Table of Contents CCIE Routing and Switching v4.0 Quick Reference Brad Ellis Jacob Uecker Steven Means ciscopress.com Chapter General Networking Theory Chapter Bridging and LAN Switching 11 Chapter IP Addressing 30 Chapter IP Routing 55 Chapter Quality of Service (QoS) 113 Chapter Network Optimization .144 Chapter WAN .157 Chapter IP Multicasting 168 Chapter Security .185 Chapter 10 MPLS 204 Chapter 11 IPv6 217 Chapter 12 Implementing Layer Technologies 226 Chapter 13 Implementing IPv4 232 Chapter 14 Implementing IPv6 241 [2] CCIE Routing and Switching v4.0 Quick Reference Chapter General Networking Theory General Routing Concepts Link-State and Distance Vector Protocols Distance Vector Examples: Routing Information Protocol Version (RIPv1), RIPv2, Interior Gateway Routing Protocol (IGRP) n Features periodic transmission of entire routing tables to directly connected neighbors n n Mathematically compares routes using some measurement of distance Features hop-count limitation Link State Examples: Open Shortest Path First (OSPF), Intermediate System-to-Intermediate System (IS-IS) n n n n Sends local connection information to all nodes in the internetwork Forms adjacencies with neighboring routers that speak the same protocol; sends local link information to these devices Although this floods of information to all nodes, the router sends only the portion of information that deals with the state of its own links Each router constructs its own complete “picture” or “map” of the network from all the information received © 2011 Cisco Systems Inc All rights reserved This publication is protected by copyright Please see page 245 for more details [3] CCIE Routing and Switching v4.0 Quick Reference Hybrid n Example: Enhanced Interior Gateway Routing Protocol (EIGRP) n Features properties of both distance vector and link-state routing protocols Path Vector Protocol Example: Border Gateway Protocol (BGP) n n Path vector protocols are a subset of distance vector protocols; BGP uses path vectors or a list of all the autonomous systems a prefix has crossed to make metric decisions and to ensure a loop-free environment In addition to the autonomous system path list, an administrator can use many other factors to affect the forwarding or receipt of traffic using BGP Split Horizon n Routing protocols use the Split horizon technique to help prevent routing loops The split-horizon rule states that an interface will not send routing information out an interface from which the routing information was originally received Split horizon can cause problems in some topologies, such as hub-and-spoke Frame Relay configurations Summarization Summarization is the process in which the administrator collapses many routes with a long mask to form another route with a shorter mask Route summarization reduces the size of routing tables and makes the routing function more efficient Route summarization also helps to make networks more stable by reducing the number of updates sent when subnets change state Route summarization makes classless interdomain routing (CIDR) possible Variable-length subnet masking (VLSM) promotes the use of route summarization Some dynamic routing protocols engage in route summarization automatically for changes in a major classful network, whereas others not For any routing protocol within the scope of the CCIE written exam, an administrator can disable any automatic summarization that might occur and configure manual summarization To engage in route summarization, find all the left-most bits that are in common and create a mask that encompasses them An example follows © 2011 Cisco Systems Inc All rights reserved This publication is protected by copyright Please see page 245 for more details [4] CCIE Routing and Switching v4.0 Quick Reference The following routes exist in the routing table—all routes use a 24-bit mask: 10.108.48.0 = 00001010 01101100 00110000 00000000 10.108.49.0 = 00001010 01101100 00110001 00000000 10.108.50.0 = 00001010 01101100 00110010 00000000 10.108.51.0 = 00001010 01101100 00110011 00000000 10.108.52.0 = 00001010 01101100 00110100 00000000 10.108.53.0 = 00001010 01101100 00110101 00000000 10.108.54.0 = 00001010 01101100 00110110 00000000 10.108.55.0 = 00001010 01101100 00110111 00000000 Notice that the first 21 bits of the subnetwork IDs are all common These can be masked off You can use the single route entry for all these subnetworks as follows: 10.108.48.0/21 Classful and Classless Routing Protocols Classful routing protocols are considered legacy and not include subnet mask information with routing updates Examples of classful routing protocols are RIPv1 and IGRP Because subnet mask information is not included in updates, consistency of the mask is assumed throughout the network Classful routing protocols also feature automatic summarization of routing updates when sent across a major classful network boundary For example, the 10.16.0.0/16 network would be advertised as 10.0.0.0/8 when sent into a 172.16.0.0 domain Although BGP and EIGRP are not classful routing protocols, both engage in automatic summarization behavior by default, and in that sense they act classful The no auto-summary command is used to disable this behavior Classful routing protocols feature a fixed-length subnet mask (FLSM) because of their inherent limitations The FLSM leads to inefficient use of addresses and limits the network’s overall routing efficiency By default, classful routing protocols discard traffic bound for any unknown subnet of the major classful network For example, if your classful routing protocol receives traffic destined for 10.16.0.0 and it knows of only the 10.8.0.0 and 10.4.0.0 subnets in its routing table, it discards the traffic—even if a default route is present! The ip classless © 2011 Cisco Systems Inc All rights reserved This publication is protected by copyright Please see page 245 for more details [5] CCIE Routing and Switching v4.0 Quick Reference command was introduced to change this behavior The ip classless command enables the protocol to use the default route in this case This command is on by default with Cisco IOS Release 12.0 and later routers As a classic example of a classless routing protocol, OSPF carries subnet mask information in updates Wireless LAN Services Module (WLSM) is possible with such protocols Routing Decision Criteria Routers must determine the best route to send traffic on toward its destination This is accomplished as follows (note that the order of operations is critical and fixed): Valid next-hop IP address: When updates are received, the router first verifies that the next-hop IP address to reach the potential destination is valid Metric: The router then examines the metrics for the various routes that might exist from a particular protocol For example, if OSPF has several routes to the destination, the router tries to install the route with the best metric (in this case, cost) into the routing table Administrative distance: If multiple routing protocols run on the device, and multiple protocols all present routes to the destination with valid next hops, the router examines administrative distance The route sourced from the lowest administrative distance protocol or mechanism is installed in the routing table Prefix: The router examines the route’s prefix length If no exact match exists in the routing table, the route is installed This might cause the routing table to fill with the following entries: EIGRP 172.16.2.0/24 and RIP 172.16.2.0/19 For the prefix length and the routing table, remember that when a router looks for a match in the IP routing table for the destination address, it always looks for the longest possible prefix match For example, if the routing table contains entries of 10.0.0.0/8, 10.2.0.0/16, and 10.2.1.0/24, and your traffic is destined for 10.2.1.0/24, the longest match prefix is selected This prefix length rule trumps administrative distance So a /24 prefix learned via EIGRP would be preferred over a /16 added as a static route despite the static route having a superior administrative distance © 2011 Cisco Systems Inc All rights reserved This publication is protected by copyright Please see page 245 for more details [6] CCIE Routing and Switching v4.0 Quick Reference Routing Information Base and Routing Protocol Interaction Administrative Distance If a router learns of a network from multiple sources (routing protocols or static configurations), it uses the administrative distance value to determine which route to install in the routing (forwarding) table The default administrative distance values are listed here Source Administrative Distance Connected interface Static route EIGRP summary route External BGP Internal EIGRP IGRP OSPF IS-IS RIP Exterior Gateway Protocol On-demand routing External EIGRP Internal BGP Unknown 20 90 100 110 115 120 140 160 170 200 255 Administrators can create static routes that float A floating static route means the administrator increases the administrative distance of the static route to be greater than the default of For example, if you run EIGRP on your network, the AD of a static route could be increased to 95 This would mean the static route would be used only when a dynamic EIGRP route did not exist © 2011 Cisco Systems Inc All rights reserved This publication is protected by copyright Please see page 245 for more details [7] CCIE Routing and Switching v4.0 Quick Reference Routing Table The routing table has been the principal element of IP routing and the primary goal of routing protocols to build and maintain for most of modern internetworking The main routing table model, the hop-by-hop routing paradigm, has the routing table list for each destination network of the next-hop address to reach that destination If the routing tables are consistent and accurate, with no misinformation, this simple hop-by-hop paradigm works well enough to deliver data to anywhere from anywhere in the network In recent practice, this simple hop-by-hop model is abandoned for new technologies such as Multiprotocol Label Switching (MPLS) These technologies enable a simple and efficient label lookup to dictate the next hop that data should follow to reach a specific destination Although this determination can be based on the routing table information, it can easily be based on other parameters, such as quality of service (QoS) or other traffic engineering considerations MPLS is explored in its own chapter of this Q Routing Information Base and Forwarding Information Base Interaction The routing and forwarding architecture in Cisco routers and multilayer switches used to be a centralized, cachebased system that combined a control plane and a data plane The control plane refers to the resources and technologies that create and maintain the routing table The data plane refers to those resources and technologies needed to actually move data from the ingress port to the egress port on the device This centralized architecture has migrated so that the two planes can separate to enhance scalability and availability in the routing environment The separation of routing and forwarding tasks has created the Routing Information Base (RIB) and the Forwarding Information Base (FIB) The RIB operates in software, and the control plane resources take the best routes from the RIB and place them in the FIB The FIB resides in faster hardware resources The Cisco implementation of this enhanced routing and forwarding architecture is called Cisco Express Forwarding (CEF) Redistribution Redistribution Between Routing Protocols Route redistribution might be required in an internetwork because multiple routing protocols must coexist Multiple routing protocols might be a necessity because of an interim period during conversion from one to another, application-specific protocol requirements, political reasons, or a lack of multivendor interoperability © 2011 Cisco Systems Inc All rights reserved This publication is protected by copyright Please see page 245 for more details [8] CCIE Routing and Switching v4.0 Quick Reference A major issue with redistribution is the seed metric used when the routes enter the new routing protocol Normally, the seed metric is generated from the originating interface For example, EIGRP would use the bandwidth and delay of the originating interface to seed the metric With redistributed routes, however, these routes are not connected to the router Some routing protocols feature a default seed metric for redistribution, whereas others not Following is a list of the defaults for the various protocols Infinity indicates a seed metric must be configured; otherwise, the receiving protocol will not use the route Protocol Default Seed Metric OSPF IS-IS RIP IGRP/EIGRP 20; except BGP, which is Infinity Infinity Redistribution Into RIP Remember to set a default metric, using either the redistribute command or the default-metric command Following is the command to redistribute routes into RIP: redistribute protocol [process-id] [match route-type] [metric metric-value] [route-map map-tag] The match keyword enables you to match certain route types when redistributing OSPF For example, you can specify internal, external 1, or external The route-map keyword enables you to specify a route map for controlling or altering the routes that are redistributed Redistribution Into OSPF The default seed metric is 20 The default metric type for redistributed routes is External Type (E2), meaning the metric reflects only the cost from the redistributing router to the destination regardless of the path cost within the © 2011 Cisco Systems Inc All rights reserved This publication is protected by copyright Please see page 245 for more details [9] CCIE Routing and Switching v4.0 Quick Reference OSPF network Type (e1) can be optionally used, which means the metric will be based on the total path to the destination Subnets are not redistributed by default Following is the command for redistribution into OSPF: redistribute protocol [process-id] [metric metric-value] [metric-type type-value] [route-map map-tag] \[subnets] [tag tag-value] The subnets keyword is critical in this command and specifies that subnets should indeed be redistributed The tag value enables the administrator to configure an optional tag value that can be used later to easily identify these routes Redistribution into EIGRP Remember that like RIP, you must set a default seed metric when redistributing into EIGRP Following is the command for redistribution into EIGRP: redistribute protocol [process-id] [match {internal | external | external 2}] [metric metric-value] [route-map map-tag] Troubleshooting Routing Loops You can perform one-way or two-way redistributions You can also perform redistribution in multiple locations throughout the topology With one-way redistribution, you typically pass a default route into the edge protocol, and take all the edge protocol routes and redistribute them into the core protocol of the network With two-way redistribution, all routes from each routing protocol pass into each other If two-way redistribution is © 2011 Cisco Systems Inc All rights reserved This publication is protected by copyright Please see page 245 for more details ... 00 001 0 10 01 101 100 00 1 100 01 00 000 000 10. 108 . 50. 0 = 00 001 0 10 01 101 100 00 1 100 10 000 000 00 10. 108 .51 .0 = 00 001 0 10 01 101 100 00 1 100 11 00 000 000 10. 108 .52 .0 = 00 001 0 10 01 101 100 00 1 101 00 000 000 00 10. 108 .53 .0. .. 00 1 101 00 000 000 00 10. 108 .53 .0 = 00 001 0 10 01 101 100 00 1 101 01 00 000 000 10. 108 .54 .0 = 00 001 0 10 01 101 100 00 1 101 10 000 000 00 10. 108 .55 .0 = 00 001 0 10 01 101 100 00 1 101 11 00 000 000 Notice that the first 21 bits... Routing and Switching v4. 0 Quick Reference The following routes exist in the routing table—all routes use a 24-bit mask: 10. 108 .48 .0 = 00 001 0 10 01 101 100 00 1 100 00 000 000 00 10. 108 .49 .0 = 00 001 010