that of updating and keeping in sync all the tables.
Software based routers have a single cpu but they take advantage from having the job prepared in advance.
It stores in a specialized table all the information needed to forward packets that have the same destination (or the same source and destination if doing load balancing).
This avoids to process all packets as stand alone.
To forward packets that belong to a flow certain operations has to be done only once:
for example an ARP request to resolve the IP address of next-hop in a valid MAC address.
fast switching that is traffic driven does this operations on first packet of the flow
CEF that is topology based does this even before seeing the first packet of the flow (it keeps a table of Neighbors and resolves all of them).
So a packet rewrite operation can be done efficiently:
the IP packet is extracted from the received frame, IP header is modified accordingly (TTL decremented IP header checksum recomputed) and the the packet is encapsulated in the resulting outgoing frame and queued to the outgoing interface
sorry about this, so you say it wont have to re-write everypackets layer 2 destination address when routing, will it just od it for the first packet? if so how does the router know they are part of the same flow ?
This is actually a pretty cool feature, i didn't even know it existed until I was looking for a solution to advertise a subnet (prefix in BGP talk), only if a certain condition existed. This is exactly what conditional advertisements does
j ai une question j ai achete un routeur cisco 887VA-k9 , je le configuré avec la configuration ci- dessous
si je le lier avec mon pc portable sur l un de ses ports directement ça marche toute est bien ( la connexion internet + m...
Attached policy provides CLI access to the Cisco 4G router over text messaging. Two files are in the attached .tar file:
2. PDF with instructions on how to load and use the .tcl file.