Lesson 5: Installing and Configuring Routed Networks

An Exterior Gateway Protocol (EGP)

Is one that can perform routing between autonomous systems.

Flapping interface

Is one that frequently changes from online to offline and offline to online. Flapping can cause serious convergence problems.

Maximum Transmission Unit (MTU)

How large a packet can be sent without the need for fragmentation.

Learned route

A _________ is one that was communicated to the router by another router by using a dynamic routing protocol. Routers use these protocols to exchange information about connected networks periodically and select the best available route to a destination.

Load (link utilization)

A metric that bases routing decisions on how busy a route is.

Steady state

A network where all the routers share the same topology is described as _________. The time taken to reach _________ is a measure of a routing protocol's convergence performance.

Metric

A preference value assigned to the route, with low values being preferred over high ones. The value of the _____ may be determined by different parameters, such as how far the next hop router is, how long it will take to route a packet to the subsequent routers, what bandwidth is available on the selected path, how large a packet can be sent without fragmentation, and so on.

Border router

A router designed to connect a private network to the Internet is called an _________. These routers can perform framing to repackage data from the private LAN frame format to the WAN Internet access frame format. Routers very often also support the functions of a firewall.

Virtual Router Redundancy Protocol (VRRP)

An open standard protocol known as ______________________ was developed in 2004. The current version is _____ version 3, defined in RFC 5798, which adds support for IPv6. ______ is very similar to HSRP, the differences mainly being in terminology and packet formats. In _________ , the active router is known as the master, and all other routers in the group are known as backup routers. There is no specific standby router; instead, all backup routers monitor the status of the master, and in the event of a failure, a new master router is selected from the available backup routers based on priority. One advantage of ____ over HSRP is that it does not require each router interface to be assigned a unique IP address. It is possible to configure _______routers to use only the virtual IP address. This can be useful on subnets where address space utilization is high.

Loopback interface

Any given physical interface could suffer a hardware fault or be temporarily unavailable for various reasons, it is considered best practice to create a virtual interface, known as a _________, in the router's operating system and assign it an IP address for use in remotely managing the router. This is a way of giving the router an internal IP address, not connected to any physical network, that is therefore not reliant on a specific network being available.

Autonomous System Numbers (ASN)

Are allocated to ISPs by IANA via the various regional registries.

Blocked

ICMP is often _____ by firewalls and routers, so the output from tools such as tracert and pathping is not always completely accurate.

Non-persistent

For example, routes added in a manner are __________ by default. This means that they are stored in memory and will be discarded if the machine is restarted.

End Systems (ES)

Hosts with no capacity to forward packets to other IP networks.

Hierarchical

In ________ routing systems, certain routers form a routing backbone. Other routers are grouped into logical collections, sometimes called areas or domains. Backbone or border routers can communicate with routers in other domains, while internal routers are limited to communications with routers within the current domain or the border router(s) serving it. The __________ approach allows for route summarization.

Route aggregation

Internet routers must be able to locate any host on the Internet. As there are millions of networks and hosts, it is impossible to do this by storing routes to each of them. Instead, an Internet core router will consolidate the route to a group of networks, as identified by their shared network routing prefix, to a single routing table entry.

Routing Information Protocol (RIP)

Is a long-established distance vector-based routing protocol. It uses a hop count metric to determine the distance to the destination network. Generally speaking, each router is assigned a hop count value of 1. _______ considers only one route to a given destination network, that with the lowest hop count. To help prevent looping, the maximum hop count allowed is 15. Consequently, this limits the maximum size of a ______ network, since networks that have a hop count of 16 or higher are unreachable. Because it is widely adopted, well understood, and simple, __________ is ideally suited to small networks with limited failover routes. ____ uses the User Datagram Protocol (UDP port 520) for routers to exchange messages.

Border Gateway Protocol (BGP)

Is designed to be used between routing domains, or Autonomous Systems (ASes), and as such is used as the routing protocol on the Internet, primarily between ISPs. _____ is a type of exterior gateway protocol (in fact, it replaced a protocol named EGP) for communications between routers in diverse ASes. When ______ is used within an AS, it is referred to as Interior ____ (I___), and when implemented between ASes, it is referred to as Exterior ___ (E___). _____ works with Classless Inter-Domain Routing (CIDR) IP network prefixes called Network Layer Reachability Information (NLRI). Route selection is based on multiple metrics, including hop count, weight, local preference, origin, and community. ___ is not a pure distance-vector algorithm but uses a hybrid approach. In fact, ___ is more usually classed as a path vector routing protocol. ___ works over TCP on port 179.

Looking glass site

Is one that hosts a server that exposes its routing table to public queries via HTTP. Some ______________ can be accessed via Telnet. An ISP can use these to verify that information about routes to its networks is being properly propagated to the routers of other ISPs. You could also use a _______________ to troubleshoot problems accessing a particular website. If a site is available from one location but not another, you can use the ____________ to work out the route(s) from the site to an IP address.

Interior Gateway Protocol (IGP)

Is one that performs routing within a network under the administrative control of a single owner, also referred to as an Autonomous System (AS).

Latency

It takes time for a packet to traverse an internetwork. Delay-based metrics measure transit time. This metric is most important if the route is used to carry time-sensitive data, such as voice or video.

Enhanced IGRP (EIGRP)

Like RIP, __________ is usually classed as a distance vector-based routing protocol. Unlike RIP, _________ uses a metric composed of several administrator weighted elements, including reliability, bandwidth, delay, and load. _________ also supports multiple paths to the destination network; again, unlike RIP. __________ may also, therefore, be described as an advanced distance vector protocol or as a hybrid routing protocol. _________ builds on the strengths of RIP while providing for more efficient route selection, better administrative control, and better fault tolerance. Unlike RIP, _________ is a native IP protocol, which means that it is encapsulated directly in IP datagrams, rather than using TCP or UDP. It is tagged with the protocol number 88 in the Protocol field of the IP header.

Bandwidth

Metrics based on ________ look at the maximum achievable ___________ on a link and do not consider the available ________. This is a less efficient metric than delay-based metrics.

Route command

On a Windows or Linux/UNIX host, the _________, is used to view and modify the routing table.

Routing table

On a router, information about the location of other IP networks and hosts is stored in a ____________.

Reliability

Over time, it might become obvious that some links between routers are more reliable than others. You can assign a value for this ________ that routers can assess when determining an effective path.

Layer 3

Routers are always described as working at _______, even if they contain modules that work at layers 1 and 2 as well.

Link-state

Routers implementing a ________ algorithm propagate information about only their own links to other routers on the internetwork. Routers update each other only when one of their links changes state. Otherwise, they do not communicate except for sending a periodic "hello" packet to assure their neighboring routers that they are still functioning on the network. These smaller, frequent updates lead to more rapid convergence and more efficiently support larger networks. However, __________ algorithms can be more expensive to implement because they require more power and memory.

Intermediate systems (IS)

Routers that interconnect IP networks and can perform this packet forwarding process

Routing

Takes place when a host needs to communicate with a host on a different IP network or a different subnet.

Gateway (next hop)

The IP address of the next router along the path.

Jitter

The StDev field in the output shows standard deviation in the latency measurements. Standard deviation is a measure of how dispersed the highest and lowest values are from the mean (or average) sample value. If StDev is high, latency values vary widely across the samples. This sort of inconsistent latency is called ________.

Path length

The end-to-end cost of using a route (hop count). You might assign a value to a path between router A and B and between B and C. The end-to-end _________ is the sum of A-to-B and B-to-C.

Open Shortest Path First (OSPF)

The hierarchical link-state routing protocol is suited to large organizations with multiple redundant paths between networks. It has high convergence performance compared to RIP. It was designed from the outset to support classless addressing. Networks and their connected hosts and routers within an autonomous system are grouped into _____ areas. Routers within a given area share the same topological database of the networks they serve. Routers that can connect to multiple areas are known as area border routers. A backbone (always called Area 0) is created by the collection of border routers. This backbone is only visible to the border routers and invisible to the routers within a specific area. Routers use a Link State Advertisement (LSA) to update their routing tables. In a given area, routers exchange ____ "hello" messages, both as a form of a keep-alive packet and in order to acquire neighbors with which to exchange routing information. These exchanges of routing information enable the routers to each build a topological routing tree (a shortest path tree) and keep it up to date. Messages are sent using _______'s own datagram format. This is tagged as protocol number 89 in the IP datagram's Protocol field.

Interface

The local port to use to forward a packet along the chosen route.

Autonomous System

The uses of ____ are discussed in RFC 1930. _____ are designed to hide the complexity of private networks from the public Internet. If all Internet locations had to be propagated to all Internet routers, the routing tables would become too large to process. Border (or edge) routers for each _____exchange only as much network-reachability information as is required to access other _______ (the ____ path), rather than networks and hosts within each ___.

Distance vector

Their algorithms require that routers periodically propagate their entire routing table to their immediate neighbors. __________ algorithms provide for slower convergence than link-state algorithms.

Dynamic routing protocol

These days, almost all routers use some sort of ______________ to learn about remote networks and the most efficient route to those networks. Different routing algorithms, as implemented in the various protocols, may use different metrics and make comparisons of available paths in different ways. However, the desired result is always the same—to choose the optimal path for a specific packet at a given moment.

Hot Standby Router Protocol (HSRP)

To address this problem, Cisco developed the proprietary ___________, standardized in 1998 as RFC 2281. ______ allows for multiple physical routers to serve as a single default gateway for a subnet. To do this, each router must have an interface connected to the subnet, with its own unique MAC address and IP address. In addition, they also need to be configured to share a common virtual IP address and a common MAC address.

Standby group

To do this, each router must have an interface connected to the subnet, with its own unique MAC address and IP address. In addition, they also need to be configured to share a common virtual IP address and a common MAC address. The group of routers configured in this way is known as a ________. They communicate among themselves using IP multicasts and choose an active router based on priorities configured by an administrator. The active router responds to any traffic sent to the virtual IP address. Of the remaining routers in the ________, the router with the next highest priority is chosen as the standby router. The standby router monitors the status of the active router and takes over the role if the active router becomes unavailable, also triggering the selection of a new standby router from the remaining routers in the group.

Cost

You can assign a monetary ______ to various links, and the router using a ________ metric will try to select the cheapest link available. This is useful for organizations routing on a budget.

Flat

a situation where all routers can inter-communicate with one another. Each network ID requires a separate entry in the routing table, which can be problematic in very large internetworks.

Loop

causes a packet to be forwarded around the network until its TTL expires.

Host route

for routes to a specific IP address.

Remote network route

for subnets and IP networks that are not directly attached.

Direct network route

for subnets to which the router is directly attached.

Convergence

is the process whereby routers running dynamic routing algorithms agree on routes through the internetwork. As the internetwork topology changes constantly (with router failures, addressing changes, and unforeseen events), routers must be capable of adapting to these changes and communicating them quickly to other routers to avoid loops and black holes.

Black hole

means that a packet is discarded without notification back to the source.

Static routing

means that you manually add routes to a routing table, and they change only if you edit them. Configuring ________ entries can be useful in some circumstances, but it can be problematic if the routing topology changes often, as each route on each affected router needs to be updated manually.

Route flapping

refers to a router changing the properties of a route it is advertising quickly and often. Flapping can cause serious convergence problems.

Default route

which are used when a better network or host route is not found. It is a special type of static route that identifies the next hop router for an unknown destination. The destination address 0.0.0.0/0 (IPv4) or ::/0 (IPv6) is used.