Chap 1 - Forwarding Flashcards
What is Unknown Unicast Flooding?
A destination MAC address is not in the MAC table so the switch floods the packet out all ports.
What separates broadcast domains?
A router.
What do VLANs do?
They create multiple broadcast domains on a single switch.
What 4 fields did IEEE 802.1q add to the VLAN packet structure?
- Tag Protocol Identifier (TID)
- Priority Code Point (PCP)
- Drop Eligible Indicator (DEI)
- VLAN Identifier (VLAN ID)
How many bits in the TID field, what is its hex value, and what does it identify?
- 16 bits
- set to 0x8100 identifying the packet as an 802.1q packet
What does the Priority Code Point field do?
- 3 bits
- indicates Class of Service for QOS between switches
How many bits in the Drop Eligible Indicator and what does it do?
- 1 bit
- indicates whether the packet can be dropped due to congestion
How many bits in the VLAN ID field and what does it do?
- 12 bits
- specifies the VLAN associated with the packet
What is VLAN 0 reserved for?
802.1P traffic
What is different about VLAN 1?
- default VLAN
- It can’t be deleted.
What is the range of normal VLANs?
2 - 1001
What VLANs above 1000 are reserved?
1002 - 1005
What is the range of extended VLANs?
1006 - 4094
After being in VLAN configuration mode and creating a VLAN at what point does the VLAN actually get created?
- when you leave VLAN config mode
- when you move to a different VLAN configuration
What VLAN are ports put in by default?
VLAN 1
What is the default Native VLAN?
VLAN 1
What is the CAM?
- Content addressable memory
- MAC address table resides here
What is the fallback for CEF?
Process switching
What is process switching?
Switching in software
What are the 3 types of packets that need to be process switched?
- Packets sourced by or destined for the router itself
- Packets too complex for hardware (packets with IP options)
- Packets that need extra information that isn’t already known (like ARP)
What 5 things happen when a packet is punted by CEF to be process switched?
- ip_input process checks the routing table and ARP for next hop routers IP address, outgoing interface and MAC
- Overwrites source MAC with its own MAC, and destination MAC with next hop router’s MAC
- Decrements TTL
- Recomputes checksum of IP header
- Delivers packet to next hop router
What is another name for the routing table?
Routing Information Base (RIB)
What 3 sources build the RIB?
- info from dynamic routing protocols
- static routes
- connected routes
How is the ARP table built?
From info from the ARP protocol
Where is CEF used?
- Default switching mechanism used on Cisco routers that do packet switching on general purpose CPU
- Default switching machinism on platforms with ASICs and hardware-based routers
Why do most switches contain multiple TCAM partitions?
So that inbound/outbound security, QOS, L2/L3 forwarding decisions occur all at once
Concerning ACLs what does TCAM make possible?
ACLs can be processed at the same speed whether the ACL has 10 lines or 500 lines
What is Centralized Forwarding Architecture?
When a Route Processor is equipped with a Forwarding Engine so it can make all forwarding decisions.
What is Distributed Forwarding Architecture?
When Line Cards are equipped with Forwarding Engines and can forward without intervention of the Route Processor (RP()
What 3 steps occur in Centralized Forwarding?
- Packet received on ingress Line Card, transmitted to Forwarding Engine on RP
- Forwarding Engine determines where packet will exit
- Forwards packet to Egress Line Card
What are 4 steps in Distributed Forwarding?
- Packet received on Ingress Line Card,
- Transmitted to Local Forwarding Engine
- If destination is local Forwarding Engine sends packet out Egress Port
- If not local, Forwarding Engine sends packet across Switch Fabric directly to Egress Line Card
What is another name for Software CEF?
Software Forwarding Information Base
What does Software CEF consist of?
- Forwarding Information Base (FIB)
- Adjacency Table (Adjacency Information Base)
How is the Forwarding Information Base (FIB) built and what does it contain?
- Built from Routing Table to have a mirror image
- Contains next hop IP address for each destination in routing table
How does CEF use the FIB?
CEF uses the FIB to make switching decisions based on IP destination prefixes
How is the Adjacency Table (AIB) built and what does it contain?
- Built from ARP and other L2 protocols
- Contains directly-connected next hop IP/MAC addresses and egress interfaces MAC
What is a Glean Adjacency in CEF and when does it occur?
- CEF punts to CPU where ARP is invoked
- Glean Adjacency occurs when a packet arrives but no entry in FIB
In CEF switching what 6 things happen during packet forwarding process?
- Packets headers are re-written
- Destination MAC overwritten with info from Adjacency Table
- Source MAC overwritten with MAC from egress interface
- TTL is decremented
- Header checksum is re-computed
- Packet delivered to next hop router
What happens when a router receives a packet with TTL = 0 ?
Packet is discarded
What is done to prevent starving of CPU cycles by packets with an invalid or imcomplete adjacency?
Rate limiter
What is an NPU?
Network Processing Unit
What is the difference between an NPU and an ASIC?
The NPU is programmable but the ASIC is not.
In a Distributed Forwarding Architecture what mechanism is used for packet switching?
It uses Distributed CEF (dCEF)
What does dCEF stand for and what 2 things are true about it?
- Distributed CEF
- CEF data structures are downloaded to ASICs and CPUs of all the Line Cards
- Increases packet throughput of the router
What is Hardware CEF?
CEF switching done in ASICs and NPUs
In hardware-based platforms what is Software CEF used for?
For programming Hardware CEF in ASICs / NPUs
What is Stateful Switchover?
Redundancy feature that allows 2 RPs to synchronize configuration and control plane info
What is the process called for mirroring information between RPs?
Checkpointing
What does Non-Stop Forwarding (NSF) do?
Informs routers to maintain CEF entries and to continue forwarding packets during an RP failure until the control plane recovers
What is SDM
Switching Database Manager
What do SDM templates do?
Used to adjust the Allocation Ratios of memory allocated to various TCAM tables
What is particular about SDM templates when used in a switch stack?
The switches all need to use the same SDM template.
What is the ‘show’ command that allows you to see the current SDM template?
show sdm prefer’
What 2 components make up CEF?
- Forwarding Information Base (FIB)
- Adjacency Table
Display the MAC address table for a switch
- show mac address-table address (mac_address)
- show mac address-table dynamic
- show mac address-table vlan (vlan_id)
Display the Layer 2 configuration for a specific switchport.
show interfaces (interface_id) switchport
Display the ARP table.
- show ip arp (mac_address)
- show ip arp (ip_address)
- show ip arp vlan (vlan_id)
- show ip arp (interface_id)
Display the SDM template.
show sdm prefer
How is TCAM different than CAM?
TCAM allows for the matching and evaluation of a packet on more than one field.
Architecturally, describe what the TCAM is.
It is an extension of the CAM table but enhanced so that it can do upper-layer processing.
Name 3 things TCAM can identify.
- L2 / L3 source/destination address
- Protocol
- QOS markings
Name 2 differences between TCAM and CAM.
- TCAM (ternary) provides more flexibility in searching than CAM (binary)
- TCAM does upper-layer processing, CAM does not
What are the states TCAM can match on?
1’s, 0’s, and X’s where X is ‘don’t care’
example:
TCAM can search for a value that has 0 1 1 1 X X 1 0 . This will match:
01110010
01110110
01111010
01111110
What 3 components make up a TCAM entry?
- Value 172.19.0.0
- Mask 255.255.0.0
- Result Deny
What are 3 things that the TCAM Result can be?
- Permit or deny
- Pointer to a next hop in the routing table
- An index to a QOS Policer
What 2 factors allow TCAM to be really fast?
- TCAM operates in hardware
- Each of the 5 TCAM partitions can be processing simultaneously allowing multiple operations to be done at the same time
