CCNP Route BoS Qs Flashcards
Which of the following is most likely to cause a CEF routing loop? (Select the best answer.)
A. a dynamic EIGRP route
B. a dynamic OSPF route
C. a floating static route that points to an IP address
D. a floating static route that points to a multiaccess interface
Correct Answer: D
Explanation: Of the choices provided, a floating static route that points to a multiaccess interface is most likely to cause a Cisco Express Forwarding (CEF) routing loop. Consider the following network topology:
RouterA has a static default route that points to RouterC. RouterB has a static default route that points to RouterD. RouterA also has a backup floating static route that points to E0/0. RouterA learns about 192.168.1.0/24 from RouterC from external Border Gateway Protocol (eBGP) and advertises the route to RouterB through internal Border Gateway Protocol (iBGP).
Normally, RouterA will send packets to 192.168.1.55 over S1/0 to RouterC. However, when the link between RouterA and RouterC goes down, RouterA will use the backup default route to RouterB to send packets to 192.168.1.55. RouterA will send an Address Resolution Protocol (ARP) request for 192.168.1.55 to RouterB, which will give a proxy reply with its own Media Access Control (MAC) address. RouterA will then create an ARP entry for 192.168.1.55 with the MAC address of RouterB. RouterA will create a CEF adjacency for 192.168.1.55, and an entry for 192.168.1.55/32 will be installed in the CEF Forwarding Information Base (FIB) that indicates that it is accessible over the E0/0 interface.
When the link between RouterA and RouterC comes back up, RouterA will again install the eBGP route into the routing table and again advertise it to RouterB. When RouterB has a packet that is destined for 192.168.1.55, it will forward the packet to RouterA. RouterA has a CEF entry for 192.168.1.55/32 that points to the E0/0 interface, so RouterA will send the packet back to RouterB without checking the routing table, thereby creating a routing loop.
You should configure RouterA by removing the floating static route that points to the multiaccess E0/0 interface and replacing it with a floating static route that points to the 10.1.1.2 next-hop IP address on RouterB. When RouterA has a static route that points to a next-hop IP address, RouterA will no longer send ARP requests for all destinations, which will prevent CEF entries from being created. Dynamic routing protocols, such as Open Shortest Path First (OSPF), Enhanced Interior Gateway Routing Protocol (EIGRP), and Border Gateway Protocol (BGP), are unlikely to cause CEF routing loops.
Reference: Cisco: Troubleshooting Cisco Express Forwarding Routing Loops
Category: 1. Network Principles
RouterA is configured with a VRF instance named BSNVRF. You want to configure IPv4 EIGRP so that the VRF operates in AS 11. You also want to keep administrative effort to a minimum. You issue the following command on RouterA:
RouterA(config)#router eigrp 10
Which of the following command sets are you most likely to issue next? Each choice represents a complete solution. (Select 2 choices.)
- RouterA(config-router)#address-family ipv4 vrf BSNVRF autonomous-system 11
- RouterA(config-router)#address-family ipv4 vrf BSNVRF 11
- RouterA(config-router)#autonomous-system 11
- RouterA(config-router)#address-family ipv4 vrf BSNVRF RouterA(config-router-af)#autonomous-system 11
- RouterA(config-router)#exit
RouterA(config)#no router eigrp 10
RouterA(config)#router eigrp 11
RouterA(config-router)#address-family ipv4 vrf BSNVRF autonomous-system 11
Correct Answers: A, D
Explanation:
You could issue the following command set to configure IP version 4 (IPv4) Enhanced Interior Gateway Routing Protocol (EIGRP) so that the Virtual Routing and Forwarding (VRF) instance named BSNVRF operates in autonomous system (AS) 11:
RouterA(config)#router eigrp 10
RouterA(config-router)#address-family ipv4 vrf BSNVRF RouterA(config-router-af)#autonomous-system 11
The autonomous-system autonomous-system-number command configures the AS for the VRF specified in the address-family command. The autonomous-system autonomous-system**number command is available only when configuring an EIGRP AS for a VRF instance in classic EIGRP configuration mode, which is also known as autonomous system mode.
Alternatively, you could issue the following command to configure IPv4 EIGRP so that the VRF instance named BSNVRF operates in AS 11:
RouterA(config)#router eigrp 10
RouterA(config-router)#address-family ipv4 vrf BSNVRF autonomous-system 11
Similar to EIGRP named configuration mode, you can issue the address-family command with the autonomous-system keyword to configure the AS in which the VRF instance should operate. Unlike a classic EIGRP configuration, named configuration mode enables you to consolidate all EIGRP configuration commands under a single section of the running configuration. However, you cannot issue the autonomous-system autonomous-system command in EIGRP named configuration mode; therefore, you must specify the AS by issuing the autonomous-system keyword along with the address-family command.
You do not need to issue the following command set:
RouterA(config)#router eigrp 10 RouterA(config-router)#exit RouterA(config)#no router eigrp 10 RouterA(config)#router eigrp 11
RouterA(config-router)#address-family ipv4 vrf BSNVRF autonomous-system 11
Although there is nothing that will prevent you from configuring the VRF instance named BSNVRF to operate in the same AS that is configured for the EIGRP process, it is not necessary to configure the VRF to use the same AS. Removing and replacing the EIGRP process configuration adds administrative overhead, especially if the EIGRP process is already configured with other address families or networks.
You cannot issue the autonomous-system autonomous-system-number command in router configuration mode. Therefore, the following command contains invalid syntax: RouterA(config-router)#autonomous-system 11
In addition, the following command contains invalid syntax because the autonomous-system keyword is missing:
RouterA(config-router)#address-family ipv4 vrf BSNVRF 11
Reference:
Boson ROUTE Curriculum, Module 5: Understanding EIGRP, Configuring a Named Mode Configuration
Cisco: Cisco IOS IP Routing: EIGRP Command Reference: autonomous-system (EIGRP)
