Section III: Routing Technologies – Test Online

Hint

When one route is advertised by more than one routing protocol, the router will choose to use the routing protocol which has lowest Administrative Distance. The Administrative Distances of popular routing protocols are listed below:

Hint

Below is an example of the “show ip eigrp neighbors” output. Let’s analyze these columns: + H: lists the neighbors in the order this router was learned + Address: the IP address of the neighbors + Interface: the interface of the local router on which this Hello packet was received + Hold (sec): the amount of time left before neighbor is considered in “down” status + Uptime: amount of time since the adjacency was established + SRTT (Smooth Round Trip Timer): the average time in milliseconds between the transmission of a packet to a neighbor and the receipt of an acknowledgement. + RTO (Retransmission Timeout): if a multicast has failed, then a unicast is sent to that particular router, the RTO is the time in milliseconds that the router waits for an acknowledgement of that unicast. + Queue count (Q Cnt): shows the number of queued EIGRP packets. It is usually 0. + Sequence Number (Seq Num): the sequence number of the last update EIGRP packet received. Each update message is given a sequence number, and the received ACK should have the same sequence number. The next update message to that neighbor will use Seq Num + 1. In this question we have to check the RTO and Q cnt fields.

Hint

This command can be used to verify if a BGP connection to a BGP neighbor is good or not. Let’s see an example: Please pay attention to the “State/PfxRcd” column of the output. It indicates the number of prefixes that have been received from a neighbor. If this value is a number (including “0”, which means BGP neighbor does not advertise any route) then the BGP neighbor relationship is good. If this value is a word (including “Idle”, “Connect”, “Active”, “OpenSent”, “OpenConfirm”) then the BGP neighbor relationship is not good. In the outputs above we see the BGP neighbor relationship between R1 & 10.1.1.1 is good with 2 Prefix Received (PfxRcd) while the BGP neighbor relationships between R1 & 10.2.2.2; R1 & 10.3.3.3 are not good (they are in “Active” and “Idle” state).

Hint

Check the configuration of the interface that is connected to Server1 and Server2 on R2 with “show runningconfig” command. We see that subinterface E0/1.100 has been configured with VLAN 200 (via “encapsulation dot1Q 200” command) while Server1 belongs to VLAN 100. Therefore this configuration is not correct. It should be “encapsulation dot1Q 100” instead. The same thing for interface E0/1.200, it should be “encapsulation dot1Q 200” instead.

Hint

First we should check the routing table of R2 with the “show ip route” command. In this table we cannot find the subnet “10.10.12.0/24” (R3 LAN network) so R2 will use the default route advertised from R1 (with the command “default-information originate” on R1) to reach unknown destination, in this case subnet 10.10.12.0/24 -> R2 will send traffic to 10.10.12.0/24 to R1. Next we need to find out why R3 did not advertise this subnet to R2. A quick check with the “show runningconfig” on R3 we will see that R3 was not configured with RIP ( no “router rip” section). Therefore, we can conclude RIPv2 was not enabled on R3.

Hint

First use the “show ip route” command to check the R1 routing table: As we see here, 10.10.10/24 is learned from RIP. Notice that although there is a static route on R1 to this destination (you can check with the “show running-config” on R1 to see the line “ip route 10.10.10.0 255.255.255.0 172.16.14.2 200”), this static route is not installed to the routing table because it is not the best path because the Administrative Distance (AD) of this static route is 200 while the AD of RIP is 120 so R1 chose the path with lowest AD so it chose path advertised via RIP.

Hint

When all the users cannot reach internet sites we should check on the router connecting to the ISP to see if it has a default route pointing to the ISP or not. Use the “show ip route” command on R1: We cannot find a default route on R1 (something like this: S* 0.0.0.0/0 [1/0] via 209.165.201.2) so maybe R1 was not configured with a default route. We can check with the “show running-config” on R1: https://itexamanswers.net/wp-content/uploads/2019/08/434gt2.png We need a configure a default route (“ip route 0.0.0.0 0.0.0.0 209.165.201.2”) but we cannot find here so we can conclude R1 was not be configured with a default route pointing to the ISP router.

Hint

192.168.10.45 belongs to 192.168.10.32/27 subnet (range from 192.168.10.32 to 192.168.10.63) so the router will use FastEthernet0/1 as the exit interface.

Hint

Floating static routes are static routes that have an administrative distance greater than the administrative distance (AD) of another static route or dynamic routes. By default a static route has an AD of 1 then floating static route must have the AD greater than 1. Floating static route has a manually configured administrative distance greater than that of the primary route and therefore would not be in the routing table until the primary route fails.

Hint

First please notice that unlike other routing protocols like OSPF or EIGRP, we have to use subnet mask, not wildcard mask, to advertise the routes in the “network” command -> C is not correct.Secondly, with BGP, you must advertise the correct network and subnet mask in the “network” command ( in this case network 172.16.1.32/27). BGP is very strict in the routing advertisements. In other words, BGP only advertises the network which exists exactly in the routing table (in this case network x.x.x.32/27 exists in the routing table as the Fa0/0 interface). If you put the command “network x.x.0.0 mask 255.255.0.0” or “network x.0.0.0 mask 255.0.0.0” or “network x.x.x.33 mask 255.255.255.255” then BGP will not advertise anything.Therefore the full command in this question is “network 172.16.1.32 mask 255.255.255.224”. For more information about BGP configuration, please read our Basic BGP Configuration tutorial.

Hint

From the routing table we learn that network 192.168.10.0/30 is learned via 2 equal-cost paths (192.168.10.9 &192.168.10.5) -> traffic to this network will be load-balancing.

Hint

A host route for IPv4 has the mask /32, and a host route for IPv6 has the mask /128. If an IPv4 address is configured with a mask of /32 on an interface of the router, which is typical for loopback interfaces, the host route appears in the routing table only as connected (for example in the routing table we will see this line: “C 10.10.10.1/32 is directly connected, Loopback0”. Reference: https://www.cisco.com/c/en/us/support/docs/ip/ip-routing/116264-technote-ios-00.html

Hint

To configure EIGRP for IPv6 we must explicitly specify a router ID before it can start running. For example:

ipv6 router eigrp 1 
eigrp router-id 2.2.2.2 
no shutdown

EIGRPv3 also uses the AS number (for example: ipv6 eigrp 1 under interface mode).Notice that EIGRP for IPv6 router-id must be an IPv4 address. EIGRP for IPv4 can automatically pick-up an IPv4 to use as its EIGRP router-id with this rule: + The highest IP address assigned to a loopback interface is selected as the router ID. + If there are not any loopback addresses configured, the highest IP address assigned to any other active interface is chosen as the router ID

Hint

To form an adjacency (become neighbor), router A & B must have the same Hello interval, Dead interval and AREA number.

Hint

Before you enable OSPF for IPv6 on an interface, you must perform the following:+ Complete the OSPF network strategy and planning for your IPv6 network. For example, you must decide whether multiple areas are required. + Enable IPv6 unicast routing. + Enable IPv6 on the interface.Reference: https://www.cisco.com/c/en/us/support/docs/ip/ip-version-6-ipv6/112100-ospfv3-config-guide.html Note: If we have already had an active interface, we don’t need to configure the router ID for OSPFv3 anymore because the device will automatically choose that IPv4 address for its router ID).

Hint

Below lists some reserved and well-known IPv6 multicast address in the reserved multicast address range:FF01::1 All IPv6 nodes within the node-local scope FF01::2 All IPv6 routers within the node-local scope FF02::1 All IPv6 nodes within the link-local scope FF02::2 All IPv6 routers within the link-local scope FF02::5 All OSPFv3 routers within the link-local scope FF02::6 All OSPFv3 designated routers within the link-local scope FF02::9 All RIPng routers within the link-local scope FF02::A All EIGRP routers within the link-local scope FF02::D All PIM routers within the link-local scope FF02::1:2 All DHCPv6 agents (servers and relays) within the link-local scope FF05::2 All IPv6 routers within the site-local scope FF02::1:FF00:0/104 IPv6 solicited-node multicast address within the link-local scope

Hint

Hierarchical design of OSPF (basically means that you can separate the larger internetwork into smaller internetworks called areas) helps us create a network with all features listed above (decrease routing overhead, speed up convergence, confine network instability to single areas of the network).

Hint

http://www.cisco.com/c/en/us/support/docs/ip/enhanced-interior-gateway-routing-protocol-eigrp/113267-eigrp-ipv6-00.html

Hint

The “show ip ospf database” command displays the link states. Here is an example: Here is the lsa database on R2. R2#show ip ospf database OSPF Router with ID (2.2.2.2) (Process ID 1) Router Link States (Area 0) Link ID ADV Router Age Seq# Checksum Link count2.2.2.2 2.2.2.2 793 0x80000003 0x004F85 210.4.4.4 10.4.4.4 776 0x80000004 0x005643 1111.111.111.111 111.111.111.111 755 0x80000005 0x0059CA 2133.133.133.133 133.133.133.133 775 0x80000005 0x00B5B1 2 Net Link States (Area 0) Link ID ADV Router Age Seq# Checksum10.1.1.1 111.111.111.111 794 0x80000001 0x001E8B10.2.2.3 133.133.133.133 812 0x80000001 0x004BA910.4.4.1 111.111.111.111 755 0x80000001 0x007F1610.4.4.3 133.133.133.133 775 0x80000001 0x00C31F

Hint

Both the “ip default-network” and “ip route 0.0.0.0 0.0.0.0 (next hop)” commands can be used to set the default gateway in a Cisco router.

Hint

By default, the administrative distance of a static route is 1, meaning it will be preferred over all dynamic routing protocols. If you want to have the dynamic routing protocol used and have the static route be used only as a backup, you need to increase the AD of the static route so that it is higher than the dynamic routing protocol.

Hint

The “network 192.168.12.64 0.0.0.63 equals to network 192.168.12.64/26. This network has: + Increment: 64 (/26= 1111 1111.1111 1111.1111 1111.1100 0000) + Network address: 192.168.12.64 + Broadcast address: 192.168.12.127 Therefore, all interface in the range of this network will join OSPF.

Hint

The 10.4.0.0/22 route includes 10.4.0.0/24, 10.4.1.0/24, 10.4.2.0/24 and 10.4.3.0/24 only.

Hint

The simple syntax of static route: ip route destination-network-address subnet-mask {next-hop-IP-address | exit-interface} + destination-networkaddress: destination network address of the remote network + subnet mask: subnet mask of the destination network + next-hop-IP-address: the IP address of the receiving interface on the next-hop router + exit-interface: the local interface of this router where the packets will go out in the statement “ip route 10.5.6.0 255.255.255.0 fa0/0: + 10.5.6.0 255.255.255.0: the destination network +fa0/0: the exit-interface

Hint

Neighbor discovery is the first step in getting a link state environment up and running. In keeping with the friendly neighbor terminology, a Hello protocol is used for this step. The protocol will define a Hello packet format and a procedure for exchanging the packets and processing the information the packets contain. After the adjacencies are established, the routers may begin sending out LSAs. As the term flooding implies, the advertisements are sent to every neighbor. In turn, each received LSA is copied and forwarded to every neighbor except the one that sent the LSA.

Hint

The OSPF protocol is based on link-state technology, which is a departure from the Bellman-Ford vector based algorithms used in traditional Internet routing protocols such as RIP. OSPF has introduced new concepts such as authentication of routing updates, Variable Length Subnet Masks (VLSM), route summarization, and so forth. OSPF uses flooding to exchange link-state updates between routers. Any change in routing information is flooded to all routers in the network. Areas are introduced to put a boundary on the explosion of link-state updates. Flooding and calculation of the Dijkstra algorithm on a router is limited to changes within an area.

Hint

Administrative distance is the feature that routers use in order to select the best path when there are two or more different routes to the same destination from two different routing protocols. Administrative distance defines the reliability of a routing protocol. Each routing protocol is prioritized in order of most to least reliable (believable) with the help of an administrative distance value. Default Distance Value Table This table lists the administrative distance default values of the protocols that Cisco supports: Route Source Default Distance Values Connected interface Static route Enhanced Interior Gateway Routing Protocol (EIGRP) summary route External Border Gateway Protocol(BGP) Internal EIGRP IGRP OSPF Intermediate System-to-Intermediate System (IS-IS) Routing Information Protocol (RIP) Exterior Gateway Protocol (EGP) On Demand Routing (ODR) External EIGRP Internal BGP Unknown*

Hint

Since all the same router (C-router) is the default gateway for all three VLANs, all traffic destined to a different VLA will be sent to the C-router. The C-router will have knowledge of all three networks since they will appear as directly connected in the routing table. Since the C-router already knows how to get to all three networks, no routing protocols need to be configured.

Hint

The destination IP address 10.1.5.65 belongs to 10.1.5.64/28, 10.1.5.64/29 & 10.1.5.64/27 subnets but the “longest prefix match” algorithm will choose the most specific subnet mask -> the prefix “/29 will be chosen to route the packet. Therefore, the next-hop should be 10.1.3.3 ->.

Hint

The binary version of 20 is 10100. The binary version of 16 is 10000. The binary version of 24 is 11000. The binary version of 28 is 11100. The subnet mask is /28. The mask is 255.255.255.240. Note: From the output above, EIGRP learned 4 routes and we need to find out the summary of them: + 192.168.25.16 + 192.168.25.20 + 192.168.25.24 + 192.168.25.28 -> The increment should bE. 28 ?16 = 12 but 12 is not an exponentiation of 2 so we must choose 16 (24). Therefore, the subnet mask is /28 (=1111 1111.1111 1111.1111 1111.11110000) = 255.255.255.240 So the best answer should be 192.168.25.16 255.255.255.240

Hint

The highest IP address of all loopback interfaces will be chosen -> Loopback 0 will be chosen as the router ID.

Hint

Multiple OSPF processes can be configured on a router using multiple process ID’s. The valid process ID’s are shown below: Edge-B(config)#router ospf ? <1-65535> Process ID

Hint

In the router ospf command, the ranges from 1 to 65535 so o is an invalid number -> but To configure OSPF, we need a wildcard in the “network” statement, not a subnet mask. We also need to assgin an area to this process -> .

Hint

EIGRP uses the Neighbor Table to list adjacent routers. The Topology Table list all the learned routers to destination whilst the Routing Table contains the best route to a destination, which is known as the Successor. The Feasible Successor is a backup route to a destination which is kept in the Topology Table.

Hint

Host on the LAN attached to router R1 belongs to 192.168.10.64/26 subnet. From the output of the routing table of R3 we learn this network can be reach via 192.168.10.9, which is an IP address in 192.168.10.8/30 network (the network between R1 & R3) -> packets destined for 192.168.10.64 will be routed from R3 -> R1 -> LAN on R1.

Hint

We use default routing to send packets with a remote destination network not in the routing table to the nexthop router. You should generally only use default routing on stub networks–those with only one exit path out of the network. According to exhibit, all traffic towards Internet that originates from workstations should forward to Router R1. Syntax for default route is: ip route .

Hint

This static route will allow for communication to the Manchester workstations and it is better to use this more specific route than a default route as traffic destined to the Internet will then not go out the London Internet connection.

Hint

The use of static routes will provide the necessary information for connectivity while producing no routing traffic overhead.

Hint

http://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/7039-1.html#t6OSPF Cost The cost (also called metric) of an interface in OSPF is an indication of the overhead required to send packets across a certain interface. The cost of an interface is inversely proportional to the bandwidth of that interface. A higher bandwidth indicates a lower cost. There is more overhead (higher cost) and time delays involved in crossing a 56k serial line than crossing a 10M ethernet line. The formula used to calculate the cost is: cost= 10000 0000/bandwith in bps For example, it will cost 10 EXP8/10 EXP7 = 10 to cross a 10M Ethernet line and will cost 10 EXP8/1544000 = 64 to cross a T1 line. By default, the cost of an interface is calculated based on the bandwidth; you can force the cost of an interface with the ip ospf cost interface subconfiguration mode command.

Hint

Distance means how far and Vector means in which direction. Distance Vector routing protocols pass periodic copies of routing table to neighbor routers and accumulate distance vectors. In distance vector routing protocols, routers discover the best path to destination from each neighbor. The routing updates proceed step by step from router to router.

Hint

This question is to examine the understanding of the interaction between EIGRP routers. The following information must be matched so as to create neighborhood. EIGRP routers to establish, must match the following information: 1. AS Number; 2. K value.

Hint

When configuring OSPF, the mask used for the network statement is a wildcard mask similar to an access list. In this specific example, the correct syntax would have been “network 10.0.0.0 0.0.0.255 area 0.

Hint

A is not correct because the backbone area of OSPF is always Area 0. B is not correct because R1 or R3 must be the DR or BDR -> it has to establish neighbor adjacency with the other. C is not correct because OSPF neighbor relationship is not established based on static routing. It uses multicast address 224.0.0.5 to establish OSPF neighbor relationship. E is not correct because configure EIGRP on these routers (with a lower administrative distance) will force these routers to run EIGRP, not OSPF.D and F are correct because these entries must match on neighboring routers:– Hello and dead intervals – Area ID (Area 0 in this case) – Authentication password – Stub area flag

Hint

When there is more than one way to reach a destination, it will choose the best one based on a couple of things. First, it will choose the route that has the longest match; meaning the most specific route. So, in this case the /24 routes will be chosen over the /16 routes. Next, from all the /24 routes it will choose the one with the lowest administrative distance. Directly connected routes have an AD of 1 so this will be the route chosen.

Hint

maximum-paths (OSPF) To control the maximum number of parallel routes that Open Shortest Path First (OSPF) can support, use the maximum-paths command. Syntax Description maximum Maximum number of parallel routes that OSPF can install in a routing table. The range is from 1 to 16 routes. Command Default 8 paths

Hint

What you need to read up on is around the Count to infinity routing loops, the holddown timers are used to ensure that routing loops dont occur and the hold down timer only occurs after 180s then at 240s the bad route is flushed/deleted. Coming back to the question where it asks for the time elapsed before it gets removed. So once the holddown timer passes thats when it knows there is a bad route, then the question is asking how long does it stay in the routing table before it gets “flushed” so that is 240-180=60s

Hint

The “show ipv6 eigrp neighbors” command displays the neighbors discovered by the EIGRPv6. Notice that the neighbors are displayed by their link-local addresses.

Hint

In order to become OSPF neighbor following values must be match on both routers:+ Area ID + Authentication + Hello and Dead Intervals + Stub Flag + MTU SizeTherefore we need to adjust the MTU size on one of the router so that they are the same. Or we can tell OSPF to ignore the MTU size check with the command “ip ospf mtu-ignore”.

Hint

The “show ipv6 route” displays the current contents of the IPv6 routing table. This device is running OSPF so we can deduce it is running OSPFv3 (OSPF for IPv6). An example of the “show ip v6 route” is shown below:

Hint

The Administrative Distance (AD) of popular routing protocols is shown below. You should learn them by heart: Note: The AD of iBGP is 200 The smaller the AD is, the better it is. The router will choose the routing protocol with smallest AD. In this case EIGRP with AD of 90 is the smallest one.

Hint

Static routing can be used to define an exit point from a router when no other routes are available or necessary. This is called a default route.

Hint

Link State Routing Protocols Link state protocols are also called shortest-path-first protocols. Link state routing protocols have a complete picture of the network topology. Hence they know more about the whole network than any distance vector protocol. Three separate tables are created on each link state routing enabled router. One table is used to hold details about directly connected neighbors, one is used to hold the topology of the entire internetwork and the last one is used to hold the actual routing table. Link state protocols send information about directly connected links to all the routers in the network. Examples of Link state routing protocols include OSPF – Open Shortest Path First and IS-IS – Intermediate System to Intermediate System. There are also routing protocols that are considered to be hybrid in the sense that they use aspects of both distance vector and link state protocols. EIGRP – Enhanced Interior Gateway Routing Protocol is one of those hybrid routing protocols.

Hint

Making a forwarding decision actually consists of three sets of processes: the routing protocols, the routing table, and the actual process which makes a forwarding decision and switches packets. The longest prefix match always wins among the routes actually installed in the routing table, while the routing protocol with the lowest administrative distance always wins when installing routes into the routing table. Reference: http://www.cisco.com/c/en/us/support/docs/ip/enhanced-interior-gateway-routing-protocol-eigrp/8651-21.html

Hint

http://www.ciscopress.com/articles/article.asp?p=24090&seqNum=4 The reason is that unlike the routing-by-rumor approach of distance vector, link state routers have firsthand information from all their peer routers. Each router originates information about itself, its directly connected links, and the state of those links (hence the name). This information is passed around from router to router, each router making a copy of it, but never changing it. The ultimate objective is that every router has identical information about the internetwork, and each router will independently calculate its own best paths.

Hint

Reference: https://www.cisco.com/c/en/us/td/docs/ios/12_2/iproute/command/reference/fiprrp_r/1rfrip.html

Hint

EIGRP provides a mechanism to load balance over unequal cost paths (or called unequal cost load balancing) through the “variance” command. In other words, EIGRP will install all paths with metric < variance * best_metric into the local routing table, provided that it meets the feasibility condition (to prevent routing loop). The feasibility condition states that, the Advertised Distance (AD) of a route must be lower than the feasible distance of the current successor route.

Hint

You can use the passive-interface command in order to control the advertisement of routing information. The command enables the suppression of routing updates over some interfaces while it allows updates to be exchanged normally over other interfaces. With most routing protocols, the passive-interface command restricts outgoing advertisements only. But, when used with Enhanced Interior Gateway Routing Protocol (EIGRP), the effect is slightly different. This document demonstrates that use of the passive-interface command in EIGRP suppresses the exchange of hello packets between two routers, which results in the loss of their neighbor relationship. This stops not only routing updates from being advertised, but it also suppresses incoming routing updates. This document also discusses the configuration required in order to allow the suppression of outgoing routing updates, while it also allows incoming routing updates to be learned normally from the neighbor

Hint

Since static routing is a manual process, it can be argued that it is more secure (and more prone to human errors) since the network administrator will need to make changes to the routing table directly. Also, in stub networks where there is only a single uplink connection, the load is reduced as stub routers just need a single static default route, instead of many routes that all have the same next hop IP address.

Hint

The difference between RIPv1 and RIPv2 is RIPv1 is a classful protocol (no support for VLSM or CIDR) while RIPv2 is a classless protocol (which supports VLSM and CIDR). Therefore in this question if we forget to enable RIPv2 then the router will use RIPv1 and it only advertise major network 172.16.0.0/16 to other routers. By enabling RIPv2 (via the “version 2” command) the router will advertise two subnets 172.16.10.0/27 & 172.16.10.32/28.

Hint

Distance vector protocols (like RIP) exchanges the entire routing information each time the routers send the updates.Note: EIGRP is considered an advanced distance vector protocol so it does not send the whole routing table for each update.

Hint

In the configuration above, the “no shutdown” command was missing so interface Vlan 1 is still inactive. Notice that only the loopback command does not need the “no shutdown” command to work.

Hint

To enable RIP surely we have to enable it first (with the “router rip” command in global configuration mode) -> E is correct. RIPv2 sends its updates via multicast but in Nonbroadcast Multiple Access (NBMA) environment, multicast is not allowed so we have to use unicast to send RIPv2 updates -> D is correct.

Hint

This question only asks about enable router-on-stick on a switch, not a router. We don’t have subinterface on a switch so B is not a correct answer.

Hint

http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_ospf/configuration/15-sy/iro-15-sy-book/ip6-routeospfv3.html#GUID-05F3F09C-FE3E-41D6-9845-111FB17AD030 “In IPv6, you can configure many address prefixes on an interface. In OSPFv3, all address prefixes on an interface are included by default. You cannot select some address prefixes to be imported into OSPFv3; either all address prefixes on an interface are imported, or no address prefixes on an interface are imported.”