Cramsession for Cisco Certified Internetwork Expert

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  1. TM BrainBuzz Cramsession Last updated November, 2000. Click here for updates. Cramsession™ for Click here to see additional Cisco Certified documents related to this study guide. Internetwork Expert Contents Abstract: Contents .............................. 1 This Cramsession will help you to Cisco Device Operation........... 2 prepare for Cisco exam #350-001, the CCIE written exam. Exam topics include, General Networking Theory .... 5 Cisco Device Operation, General Bridging & LAN Switching ....... 8 Networking Theory, Bridging & LAN Switching, Internet Protocol, IP Routing IP Routing Protocols..............17 Protocols, Desktop Protocols, Desktop Protocols.................26 Performance Management, WAN, LAN, Security, and Multiservice. Performance Management .....28 WAN ...................................29 LAN ....................................33 Security ..............................35 TACACS (Terminal Access Controller Access Control System) ..............................35 Multiservice .........................36 Notice: While every precaution has been taken in the preparation of this material, neither the author nor BrainBuzz.com assumes any liability in the event of loss or damage directly or indirectly caused by any inaccuracies or incompleteness of the material contained in this document. The information in this document is provided and distributed “as-is”, without any expressed or implied warranty. Your use of the information in this document is solely at your own risk, and Brainbuzz.com cannot be held liable for any damages incurred through the use of this material. The use of product names in this work is for information purposes only, and does not constitute an endorsement by, or affiliation with BrainBuzz.com. Product names used in this work may be registered trademarks of their manufacturers. This document is protected under US and international copyright laws and is intended for individual, personal use only. For more details, visit our legal page. © 2000 All Rights Reserved – BrainBuzz.com
  2. BrainBuzz Cramsession: Cisco Certified Internetwork Expert 2 Cisco Device Operation Router Components ROM (Read-Only Memory) – Hosts the basic commands of the router and sometimes a limited version of the IOS (Internet Operating System). ROM is non- volatile, meaning it is hard-coded and does not change. Contains power-on diagnostics, a bootstrap program, and operating system software. RAM (Random Access Memory) – Contains the running version of the IOS and the current running configuration. This is extremely volatile; when the router is shutdown, anything in RAM is lost. Stores routing tables, ARP cache, fast-switching cache, packet buffering (shared RAM), and packet hold queues. NVRAM (Non-Volatile Random Access Memory) - As the name implies, files can be written to this memory and will not be lost when the system is powered down. This is where the startup version of the router configuration is stored. Flash memory (EEPROM – Electronic Erasable Programmable Read Only Memory) – This is where the IOS version for the router is stored. It is important when determining what version of IOS to load on a router, that you ascertain how much flash is installed. Different versions of IOS require more flash to be loaded. Flash memory holds the operating system image and microcode. Ways to Configure a New Router • By connecting to the console port and using TFTP to download a configuration file that has been created ahead of time. • By connecting to the console port and running the Setup dialog. • By connecting to the console port and directly typing in configuration commands. • Using bootp in conjunction with SLARP/RARP to download a configuration file that has been created ahead of time. Determine Hardware Configuration The EXEC commands that will show hardware configuration of a Cisco router are “show hardware” and “show version” More study guides and information available at www.cramsession.brainbuzz.com. © 2000 All Rights Reserved – BrainBuzz.com
  3. BrainBuzz Cramsession: Cisco Certified Internetwork Expert 3 Mode Prompts Monitor mode rommon 1 > User mode router> Privileged mode router# Global configuration mode router(config)# Interface configuration mode router(config-if)# Sub-interface configuration mode router(config-subif)# Line configuration mode router(config-line)# Router configuration mode router(config-router)# IPX router configuration mode router(config-ipx-router)# Really Delete Files from Flash When you delete a file from flash, it is not removed from flash, and you will not regain the space - it is simply marked for deletion. Once a file is marked for deletion, issuing the “squeeze” command will perform a function similar to a hard drive defrag and move the files on flash to reclaim the space occupied by the “deleted” file. Passwords Things to know: • The user mode password is the only one that cannot be created in the setup dialogue. • All passwords can be encrypted. • A password can be set for individual lines. • If no password is set on the vty lines there is no telnet access into this router. • Router(config)# service password-encryption – encrypts all passwords in the configuration file. Procedure to Recover a Lost Password: • Reboot the router • Issue the break command in the first 60 seconds (CTRL-Break) • Enter the appropriate register value (0x2142) • Reboot the router again • Avoid the startup script • Copy startup to running configuration • Change the passwords • Copy running to startup configuration More study guides and information available at www.cramsession.brainbuzz.com. © 2000 All Rights Reserved – BrainBuzz.com
  4. BrainBuzz Cramsession: Cisco Certified Internetwork Expert 4 Types of passwords: • Exec – used to restrict access to the EXEC mode, the basic console on the router. • Enable – used to restrict access to the privileged EXEC mode where changes to the router configuration can be made. • Enable Secret – Similar to the Enable password, but they are encrypted so they cannot be read. Setting different types of passwords: • Console password – used with the routers console port. router(config)# line con 0 router(config-line)# login router(config-line)# password {password} • Auxiliary password - used for the router’s auxiliary port. router(config)# line aux 0 router(config-line)# login router(config-line)# password {password} • Virtual terminal password – used for telnet sessions to router. router(config)# line vty 0 4 router(config-line)# login router(config-line)# password {password} • Enable password – used when enable secret is not configured or software revision is too old. router(config)# enable password {password} • Enable secret password – encrypted password that provides enable privileges. router(config)# enable secret {password} Register Values • 0x2102 – Default mode • 0x2142 - The value used to recover passwords Debug By default, all debug information goes to the console port on a Cisco router only. To view debug messages from a VTY session, you must issue the “terminal monitor” command. Router# debug serial interface - monitors keepalives on an interface. To reduce the impact of a debug command on the CPU of the router, use the scheduler-interval command and be sure to use the debug command as specifically as possible. More study guides and information available at www.cramsession.brainbuzz.com. © 2000 All Rights Reserved – BrainBuzz.com
  5. BrainBuzz Cramsession: Cisco Certified Internetwork Expert 5 SNMP (Simple Network Management Protocol) SNMP is a standard method for Network Management Stations (such as CiscoWorks) to gather information about networked devices. This UDP-based protocol uses MIBs (Management Information DataBases) defined for each type of device to interpret the information provided by the SNMP enabled equipment To enable SNMP on a router the command is "snmp-server community" General Networking Theory OSI Model The OSI is a common tool for conceptualizing how network traffic is handled. In the CCIE track, we will be interested primarily in the lower three levels. Just a reminder, that you can use the old mnemonic “All People Seem To Need Data Processing” as a way to help remember the sequence. 7. Application – User interface tools (such as Telnet, SMTP, FTP, etc.) 6. Presentation – Encoding/Decoding (such as ASCII, MPEG, GIF, JPEG, etc.) 5. Session – Creating, managing and terminating Presentation layer 4. Transport – Error checking and recovery, flow control and multiplexing (TCP, SPX, etc.) 3. Network – Routing (IP, IPX, etc.) 2. Data Link (LLC/MAC) • LLC – Manages communications • MAC – Manages addressing and access to the physical layer 1. Physical – Establish and maintain physical connectivity Cisco Hierarchical Internetworking Model • Core – Concentrates all traffic traversing the network. The focus in on speed and fast switching. Gigabit Ethernet and ATM are seen here. • Distribution – Control layer; Aggregation of traffic, access lists, compression, encryption and other services that provide the glue between Access and Core layers. • Access – The point at which users join the network. VLANs, WAN connections, RAS services are all at this layer. Connection-oriented vs. Connectionless Service Connection-oriented: Similar to HDLC • Connection establishment and termination required • Sequenced, acknowledged data delivery • Built-in error recovery • Sliding window flow control Connectionless: Data transfer without virtual circuit More study guides and information available at www.cramsession.brainbuzz.com. © 2000 All Rights Reserved – BrainBuzz.com
  6. BrainBuzz Cramsession: Cisco Certified Internetwork Expert 6 • No message sequencing • No delivery guarantee • Higher layer is responsible for error recovery, flow control, and reliability Routing / Switching • Routing is defined as a Layer-3 activity. • Bridging is defined as a Layer-2 activity. • Switching is defined as a Layer-2 activity. Switching is often called micro- segmentation, in that each switched port is basically its own bridged domain. Routing and Routed Protocols A routing protocol, such as BGP or OSPF communicates between routers which paths to follow in order to get data delivered to desired destinations. A routed protocol, such as IP or IPX is the method for passing data, and travels the paths defined by the routing protocol. Reliability of Protocols • TCP and LLC Type 2 are reliable protocols because they are layer four protocols • IP, UDP, and Frame Relay are NOT reliable protocols because they are layer three protocols 802.x Protocols 802.2 Link Layer Control (LLC) 802.3 CSMA/CD Access Method (Ethernet) 802.4 Token Ring Bus 802.5 Token Ring 802.6 MAN (Metropolitan Area Network) 802.7 Broadband 802.8 Fiber-optic LANs 802.9 Integrated Voice & Data 802.10 LAN/MAN Security 802.11 Wireless 802.12 VGAnyLAN More study guides and information available at www.cramsession.brainbuzz.com. © 2000 All Rights Reserved – BrainBuzz.com
  7. BrainBuzz Cramsession: Cisco Certified Internetwork Expert 7 Passive Interface When enabled on an interface this command allows the interface to hear routing updates, but not repeat them. This helps to control routing updates. Example: Router(config-router)# passive-interface s0 Connectivity (Diagram A-1) Bridged Environment (Refer to Diagram A-1) In a bridged environment, a Cisco router will not modify the layer-2 MAC address of a frame when bridging. In other words, a packet retains the true source and destination MAC addresses when crossing a bridge. For example, if in the diagram above Devices B and C were bridges, packets sent from Host A to Host B would have the Source MAC Address of Host A’s Ethernet adapter and the Destination MAC Address of Host B’s Ethernet adapter, regardless of what segment they were passing through. If a packet were to be lost anywhere between Host A and Host D, the originator would rebroadcast. Routed Environment (Refer to diagram A-1) In a routed environment, when a host sends a packet it has the Source MAC Address of either the originating host (if on the first segment) or the last router port it was processed by. It would have the Destination MAC Address of the next hop router port or the destination host, if on the final segment. In other words, a host sending a packet to a router for processing to a remote destination will have the routers local port as a destination address; a host receiving a packet from the router will see a source address of the local router port. More study guides and information available at www.cramsession.brainbuzz.com. © 2000 All Rights Reserved – BrainBuzz.com
  8. BrainBuzz Cramsession: Cisco Certified Internetwork Expert 8 For example, if in the devices in diagram A-1 were routers: • Packet from Host A to Host D will have source MAC address of Host A and destination MAC address of Router B’s local Ethernet port on Segment 1. • Packet from Host A to Host D will have source MAC address of Router B’s Serial port and destination MAC address of Router C’s local Serial port on Segment 2. • Packet from Host A to Host D will have source MAC address of Router C’s Ethernet port and a destination MAC address of Host D on Segment 3. If a packet sent from Host A to Host D were to be lost: • On segment 1, Host A would rebroadcast • On segment 2, Router B would rebroadcast • On segment 3, Router C would rebroadcast Bridging & LAN Switching A View of Bridging When non-routable protocols, such as NetBEUI, LAT or SNA were developed all devices on a network resided locally. As networks matured and bridges were introduced to segment LANs there was the need for these devices to communicate across networks, especially WAN links. Because these protocols did not have the mechanisms to allow this connectivity, bridging techniques were developed to allow the communication between devices at the Data Link Layer (layer 2 of the OSI model). By default, bridging is disabled on all Cisco routers. However, these services are still an important component of the real-world networks you will be asked to deal with in your professional life. For the purposes of the current discussion, you will also need to know them for the CCIE certification exams, both written and lab. Keep in mind that many non-routable protocols, most importantly SNA, are very time sensitive, and delays can cause loss of data or session connectivity. It is also important to understand that bridging techniques are broadcast intensive, and that this can flood slower WAN links. Bridging techniques Transparent Bridging (TB) – As the name implies, this type of bridging is transparent to the end devices. The end devices are unaware that when they communicate they are not local to one another. This functionality is not enabled by default on Cisco routers, but can be turned on when needed. When a device wishes to communicate, it will send out a broadcast to search for the requested destination address. When a Transparent Bridge sees the first broadcast from a device, it extracts the MAC address from the packet and enters it into its forwarding table, the list of devices on each interface. This process of determining what devices exist on each of the bridge’s ports is called learning. More study guides and information available at www.cramsession.brainbuzz.com. © 2000 All Rights Reserved – BrainBuzz.com
  9. BrainBuzz Cramsession: Cisco Certified Internetwork Expert 9 If the bridge receives a broadcast with a destination address that is in its forwarding table it forwards the broadcast only to that one interface. If it is not in the table, it repeats the broadcast out of all of its interfaces (except the one on which it was received). This process is called flooding. Source-Route Bridging (SRB) – Source routing is called that because instead of an intermediate device determining a path, the originating device creates its own. Routing Information Fields (RIF) are used to define paths for SRB frames to traverse a network. They are easy to read if you understand their function. For the current discussion it’s important that you understand how to understand how a RIF works. Later we’ll come back to how to rip ‘um up and read ‘um. When an SNA device needs to access a remote unit, it sends out a test frame that attempts to find the destination. You can think of this as a broadcast in the IP world; it isn’t, but that will help you to conceptualize. If the destination is not found, the source device sends out a single-route or all- routes explorer frame. Any bridges that the frame comes across in its travels add their local bridge and ring numbers to the RIF. Eventually the frame either finds its target or dies on the vine. IBM bridges support 8 rings and 7 bridges; IEEE 802.5 bridges support 14 bridges and 13 rings. Once one of the explorer frames finds the destination, it returns to its creator to announce its success. If multiple frames return, the source device takes the route of the first frame to return, assuming this is the best path. Think of it as a race in a maze; the first one to grab the cheese and get home first, wins. Ripping up a RIF This will seem complicated, but once you understand how RIFs are defined, simple practice will drive home the necessary techniques The first bit of the first byte of the source address is the Routing Information Indicator (RII), which is exactly what it sounds like; it indicates that what follows is a RIF. If this bit is a 1, the frame is a RIF; if the bit is a 0, it is not. Here are the component parts of the first 2 bytes of a RIF, called the RCF (Routing Control Field): 1. The first 3 bits define what kind of RIF is being examined: • 0xx – single route frame • 10x – all-routes explorer frame • 11x – spanning explorer frame 2. The next 5 bits show the length of the RIF. This indicates how many bytes of bridge/ring numbers follow. 3. The next single bit shows direction: • 0 – read from left-to-right • 1 – read from right-to-left 4. The last 3 bits indicate the maximum frame length More study guides and information available at www.cramsession.brainbuzz.com. © 2000 All Rights Reserved – BrainBuzz.com
  10. BrainBuzz Cramsession: Cisco Certified Internetwork Expert 10 • 000 – up to 512 bytes • 001 – up to 1,500 bytes • 010 – up to 2,052 bytes • 011 – up to 4,472 bytes • 100 – up to 8,144 bytes • 101– up to 11,407 bytes • 110 – up to 17,800 bytes • 111 – broadcast frame 5. The last four bits are not relevant. They are reserved for future use. Here’s an example of a RIF: 0810.0011.0023.0040 Translating the first two bytes (0810) to binary gives us: 0000.1000.0001.0000 Rip it up to define: Type RIF Length Direction Frame Length Not used 000 01000 0 000 0000 From this we know: • The RIF type is: single route frame • The RIF Length: 8 bytes (01000 binary = 8 decimal) • Direction to read the RIF: right-to-left • The maximum frame length: up to 512 bytes The rest of the RIF is called the RDF (Route Descriptor Field) and reading it is easy. The first three digits of each two-byte grouping are the ring number (in hexadecimal). The last digit is the bridge number (again, in hex). A zero in the bridge number designation indicates that the destination ring has been reached. Notice that since only four bits are used for the ring number, and zero is already taken, the only bridge numbers available are hex 1 through F (1 to 15 in decimal). Looking at our example again (0810.0011.0023.0040), (remember that the 0x indicates that the number that follows is in Hex) we find that the path is: • Ring 0x1 to bridge 0x1 • Ring 0x2 to bridge 0x3 • Ring 0x4 to the destination Taking another example: 0A10.0021.00B1.0101.0020 Translating the first two bytes (0A10) to binary gives us: 0000.1010.0001.0000 More study guides and information available at www.cramsession.brainbuzz.com. © 2000 All Rights Reserved – BrainBuzz.com
  11. BrainBuzz Cramsession: Cisco Certified Internetwork Expert 11 Rip it up to define: Type RIF Length Direction Frame Length Not used 000 01010 0 001 0000 From this we know: • The RIF type is: single-route • The RIF Length: 10 bytes (01010 binary = 10 decimal) • Direction to read the RIF: right-to-left • The maximum frame length: up to 512 bytes Following the rest of the RIF • Ring 0x2 to bridge 0x1 • Ring 0xB to bridge 0x1 • Ring 0x10 to bridge 0x1 • Ring 0x2 to the destination Tricky RIF (Common errors when reviewing RIFs) The Apples-to-Oranges rule: If you see 0x before a number, remember that what follows is a hexadecimal. For example, what’s the difference between ring 0x14 and ring 14? Since decimal 14 is 0xE, they’re obviously not the same ring designation. The Nice-try rule: SRB only runs on Token Ring networks, so Ethernet devices do not use RIFs. If you are looking at a network diagram and see that one of the hosts is on an Ethernet segment, remember that RIFs are irrelevant. The Roadblock rule: In a DLSw environment the RIF is terminated at the DLSw router (the definition of DLSw occurs later in this document). Source-Route Transparent Bridging (SRT) Since you now have an understanding of both TB and SRB, this next technique will come easy. An SRT bridge looks at each frame to see if it finds a RIF (looking for the RII). If there’s an RII, the frame is processed like SRB; if not, like TB. Some devices, such as Windows 95 workstations do not support RIFs. SRT allows them to communicate through bridges between LAN segments. This all takes place on Token Ring devices. The next technique will address Ethernet translations. Source-Route Translational Bridging (SR/TLB) This Cisco proprietary bridging technique allows bridging to take place between Ethernet domains and Token Ring domains. Ethernet frames are not capable of supporting RIFs. This bridging method, when enabled on Cisco routers, handles the More study guides and information available at www.cramsession.brainbuzz.com. © 2000 All Rights Reserved – BrainBuzz.com
  12. BrainBuzz Cramsession: Cisco Certified Internetwork Expert 12 conversion from Ethernet frames to Token Ring frames (bit ordering); adjusts the MTU sizes (default for Token Ring is 4,464 bytes, Ethernet 1.500 bytes); and adds and removes RIFs, as necessary. To the Token Ring devices the Ethernet segment looks like an SRB domain using a pseudo ring. Remote-Source Route Bridging (RSRB) An advanced bridging technique that allows legacy protocols, predominantly SNA, to communicate over large bridged environments using IP tunnels as a transport mechanism. Frames from Token Ring networks are encapsulated and sent over the IP network. The methods of encapsulation are: • Direct Encapsulation – This method uses HDLC (High-Level Data Link Control) and adds little overhead, but lacks reliability. This is usually used over a single network connection between two routers attached to Token Ring networks. • Fast-Sequenced Transport (FST) – This method uses IP encapsulation, which adds some overhead, but is still connectionless. • Transport Control Protocol (TCP) – This method uses TCP connection, which adds significant overhead, but ensures reliable transport. The IP network being traversed is considered one hop, using the concept of a virtual ring. Though RIFs pass through the network, they are calculated as if the entire IP network is one hop using this concept, and all acknowledgements are local, conserving valuable WAN bandwidth. Ethernet networks can be traversed as long as the local router is running SR/TLB. Data-Link Switching Plus (DLSw+) DLSw was developed as an advanced tool for the transport of SNA and other non- routable protocols over IP backbones. DLSw+ is Cisco’s enhanced version of DLSw, and provides additional functionality over previous versions. DLSw+ has more options and greater functionality then RSRB. The methods of encapsulation include: • Direct Encapsulation – This method uses HDLC (High-Level Data Link Control) and adds little overhead, but lacks reliability. This is usually used over a single network connection between two routers attached to Token Ring networks. (Same as RSRB). • Fast-Sequenced Transport (FTS) – This method uses IP encapsulation, which adds some overhead, but is still connectionless. (Same as RSRB). • Transport Control Protocol (TCP) – This method uses TCP connection, which adds significant overhead, but ensures reliable transport. (Same as RSRB). • Frame Relay RIFs are generally terminated at the DLSw router. DLSw+ can support Ethernet without SR/TLB being loaded. More study guides and information available at www.cramsession.brainbuzz.com. © 2000 All Rights Reserved – BrainBuzz.com
  13. BrainBuzz Cramsession: Cisco Certified Internetwork Expert 13 Additional tools available with DLSw+ include: Dynamic peers, peers on demand, backup peers and the ability to load balance connections. Encapsulated Bridging Used to bridge over an IP Backbone or FDDI Backbone. IRB (Integrated Routing and Bridging) A BVI (Bridged Virtual Interface) is created that acts as a member of a bridge-group to allow traffic to be routed. The BVI number must match the bridge-group number. CRB (Concurrent Routing and Bridging) Concurrent routing of one group of interfaces, while bridging another. LAN Switching All nodes on an Ethernet network can transmit at the same time, so the more nodes you have the greater the possibility of collisions happening, which can slow the network down. LAN Segmentation: breaking up the collision domains by decreasing the number of workstations per segment. Switching – examines MAC address. Works like a massive multiport bridge. Switching types: • Store-and-Forward – copies entire frame into buffer, checks for CRC errors. Higher latency. Used by Catalyst 5000 switches • Cut-Through – reads only the destination address into buffer, and forwards immediately. Low latency Spanning Tree and Root Bridge Developed to prevent routing loops. The STA (Spanning-Tree Algorithm) is used by the STP (Spanning Tree Protocol) to calculate a loop-free network topology. • There is one root bridge for Ethernet and switching environments. • There is one root bridge per VLAN, with 1 for all VLANs. • Route bridge calculation is determined by lowest MAC address. VLAN (Virtual LAN) Broadcast domains defined on Cisco switches. Since each VLAN is a separate domain, routing must be enabled between them if data is to be passed. If multiple VLANs exist on a switch, a trunk can be setup on a Fast or Gigabit Ethernet port to pass the separated data between network devices. A trunk passes data from device to device; it does not route data between VLANs. Trunking encapsulations include: More study guides and information available at www.cramsession.brainbuzz.com. © 2000 All Rights Reserved – BrainBuzz.com
  14. BrainBuzz Cramsession: Cisco Certified Internetwork Expert 14 • ISL – Used with Ethernet, and is Cisco Proprietary • 802.1Q – Used with Ethernet and is IEEE standard VTP (VLAN Transport Protocol) VLANs definitions can span switches. VTP is the method for communicating these definitions. Switches can be defined as: • Server – Listens to, stores and broadcasts VLAN configurations. Can create and delete VLANs. • Client – Listens to configurations. Can assign ports to participating VLANs. • Transparent – Forwards VTP traffic, but doesn’t participate in the VLANs. CDP (Cisco Discovery Protocol) A proprietary Data Link layer protocol used between Cisco devices to pass information about local conditions. CDP uses a data-link, multicast address with no protocol ID or network layer field, and cannot be filtered. The only way to prevent their being passed is to configure “no cdp enable” on those interfaces on which you do not want to run CDP. You can configure a MAC-layer filter to deny a multicast address as an alternative method to block these packets. Internet Protocol (IP) IP is a layer-3 routed protocol that provides addressing, fragmentation and reassembly. The minimum and maximum packet headers are 20 and 24 bytes, respectively. An IP address is 32 bits long, and the network and host sections are defined by the subnet mast associate with the address. An IP address can be bound to a host name on a router using the “ip host” command Example: Router(config)# ip host my-example 10.10.10.1 10.10.10.2 – binds name to both addresses IP Routing Protocols • Static Routes • OSPF • ISIS • EIGRP • RIP • IGRP More study guides and information available at www.cramsession.brainbuzz.com. © 2000 All Rights Reserved – BrainBuzz.com
  15. BrainBuzz Cramsession: Cisco Certified Internetwork Expert 15 Common IP Ports 20/21 FTP 23 Telnet 25 SMTP 37 Time Service 49 TACACs 53 DNS 68 BootP Client 67 BootP Server 69 TFTP 161 SNMP DHCP (Dynamic Host Configuration Protocol) To get away from statically configuring workstations addresses (which is a royal pain in the tuchis), a DHCP server can be configured which will allocate addresses dynamically. To configure a router to pass bootp packets (DHCP requests) you can use the “ip helper-address x.x.x.x” command. NAT (Network Address Translation) Used to translate one set of IP network numbers to another. The primary use for NATing is to translate external valid IP addresses to internal private addresses when connecting a network to the Internet. It can also be used to temporarily merge two networks that have different addressing schemes. Addressing issues CIDR - Classless Inter-domain Routing, CIDR used by BGP ver4. Route Summarization Reducing the number of networks being advertised between routers simplifies the routing table, reduces memory and CPU requirements, and makes the network more logical. This results in enhancing network performance and reclaiming bandwidth that would otherwise be used to pass routes back and forth. Access Lists Used to permit or deny traffic based on the source network/subnet/host address. Things to know: More study guides and information available at www.cramsession.brainbuzz.com. © 2000 All Rights Reserved – BrainBuzz.com
  16. BrainBuzz Cramsession: Cisco Certified Internetwork Expert 16 • The wildcard mask, which looks like a reversed subnet mask, defines which bits of the address are used for the access list decision-making process. • Lists are processed top-down. In other words, the first matching rule preempts further processing. • Only one access list is allowed per port/per direction/per protocol. • Remember that there is an implicit deny at the end of all access lists. • The last configured line should always be a permit statement. • Standard lists will most likely be placed close to the destination. • Extended lists will most likely be placed close to the source. • If the access-group command is configured on an interface and there is no corresponding access-list created, the command will be executed and permit all traffic in and out. • An Access Class limits VTY (telnet) access. • A Distribution List filters incoming or outgoing routing updates. Access Lists Numbers 1-99 IP standard 100-199 Extended IP 200-299 Protocol type-code 300-399 DECNet 400-499 XNS standard 500-599 XNS extended 600-699 AppleTalk 700-799 48-bit Mac Address 800-899 IPX standard 900-999 IPX extended 1000-1099 IPX Sap 1100-1199 Extended 48-bit Mac Address 1200-1299 IPX Summary Address HSRP (Hot Standby Routing Protocol) Provides a means of having two default gateways to protect against an equipment failure locking out a group of users from the wider internetwork. The default priority for each router is 100, but can be change to give one priority as the most likely default gateway (if say, one unit were faster than another). More study guides and information available at www.cramsession.brainbuzz.com. © 2000 All Rights Reserved – BrainBuzz.com
  17. BrainBuzz Cramsession: Cisco Certified Internetwork Expert 17 IP Routing Protocols Methods for avoiding routing loops Holddowns – Learned routes are held incommunicado for a period of time to prevent updates advertising networks that are misbehaving. Triggered updates – Configuring routing updates to occur after a triggering event, such as a topology change. This allows quicker convergence. Split horizon – If a router has received a route advertisement from another router, it will not re-advertise it back to the sending router. Think of this as a sphincter - things are not sent back to where they came from (gross, but you won’t forget it, and that’s the point). Poison reverse – Similar to split horizon, but instead of ignoring the update, the route is advertised back to the originating interface as a poisoned reverse update. The originating router gets its own route back, but with the time-to-live field exceeded, so the route is removed from the table. When the routers re-converge, the holddown timers have expired. This helps to more quickly clear bad routes from the list being passed back and forth between the routers. Administrative Distance Determines the level of trust each routing protocol will be given when a route is advertised from more than one. The primary AD’s are: Directly Connected 0 Static 1 EBGP 20 EIGRP (Internal) 90 IGRP 100 OSPF 110 ISIS 115 RIP 120 EGP 140 EIGRP (External) 170 IBGP 200 BGP Local 200 Unknown 255 More study guides and information available at www.cramsession.brainbuzz.com. © 2000 All Rights Reserved – BrainBuzz.com
  18. BrainBuzz Cramsession: Cisco Certified Internetwork Expert 18 RIP There are two versions of RIP - version 1 and 2. RIPv2 is classless and supports a variable subnet mask. Both use hop count as the only metric and have a time to live of 15 hops. A hop is basically one pass through a router. Updates include the entire routing table, and are sent out every 30 seconds. RIP requires neither an AS or Process ID number. Example: Router(config)# router rip Configuring a default route in RIP: Example: Router(config)# ip route 0.0.0.0 0.0.0.0 10.10.10.1 OSPF (Open Shortest Path First) OSPF Areas Types: • Backbone (transit area) - Always labeled area “0”, it accepts all LSAs and is used to connect multiple areas. All other areas must connect to this area in order to exchange and route information. When interconnecting multiple areas, the backbone area is the central entity to which all other areas must connect. • Standard - Accepts internal and external LSAs and also summary information. • Stub - Refers to an area that does not accept Type-5 LSAs to learn of external ASs. If routers need to route to networks outside the autonomous system, they use a default route (0.0.0.0). • Totally Stub - Further reduces routing tables by blocking external Type-5 LSAs and summary (Type-3-and-4) LSAs. Intra-area routes and the default of 0.0.0.0 are the only routes known to this area. Cisco proprietary. Peer Relationships: OSPF hello packet information must be the same on all routers in an area for peering relationships to be formed. This information includes: • Hello/Dead Interval • Area ID • Authentication Password • Stub Area Flag Router Types: • Internal Router (LSA Type 1 or 2) – Routers that have all their interfaces in the same area. They have identical link-state databases and run single copies of the routing algorithm. • Backbone Routers (LSA Type 1 or 2) – Routers that have at least one interface connected to area 0. • Area Border Router (LSA Type 3 or 4) – Routers that have interfaces attached to multiple areas. They maintain separate link-state databases for each area. More study guides and information available at www.cramsession.brainbuzz.com. © 2000 All Rights Reserved – BrainBuzz.com
  19. BrainBuzz Cramsession: Cisco Certified Internetwork Expert 19 • Autonomous System Boundary Router (LSA Type 5) – Routers that have at least one interface into an external internetwork, such as a non-OSPF network. These routers can redistribute non-OSPF network information to and from an OSPF network. Connection to area 0 - The main dictate in OSPF is that multiple areas all connect directly to the backbone area. The connection to the backbone area is via an ABR, which is resident in both areas and holds a full topological database for each area. A remote network can connect to area 0 via a virtual link, essentially a tunnel through the ABR in the intermediate area. From the viewpoint of OSPF, it has a direct connection. Packets designated to a different AS (Autonomous System) are forwarded to an ABR, which sends the packet through the backbone area (area 0) to the destination network ABR, which forwards it to the appropriate host. All packets that pass between ASs must pass through the backbone area when being forwarded from one area to another. The ABRs have the responsibility for maintaining the routing information between areas. To set a designated router in an OSPF network, you can set the priority or use the router with the highest loopback address. Setting the priority to 0 makes the router ineligible to become the DR. To make an OSPF router the designated router, set the priority with the highest value: Example: Router(config)# ip ospf priority 100 Recalculating the database: • Calculate routes within the local AS using Type-1 and Type-2 LSAs. • Calculate routes to other ASs (inter-area routes) using Type-3 and Type-4 summary LSAs. Remember that a totally stubby area will not accept summary LSAs. • Calculate routes to external networks using Type-5 LSAs. Remember that stub and totally stubby areas will not accept Autonomous system LSAs. Stub and Totally Stubby Area Similarities: • Inter-area routing is based on a default route (0.0.0.0). • The backbone area cannot be a stub area. • There can only be one exit point for a stub or totally stubby area - a single ABR. • Routers within the area must be configured as stub routers or they will not form neighbor adjacencies. • The area cannot be used for a transit area for virtual links to the backbone. • An ASBR cannot be internal to a stub area. • Typically used in a hub and spoke topology, with the spokes being remote offices set-up as stub or totally stubby areas. More study guides and information available at www.cramsession.brainbuzz.com. © 2000 All Rights Reserved – BrainBuzz.com
  20. BrainBuzz Cramsession: Cisco Certified Internetwork Expert 20 Stub and Totally Stubby Area Differences: • Both reduce the size of routing tables, although more so with totally stubby areas. • Both will not accept Type-5 (autonomous system entries), and totally stubby will also not accept Summary LSAs either (Type-3 and Type-4). • Totally stubby is Cisco proprietary. • Totally stubby area is preferable, since it increases stability and scalability while reducing the routing information that must be maintained. Remember that you cannot use totally stubby areas if there is a mixture of Cisco and non-Cisco routers. LSA Types: • Router link entry - This is a Type 1 LSA. Broadcast only in a specific area. Contains all the default link state information. Generated by each router for each area to which it belongs. It describes the states of the router’s link to the area. These are only flooded within a particular area. The link status and cost are two of the descriptors provided. • Network entry - This is a Type 2 LSA. Multicast to all area routers in a multi- access network by the DR (Designated Router). Contains network specific information. They describe the set of routers attached to a particular network and are flooded only within the area that contains the network. • Summary entry - Type 3 LSAs have route information for the internal networks and are sent to the backbone routers. Type 4 LSAs have information about the ASBRs. This information is broadcast by the ABR, and it will reach all the backbone routers. • Autonomous system entry - This is a Type 5 LSA. It come from the ASBR and has information relating to the external networks. No special commands are required to turn a router into an ABR or ASBR. The router takes on this role by virtue of the areas to which it is connected. As a reminder, the basic OSPF configuration steps are as follows and you would simply add another network statement for the ABR or ASBR to cover another area. To configure OSPF, do the following: 1. Enable global configuration mode 2. Enable OSPF on the ABR or ASBR router 3. Identify the ip networks and their areas Enable OSPF on the router Router(config)# router ospf process-id More study guides and information available at www.cramsession.brainbuzz.com. © 2000 All Rights Reserved – BrainBuzz.com
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