MPLS Questions
Note: If you are not sure about MPLS, please read our Basic MPLS tutorial (on 9tut.com).
Question 1
Explanation
LDP uses TCP as a reliable transport for sessions. When multiple LDP sessions are required between two LSRs, there is one TCP session for each LDP session.
Reference: https://tools.ietf.org/html/rfc5036
Question 2
Question 3
Explanation
The command “maximum-paths [ ibgp ] number-of-paths” configures the maximum number of multipaths allowed. Use the ibgp keyword to configure iBGP load balancing. This question does not tell which BGP load-balance it wants (iBGP or eBGP) so in fact answer B is also correct.
Question 4
Explanation
The “imp-null” (implicit null) tag instructs the upstream router to pop the tag entry off the tag stack before forwarding the packet.
Note: pop means “remove the top MPLS label”
Question 5
Explanation
MPLS uses a 32-bit label field that contains the information that follows:
+ 20-bit label (a number)
+ 3-bit class of service (or experimental field, typically used to carry IP precedence value)
+ 1-bit bottom-of-stack indicator (indicates whether this is the last label before the IP header)
+ 8-bit TTL (equal to the TTL in the IP header)
Question 6
Explanation
First we thought answer B is the best answer but in fact it is not. Please read the paragraph below (quoted from “Designing for Cisco Network Service Architectures (ARCH) Foundation Learning Guide CCDP ARCH 300-320 Book) to understand more about RD role in an MPLS network:
“Route Distinguishers
In MPLS VPN, the PE router provides isolation between customers by using VRFs. The prefixes are propagated to the other PE routers with MP-BGP. The problem is that the prefixes need to be unique across the MPLS VPN provider network. For instance, if different customers have overlapping IP addressing (and this is very common and typical for a shared infrastructure providing transit service for many different customers), this situation would cause a serious problem for BGP. As mentioned earlier, to solve this problem, the concept of route distinguisher (RD) was conceived.
The basic idea behind the RD is that every customer receives a unique identifier to distinguish between the same prefix from different customers. To create a unique prefix, you combine the RD with the IPv4 prefix. The combination is called a VPNv4 prefix. MP-BGP needs to carry these VPNv4 prefixes between the PE routers.
An RD is a 64-bit unique identifier prepended to the 32-bit customer prefix learned from the CE router. The combination of the RD and the prefix will generate a unique 96-bit-long IP prefix that can be carried across the MPLS-BGP domain as a unique prefix (to overcome customers’ overlapping IP addresses). There are two formats for the RD. The first one is ASN:nn, where ASN represents the autonomous system number and nn represents a number. The second format is IP-address:nn. The first format is the most commonly used.”
Question 6
The answer should be “A. Route distinguishers make a unique VPNv4 address across the MPLS network”
“Virtual routing and forwarding (VRF) is a technology that creates separate virtual
routers on a physical router.” Option B is describing a VRF not RD.
BGP, how does BGP handle identical network prefixes
that belong to different customers? By using a Route
Distinguisher (RD). The RD is used to expand the
customers IP prefix so that it includes a unique value
that distinguishes it from the other identical prefixes.
The RD is generated and used by the PE routers on a per
customer VRF basis and to keep things simple, the RD
will be used regardless of overlapping or none
overlapping address spaces. So, the RD will be used all
the time.
The unique 64 bit RD is prepended to the 32 bit
customer prefix (IPv4 route) to create a 96 bit unique
prefix called a VPNv4 address
SOURCES: CCNP Enterprise Advanced Routing ENARSI 300-410 Official Cert Guide AND
CCNP and CCIE Enterprise Core ENCOR 300-401
Question 3, Answer: B. Maximum-paths ibgp 2 Reference:https://www.cisco.com/c/en/us/td/docs/switches/datacenter/sw/5_x/nx-os/mpls/configuration/guide/mpls_cg/mp_vpn_multipath.html
Example: MPLS Layer 3 VPN Load Balancing
The following example shows how to configure iBGP load balancing:
configure terminal
feature-set mpls
feature mpls l3vpn
feature bgp
router bgp 1.1
bestpath cost-community ignore
address-family ipv6 unicast
maximum-paths ibgp 4
Hi guys,
did anyone knows correct answer ot Question 3, B or D.
thanks
q3 i agree with Heamgu
Configuring BGP Load Balancing for eBGP and iBGP
You can configure a Layer 3 VPN load balancing for an eBGP or iBGP network.
Prerequisites
Ensure that you are in the correct VDC (or use the switchto vdc command).
SUMMARY STEPS
1. configure terminal
2. feature- s et mpls
3. feature mpls l3vpn
4. feature bgp
5. router bgp as-number
6. (Optional) bestpath cost-community ignore
7. address-family { ipv4 | ipv6 } unicast
8. maximum-paths [ibgp] number-of-paths – ANSWER is B
9. (Optional) show running-config bgp
10. (Optional) copy running-config startup-config
DETAILED STEPS
@Networktut: can you please fix the answer for Q3 to B
@Dave: We can have “ibgp” keyword or not so both answers are correct.
Q3 is not clear. B and D are correct based on that question. iBGP requires ibgp command at the end of maximum-path. Correct me if I am wrong yet eBGP does not.
Q3 “Use the ibgp keyword to configure iBGP load balancing.” So the way I read it is “maximum-paths 2” is just to allow a maximum of “2” but if you want to load balance between the 2, you need “ibgp” key word option.
https://www.cisco.com/c/en/us/td/docs/switches/datacenter/nexus9000/sw/93x/label-switching/b-cisco-nexus-9000-series-nx-os-label-switching-configuration-guide-93x/b-cisco-nexus-9000-series-nx-os-label-switching-configuration-guide-93x_chapter_0111.pdf
Q3 is confusing.
For me, B and D are correct.
By [1]: “To configure multipath load sharing for external BGP (eBGP) and internal (iBGP) routes, use the ‘maximum-paths eibgp’ command in address family configuration mode.”
“The ‘maximum-paths eibgp’ command cannot be configured with the ‘maximum-paths’ or ‘maximum-paths ibgp’ command because the ‘maximum-paths eibgp’ command is a superset of these commands.”
By [2]: “The BGP Diverse Path feature can be enabled on a route reflector to calculate a bestpath and an additional path per address family.
Computation of a diverse path per address family is triggered by any of the following commands:
• bgp additional-paths install
• bgp additional-paths select
• maximum-paths ebgp \\ (Configures multipath load sharing for eBGP and iBGP routes)
• maximum-paths ibgp \\ (Controls the maximum number of parallel iBGP routes that can be installed in a routing table) ”
By [3]: “Use the ‘maximum-paths’ command to allow EIGRP to install multiple paths into the routing table for each prefix. Multiple paths are installed for both internal and external routes that are learned in the same autonomous system and that have an equal cost (according to the EIGRP best path algorithm). ”
By [4]: ”However, you can change the maximum number of parallel equal-cost paths that are allowed. In order to make this change, include the ‘maximum-paths paths’ command under the BGP configuration. Use a number between 1 and 6 for the paths argument. ”
By [5]: “Command: maximum-paths [ibgp] number-of-paths. Configures the maximum number of multipaths allowed. Use the ibgp keyword to configure iBGP load balancing. The range is from 1 to 16.”
[1] –
https://www.cisco.com/c/en/us/td/docs/ios/12_2sx/feature/guide/fsxeibmp.html
[2] –
https://www.cisco.com/c/en/us/td/docs/ios/iproute_bgp/command/reference/irg_book/irg_bgp1.html
[3] –
https://www.cisco.com/c/m/en_us/techdoc/dc/reference/cli/n5k/commands/maximum-paths-eigrp.html
[4] –
https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13762-40.html
[5] –
https://www.cisco.com/c/en/us/td/docs/switches/datacenter/sw/5_x/nx-os/mpls/configuration/guide/mpls_cg/mp_vpn_multipath.html
Hello Guys, I am a bit confused with question 5. I think answer A makes more sense: ” 20-bit label; 3-bit traffic class; 1 -bit bottom stack; 8-bit TTL ” . The 3 bits are used for QoS which associates with traffic class. Sounds better to me, instead of “flow label” . Any clue? Thanks
Are these questions still valid for certification?