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QUESTION 45
What must occur before an (S,G) entry can be populated in the multicast routing table?
A. The (*,G) entry must have timed out
B. The (*,G) entry OIL must be null
C. The router must be directly connected to the multicast source
D. The parent (*,G) entry must be created first
Correct Answer: D Section: (none) Explanation
QUESTION 46
Which field in the IPv6 header can be used to set the DSCP value?
A. Flow Label
B. Type of Service
C. Traffic Class “First Test, First Pass” – www.lead2pass.com 32 Cisco 642-885 Exam
D. Precedence
E. EXP
Correct Answer: C Section: (none) Explanation
Explanation/Reference: Traffic Class
The Traffic Class field is an 8 bit field that is used to signify the importance of the data contained within this specific packet. With IPv4, this information was signified with the TOS field and supported both IP precedence and Differentiated Services Code Point (DSCP). The Traffic Class field used with IPv6 supports DSCP solely; this specification uses the first 6 bits to indicate the Per Hop Behavior (PHB) of the contained data; these PHB＊s are defined in RFC 2474 and its additions.
QUESTION 47
Which mechanism is used by an IPv6 multicast receiver to join an IPv6 multicast group?
A. IGMP report
B. IGMP join
C. MLD report
D. General query
E. PIM join
Correct Answer: C Section: (none) Explanation
Explanation/Reference: MLD Reports
The processing of MLDv1 join messages is essentially the same as with IGMPv2. When no IPv6 multicast routers are detected in a VLAN, reports are not processed or forwarded from the switch. When IPv6 multicast routers are detected and an MLDv1 report is received, an IPv6 multicast group address and an IPv6 multicast MAC address are entered in the VLAN MLD database. Then all IPv6 multicast traffic to the group within the VLAN is forwarded using this address. When MLD snooping is disabled, reports are flooded in the ingress VLAN.
When MLD snooping is enabled, MLD report suppression, called listener message suppression, is automatically enabled. With report suppression, the switch forwards the first MLDv1 report received by a group to IPv6 multicast routers; subsequent reports for the group are not sent to the routers. When MLD snooping is disabled, report suppression is disabled, and all MLDv1 reports are flooded to the ingress VLAN.
The switch also supports MLDv1 proxy reporting. When an MLDv1 MASQ is received, the switch responds with MLDv1 reports for the address on which the query arrived if the group exists in the switch on another port and if the port on which the query arrived is not the last member port for the address.
QUESTION 48
Which of the following can be used by dual-stack service providers supporting IPv4/IPv6 customers with dual-stack hosts using public IPv6 addresses and private IPv4 addresses?
A. NAT64
B. 6RD
C. 6to4 tunnels
D. Carrier-grade NAT
Correct Answer: D Section: (none) Explanation Explanation/Reference:
Carrier Grade NAT is a large-scale NAT, capable of providing private-IPv4-to-public-IPv4 translation in the order of millions of translations. Carrier Grade NAT can support several hundred thousand subscribers with the bandwidth throughput of at least 10Gb/s full-duplex. With IPv4 addresses reaching depletion, Carrier Grade NAT is vital in providing private IPv4 connectivity to the public IPv4 internet. In addition, Carrier Grade NAT is not limited to IPv4 NAT; it can also translate between IPv4 and IPv6 addresses
QUESTION 49
Refer to the Cisco IOS DHCPv6 configuration shown in the exhibit. Which statement is correct?
A. The configuration is missing a command under interface Gi0/1 to indicate to the attached hosts to use stateful DHCPv6 to obtain their IPv6 addresses
B. The IPv6 router advertisements indicate to the attached hosts on the Gi0/1 interface to get other information besides their IPv6 address via stateless auto configuration “First Test, First Pass” -www.lead2pass.com 33 Cisco 642-885 Exam
C. The IPv6 DHCPv6 server pool configuration is misconfigured
D. The DNS server address can also be imported from another upstream DHCPv6 server
Correct Answer: A Section: (none) Explanation
Explanation/Reference: Server Configuration
In Global Configuration Mode
ipv6 unciast-routing
ipv6 dhcp pool <pool name>
address prefix <specify address prefix> lifetime <infinite> <infinite>
dns-server <specify the dns server address>
domain-name <specify the domain name>
exit
In Interface Configuration Mode
ipv6 address <specify IPv6 Address>
ipv6 dhcp server <server name>rapid-commit
Client Configuration
In Global Configuration Mode enable configure terminal ipv6 unicast-routing In Interface Configuration Mode ipv6 address dhcp rapid commitipv6 enable exit
QUESTION 50
Which IPv6 mechanism occurs between a provider edge router and the customer premises equipment router to allow an ISP to automate the process of assigning a block of IPv6 addresses to a customer for use within the customer network?
A. Router Advertisement
B. DHCPv6 Prefix Delegation
C. DHCPv6 Lite
D. Stateful DHCPv6
Correct Answer: B Section: (none) Explanation
Explanation/Reference:
http://www.cisco.com/en/US/tech/tk872/technologies_configuration_example09186a0080b8a116.shtml
QUESTION 51
Which three statements regarding NAT64 operations are correct? (Choose three.)
A. With stateful NAT64, many IPv6 address can be translated into one IPv4 address, thus IPv4 address conservation is achieved
B. Stateful NAT64 requires the use of static translation slots so IPv6 hosts and initiate connections to IPv4 hosts.
C. With stateless NAT64, the source and destination IPv4 addresses are embedded in the IPv6 addresses
D. NAT64 works in conjunction with DNS64
E. Both the stateful and stateless NAT64 methods will conserve IPv4 address usage
Correct Answer: ACD Section: (none) Explanation
Explanation/Reference:
Stateful NAT64-Network Address and Protocol Translation from IPv6 Clients to IPv4 Servers Stateful NAT64 multiplexes many IPv6 devices into a single IPv4 address. It can be assumed that this technology will be used mainly where IPv6-only networks and clients (ie. Mobile handsets, IPv6 only wireless, etc…) need access to the IPv4 internet and its services. The big difference with stateful NAT64 is the elimination of the algorithmic binding between the IPv6 address and the IPv4 address. In exchange, state is created in the NAT64 device for every flow. Additionally, NAT64 only supports IPv6-initiated flows. Unlike stateless NAT64, stateful NAT64 does `not’ consume a single IPv4 address for each IPv6 device that wants to communicate to the IPv4 Internet. More practically this means that many IPv6-only users consume only single IPv4 address in similar manner as IPv4-to-IPv4 network address and port translation works. This works very well if the connectivity request is initiated from the IPv6 towards the IPv4 Internet. If an IPv4-only device wants to speak to an IPv6-only server for example, manual configuration of the translation slot will be required, making this mechanism less attractive to provide IPv6 services towards the IPv4 Internet. DNS64 is usually also necessary with a stateful NAT64, and works the same with both stateless and stateful NAT64
Stateless NAT64-Stateless translation between IPv4 and IPv6 RFC6145 (IP/ICMP Translation Algorithm) replaces RFC2765 (Stateless IP/ICMP Translation Algorithm (SIIT)) and provides a stateless mechanism to translate a IPv4 header into an IPv6 header and vice versa. Due to the stateless character this mechanism is very effective and highly fail safe because more as a single-or multiple translators in parallel can be deployed and work all in parallel without a need to synchronize between the translation devices. The key to the stateless translation is in the fact that the IPv4 address is directly embedded in the IPv6 address. A limitation of stateless NAT64 translation is that it directly translates only the IPv4 options that have direct IPv6 counterparts, and that it does not translate any IPv6 extension headers beyond the fragmentation extension header; however, these limitations are not significant in practice. With a stateless NAT64, a specific IPv6 address range will represent IPv4 systems within the IPv6 world. This range needs to be manually configured on the translation device. Within the IPv4 world all the IPv6 systems have directly correlated IPv4 addresses that can be algorithmically mapped to a subset of the service provider’s IPv4 addresses. By means of this direct mapping algorithm there is no need to keep state for any translation slot between IPv4 and IPv6. This mapping algorithm requires the IPv6 hosts be assigned specific IPv6 addresses, using manual configuration or DHCPv6. Stateless NAT64 will work very successful as proven in some of the largest networks, however it suffers from some an important side-effect: Stateless NAT64 translation will give an IPv6-only host access to the IPv4 world and vice versa, however it consumes an IPv4 address for each IPv6-only device that desires translation — exactly the same as a dual-stack deployment. Consequentially, stateless NAT64 is no solution to address the ongoing IPv4 address depletion. Stateless NAT64 is a good tool to provide Internet servers with an accessible IP address for both IPv4 and IPv6 on the global Internet. To aggregate many IPv6 users into a single IPv4 address, stateful NAT64 is required. NAT64 are usually deployed in conjunction with a DNS64. This functions similar to, but different than, DNS-ALG that was part of NAT-PT. DNS64 is not an ALG; instead, packets are sent directly to and received from the DNS64’s IP address. DNS64 can also work with DNSSEC (whereas DNS-ALG could not).
QUESTION 52
Which type of DNS record is used for IPv6 forward lookups?
A. A records
B. AAAA records
C. PTR records
D. MX records
Correct Answer: B Section: (none) Explanation
Explanation/Reference: QUESTION 53
What is enabled by default on Cisco IOS-XR routers and cannot be disabled?
A. SSH server
B. Multicast routing
C. IPv4 and IPv6 CEF
D. IPv6 routing
E. CDP
F. BFD “First Test, First Pass” – www.lead2pass.com 34 Cisco 642-885 Exam
Correct Answer: C Section: (none) Explanation
Explanation/Reference:
Before using the BGP policy accounting feature, you must enable BGP on the router (CEF is enabled by
default).
QUESTION 54
The IPv6 2002::/16 prefix is used in which kind of implementations?
A. 6RD
B. 6to4
C. NAT64
D. IPv6 Multicast
Correct Answer: B Section: (none) Explanation
QUESTION 55
When implementing IP SLA icmp-echo probes on Cisco IOS-XE routers, which two options are available for IPv6? (Choose two.)
A. flow-label
B. hop-limit
C. DSCP
D. traffic-class
E. TOS
Correct Answer: AD Section: (none) Explanation
Explanation/Reference:
QUESTION 56
With IPv6 multicast, which feature can be used as a replacement method for static RP configuration?
A. PIM Snooping
B. MLD
C. MLD Snooping
D. Embedded RP
E. DHCPv6
Correct Answer: D Section: (none) Explanation Explanation/Reference:
QUESTION 57
Which additional feature is provided using MLDv2 that is not available in MLDv1?
A. Multicast Address Specific Queries
B. Source filtering
C. Done messages
D. Report messages
Correct Answer: B Section: (none) Explanation
Explanation/Reference:
. PIM-SSM is made possible by IGMPv3 and MLDv2. Hosts can now indicate interest in specific sources using IGMPv3 and MLDv2. SSM does not require a rendezvous point (RP) to operate.
QUESTION 58
“First Test, First Pass” – www.lead2pass.com 35 Cisco 642-885 Exam
When implementing high-availability stateful switchover BGP routing, in which situation would Cisco NSR be required?
A. On the PE routers connecting to the CE routers which are not NSF aware or are not NSF capable
B. On the PE routers connecting to the CE routers which support graceful restart
C. On the PE routers connecting to the CE routers which are incapable of performing stateful switchover operations because the CE routers are only NSF aware but not NSF capable
D. On the PE routers connecting to the CE routers which are incapable of performing stateful switchover operations because the CE routers are only NSF capable but not NSF aware
E. On the service provider core P routers which are also NSF aware
F. On the service provider core P routers which are also NSF capable
Correct Answer: A Section: (none)
Explanation
QUESTION 59
What are three BGP configuration characteristics of a multihomed customer that is connected to multiple service providers? (Choose three.)
A. The multihomed customer can use local preference to influence the return traffic from the service providers
B. The multihomed customer announces its assigned IP address space to its service providers through BGP
C. The multihomed customer has to decide whether to perform load sharing or use a primary/backup implementation
D. The multihomed customer must use private AS number
E. The multihomed customer configures outbound route filters to prevent itself from becoming a transit AS
Correct Answer: BCE Section: (none) Explanation
QUESTION 60
Refer to the EBGP configuration on a PE IOS-XR router exhibit. After the EBGP configuration, no routes
are accepted from the EBGP peer, nor are any routes advertised to the EBGP peer.
What could be the problem?
A. The update-source neighbor configuration command must also be configured
B. The next-hop-self neighbor configuration command must also be configured
C. EBGP neighbors must have an inbound and outbound route policy configured
D. An access list is blocking IP protocol 179 packets between the two EBGP peers
E. The maximum-prefix neighbor configuration command must also be configured
Correct Answer: C Section: (none) Explanation
Explanation/Reference:
“First Test, First Pass” – www.lead2pass.com 36 Cisco 642-885 Exam
QUESTION 61
Refer to the exhibit. The following multicast IP addresses map to which multicast MAC address?
A. 01:00:5E:8A:00:01
B. 01:00:5E:0A:00:01
C. 01:00:5E:7A:00:01
D. 01:00:5E:05:00:01
Correct Answer: B Section: (none) Explanation
QUESTION 62
A junior network engineer has just configured a new IBGP peering between two Cisco ASR9K PE routers in the network using the loopback interface of the router, but the IBGP neighborship is not able to be established. Which two verification steps will be helpful in troubleshooting this problem? (Choose two.)
A. Verify that the network command under router BGP is configured correct on each router for announcing the router’s loopback interface in BGP
B. Verify that the ibgp-multihop command under the BGP neighbor is configured correctly on each router “First Test, First Pass” – www.lead2pass.com 37 Cisco 642-885 Exam
C. Verify that the loopback interfaces are reachable over the IGP
D. Verify that the update-source loopback command under the BGP neighbor is configured correctly on each router
E. Verify that the ttl-security command under the BGP neighbor is configured correctly on each router to enable the router to send the BGP packets using a proper TTL value
F. Verify that the UDP port 179 traffic is not being blocked by an ACL or firewall between the two IBGP peers
Correct Answer: CD Section: (none) Explanation
QUESTION 63
Refer to the exhibit for the outputs from an ASR9K router. Why did the ping fail?
A. The ping command is missing the ipv6 option: ping ipv6 2001:db8:10:1:10::1/128
B. There is a problem with the IS-IS configurations
C. The fe80::eab7:48ff:fe2c:a180 next-hop is not reachable
D. The prefix length should be removed from the IPv6 address in the ping command: ping ipv6 2001:db8:10:1:10::1
E. IPv6 is not enabled on the Gi0/0/0/0 interface
F. The IPv6 neighbor discovery protocol is not enabled on the Gi0/0/0/0 interface
Correct Answer: D Section: (none) Explanation
Explanation/Reference:
“First Test, First Pass” – www.lead2pass.com 38 Cisco 642-885 Exam
QUESTION 64
Which multicast routing protocol supports dense mode, sparse mode and bidirectional mode?
A. DVMRP
B. MOSPF
C. PIM
D. MP-BGP
E. MSDP
Correct Answer: C Section: (none) Explanation
QUESTION 65
When configuring BFD, the multiplier configuration option is used to determine which value?
A. The retry interval
B. The number of BFD packets that can be lost before the BFD peer is declared “down”
C. The minimum interval between packets accepted from the BFD peers
D. The number of BFD echo packets that will be originated by the router
E. The number of routing protocols that will use BFD for fast peer failure detection
Correct Answer: B Section: (none) Explanation
Explanation/Reference:
QUESTION 66
After configuring the tunnel interface as shown in the exhibit, no IPv6 traffic is passed over the IPv4 network. Which additional configuration is required to pass the IPv6 traffic over the IPv4 network?
A. Configure an IPv4 address on the tunnel0 interface
B. Configure an IPv6 static route to send the required IPv6 traffic over the tunnel0 interface
C. The tunnel destination should be pointing to an IPv6 address instead of an IPv4 address
D. The tunnel0 interface IPv6 address must use the 2002:://16 prefix
Correct Answer: B Section: (none) Explanation
QUESTION 67
Refer to the Cisco IOS configuration exhibit. Which statement is correct?
“First Test, First Pass” – www.lead2pass.com 39 Cisco 642-885 Exam
A. This configuration is typically configured on the boundary routers within a PIM SM domain to filter out malicious candidate-RP-announce and candidate-RP-discovery packets
B. This configuration is typically configured on the RPs within a PIM-SM domain to restrict the candidate-RP-announce packets
C. This configuration is typically configured on the mapping agents within a PIM-SM domain to restrict the candidate-RP-discovery packets
D. This configuration is typically configured on the MSDP peering routers within a PIM-SM domain to filter out malicious MSDP SA packets
Correct Answer: A Section: (none) Explanation
Explanation/Reference: QUESTION 68
Given the IPv6 address of 2001:0DB8::1:800:200E:88AA, what will be its corresponding the solicited-node multicast address?
A. FF01::1:200E:88AA
B. FF01::1:FF0E:88AA
C. FF01:0DB8::1:800:200E:88AA
D. FF02::1:FF0E:88AA
E. FF02::1:200E:88AA
F. FF02:0DB8::1:800:200E:88AA
Correct Answer: D Section: (none) Explanation
Explanation/Reference:
IPv6 nodes (hosts and routers) are required to join (receive packets destined for) the following multicast groups:
.
All-nodes multicast group FF02:0:0:0:0:0:0:1 (scope is link-local)
.
Solicited-node multicast group FF02:0:0:0:0:1:FF00:0000/104 for each of its assigned unicast and anycast addresses
IPv6 routers must also join the all-routers multicast group FF02:0:0:0:0:0:0:2 (scope is link-local).
The solicited-node multicast address is a multicast group that corresponds to an IPv6 unicast or anycast address. IPv6 nodes must join the associated solicited-node multicast group for every unicast and anycast address to which it is assigned. The IPv6 solicited-node multicast address has the prefix FF02:0:0:0:0:1:FF00:0000/104 concatenated with the 24 low-order bits of a corresponding IPv6 unicast or anycast address (see Figure 2). For example, the solicited-node multicast address corresponding to the IPv6 address 2037::01:800:200E:8C6C is FF02::1:FF0E:8C6C. Solicited-node addresses are used in neighbor solicitation messages
QUESTION 69
With PIM-SM operations, which four pieces of information are maintained in the multicast routing table for each (*,G) or (S,G) entry? (Choose four.)
A. RPF Neighbor
B. RP Set
C. Incoming Interface
D. OIL
E. DF priority
F. PIM SM state flags
Correct Answer: ACDF Section: (none) Explanation
Explanation/Reference: The following is sample output from the show ip mroute command for a router operating in sparse mode: show ip mroute IP Multicast Routing Table Flags: D – Dense, S – Sparse, C – Connected, L – Local, P – Pruned R – RP-bit set, F – Register flag, T – SPT-bit set Timers: Uptime/Expires Interface state: Interface, Next-Hop, State/Mode (*, 224.0.255.3), uptime 5:29:15, RP is 198.92.37.2, flags: SC Incoming interface: Tunnel0, RPF neighbor 10.3.35.1, Dvmrp Outgoing interface list: Ethernet0, Forward/Sparse, 5:29:15/0:02:57 (198.92.46.0/24, 224.0.255.3), uptime 5:29:15, expires 0:02:59, flags: C Incoming interface: Tunnel0, RPF neighbor 10.3.35.1 Outgoing interface list: Ethernet0, Forward/Sparse, 5:29:15/0:02:57
QUESTION 70
What is one of the configuration errors within an AS that can stop a Cisco IOS-XR router from announcing certain prefixes to its EBGP peers?
A. Some prefixes were mistagged with the no-export BGP community
B. Some prefixes were set with an MED of 0
C. The outbound BGP route policy only has set actions defined without any pass actions defined
D. The inbound BGP route policy only has set actions defined without any pass actions defined
Correct Answer: A Section: (none) Explanation
Explanation/Reference:
“First Test, First Pass” – www.lead2pass.com 40 Cisco 642-885 Exam
QUESTION 71
Which three statements are correct regarding PIM-SM? (Choose three.)
A. There are three ways to configure the RP: Static RP, Auto-RP, or BSR
B. PIM-SM only uses the RP rooted shared tree and has no option to switch over to the shortest path tree
C. Different RPs can be configured for different multicast groups to increase RP scalability
D. Candidate RPs and RP mapping agents are configured to enable Auto-RP
E. PIM-SM uses the implicit join model
Correct Answer: ACD Section: (none) Explanation
QUESTION 72
Which of the following is a feature added in IGMPv3?
A. Support for source filtering
B. Support for Host Membership Report and a Leave Group message
C. Uses a new variation of the Host Membership Query called the Group-Specific Host Membership Query
D. Uses an election process to determine the querying router on the LAN
E. Uses an election process to determine the designated router on the LAN
F. IPv6 support
Correct Answer: A Section: (none) Explanation
Explanation/Reference:
.IGMP Version 3 permits joins and leaves for certain source and group pairs instead of requesting traffic from all sources in the multicast group.
MLDv2 provides the same functionality (under IPv6) as IGMP Version 3.
QUESTION 73
Which types of multicast distribution tree can PIM-SM use?
A. Only shared tree rooted at the source
B. Only shared tree rooted at the RP
C. Only shortest path tree rooted at the RP
D. Shared tree rooted at the source and shortest path tree switchover
E. Shared tree rooted at the RP and shortest path tree switchover
F. Shared tree rooted at the first-hop router and shortest path tree rooted at the RP
Correct Answer: E Section: (none) Explanation
QUESTION 74
Which multicast routing protocol is most optimal for supporting many-to-many multicast applications?
A. PIM-SM
B. PIM-BIDIR
C. MP-BGP
D. DVMRP
E. MSDP
Correct Answer: B Section: (none) Explanation
Explanation/Reference: PIM-Bidirectional Operations
PIM Bidirectional (BIDIR) has one shared tree from sources to RP and from RP to receivers. This is unlike the PIM-SM, which is unidirectional by nature with multiple source trees – one per (S,G) or a shared tree from receiver to RP and multiple SG trees from RP to sources. Benefits of PIM BIDIR are as follows:
.
As many sources for the same group use one and only state (*, G), only minimal states are required in each router.
.
No data triggered events.
.
Rendezvous Point (RP) router not required. The RP address only needs to be a routable address and need not exist on a physical device.
QUESTION 75
“First Test, First Pass” – www.lead2pass.com 41 Cisco 642-885 Exam
Which statement is correct regarding using the TTL threshold to define the delivery boundaries of multicast traffic?
A. If a packet TTL is less than the specified TTL threshold, the packet is forwarded out of the interface
B. If a packet TTL is greater or equal to the specified TTL threshold, the packet is forwarded out of the interface
C. If a packet TTL is equal to the specified TTL threshold, the packet is dropped
D. When a multicast packet arrives, the TTL threshold value is decremented by 1. If the resulting TTL threshold value is greater than or equal to 0, the packet is dropped
Correct Answer: B Section: (none) Explanation
QUESTION 76
Refer to the exhibit. Which three statements are correct regarding the Cisco IOS-XR configuration? (Choose three.)
A. This router, acting as the RP mapping agent, will send RP announcement messages to the 224.0.1.40 group
B. This router, acting as the RP mapping agent, will send RP discovery messages to the 224.0.1.39 group
C. This router is the RP mapping agent only for the 224.11.11.11 and 224.99.99.99 multicast groups
D. This router is a candidate PIM-SM RP for the 224.99.99.99 multicast group
E. This router is a candidate PIM-BIDIR RP for the 224.11.11.11 multicast group
F. IGMPv3 is enabled on all interfaces
G. Other routers will recognize this router as the RP for all multicast groups with this router loopback 0 IP address
Correct Answer: DEF Section: (none) Explanation
QUESTION 77
“First Test, First Pass” – www.lead2pass.com 42 Cisco 642-885 Exam
Which statement is correct regarding MP-BGP?
A. MP-BGP can indicate whether an advertised prefix (NLRI) is to be used for unicast routing, multicast RPF checks or for both using different SAFIs.
B. MP-BGP uses a single BGP table to maintain all the unicast prefixes for unicast forwarding and all the unicast prefixes for RPF checks.
C. MP-BGP can be used to propagate multicast state information, which eliminates the need to use PIM for building the multicast distribution trees.
D. MP-BGP enables BGP to carry IP multicast routes used by MSDP to build the multicast distribution trees.
Correct Answer: A Section: (none) Explanation
Explanation/Reference:
Protocol Independent Multicast
Protocol Independent Multicast (PIM) is a routing protocol designed to send and receive multicast routing updates. Proper operation of multicast depends on knowing the unicast paths towards a source or an RP. PIM relies on unicast routing protocols to derive this reverse-path forwarding (RPF) information. As the name PIM implies, it functions independently of the unicast protocols being used. PIM relies on the Routing Information Base (RIB) for RPF information. If the multicast subsequent address family identifier (SAFI) is configured for Border Gateway Protocol (BGP), or if multicast intact is configured, a separate multicast unicast RIB is created and populated with the BGP multicast SAFI routes, the intact information, and any IGP information in the unicast RIB. Otherwise, PIM gets information directly from the unicast SAFI RIB. Both multicast unicast and unicast databases are outside of the scope of PIM. The Cisco IOS XR implementation of PIM is based on RFC 4601 Protocol Independent Multicast – Sparse Mode (PIM-SM): Protocol Specification. For more information, see RFC 4601 and the Protocol Independent Multicast (PIM): Motivation and Architecture Internet Engineering Task Force (IETF) Internet draft
QUESTION 78
Select and Place:
Correct Answer:
Section: (none) Explanation
Explanation/Reference:
Download this chapter Implementing Tunnels Download the complete book Interface and Hardware Component Configuration Guide, Cisco IOS XE Release 3S (PDF – 1 MB) Feedback
Contents Implementing Tunnels Finding Feature Information Restrictions for Implementing Tunnels Information About Implementing Tunnels Tunneling Versus Encapsulation Tunnel ToS Generic Routing Encapsulation GRE Tunnel IP Source and Destination VRF Membership GRE IPv4 Tunnel Support for IPv6 Traffic EoMPLS over GRE Provider Edge to Provider Edge Generic Routing EncapsulationTunnels Provider to Provider Generic Routing Encapsulation Tunnels Provider Edge to Provider Generic Routing Encapsulation Tunnels Features Specific to Generic Routing Encapsulation Features Specific to Ethernet over MPLS Features Specific to Multiprotocol Label Switching Virtual Private Network Overlay Tunnels for IPv6 IPv6 Manually Configured Tunnels Automatic 6to4 Tunnels ISATAP Tunnels Path MTU Discovery QoS Options for Tunnels How to Implement Tunnels Determining the Tunnel Type Configuring an IPv4 GRE Tunnel GRE Tunnel Keepalive What to Do Next Configuring GRE on IPv6 Tunnels What to Do Next Configuring GRE Tunnel IP Source and Destination VRF Membership What to Do Next Manually Configuring IPv6 Tunnels What to Do Next Configuring 6to4 Tunnels What to Do Next Configuring ISATAP Tunnels Verifying Tunnel Configuration and Operation Configuration Examples for Implementing Tunnels Example: Configuring a GRE IPv4 Tunnel Example: Configuring GRE on IPv6 Tunnels Example: Configuring GRE Tunnel IP Source and Destination VRF Membership Example: Configuring EoMPLS over GRE Example: Manually Configuring IPv6 Tunnels Example: Configuring 6to4 Tunnels Example: Configuring ISATAP Tunnels Configuring QoS Options on Tunnel Interfaces Examples Policing Example Additional References Feature Information for Implementing Tunnels
Implementing Tunnels
Last Updated: September 17, 2012
This module describes the various types of tunneling techniques. Configuration details and examples are provided for the tunnel types that use physical or virtual interfaces. Many tunneling techniques are implemented using technology-specific commands, and links are provided to the appropriate technology modules. Tunneling provides a way to encapsulate arbitrary packets inside a transport protocol. Tunnels are implemented as virtual interfaces to provide a simple interface for configuration purposes. The tunnel interface is not tied to specific “passenger” or “transport” protocols, but rather is an architecture to provide the services necessary to implement any standard point-to-point encapsulation scheme.
Note
Cisco ASR 1000 Series Aggregation Services Routers support VPN routing and forwarding (VRF)-aware generic routing encapsulation (GRE) tunnel keepalive features. Finding Feature Information Restrictions for Implementing Tunnels Information About Implementing Tunnels How to Implement Tunnels Configuration Examples for Implementing Tunnels Additional References Feature Information for Implementing Tunnels
Finding Feature Information
Your software release may not support all the features documented in this module. For the latest caveats and feature information, see Bug Search Tool and the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the feature information table at the end of this module. Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.
Restrictions for Implementing Tunnels
It is important to allow the tunnel protocol to pass through a firewall and access control list (ACL) check. Multiple point-to-point tunnels can saturate the physical link with routing information if the bandwidth is not
configured correctly on a tunnel interface.
A tunnel looks like a single hop link, and routing protocols may prefer a tunnel over a multihop physical
path. The tunnel, despite looking like a single hop link, may traverse a slower path than a multihop link. A
tunnel is as robust and fast, or as unreliable and slow, as the links that it actually traverses. Routing
protocols that make their decisions based only on hop counts will often prefer a tunnel over a set of
physical links. A tunnel might appear to be a one-hop, point-to-point link and have the lowest-cost path, but
the tunnel may actually cost more in terms of latency when compared to an alternative physical topology.
For example, in the topology shown in the figure below, packets from Host 1 will appear to travel across
networks w, t, and z to get to Host 2 instead of taking the path w, x, y, and z because the tunnel hop count
appears shorter. In fact, the packets going through the tunnel will still be traveling across Router A, B, and
C, but they must also travel to Router D before coming back to Router C.
Figure 1
Tunnel Precautions: Hop Counts
A tunnel may have a recursive routing problem if routing is not configured accurately. The best path to a
tunnel destination is via the tunnel itself; therefore recursive routing causes the tunnel interface to flap. To
avoid recursive routing problems, keep the control-plane routing separate from the tunnel routing by using
the following methods:
Use a different autonomous system number or tag.
Use a different routing protocol.
Ensure that static routes are used to override the first hop (watch for routing loops).
The following error is displayed when there is recursive routing to a tunnel destination:
%TUN-RECURDOWN Interface Tunnel 0 temporarily disabled due to recursive routing
Information About Implementing Tunnels
Tunneling Versus Encapsulation Tunnel ToS Generic Routing Encapsulation EoMPLS over GRE Overlay Tunnels for IPv6 IPv6 Manually Configured Tunnels Automatic 6to4 Tunnels ISATAP Tunnels Path MTU Discovery QoS Options for Tunnels
Tunneling Versus Encapsulation
To understand how tunnels work, you must be able to distinguish between concepts of encapsulation and
tunneling. Encapsulation is the process of adding headers to data at each layer of a particular protocol
stack. The Open Systems Interconnection (OSI) reference model describes the functions of a network. To
send a data packet from one host (for example, a PC) to another on a network, encapsulation is used to
add a header in front of the data packet at each layer of the protocol stack in descending order. The
header must contain a data field that indicates the type of data encapsulated at the layer immediately
above the current layer. As the packet ascends the protocol stack on the receiving side of the network,
each encapsulation header is removed in reverse order.
Tunneling encapsulates data packets from one protocol within a different protocol and transports the
packets on a foreign network. Unlike encapsulation, tunneling allows a lower-layer protocol and a same-
layer protocol to be carried through the tunnel. A tunnel interface is a virtual (or logical) interface. Tunneling
consists of three main components:
Passenger protocol–The protocol that you are encapsulating. For example, IPv4 and IPv6 protocols.
Carrier protocol–The protocol that encapsulates. For example, generic routing encapsulation (GRE) and
Multiprotocol Label Switching (MPLS).
Transport protocol–The protocol that carries the encapsulated protocol. The main transport protocol is IP.
The figure below illustrates IP tunneling terminology and concepts:
Figure 2
IP Tunneling Terminology and Concepts
Tunnel ToS
Tunnel type of service (ToS) allows you to tunnel network traffic and group all packets in the same ToS byte value. The ToS byte values and Time-to-Live (TTL) hop-count value can be set in the encapsulating IP header of tunnel packets for an IP tunnel interface on a router. Tunnel ToS feature is supported for Cisco Express Forwarding (formerly known as CEF), fast switching, and process switching. The ToS and TTL byte values are defined in RFC 791. RFC 2474, and RFC 2780 obsolete the use of the ToS byte as defined in RFC 791. RFC 791 specifies that bits 6 and 7 of the ToS byte (the first two least significant bits) are reserved for future use and should be set to 0. For Cisco IOS XE Release 2.1, the Tunnel ToS feature does not conform to this standard and allows you to use the whole ToS byte value, including bits 6 and 7, and to decide to which RFC standard the ToS byte of your packets should conform.
Generic Routing Encapsulation
GRE is defined in RFC 2784. GRE is a carrier protocol that can be used with many different underlying transport protocols and can carry many passenger protocols. RFC 2784 also covers the use of GRE with IPv4 as the transport protocol and the passenger protocol. Cisco software supports GRE as the carrier protocol with many combinations of passenger and transport protocols. GRE tunnels are described in the following sections: GRE Tunnel IP Source and Destination VRF Membership GRE IPv4 Tunnel Support for IPv6 Traffic
GRE Tunnel IP Source and Destination VRF Membership
The GRE Tunnel IP Source and Destination VRF Membership feature allows you to configure the source and destination of a tunnel to belong to any VPN routing and forwarding (VRFs) tables. A VRF table stores routing data for each VPN. The VRF table defines the VPN membership of a customer site that is attached to the network access server (NAS). Each VRF table comprises an IP routing table, a derived Cisco Express Forwarding table, and guidelines and routing protocol parameters that control the information that is included in the routing table. Prior to Cisco IOS XE Release 2.2, GRE IP tunnels required the IP tunnel destination to be in the global routing table. The implementation of this feature allows you to configure a tunnel source and destination to belong to any VRF. As with existing GRE tunnels, the tunnel becomes disabled if no route to the tunnel destination is defined.
GRE IPv4 Tunnel Support for IPv6 Traffic
IPv6 traffic can be carried over IPv4 GRE tunnels by using the standard GRE tunneling technique to provide the services necessary to implement a standard point-to-point encapsulation scheme. GRE tunnels are links between two points, with a separate tunnel for each point. GRE tunnels are not tied to a specific passenger or transport protocol, but in case of IPv6 traffic, IPv6 is the passenger protocol, GRE is the carrier protocol, and IPv4 is the transport protocol. The primary use of GRE tunnels is to provide a stable connection and secure communication between two edge devices or between an edge device and an end system. The edge device and the end system must have a dual-stack implementation. GRE has a protocol field that identifies the passenger protocol. GRE tunnels allow intermediate system to intermediate system (IS-IS) or IPv6 to be specified as the passenger protocol, thereby allowing both IS-IS and IPv6 traffic to run over the same tunnel. If GRE does not have a protocol field, it becomes impossible to distinguish whether the tunnel is carrying IS-IS or IPv6 packets.
EoMPLS over GRE
Ethernet over MPLS (EoMPLS) is a tunneling mechanism that allows you to tunnel Layer 2 traffic through a Layer 3 MPLS network. EoMPLS is also known as Layer 2 tunneling. EoMPLS effectively facilitates Layer 2 extension over long distances. EoMPLS over GRE helps you to create the GRE tunnel as hardware-based switched, and encapsulates EoMPLS frames within the GRE tunnel. The GRE connection is established between the two core routers, and then the MPLS label switched path (LSP) is tunneled over. GRE encapsulation is used to define a packet that has header information added to it prior to being forwarded. De-encapsulation is the process of removing the additional header information when the packet reaches the destination tunnel endpoint. When a packet is forwarded through a GRE tunnel, two new headers are added to the front of the packet and hence the context of the new payload changes. After encapsulation, what was originally the data payload and separate IP header are now known as the GRE payload. A GRE header is added to the packet to provide information on the protocol type and the recalculated checksum. A new IP header is also added to the front of the GRE header. This IP header contains the destination IP address of the tunnel. The GRE header is added to packets such as IP, Layer 2 VPN, and Layer 3 VPN before the header enters into the tunnel. All routers along the path that receives the encapsulated packet use the new IP header to determine how the packet can reach the tunnel endpoint. In IP forwarding, on reaching the tunnel destination endpoint, the new IP header and the GRE header are removed from the packet and the original IP header is used to forward the packet to the final destination. The EoMPLS over GRE feature removes the new IP header and GRE header from the packet at the tunnel destination, and the MPLS label is used to forward the packet to the appropriate Layer 2 attachment circuit or Layer 3 VRF. The scenarios in the following sections describe the L2VPN and L3VPN over GRE deployment on provider edge (PE) or provider (P) routers: Provider Edge to Provider Edge Generic Routing EncapsulationTunnels Provider to Provider Generic Routing Encapsulation Tunnels Provider Edge to Provider Generic Routing Encapsulation Tunnels Features Specific to Generic Routing Encapsulation Features Specific to Ethernet over MPLS Features Specific to Multiprotocol Label Switching Virtual Private Network
Provider Edge to Provider Edge Generic Routing EncapsulationTunnels
In the Provider Edge to Provider Edge (PE) GRE tunnels scenario, a customer does not transition any part of the core to MPLS but prefers to offer EoMPLS and basic MPLS VPN services. Therefore, GRE tunneling of MPLS traffic is done between PEs.
Provider to Provider Generic Routing Encapsulation Tunnels
In the Provider to Provider (P) GRE tunnels scenario, Multiprotocol Label Switching (MPLS) is enabled between Provider Edge (PE ) and P routers but the network core can either have non-MPLS aware routers or IP encryption boxes. In this scenario, GRE tunneling of the MPLS labeled packets is done between P routers.
Provider Edge to Provider Generic Routing Encapsulation Tunnels
In a Provider Edge to Provider GRE tunnels scenario, a network has MPLS-aware P to P nodes. GRE tunneling is done between a PE to P non-MPLS network segment.
Features Specific to Generic Routing Encapsulation
You should understand the following configurations and information for a deployment scenario:
Tunnel endpoints can be loopbacks or physical interfaces.
Configurable tunnel keepalive timer parameters per endpoint and a syslog message must be generated
when the keepalive timer expires.
Bidirectional forwarding detection (BFD) is supported for tunnel failures and for the Interior Gateway
Protocol (IGP) that use tunnels.
IGP load sharing across a GRE tunnel is supported.
IGP redundancy across a GRE tunnel is supported.
Fragmentation across a GRE tunnel is supported.
Ability to pass jumbo frames is supported.
All IGP control plane traffic is supported.
IP ToS preservation across tunnels is supported.
A tunnel should be independent of the endpoint physical interface type; for example, ATM, Gigabit, Packet
over SONET (POS), and TenGigabit.
Up to 100 GRE tunnels are supported.
Features Specific to Ethernet over MPLS
Any Transport over MPLS (AToM) sequencing.
IGP load sharing and redundancy.
Port mode Ethernet over MPLS (EoMPLS).
Pseudowire redundancy.
Support for up to to 200 EoMPLS virtual circuits (VCs).
Tunnel selection and the ability to map a specific pseudowire to a GRE tunnel.
VLAN mode EoMPLS.
Features Specific to Multiprotocol Label Switching Virtual Private Network
Support for the PE role with IPv4 VRF.
Support for all PE to customer edge (CE) protocols.
Load sharing through multiple tunnels and also equal cost IGP paths with a single tunnel.
Support for redundancy through unequal cost IGP paths with a single tunnel.
Support for the IP precedence value being copied onto the expression (EXP) bits field of the Multiprotocol
Label Switching (MPLS) label and then onto the precedence bits on the outer IPv4 ToS field of the generic
routing encapsulation (GRE) packet.
See the section, “Example: Configuring EoMPLS over GRE” for a sample configuration sequence of
EoMPLS over GRE. For more details on EoMPLS over GRE, see the Deploying and Configuring MPLS
Virtual Private Networks In IP Tunnel Environments document.
Overlay Tunnels for IPv6
The figure below illustrates how overlay tunneling encapsulates IPv6 packets in IPv4 packets for delivery across an IPv4 infrastructure (a core network or the Internet). By using overlay tunnels, you can communicate with isolated IPv6 networks without upgrading the IPv4 infrastructure between them. Overlay tunnels can be configured between border routers or between a border router and a host; however, both tunnel endpoints must support, IPv4 and IPv6 protocol stacks. IPv6 supports the following types of overlay tunneling mechanisms:
6to4 GRE Intra-Site Automatic Tunnel Addressing Protocol (ISATAP) IPv4-compatible Manual
Figure 3 Overlay Tunnels
Note
If the basic IPv4 packet header does not have optional fields, overlay tunnels can reduce the maximum transmission unit (MTU) of an interface by 20 octets. A network that uses overlay tunnels is difficult to troubleshoot. Therefore, overlay tunnels that connect isolated IPv6 networks should not be considered as the final IPv6 network architecture. The use of overlay tunnels is considered as a transition technique for a network that supports either both IPv4 and IPv6 protocol stacks or just the IPv6 protocol stack. Consult the table below to determine which type of tunnel you want to configure to carry IPv6 packets over an IPv4 network.
Table 1 Suggested Usage of Tunnel Types to Carry IPv6 Packets over an IPv4 Network
Tunneling Type Suggested Usage Usage Notes
6to4
Point-to-multipoint tunnels that can be used to connect isolated IPv6 sites.
Sites use addresses that begin with the 2002::/16 prefix.
GRE/IPv4
Simple point-to-point tunnels that can be used within a site or between sites.
Tunnels can carry IPv6, Connectionless Network ServiceCLNS, and many other types of packets.
ISATAP
Point-to-multipoint tunnels that can be used to connect systems within a site.
Sites can use any IPv6 unicast addresses.
Manual
Simple point-to-point tunnels that can be used within a site or between sites.
Tunnels can carry IPv6 packets only.
Individual tunnel types are discussed in detail in the following concepts, and we recommend that you
review and understand the information on the specific tunnel type that you want to implement. Consult the
table below for a summary of the tunnel configuration parameters that you may find useful.
Table 2 Overlay Tunnel Configuration Parameters by Tunneling Type
Overlay Tunneling Type Overlay Tunnel Configuration Parameter
Tunnel Mode Tunnel Source Tunnel Destination Interface Prefix/Address 6to4 ipv6ip 6to4 An IPv4 address or a reference to an interface on which IPv4 is configured. Not required. These are all point-to-multipoint tunneling types. The IPv4 destination address is calculated, on a per-packet basis, from the IPv6 destination. An IPv6 address. The prefix must embed the tunnel source IPv4 address. GRE/IPv4 gre ip An IPv4 address. An IPv6 address. ISATAP ipv6ip isatap Not required. These are all point-to-multipoint tunneling types. The IPv4 destination address is calculated on a per-packet basis from the IPv6 destination. An IPv6 prefix in modified eui-64 format. The IPv6 address is generated from the prefix and the tunnel source IPv4 address. Manual ipv6ip An IPv4 address. An IPv6 address.
IPv6 Manually Configured Tunnels
A manually configured tunnel is equivalent to a permanent link between two IPv6 domains over an IPv4 backbone. The primary use of a manually configured tunnel is to stabilize connections that require secure communication between two edge routers, or between an end system and an edge router. The manual configuration tunnel also stabilizes connection between remote IPv6 networks. An IPv6 address is manually configured on a tunnel interface. Manually configured IPv4 addresses are assigned to the tunnel source and destination. The host or router at each end of a configured tunnel must support both the IPv4 and IPv6 protocol stacks. Manually configured tunnels can be configured between border routers or between a border router and a host. Cisco Express Forwarding switching can be used for manually configured IPv6 tunnels. Switching can be disabled if process switching is required.
Automatic 6to4 Tunnels
An automatic 6to4 tunnel allows isolated IPv6 domains to be connected over an IPv4 network to remote IPv6 networks. The key difference between automatic 6to4 tunnels and manually configured tunnels is that the tunnel is not point-to-point; it is point-to-multipoint. In automatic 6to4 tunnels, routers are not configured in pairs because they treat the IPv4 infrastructure as a virtual nonbroadcast multiaccess (NBMA) links. The IPv4 address embedded in the IPv6 address is used to find the other end of the automatic tunnel. An automatic 6to4 tunnel may be configured on a border router in an isolated IPv6 network, which creates a tunnel on a per-packet basis on a border router in another IPv6 network over an IPv4 infrastructure. The tunnel destination is determined by the IPv4 address of the border router extracted from the IPv6 address
that starts with the prefix 2002::/16, where the format is 2002:border-router-IPv4-address ::/48.The embedded IPv4 addresses are 16 bits and can be used to number networks within the site. The border router at each end of a 6to4 tunnel must support both IPv4 and IPv6 protocol stacks. 6to4 tunnels are configured between border routers or between a border router and a host. The simplest deployment scenario for 6to4 tunnels is to interconnect multiple IPv6 sites, each of which has at least one connection to a shared IPv4 network. This IPv4 network could either be the Internet or a corporate backbone. The key requirement is that each site have a globally unique IPv4 address; the Cisco software uses this address to construct a globally unique 6to4/48 IPv6 prefix. A tunnel with appropriate entries in a Domain Name System (DNS) that maps hostnames and IP addresses for both IPv4 and IPv6 domains, allows the applications to choose the required address IPv6 traffic can be carried over IPv4 GRE tunnels by using the standard GRE tunneling technique to provide the services necessary to implement a standard point-to-point encapsulation scheme. GRE tunnels are links between two points, with a separate tunnel for each point. GRE tunnels are not tied to a specific passenger or transport protocol, but in case of IPv6 traffic, IPv6 is the passenger protocol, GRE is the carrier protocol, and IPv4 is the transport protocol. The primary use of GRE tunnels is to provide a stable connection and secure communication between two edge devices or between an edge device and an end system. The edge device and the end system must have a dual-stack implementation. GRE has a protocol field that identifies the passenger protocol. GRE tunnels allow intermediate system to intermediate system (IS-IS) or IPv6 to be specified as the passenger protocol, thereby allowing both IS-IS and IPv6 traffic to run over the same tunnel. If GRE does not have a protocol field, it becomes impossible to distinguish whether the tunnel is carrying IS-IS or IPv6 packets.
QUESTION 79
Select and Place:
Correct Answer: Section: (none) Explanation
Explanation/Reference:
i) Dense Mode Flood-and-Prune Protocols (DVMRP / MOSPF / PIM-DM)
In dense mode protocols, all routers in the network are aware of all trees, their sources and receivers. Protocols such as DVMRP and PIM dense mode flood ※active source§ information across the whole network and build trees by creating ※Prune State§ in parts of the topology where traffic for a specific tree is unwanted. They are also called flood-and-prune protocols. In MOSPF, information about receivers is flooded throughout the network to support the building of trees. Dense mode protocols are undesirable because every tree built in some part of the network will always cause resource utilization (with convergence impact) on all routers in the network (or within the administrative scope, if configured). We will not be discussing these protocols in the rest of this paper.
ii) Sparse Mode Explicit Join Protocols (PIM-SM/PIM-BiDir)
With sparse mode explicit join protocols we do not create a group-specific forwarding state in the network unless a receiver has sent an explicit IGMP/MLD membership report (or ※join§) for a group. This variant of ASM is known to scale well and is the multicast paradigm we will mainly be discussing. This is the basis for PIM-Sparse Mode, which most multicast deployments have used to this point. This is also the basis for PIM-BiDir, which will be increasingly deployed for MANY (sources) TO MANY (receivers) applications. These protocols are called sparse mode because they efficiently support IP multicast delivery trees with a ※sparse§ receiver population 每 creating control plane state only on routers in the path between sources and receivers, and in PIM-SM/BiDir, the Rendezvous Point (RP). They never create state in other parts of the network. State in a router is only built explicitly when it receives a join from a downstream router or receiver, hence the name ※explicit join protocols§. Both PIM-SM and PIM-BiDir employ ※SHARED TREES§, which allow traffic from any source to be forwarded to a receiver. The forwarding state on a shared tree is referred to as (*,G) forwarding state, where the * is a wild card for ANY SOURCE. Additionally, PIM-SM supports the creation of forwarding state that relates to traffic from a specific source. These are known as SOURCE TREES, and the associated state is referred to as (S,G) forwarding state SSM is the model used when the receiver (or some proxy) sends (S,G) ※joins§ to indicate that it wants to receive traffic sent by source S to group G. This is possible with IGMPv3/MLDv2 ※INCLUDE§ mode membership reports. We therefore refer to this model as the Source-Specific Multicast (SSM) model. SSM mandates the use of an explicit-join protocol between routers. The standard protocol for this is PIM-SSM, which is simply the subset of PIM-SM used to create (S,G) trees. There are no shared trees (*,G) state in SSM. Multicast receivers can thus ※join§ an ASM group G, or ※join§ (or more accurately ※subscribe§ to) an SSM (S,G) channel. To avoid having to repeat the term ※ASM group or SSM channel§, we will use the term (multicast) flow in the text, implying that the flow could be an ASM group or an SSM channel
QUESTION 80
Select and Place:
Correct Answer: Section: (none) Explanation
Explanation/Reference:
SO bgp dampening 60 600 2400 240 is:
60 half life 600 reuse 2400 suppress 240 max-suppress-time
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