Cisco CEF - Revision history

Pio2pio: /* Troubleshooting */

2015-03-04T14:23:24Z

Troubleshooting

← Older revision		Revision as of 16:23, 4 March 2015
Line 35:		Line 35:

	= Troubleshooting =		= Troubleshooting =
	Show FIB		Show FIB on a router
	show ip cef		show ip cef
	Show adjacency table		Show adjacency table on a router
	show adjacency		show adjacency
	show adjacency detail		show adjacency detail
	Clear adjacency table to force rebuilding it		Clear adjacency table to force rebuilding it on a router
	clear adjacency epoch		clear adjacency epoch

Pio2pio at 14:18, 4 March 2015

2015-03-04T14:18:05Z

← Older revision		Revision as of 16:18, 4 March 2015
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	*'''Cisco Express Forwarding (CEF)''' - CEF is the most recent and preferred Cisco IOS packet-forwarding mechanism. Like fast switching, CEF builds a Forwarding Information Base (FIB), and an adjacency table. However, the table entries are not packet-triggered like fast switching but change-triggered such as when something changes in the network topology. Therefore, when a network has converged, the FIB and adjacency tables contain all the information a router would have to consider when forwarding a packet. The FIB contains pre-computed reverse lookups, next hop information for routes including the interface and Layer 2 information.		*'''Cisco Express Forwarding (CEF)''' - CEF is the most recent and preferred Cisco IOS packet-forwarding mechanism. Like fast switching, CEF builds a Forwarding Information Base (FIB), and an adjacency table. However, the table entries are not packet-triggered like fast switching but change-triggered such as when something changes in the network topology. Therefore, when a network has converged, the FIB and adjacency tables contain all the information a router would have to consider when forwarding a packet. The FIB contains pre-computed reverse lookups, next hop information for routes including the interface and Layer 2 information.

	+---------------------------+		Control Plane L3 Software CPU processing
	~~\|Control Plane \| Software CPU processing~~		+---------------------------+
	Exchange of -->\| Routing Protocol \| <-> RIB		Exchange of -->\| Routing Protocol \| <-> RIB
	routing info \| \| \| Routing Information		routing info \| \| \| Routing Information
Line 21:		Line 21:
	\| CP builds FIB and cef copies IP routing table		\| CP builds FIB and cef copies IP routing table
	\| adjacency table to hardware		\| adjacency table to hardware
	\|Data Plane V		Data Plane V
	+----------------------------------+ +---------------+		+----------------------------------+ +-------------------------+
	\|FIB table \| \|Adjacency \|		\|FIB table \| \|Adjacency \| L2 Hardware
	\|Forward Information Base \|--->\|Table \|		\|Forward Information Base \|--->\|Table \| packet forwarding
	\|DST 192.168.1.1 next hop 10.1.1.1 \| \|*exit interface\|		\| \| \| \|
	+----------------------------------+ +---------------+ ~~Hardware packet forwarding~~		\|DST 192.168.1.1 next hop 10.1.1.1 \| \|exit interface and MAC \|
			+----------------------------------+ +-------------------------+
	lookup ^ \| rewrite		lookup ^ \| rewrite
	Incoming \| v Outgoing		Incoming \| v Outgoing
	IP packets ===> [ #### L3 P A C K E T #### ] ===> IP packets		IP packets ===> [ #### L3 P A C K E T #### ] ===> IP packets



			= Troubleshooting =
			Show FIB
			show ip cef
			Show adjacency table
			show adjacency
			show adjacency detail
			Clear adjacency table to force rebuilding it
			clear adjacency epoch

Pio2pio: Created page with "Routers support three packet-forwarding mechanisms: *'''Process switching''' - Legacy method, each packet that arrives on an interface, is forwarded to the control plane whe..."

2015-03-04T13:56:35Z

Created page with "Routers support three packet-forwarding mechanisms: *'''Process switching''' - Legacy method, each packet that arrives on an interface, is forwarded to the control plane whe..."

New page

Routers support three packet-forwarding mechanisms:

*'''Process switching''' - Legacy method, each packet that arrives on an interface, is forwarded to the control plane where the CPU matches the destination address with an entry in its routing table, then determines the exit interface and forwards the packet via out interface. This is done for each packet even if the destination in the stream is the same.

Destination -> next-hop-IP -> exit interface lookup

*'''Fast switching''' - after packet has been forwarded using ''process switching'' the information how to reach the destination is stored in a '''fast-switching''' cache. When a packet arrives on an interface, it is forwarded to the control plane where the CPU searches for a match in the '''fast-switching cache'''. If it is not there, it is ''process-switched'' and forwarded to the exit interface. The flow information for the packet is also stored in the '''fast-switching cache''' therefore for packets going to the same destination, the next-hop information in the cache is re-used without CPU intervention.

Destination -> fast-switching cache -> exit interface

*'''Cisco Express Forwarding (CEF)''' - CEF is the most recent and preferred Cisco IOS packet-forwarding mechanism. Like fast switching, CEF builds a Forwarding Information Base (FIB), and an adjacency table. However, the table entries are not packet-triggered like fast switching but change-triggered such as when something changes in the network topology. Therefore, when a network has converged, the FIB and adjacency tables contain all the information a router would have to consider when forwarding a packet. The FIB contains pre-computed reverse lookups, next hop information for routes including the interface and Layer 2 information.

+---------------------------+
|Control Plane | Software CPU processing
Exchange of -->| Routing Protocol | <-> RIB
routing info | | | Routing Information
| v | table
| IP Routing Table |
----------- | -------------
|
| CP builds FIB and cef copies IP routing table
| adjacency table to hardware
|Data Plane V
+----------------------------------+ +---------------+
|FIB table | |Adjacency |
|Forward Information Base |--->|Table |
|DST 192.168.1.1 next hop 10.1.1.1 | |*exit interface|
+----------------------------------+ +---------------+ Hardware packet forwarding
lookup ^ | rewrite
Incoming | v Outgoing
IP packets ===> [ #### L3 P A C K E T #### ] ===> IP packets