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ENCORE
| Question | Answer |
|---|---|
| CEF | Cisco Express Forwarding (CEF) is an advanced layer 3 switching technology to enhance the overall network performance. CEF creates a forwarding table within the hardware before any switching takes place. All packets are switched directly by hardware. CEF |
| CAM vs TCAM when useful? | CAM is useful whenever the switch needs to do a lookup and needs to be an "exact match". TCAM table is useful when you are interested in a "certain portion" of the address as a match. e.g. first 8 bits of a 32-bit address. |
| TCAM advandeces | faster processing as only certain fields need to be checked. ACL, Qos, L2 L3. |
| 1- Process Switching 2- Fast Switching | Process Switching : packet sent to the processor to perform Switching. Fast Switching: Is a Switching mechanism that Creates Table On demand, first packet in a flow is examined by CPU, forwarding decision is cached in hardware for that flow. Faster. |
| 3- CEF Switching | CEF Switching: is a Switching mechanism that Copies and Mirrors all Routing information base into the CEF Forward Information base as soon as the router starts up and its therfore a topology driven Switching mechanism.. |
| 4- Application Specific Intergrated Ciruits (CEF in ASICs) | ASIC: The Hardware Is correctly programmed to forward the packet (Switch the packet). So yes CEF is the best switching mechanism. |
| CEF switching will be applied to packets except | Packets with IP header options Packets sourced from or destined to tunnel interfaces Packets using Ethernet encapsulation types other than ARPA Packets that require fragmentation |
| Centralized forwarding. | CPU makes forwarding descison |
| Distributed forwarding architecture | Line card has forwarding engine that can make decisions without the central Route possessor. has local forwarding engine. * can send packets directly to other line card interface |
| dCEF | dCEF mode of Cisco Express Forwarding. Line cards maintain mirror copies of the FIB and adjacency tables. The line cards perform the express forwarding between port adapters; this alleviates the Route Processor of involvement in the switching operation. |
| FIB | A forwarding information base (FIB), also known as a forwarding table or MAC table. network interface controller to which the input interface should forward a packet. |
| Adjacency table (AIB) | Populated with arp data. MAC addresses for next hops and exit interfaces. |
| Software CEF | uses FIB and AIB to make quick forwarding no need to use central CPU. And also packets are rewritten (mac ttl and so on) |
| hardware or asic CEF | Cef is used to program hardware. |
| SSO | Statefull switchover on big cisco routers. two RP syncronise all the stae. |
| SDM and use? | Switching Database Manager on Cisco Catalyst switches to manage the memory usage of TCAM. switch that is used for switching won’t require any memory to store routing information. switch that is used as a router won’t need memory to store MAC addresses. |
| CAM and TCAM dif? | Content-Addressable Memory (CAM) and Ternary Content-Addressable Memory (TCAM) are two types of special computer memory used in various very-high-speed searching applications. |
| CAM and TCAM use? | when looking up an address in the MAC address table, you always require an exact match, so CAM is used. looking up a prefix in a routing table, you don’t need an exact match, and that is where TCAM is used. |
| Rootridge | L2 topo most important switch all ports are in forwarding state and designated ports. is chosen based on BrideID that is made up of mac + bridge ID |
| Root path cost. | specific path cost to root bridge. Root bridge advertises with cost 0, next one adds its path cost (so 10GB link adds 2 into BPDU) |
| System priority. high or low better? | 4bit value for bridge election default S 32,768. Lower has priority. show spannig-tree root will show 32,768 + vlan number so vlan 1 32,769. Increments of 4096 |
| ROOT BRIDGE IDENTIFIER | root bridge system MAC + system ID + system priority of local bridge. same as local bridge identifier. |
| Max age , hello time, 2. )forward delay , listening. (both 15 sec) | Max age - indicates the age in seconds at which time the current BPDU should be discarded. start new cycle. Hello time-BPDU interval 2 sec Mac address timer 300 Forward delay 15 ( when topo change flag then mac address flush in 15sec) |
| STP must fully converge before packets can be forwarded.? Normal / RSTP | No and no. |
| STP Pathcost 1GB 10 and above | 4 , 2 , 1 ,1 ,1 ,1 .... |
| Switch startup, it assumes it is RB for STP, how does it handle neighbor conf BPDUs | If BPDU is inferior it ignores it and if it is preferred, it updates its BPDU with new root bridge identifier and correlates total path cost to RB. |
| Root port selection | 1) lowest path cost 2) lowest system priority neighbor. (Switch ID value) Links go through Same switch: 4) lower port priority value 5)lowest port number |
| Spanning Tree Port Priority and Port ID Values | Each port of a switch has a Spanning Tree Port Priority value associated with it, which is equal to 128 by default |
| BPDU topo change flag set, switch action? | old mac address get flushed from mac table. MAC timer is set to foward delay 15 sec and then flush. More mac address flooding then for a while. |
| 802.1D STP port 5 states (initaliaztion takes around 50 sec to get to forwarding) + 1 broken state. | Disabled-admin shut. Blocking-port will first enter the blocking state. Listening-listen for and send BPDUs. Learning - receive a superior BPDU, will stop sending its own BPDUs, and will relay the superior BPDUs. Forwarding- forwarding traffic. |
| Blocking state | does not send traffic, but receives bpdu's. stp will block cirten ports to prevent loops. |
| root path cost | cost from switch to root switch. zero in the root bridge. local rootpath cost is advertised then local interface cost is added. |
| STP switch ID extension | 12 bit that show which vlan the BPDU correlates to |
| show spannig-tree root | shows: root ID root path cost root port. |
| BLOcked port calc, or when applied. Between to non-root ports. | 1.not root port -logic? 2. Lower path cost to root 3 system priority 4 system mac |
| STP port roles / not statuses. | Root port Designated port Blocked ports |
| show spanning tree | root info local bridge info port role and status |
| show spanning-tree interface gi0/1 | see vlans that are on this interface detail more info |
| STP topo changes steps | switch detects and send topo change note TCN. TCN is forwared upstreem to ROOtbridge. Root creates new Conf bpdu with topo change flag set. TCN is on vlan bases. More hosts, more likely topo changes. show spanning tree detail. shows TCN count and time |
| Topo cahgne dp- block port fail | both sw send tcn to root, then root send conf bpdu with topo cahnge falg and all macs are fulshed. |
| Time after loosing root port. Connection to root bridge. | Max age 20 sec- then can start listening on blocked ports. 15 se listening state 15 sec learning state. |
| Indirect stp failure | filtered BPDU or impaired traffic. >max age> bloacked port to listening and then to root. |
| RSTP ieee 802.1w , port states. | Discard - (stp = blocking, listenig, blocking ) Learing- listens to bpdu info and MAC addresses and forwards only bpdus Forward - fully functional. |
| RSTP handshake sucess yes no | If not then switch will revert to and assume normal STP |
| RSTP port roles | Root port -one rport per vlan connects network upstream to Rootbridge Desigatned port - provides connectivity down stream Altrenate port- altreant route to root. Like blocked, recive bpdu only. Backaup port - redundacy ????????only hubs in networ |
| port types | Edge port - for hosts, no loop danger, portfast Non-edge ports - recive bpdus. Point-to-point- function only as full-duplex, quickest way to make sure other end is switch. hub only half duplex |
| BPDu guard function and global command? | When receives bpdu then shuts down port. Use on access ports, specially with portfast. spanning-tree portfast bpdufilter default |
| root guard #spannig- tree guard root | superior bpdu received >port is placed into root inconsistent state. to protect I guess |
| UDLD | Bi directional communication functionality??? |
| stp primary and secondary values??? | 24,576 and 28,672 |
| STP priority best practices | 4096 for rp and to not allow sw then put prio to 0 zero |
| Port state, role and typs. | dif types of ports (edge , non-edge) go through dif states (discard, learn, forward) to function in specific role (root, designated, alternate) |
| gi0/1 spanning-tree cost 1 | cost will change root path cost and port role in cases. |
| gi0/1 spanning-tree port-priority 64 (default is what?) | default is 128 |
| portfast portfast trunk command default? | disables TCN topo change notifications on access ports. bypass learning and listening stages. On trunks only when server or single device. spanning-tree portfast default |
| how to see bpdu guard down ports and bring them up again? | show interface status > err-disabled fix> shut /no shut errdisable recovery cause bpduguard errdisable recovery inerval show errdisable recovery |
| BPDU filter | SW does not send bpdus, but forwards inferior ones down stream. |
| STP loop guard | prevents alternative port of becoming DPs. Puts port into errdisabled state when bpdus stop coming in . |
| UDLD modes and commands? | agressiv and normal. # UDLD ENABLE # show udld neighbors bidirectional |
| MST 802.1Q lk 124 |
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