MultiLayerSwitch-CEF Flashcards
1.1 Configure and verify switch administration address table
What is process-based switching?
Tasks are switched using the CPU and this is known as the “IP Input” process.
What is fast switching?
First packet is sent to the CPU for processing while Fast Switching cache is updated with the information.
What is CEF?
CEF is prepopulated the L2 and L3 caches while CPU is rarely bothered. THis is done by tracking the ARP and Routing tables.
What are the components CEF is composed of?
CEF is composed of the FIB and Adjacency Table.
What does CEF stand for?
Cisco Express Forwarding.
What does FIB stand for?
Forwarding Information Base.
What is the FIB?
The FIB is a shadow copy of the IP Routing Table and tracks the table for automatic updates.
What is the Adjacency Table?
The Adjacency Table is populated by L2 tables such as ARP and Frame-Relay Mapping.
What is the Glean Adjacency Type?
Glean occurs when no specific information about an address in a subnet exists in ARP. This results in the CPU being tasked to trigger an ARP reply.
What is the Null Adjacency Type?
Null occurs when there are valid packets that must be dropped as they are destined for the Null0 interface.
What is the Drop Adjacency Type?
Drop occurs when there are invalid packets that must be dropped. This can occur when there is no route or there’s bad encapsulation.
What is the Discard Adjacency Type?
Discard occurs when there are valid packets that must be dropped because there is a security policy in place such as an ACL.
What is the Punt Adjacency Type?
Punt occurs when there are packets that are destined for the CPU itself such as a routing update from a neighbor.
What are some useful show commands that can be used to look up information in CEF?
Switch# show ip cef
Switch# show adjacency
What command produced this?
switch# show ip cef
What command produced this?
switch# show adjacency
What command produced this?
switch# show adjacency detail
What command produced this?
switch# show adjecency interface t1/0/3
What command produced this?
switch# show adjacency 10.147.99.52
What command produced this
switch# show adjacency vlan 300
What command produced this?
switch# show ip cef detail
What command produced this?
switch# show ip cef 10.147.97.28 detail
What command produced this?
switch# show ip cef 10.147.97.28
What command produced this?
switch# show ip cef vlan 311
How does CEF work?
By having the hardware route all packets, saving CPU resources. This is accomplished with FIB and AT.
What’s the FIB?
Forwarding information database. Contains all of the information that the IP routing table does, but in a format usable by ASICs.
What’s the AT?
Adjacency table. Contains next-hop layer 2 information learned through ARP.
In CEF L3 switching, how is excessive information handled after the AT and TCAM tables are filled?
It’s processed by the routing engine.
Does CEF or MLS need to be configured?
No, but IP routing must be enabled.
Assume that a host sends a packet to a destination IP address and that the CEF-based switch does not yet have a valid MAC address for the destination. How is the ARP entry (MAC address) of the next-hop destination in the FIB get?
A – The sending host must send an ARP request for it
B – All packets to the destination are dropped
C – The Layer 3 forwarding engine (CEF hardware) must send an ARP request for it
D – CEF must wait until the Layer 3 engine sends an ARP request for it
Answer: D
Explanation:
If a valid MAC address for the destination is not found, the Layer 3 forwarding engine can’t forward the packet in hardware due to the missing Layer 2 next-hop address. Therefore the packet is sent to the Layer 3 Engine so that it can generate an ARP request (this is called the “CEF glean” state)
CEF_ARP
CEF is a complete new routing switch technology . Which two table types are CEF components?(Choose two)
A – adjacency tables
B – caching tables
C – neighbor tables
D – forwarding information base
Answer: A D
How does the CEF L3 engine operate?
it’s a router that maintains routes (static or dynamic), and populates the FIB
What contains the next-hop entries?
FIB
what does the version number indicate when looking at CEF entries?
the number of times the entry has been updated since the table was generated
what does the epoch number indicate when looking at cef entries?
the number of times the CEF table has bee flushed and regenerated as a whole
what is a CEF punt?
a packet can’t be switched in hardware with the FIB and must go to the L3 engine
what is accelerated CEF (aCEF)?
CEF is distributed across multiple L3 forwarding engines, typically on Catalyst 6500 line cards each having only a subset
what I s distributed CEF (dCEF)?
CEF is distributed completely among multiple L3 forwarding engines.
what is the adjacency table?
a router keeps a routing table and an ARP table. The FIB combines them for every next-hop entry
what is a CEF glean?
the L3 forwarding engine can’t forward the packet in hardware because there is no L2 next-hop address, so the packet is sent to the L3 engine to generate the ARP response
what is arp throttling?
during the time that the FIB entry is in CEF glean waiting for ARP entries, subsequent packet to that host are dropped to keep input queues from filling
what is a null adjacency?
used to switch packets destined for the null interface
what is a drop adjacency?
used to switch packets that can’t be forwarded normallly due to an encapsulation failure, unresolved address, unsupported protocol, etc.
what is a discard adjacency?
packets are discarded due to an ACL or policy action
what is a punt adjacency?
packets must be sent to the L3 engine for further processing
what does the rewrite engine do?
it updates the L2 headers with the proper src and dst MAC, L3 TTL, L3 checksum, and L2 checksum information
How do you enable CEF?
it is enabled by default on all CEF capable switches
What is the FIB in multilayer switching?
The FIB is the Forwarding Information Base, and is a table of layer-3 routing information used to hardware switch packets. It contains the next hop address for each entry.
Will a FIB contain host routes (/32) entries?
Yes. Even though most routing tables do not (unless manually configured) the FIB does.
What are the four types of adjacency’s that can be found in the FIB adjacency table?
Null, Drop (actual error), Discard (ACL or policy), and Punt.
What are the two techniques of CEF that use specialized hardware?
-aCEF, functions as a FIB cache.-dCEF, has its own FIB table and forwarding engine!
What is the command to view the FIB?
(exec)show ip cef
What is a ‘CEF Glean’?
When a L3 forwarding engine cannot forward a packet because of missing L2 info, the time it takes to have the L3 engine generate an ARP request and figure out the L2 information is the CEF Glean.
What table contains the Layer 2 information for every next-hop entry in the FIB?
The adjacency table.
What is another name for Topology-based Multilayer switching?
Cisco Express Forwarding (CEF).
When a packet is ‘punted’ what does this mean?
That the packet was unable to be forwarded naturally and is sent to the switch CPU for process switching.
What are the 9 reasons that a packet can be ‘punted’?
-ARP requests and replies-IP packets needing a response (TTL, MTU, frag-IP broadcasts-Routing protocol updates-CDP packets-IPX routing packets-Packets needing encryption-Packets triggering NAT-Non-IP and non-IPX packets.
What three things are contained in the Hex-value when looking at the FIB adjacency table?
-The first 6 octets are the Hex value of the next hop MAC address.-the second 6 octets are the Source MAC for the layer 3 engine interface (like VLAN 99).-the last two octets are the EtherType (0800 for Ethernet).
