Ch 11 - Route Selection Flashcards
Identify the architectural components of Cisco Express Forwarding (CEF). (Choose two.)
a. Routing Information Base (RIB)
b. Adjacency Table
c. Forwarding Information Base (FIB)
d. ARP Cache
B and C.
Cisco Express Forwarding (CEF) maintains its information in two tables, the Adjacency Table (which contains information about Layer 2 adjacencies) and the Forwarding Information Base (FIB) (which contains Layer 3 information).
The Routing Information Base (RIB) is a data structure used by a routing protocol such as OSPF.
The ARP Cache contains IP address to MAC address mappings. Although information from the ARP Cache is used to help populate the Adjacency Table, the ARP Cache itself is not a CEF table.
What command can be used to globally enable CEF on a router?
a. ip flow egress
b. ip route-cache cef
c. no ip route-cache
d. ip cef
D. To globally enable CEF on a router, use the ip cef command in global configuration mode. The ip flow egress interface configuration mode command is used to enable outbound NetFlow. The ip route-cache cef interface configuration mode command is used to enable CEF on an individual interface, if CEF has already been globally enabled on the router. The no ip route-cache interface configuration mode command is used to enable process switching on an interface.
Policy-Based Routing (PBR) has been enabled on Router R1’s Fa 0/0 interface. Which of the following are true regarding how PBR works? (Choose two.)
a. Packets entering Fa 0/0 will be compared based on the PBR route map.
b. Packets exiting Fa 0/0 will be compared based on the PBR route map.
c. Cisco IOS ignores the PBR forwarding directions when a packet matches a route map deny clause.
d. Cisco IOS ignores the PBR forwarding directions when a packet matches a route map permit clause.
A and C. PBR supports processing packets on an interface, for the inbound direction only. The referenced route map causes PBR to attempt policy routing of packets that match a permit clause in the route map.
Examine the following configuration on Router R1. R1’s show ip route 172.16.4.1 command lists a route with outgoing interface S0/1/1.
Host 172.16.3.3 uses Telnet to connect to host 172.16.4.1. What will Router R1 do with the packets generated by host 172.16.3.3 because of the Telnet session, assuming that the packets enter R1’s Fa0/0 interface? (Choose two.)
interface Fastethernet 0/0
ip address 172.16.1.1 255.255.255.0
ip policy route-map Q2
route-map Q2 permit match ip address 101 set interface s0/0/1
access-list 101 permit tcp host 172.16.3.3 172.16.4.0 0.0.0.255
a. The packets will be forwarded out S0/0/1, or not at all.
b. The packets will be forwarded out S0/0/1 if it is up.
c. The packets will be forwarded out S0/1/1 if it is up.
d. The packets will be forwarded out S0/1/1 if it is up, or if it is not up, out S0/0/1.
e. The packets will be forwarded out S0/0/1 if it is up, or if it is not up, out S0/1/1.
B and E.
Packets created by Telnet use TCP, so the packets will match ACL 101 with a permit action.
PBR will match the only route map clause shown in the configuration, with the permit route map clause listing a set command. The set command lists S0/0/1 as the outgoing interface and without a default parameter. So, Router R1 will first attempt to forward the packet based on the set command (interface S0/0/1), but if the interface is down, R1 will then try to forward based on the IP routing table (interface S0/1/1).
The following output occurs on Router R2. Which of the following statements can be confirmed as true based on the output?
R2# show ip policy
Interface Route map
Fa0/0 RM1
Fa0/1 RM2
S0/0/0 RM3
a. R2 will forward all packets that enter Fa0/0 per the PBR configuration.
b. R2 will use route map RM2 when determining how to forward packets that exit interface Fa0/1.
c. R2 will consider using PBR for all packets exiting S0/0/0 per route map RM3.
d. R2 will consider using PBR for all packets entering S0/0/0 per route map RM3.
D is the best answer.
The output from the show ip policy command shows the interfaces on which PBR has been enabled and the name of the route map enabled for PBR on each interface.
For the purposes of this question, the output tells us the interfaces on which PBR has been enabled. Two answers mention packets exiting the interface. Therefore, these answers cannot be correct, because PBR applies to packets entering an interface.
For the two interfaces that mention inbound packets, one suggests that all packets will be forwarded per the PBR configuration; some might not be forwarded per PBR, depending on the configuration of the route map. The correct answer specifically mentions that PBR will consider all packets with PBR, which is the more accurate statement about PBR operations.
Which of the following are examples of traffic that can be created as part of an IP Service-Level Agreement operation? (Choose two.)
a. ICMP Echo
b. VoIP (RTP)
c. IPX
d. SNMP
A and B. The IP SLA feature focuses on IP traffic. Therefore, Cisco IOS does not include Novell’s older IPX protocol as part of IP SLA. IP SLA uses SNMP MIBs to store statistics, but it does not use SNMP as an operation.
The following configuration commands exist only in an implementation plan document. An engineer does a copy/paste of these commands into Router R1’s configuration. Which of the following answers is most accurate regarding the results?
ip sla 1
icmp-echo 1.1.1.1 source-ip 2.2.2.2
ip sla schedule 1 start-time now life forever
a. The SLA operation will be configured but will not start until additional commands are used.
b. The SLA operation is not completely configured, so it will not collect any data.
c. The SLA operation is complete and working, collecting data into the RTTMON MIB.
d. The SLA operation is complete and working but will not store the data in the RTTMON MIB without more configuration.
C. The three lines shown create the operation number (first command), define the operation (second command), and start the operation (third command). All commands are correct.
After the operation is started, IP SLA stores the data in the RTTMON MIB; no additional configuration is necessary.
The following output occurs on Router R1. IP SLA operation 1 uses an ICMP echo operation type, with a default frequency of 60 seconds. The operation pings from address 1.1.1.1 to address 2.2.2.2.
Which of the following answers is true regarding IP SLA and object tracking on R1?
R1# show track
Track 2
IP SLA 1 state
State is Up 3 changes, last change 00:00:03
Delay up 45 secs, down 55 secs
Latest operation return code: OK
Latest RTT (millisecs) 6
Tracked by: STATIC-IP-ROUTING 0
a. The tracking return code fails immediately after the SLA operation results in an ICMP echo failure three times.
b. The tracking return code fails immediately after the SLA operation results in an ICMP echo failure one time.
c. After the tracking object fails, the tracking object moves back to an up state 45 seconds later in all cases.
d. After moving to a down state, the tracking object moves back to an OK state 45 seconds after the SLA operation moves to an OK state.
D. The up timer on the tracking object defines how long to wait, when in a down state, after seeing the IP SLA object transition to an OK state. Similarly, the down timer defines how long to wait, when in an OK state, after seeing the IP SLA object move to a down state, before moving the tracking object to a down state.
Which of the following is a benefit of Cisco EVN as compared to VRF-Lite?
a. Cisco EVN allows a single physical router to run multiple virtual router instances.
b. Cisco EVN allows two routers to be interconnected through an 802.1Q trunk, and traffic for different VRFs is sent over the trunk, using router subinterfaces.
c. Cisco EVN allows routes from one VRF to be selectively leaked to other VRFs.
d. Cisco EVN allows two routers to be interconnected through a VNET trunk, and traffic for different VRFs is sent over the trunk, without the need to configure router subinterfaces.
D is the best answer.
Both Cisco EVN and VRF-Lite allow a single physical router to run multiple virtual router instances, and both technologies allow routes from one VRF to be selectively leaked to other VRFs. However, a major difference is the way that two physical routers interconnect.
With VRF-Lite, a router is configured with multiple sub-interfaces, one for each VRF.
However, with Cisco EVN, routers interconnect using a VNET trunk, which simplifies configuration.
What are the 3 operational planes in a router?
- Management plane
- Control plane
- Data plane
Planes of Operation
A router is typically segmented into three planes of operation, each with a specific and clearly defined objective:
The control plane: The control plane is the brain of the router. It consists of dynamic IP routing protocols (that is OSPF, IS-IS, BGP, and so on), the RIB, routing updates, in addition to other protocols such as PIM, IGMP, ICMP, ARP, BFD, LACP, and so on. In short, the control plane is responsible for maintaining sessions and exchanging protocol information with other router or network devices.
In centralized architecture platforms, the general-purpose CPU manages all control plane protocols. In distributed architecture platforms, routing protocols, and most other protocols, always run on the core CPU in the RPs or Supervisor engines, but there are other control plane protocols such as ARP, BFD, and ICMP that in some distributed architecture platforms have now been offloaded to the line card CPU.
The data plane: The data plane is the forwarding plane, which is responsible for the switching of packets through the router (that is, process switching and CEF switching). In the data plane, there could be features that could affect packet forwarding such as quality of service (QoS) and access control lists (ACLs).
The management plane: The management plane is used to manage a device through its connection to the network. Examples of protocols processed in the management plane include Simple Network Management Protocol (SNMP), Telnet, File Transfer Protocol (FTP), Secure FTP, and Secure Shell (SSH). These management protocols are used for monitoring and for command-line interface (CLI) access.
What plane is concerned with forwarding traffic from network to network internally through a router?
Data plane
Planes of Operation
A router is typically segmented into three planes of operation, each with a specific and clearly defined objective:
The data plane: The data plane is the forwarding plane, which is responsible for the switching of packets through the router (that is, process switching and CEF switching). In the data plane, there could be features that could affect packet forwarding such as quality of service (QoS) and access control lists (ACLs).
What plane do routing protocols operate on?
Control plane
Planes of Operation
A router is typically segmented into three planes of operation, each with a specific and clearly defined objective:
The control plane: The control plane is the brain of the router. It consists of dynamic IP routing protocols (that is OSPF, IS-IS, BGP, and so on), the RIB, routing updates, in addition to other protocols such as PIM, IGMP, ICMP, ARP, BFD, LACP, and so on. In short, the control plane is responsible for maintaining sessions and exchanging protocol information with other router or network devices.
In centralized architecture platforms, the general-purpose CPU manages all control plane protocols. In distributed architecture platforms, routing protocols, and most other protocols, always run on the core CPU in the RPs or Supervisor engines, but there are other control plane protocols such as ARP, BFD, and ICMP that in some distributed architecture platforms have now been offloaded to the line card CPU.
What are the 3 modes of packet switching?
Process, Fast and CEF.
Process switching is the oldest, slowest and most processor intensive.
In Fast switching, the first packet to a destination is process switched but subsequent packets are forwarded using the information stored in the fast cache.
Finally, CEF pre-builds the cache before any packets need to be forwarded
What mode of packet switching involves the CPU directly and can impact performance dramatically?
Process switching. In process switching the router CPU does it all: removes the L2 header, examines the L3 address, decides how to forward the packet, assembles a new L2 header w/new CRC and MAC and forwards the packet out the right interface.
How to do you enable (or really fall back to) process switching?
no ip route-cache
