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SRC - NRS1
Alcatel-Lucent Service Router Certification: Scalable IP Networks
| Question | Answer |
|---|---|
| Original network that ultimately became the internet? | ARPANET |
| Primary Organization behind development of the Internet? | US Department of Defense |
| Primary design concerns of the original internet? | Reliability, Interoperability, Support for different mediums |
| What is NCP? | Like TCP but had problems with unreliable links and could only address next-hop nodes. |
| What is OSI Layer 1, what are examples? | Physical: copper, fiber, radio |
| What is OSI Layer 2, what are examples? | Data Link: Ethernet, PPP, ATM |
| What is OSI Layer 3, what are examples? | Network: IP |
| What is OSI Layer 4, what are examples? | Transport: TCP, UDP |
| What is OSI Layer 5-7, what are examples? | Session, Presentation, Application: SSH, HTTP, FTP, etc. |
| What is an RFC? | Request For Comments. Documents which are used for protocols and applications. |
| Which organization is in charge of making and improving internet standards? | IETF - Internet Engineering Task Force |
| ISPs connect to each other for content at locations called: | IXP, Internet Exchange Point |
| Tier 1 ISP: | Connects to every other tier 1 network, never need to use a transit network. AT&T, Global Crossing, NTT Communications. |
| Tier 2 ISP: | Provide transit but also sometimes requests from tier 1s. Bell Canada, British Telecom |
| Tier 3 ISP: | Often reselling services for tier 2 providers to their customers. Service single city or small regional network. Teksavy. |
| What is an SLA? | Service Level Agreement; contract between ISP and customer that define traffic guarantees and penalties resulting in payouts to the customer if stated service level cannot be met for any reason. |
| What is a Demarcation Point? | The point where the responsibility shifts between the customer and the ISP, typically a modem for a home user. ISP tests if signal is good to the modem, other side of the modem is the customers own responsibility. |
| What is a POP? | Points of Presence; ISPs connection points over multiple regions. Customers who get internet from the ISP connect via access or hosting facilities in that providers POP. |
| What is Triple Play? | Voice + Video + Data |
| Originally IP protocol functions were performed by which protocol? | TCP |
| What is an OSI protocol that is still in use? | IS-IS |
| What organization is in charge of distribution of IP addresses? | IANA – Internet Assigned Numbers Authority |
| In a given region who controls IP addressing? | RIR – Regional Internet Registry agencies |
| Port 20-21: | FTP |
| Port 22: | SSH |
| Port 23: | Telnet |
| Port 25: | SMTP |
| Port 53: | DNS |
| Port 80: | HTTP |
| Port 67: | DHCP |
| Port 69: | TFTP |
| Port 123: | NTP (Network Timing Protocol) |
| Port 161: | SNMP (Simple Network Management Protocol) |
| TCP/IP Layers? (1-4) | Network Interface, Internet Protocol, Transport, Application |
| Models of ALU Routers and Switches? | SR-(1,7,12), ESS-(1,7,12) |
| What is a SF/CPM? | Switch Fabric/Control Processor Module. One per slot in the chassis, two for redundancy. 200Gbps duplex, 400Gbps half (x2 = 800 full!) |
| What is an IOM? | Input Output Modules, provide physical network interfaces. Hot swappable module for queuing, processing and forwarding data. |
| What is MDA? | Half size of IOM. Provide different physical interface options. Accepts the physical data format, converts to internal format and provides some buffering. Then sends to the Flexible Fast Path complex (one per MDA, two per IOM) for intelligent processing. |
| What is SFP? | Small Form Factor Pluggable. Small optical modules in a variety of formats. |
| What is Flexible Fast Path complex? | Provides QoS, ACLs, forward decisions (destination IOM/MDA/port) across backplane fabric. |
| How can you connect to the ESS or SR series of equipment for management? | Console port or dedicated Ethernet port on the SF/CPM. Also could use in-band on MDAs if configured. |
| Which series of devices do NOT allow for redundant SF/CPMs? | SR-1 and ESS-1 |
| How many slots are available for the different chassis? | 12=12 (2 reserved SF/CPM), 7=7 (2 reserved SF/CPM), 1=0* *has the IOM intergrated, but can accept 2 MDAs for physical interfaces. |
| What does the ESS series not support? | Layer 3 services such as MPLS/BGP, VPNs, and multicast |
| What is BOF? | Boot Options File. Configures the system during power-on process (bof.cfg) |
| What is cf? | Compact Flashes – cf1, cf2, cf3. Of sizes 256M, 512M, 1G, and 2G. |
| Where are system files are stored? | typically cf3, also where defaults to look for them |
| What are cf1 and cf2 for? | debug and accounting logs |
| What is boot.ldr? | bootstrap image |
| What is stored in bof.cfg? | management port IP address, location of images, location of configs |
| What is TiMOS-m.n.Y.z? | directory which contains software config files to run the router / switch |
| What is stored in config.cfg? | default config file, just enough to make system run so very basic. Can create other config files and point to them with bof.cfg |
| In what order are the boot up files loaded? | Boot.ldr -> bof.cfg -> TiMOS images -> config.cfg |
| What is the purpose of persistence? | Ensures synchronization with the 5620 SAM |
| What is the prompt icon which indicates and existing and new contexts? | existing = #, new = $ |
| Default password on devices? | admin |
| What are the 4 types of logs and alarms? | Main, Security, Change, Debug/Trace |
| What are the default log-ids? | 99 = all levels, 100 = serious errors |
| Where can you configure to send logs? | Console, Session, Memory, File, Syslog, SNMP |
| Steps to setting up logs? | configure log-id (1-98), Identify sources, specify optional filter, Identify destination |
| What are most commonly used layer 2 protocols? | Ethernet, PPP, ATM, and Frame-Relay |
| What are circuit-based networks? | virtual circuits between different devices over shared infrastructure. Usually involves switches. ATM. Uses circuit ID. |
| What are shared-networks? | devices send and receive traffic to each other directly through shared medium. Ethernet. Uses MAC address. |
| What functions does PPP provide? | link control to negotiate link characteristics, transfer multiple layer 3 protocols, and authentication. |
| Why aren't some fields used in PPP? | Because it was derived from HDLC which is an IBM protocol. Address and Control fields unused. |
| What is a Virtual Circuit? | virtual connection emulating a point-to-point connection. If path fails can reconfigure to take another path. Multiple VCs share same physical infrastructure. |
| Which protocols are used in circuit-switched networks? | ATM and Frame-Relay, commonly in WAN |
| Why is ATM so complex? | Designed to handle different types of traffic (voice, data, video), so many options available. |
| What is size of ATM cell, why is it that size? | 53-byte cell, 5-byte header. Small size to support latency sensitive applications like voice. |
| What is VPI/VCI? | Virtual Path/Circuit Identification. The ID of the next destination of an ATM cell as it passes through switch routers. |
| What are AALs? | Adaptation Layers. Encapsulates ATM cells for the segmentation of higher-layer data into ATM cells and re-assembly at the receiving end |
| What is AAL1? | Constant bit rate traffic. Minimal data loss is required. |
| What is AAL2? | Variable bit rate traffic. Bit rate variable but requires bounded delay for delivery. |
| What is AAL3/4? | Connection-oriented service. Connection set up before data transfer. |
| What is AAL5? | connectionless service (usually IP). No connection set up before data transfer. Actually a footer (comes at end). |
| What are the 2 Ethernet standards? | 802.3 and Ethernet II. Ethernet II replaces the Length field with a type field, which is commonly used today. If field is => 1500 (0x600) its Ethernet II. |
| What is the size of a standard Ethernet frame? Jumbo frame? | 64-1522bytes (1518 old), not including preamble and SFD. Jumbo = 9000 bytes |
| Which Ethernet frame has an LLC header? | 802.3 has the LLC header. |
| How does a switch learn MAC addresses? | dynamically from source MAC address of a frame |
| Where does a switch store the MAC address table? | Forwarding Database (FDB) |
| What is a LAG? | Link Aggregation Group, protocol to provide link redundancy. 802.3ad. Like Cisco Etherchannel |
| What are the requirements for a LAG? | full-duplex, same speed, same hold-times, can’t reorder frames when transmitted (can’t perfectly load balance due to this) |
| What are the limits of LAGs? | 8x link per LAG, up to 64x LAGs per 7750SR or 7450ESS |
| What is LACP, what are +/-? | Dynamically negotiate LAGs. +Less management, -Chance of LAG groups you did not expect |
| What is Port-threshold used for in LAGs? | determines the behavior of LAG when # of links falls below configured threshold value. |
| What does Dynamic cost affect in LAGs? | allows the cost of the link to be recalculated when the number of active links is changed (they go down or come up). This cost affects dynamic routing protocols such as OSPF. |
| How do workstations and servers participate in STP? | They do not. |
| What is the Root Bridge in STP? | critical to building the tree, serves as the starting point |
| How is the Root Bridge selected in STP? | based on numerical value, bridge ID (BID) |
| What is the BID? | 2-byte Bridge Priority, 6-byte MAC address of bridge port with the lowest port ID on that switch, usually 0. So Bridge Priority is the predominant way to determine the root, in case of a tie it goes to the MAC address. |
| What is the default bridge Priority? | 32,768 |
| What are the default STP link costs? | 4Mb=250, 10Mb=100, 16Mb=62, 45Mb=39, 100Mb=19, 155Mb=14, 622Mb=6, 1Gb=4, 10Gb=2 |
| How are STP path calculations determined? | Port priority (has default but can be configured), Per port cost (default by bandwidth but configurable), port MAC address |
| How are BPDUs sent to switches? | Sent to multicast address 01:80:C2:00:00:00 destination, so only devices participating in STP will receive and process them. |
| How do ports in the discard state handle BPDUs? | Still transmitted and received so in the case of a topology change they might go into up state quickly. |
| When is a port set to Back-up and Alternate? | Back-up = same network segment but SAME switch that has a higher path cost to root, Alternate = same network segment but DIFFERENT switch that has a higher path cost to root |
| Advantages to VLANs? | decrease broadcast traffic, increase security |
| How can you communicate between VLANs? | Routing |
| What are the VLAN IDs? | VID = 0-4095 (0 and 4095 reservered). So 4094 possible vlans |
| What is VLAN stacking? | Also called Q-in-Q. Allows for the customers to support VLANs in their networks while the ISP supports 4,094 customers |
| Is the IP Protocol reliable? | No, in it does not itself worry about packet loss, other layers can deal with that problem. All it does is give best effort to deliver to the destination. |
| Private IP Ranges? | 192.168.0.0/16, 172.16.0.0 - 172.31.255.255/12, 10.0.0.0/8 |
| Reserved IP Ranges? | Multicast = 224-239.255.255.255, link local = 169, loopback = 127, Future = 240+ |
| When can the /31 mask be used? | In a point to point connection. |
| When can the /32 mask be used? | For loopback and system address which is set as the router ID in routing protocols like OSPF and BGP. |
| Difference between VLSM and CIDR? | VLSM used within an organization, CIDR is used throughout the internet. |
| What is the maximum size of an IP packet? | 65,535 bytes |
| How many FDB are kept on a router? | One for every VLAN. |
| What is Forwarding? | Basically same thing as routing. |
| Where is the routing table maintained on ALU equipment? | CPM (control process module) |
| What is the difference between NAT and PAT? | NAT = single private to single public, PAT = single public to many private, has to map IP and port number |
| What does DHCP configure? | IP address, gateway, DNS server, Windows Information Name Servers (WINS) |
| What are some examples of ICMP? | Pings, if TTL reaches 0 on a packet an ICMP is sent to let sender know. |
| What is ARP used for? | When you have an IP address but need to find out the MAC address |
| What are ACLs? | Filter IP packets based on security policy |
| How many ACL policies can you apply? | One policy per ingress/egress per interface |
| What is the difference between Exclusive and Template policies? | Exclusive = can only be applied to single entity, Template = can be applied to multiple entities |
| In the DHCP process after the servers broadcast an offer message what does the client respond with? | Broadcast accept message for the first DHCP offer it receives |
| What is the default behavior of a port, and default of a filter? | Port = permit all, Filter = drop all |
| What can you use ACLs to filter? | IP, MAC, protocol or port matching |
| What does ACL Entry ID determine? | The order in which entries are examined, in ascending order (so lowest first) |
| What does ACL Filter ID determine? | Uniquely IDs the filter (1-65,535) |
| In OSI model what is the difference between TP0 and TP4? | TP0 provides lowest level of service, TP4 provides highest. TP4 is like TCP, reliable, connection-oriented, end-to-end transport. |
| What is the Sequence Number in TCP? | Each byte of data assigned a sequence number, 32-bit number. |
| What is the Acknowledgement Number in TCP? | The next sequence number receiving device expects from the other station. If Seq# = 1450 and window size = 1000, Ack# = 2450 |
| What is the MSS option in TCP? | Used to try and get smallest MTU out of all hops. This is done to try and prevent IP fragmentation. |
| What is the 3 way handshake? | SYN, SYN/ACK, ACK |
| What is the ISN in TCP? | Initial Sequence Number |
| Who sets the Window Size in TCP? | The receiver sets the window size, how much data can be sent before receiving an ACK |
| How does Slow Start mode work in TCP? | Congestion window initially set to 1-2 segments. It uses the sender's congestion window and the receivers advertised window size. |
| What is the well-known port range? | 1-1023 |
| What is the Ephemeral port range? | 1024-65,535 |
| What is an IGP? | Interior Gateway Protocol. OSPF, IS-IS, RIP. Network under control of a single entity or admin group. |
| What is and EGP? | Exterior Gateway Protocol. BGP. Routes between one AS and another AS. Large number of routes. |
| What is an AS? | Autonomous System. 0-65,535 controlled by RIR. 0-64,511 = public, 54,512-65,535 = private |
| What is a Distance Vector routing? | Hop-count metric, no topological view of network. RIP. |
| What is a Link State routing? | Use a cost metric, take path of least cost to destination. Complete topological view of network. OSPF, IS-IS. |
| What is Path Vector routing? | Policy based, many different ways to influence preferred route, default is shortest AS hop-count. |
| What is a FIB? | Forward Information Base, another name for routing table |
| What is a RIB? | Routing Information Base, built from advertisements from other routers. Used to generate routing table, separate RIB for each protocol. |
| What is the RTM? | Routing Table Manager, receives best routes from each RIB. Preference value to compare routes, best routes placed in FIB. |
| Default Preference Table? | 0=direct, 5=static, 10=ospf, 15=is-is1, 18=is-is2, 100=rip, 150=ospf, 160=is-is1 ext, 165=is-is2 ext, 170=bgp |
| What is a Stub Network? | Network with single connection to backbone network |
| What is Bellman-Ford associated with? | Distance Vector algorithm |
| What is a Link State Database? | Used with link state routing, gets info from LSPs to form network topology. 3 databases, neighbor, LSD, and routing table. |
| When does Link State Database update? | Whenever there is a topology change as well as a set interval (30 minutes). |
| How does OSPF send out control data? | Sends to hello packets to multicast address 224.0.0.5 |
| What is contained in OSPF hello packet? | list of all other OSPF routers that the sending router has received Hello packets from. |
| At what point in OSPF process do routers form adjacency? | Hello exchanged, begin to send network DB info, once completed formed adjacency |
| What is an LSA? | Link State Advertisement, Type 1 contains router description and connected links. |
| What is an ABR in OSPF? | Area Border Router, when using multi-area OSPF these routers have info about networks and routers outside the area. |
| What is the Router ID in OSPF? | Uniquely identifies router. If no Router-ID set goes to system ID, then last 4 octets of router chassis MAC |
| Does changing Router ID update immediately? | No it is not preemptive, need to restart ospf to take affect. |
| What are default Hello/Dead interval in OSPF? | 10/40 seconds |
| In point-to-point links what gets taken out of OSPF hello packets? | Priority and DB/BDR fields since they are only used for ethernet links which can have multiple routers in one segment. |
| What settings need to be the same to form an adjacency? | Hello/Dead interval and OSPF MTU |
| What is the order of OSPF states in forming an adjacency? | (down, init, 2way), Exstart, Exchange, Loading, Full |
| Does master or slave send data first in OSPF? | slave sends data first. |
| Are OSPF updates sent in a reliable manner? | uses IP to send which is not reliable, but OSPF application layer makes sure it is sent reliably. |
| Max age of an LSA? | 60 minutes, one is generated every 30 even without a topology change to avoid getting flushed |
| How does OSPF use sequence # in LSAs? | If seq# is lower than DB = discard lower, send newer info to one who sent it. If seq# same = send ACK and discard. If seq# higher = Place new info into DB, send ACK, forward to neighbours since topology change occured |
| How does OSPF cost calculation work? | Lower cost = better. Divides a reference bandwidth (100Gbps default) by the link bandwidth. so 100Gbps/10Gbps link = 10 cost |
| What are the major differences between IGP and EGP? | EGP has larger routing tables, policy decisions more nuanced than finding shortest path, BGP used to exchange routing information between ASes not within one. |
| What are requirements for BGP neighbours? | Any BGP devices which can form a TCP session can be neighbours. Not just directly connected devices. |
| Is BGP route exchange information reliable? | Since it uses a TCP session which is inherently reliable, yes |
| What is the difference between BGP neighbour and BGP peers? | Peers are when there is a TCP session with two or more routers. |
| What is eBGP and iBGP? | External = BGP session between routers in different AS. Internal = BGP session between routers in the same AS. |
| How can BGP neighbours communicate if they are not directly connected? | Manually configure a static route. |
| Is BGP a discovery protocol? | No, need to be manually configured to connect to each other. |
| Differences between eBGP and iBGP? | eBGP = physical link, unless redundant extra links then loopback, iBGP = system interface |
| Who does the iBGP speaker have to connect with to get network information? | Must peer with every other iBGP speaker to get consistent view of all BGP routes reachable |
| What is the difference between Distance Vector and Path Vector? | Distance = RIP, device hop count, Path = BGP, AS hop count (can be much more complicated though) |
| What is the difference between well-known and optional attributes in BGP? | Well-known = mandatory OR discretionary (not required to use but have to support). Optional = transitive (just pass it along even if don't use) OR non-transitive (if not recognized quietly drop) |
| What are the BGP well-known attributes? | Origin (source address), AS-Path (ASes that have to be traversed), and Next-hop (next BGP router to send packets to, must be reachable by static or IGP if not directly connected). |
| How does the BGP tcp session operate? | Both routers start a TCP session, exchange OPEN messages and one sessions is dropped. Keep-alives used to keep session open and UPDATE messages sent when there is a network change. |
| What is sent in a BGP UPDATE message? | NLRI (Network Layer Reachability Information). CIDR block or prefix, full AS-Path to dest, next-hop attributes. |
| How many RIB are maintained by BGP? | 3: Loc-RIB = local, master, seperate from main. Adj-RIB-In = kept for each neighbour, stores NLRI. Adj-RIB-Out = kept for each neighbour contains NLRI to be sent. |
| What port is used for BGP TCP sessions? | 179 |
| Definition of network services? | Added tech in order to fine-tune packet forwarding based on specific information. Separate traffic based on type (QoS) or origin (VPN). |
| What is a VPN? | Virtual Private Network. Over the same core network but logically separated. |
| What is a CE Device? | Customer Edge Device. Operated by customers, provides access to PE routers, connects to PE through SAP. Unaware of tunneling in the core being done. |
| What is a PE Device? | Provider Edge Devices. These devices perform all of the QoS and VPN set up work. Interface between CE through SAP and P through SDP. |
| What is a P Device? | Provider Router. Located in the Core Network, so the main bandwidth and switching over a dispersed area. Do not connect directly to CE devices and are unaware of the services being provided to customers, just route based on what PE says. |
| What is a SAP? | Service Access Point. Provided by the PE for the CE. Connects the CE to individual service offerings. |
| What is SDP? | Service Distribution Point. PE connects to these to provide tunneling services through the core network. |
| What protocols are supported by the SDP? | MPLS/RSVP-TE, MPLS/LDP, or IP/GRE |
| What are the rules with SDP ID? | Locally unique (# on one router has nothing to do with # on separate router), Not unique to a service (many services use the same SDP) |
| How do customers gain access to network services? | Customers are given and ID which is then associated with services. |
| How did older VPNs operate? | Encrypt data to travel on public network. Used IPSec and SSL. Each device in VPN tunnel had to be configured with correct info. Controlled by customers transparent to providers (but no QoS through core). |
| Are provider based VPN encrypted? | Typically no but if the customer requires it the provider may offer it as an additional service. |
| What is LER in MPLS? | Label Edge Router. End points of MPLS tunnels, PE devices. |
| What is LSR in MPLS? | Label Switch Routers. Core network devices provide connectivity between LERs. P Devices. |
| What is LSP in MPLS? | Label Switched Paths. This is the name given to MPLS tunnels. Unidirectional, for bidirectional need to set up LSP in opposite direction. |
| How does MPLS function? | Get packet across network with info other than just IP so a LABEL (special header) is added. Turns a layer 3 routed network into a switched network. |
| How are labels distributed in a MPLS network? | LERs + LSRs use a SIGNALING PROTOCOL to distribute labels. Either LDP or RSVP. |
| What is LDP? | Label Distribution Protocol. Like in IP routing protocol except for MPLS labels. Requires IP routing protocol to function. Labels only locally significant (like MAC addresses). |
| Do LSR devices use IP addressing? | The core routers in an MPLS network are not aware of IP addressing and only look at the label. Act as though it is a switched network not a routed. |
| What is iLER? | Ingress Label Edge Router, the router which encapsulates the customer packet with a label. |
| What is eLER? | Egress Label Edge Router, termination point of LSR, removes label and forwards packet to CE device. |
| How do LSRs distribute labels with LDP? | For every LSP passing or ending on the router the LSR distributes a label to all LDP neighbours. IGP used to help determine which labels to use. |
| How does RSVP work in MPLS network? | iLER requests label for the LSP from the eLER which provides the label for the LSP. Could also have some manual set up of LSPs. Redundant paths for fast failover. |
| What is VPWS? | Virtual Private Wire Service. Simplest of VPN services, layer 2 emulates point to point link. |
| What is VPLS? | Virtual Private LAN Service. Just like VPWS except is point to multipoint. Acts like a layer 2 switch. |
| What extra requirements are needed for VPLS? | MAC Address FDB (forwarding database) maintained for EACH VPLS, learns source address like a switch, associates MAC to SAPs and SDPs, if no destination unknown will flood to other SAPs (like a switch!). |
| What is VPRN? | Virtual Private Routed Network, similar to VPLS except acts as a logical router instead of a switch. Customer sees it as all sites connected to private routed network admined by provider for that customer only |
| What extra requirements are needed for VPRN? | PE router providing VPRN maintains routing table for EACH VPRN since each customer may have private IPs being used (overlapping). This table is called VRF. |
| What is the VRF in VPRN? | VPN Routing and Forwaring Instances, logical private routing table, isolates VRF from other customers and core routing table. |
| Do CE routers exchange routing info with PE routers? | If they are using VPRN then yes they exchange routing info for that VPRN instance. The PE devices in the VPRN exchange routing info as well so that routes can be transmitted to all CE devices. |
Created by:
Datheral
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