Are you looking for job as a BGP/MPLS network professional? Or are you thinking about leaving your current position for a new job in a “BGP/MPLS Network Domain” with a new company?
If you answered yes to either of those questions, then this article is a must-read for you and any of described technologies and questions may be asked of you during the interview!
An Introduction to BGP/MPLS Network Professional’s Roles and Responsibilities:
Nowadays, almost every service provider around the world is using BGP and MPLS deployments throughout their cloud for providing and managing millions of clients’ network infrastructure with better service availability and performance. >MPLS is particularly well suited in carrier networks and commercial wide area networks. BGP/MPLS deployments ensure flawless connectivity and routine activities in a multi-protocol network infrastructure and maintain the excellent communication service approach without any barrier as both are open standard technologies.
A BGP/MPLS network professional must have moderate to excellent understanding of enterprise or carrier’s network infrastructure. Their job roles often involve 24×7 support for network services with both planned and unplanned work shifts. A BGP/MPLS network professional is responsible for performing the following functions:
Configuration and maintenance of exterior gateway routing (BGP)/MPLS Cloud
Evaluation and recommendations of BGP path selection for ingress/egress traffic
Configuration and maintenance of MPLS L2VPN/L3VPN, MPLS traffic engineering
Stability and availability of access and core network services in carrier networks
Network scalability, incident response, and review of network issues
Researching and learning new technologies adopted by the clients
Technical interaction with higher management, vendors, and clients
Installation of network hardware and software
Required Key Skills for a BGP/MPLS professional:
Good knowledge of following BGP functionalities:
- BGP fundamentals and inter-AS communication techniques
- BGP neighbor (iBGP and eBGP) peering and security
- BGP path Selection (path attributes)
- BGP synchronization rule
- BGP split-horizon:
- Route reflector (role of cluster-id and originator-id)
- BGP communities (standard and extended)
- BGP AS-path filtering with regular expressions
- MP-BGP (multiprotocol BGP) with appropriate address families
Good knowledge of following MPLS functionalities:
- VRF introduction and Implementation (with RD and RT functionalities)
- VRF-based routing and redistribution
- Functional approach of P, PE and CE routers
- Understanding of PHP, LDP, TDP, LIB, LFIB, LSR, Push, Pop, Swap functions
- MPLS L3VPN Implementation, sham link
- Any Transport over MPLS (AToM), VPLS
- MPLS TE and QoS techniques
- Ethernet VPN (EVPN) and provider backbone bridging EVPN (PBB-EVPN)
- Multicast label distribution protocol (mLDP)
- Unified MPLS (seamless MPLS)
Apart from above BGP/MPLS skill set, candidate must have strong analytical and subjective knowledge of following key skills:
- In depth understanding of carrier network architecture
- Sound knowledge of network layered approach
- Good understanding of multi-vendor network devices, such as Cisco, HP, Juniper, Nortel, etc.
- Good understanding of networking IOS (IOS, IOS-XE, IOS-XR, NX-OS)
- Knowledge about network management/ analysis/ monitoring tools
- General understanding of rollback, and failovers handling
- Ability to handle multiple assignments with deadlines
- Able to communicate network issues effectively and in a comprehensible manner with clients and team members
My Best Questions for an Interview about BGP/MPLS Network Architecture
All of the questions below are very common and must be well prepared for before facing any interview in BGP/MPLS network environment so you can be honest and confident while answering.
Most of interviews in BGP/MPLS network environment start with following set of common questions:
“Tell me something about yourself.”
Questions related to current/previous job roles.
“What is your understanding about enterprises or carrier’s (service provider’s) network?”
“Do you have any experience in a carrier network?”
“What are your major strengths and weaknesses?”
“Share some of your achievements as a network professional?”
“What is your salary expectation for this position?”
While answering questions about your current and previous jobs, don’t forget to mention your responsibilities with recent activities and achievements. Apart from the above HR/Personal questions, let’s discuss some important technical questions as well.
Q. What do you mean by MPLS? What are the benefits of using MPLS?
A. This is one of the most common questions asked during the MPLS interviews, so this question must be prepared for before facing the interview. Here you should start answering by introducing MPLS.
Multiprotocol label switching (MPLS) offer a mechanism to process packets on the basis of labels. It is a data forwarding service that provides high-end network communications from one network node to the next on the basis of labels rather than routing table lookups. Routing table lookup for each packet makes communication sluggish but MPLS uses LFIB/forwarding table to process label from one node to another.
The main benefits of using MPLS are:
QoS integration: supports multiple levels of QoS, to manage latency, jitter and packet loss for various types of traffic (voice, video, email, bulk file transfers, etc.)
Fast convergence: label-based switching, no need to look up routing tables
Scalability to IP VPNs
Low network congestion using MPLS TE
Reliable and secure
For further understanding, use the following link:
Q. Describe PUSH, SWAP, POP functions in MPLS and also explain penultimate hop popping (PHP).
A. Label switching is a discrete mechanism from IP forwarding; here LSR regulates what label function needs to be performed, such as Push, Pop, or Swap, to process a packet throughout the MPLS backbone using label switching.
Push: Add a label
Pop: Remove a label
Swap: Exchange/ replace a label
To know more about these functions, click the following link:
Penultimate hop popping is a technique to remove the label one hop before its destination so that untagged packet will be received by the destination PE router and only IP lookup will be performed by destination PE router to forward it to the directly connected CE interface.
Q. What is Any Transport over MPLS (AToM)? Do we require VRF or BGP-VPN to form AToM connectivity?
A. Any Transport over MPLS (AToM) enables service providers to deliver Layer 2 connectivity over a MPLS backbone to connect different clients’ sites with existing data link layer (Layer 2) networks. BGP-VPN or VRF is not required to configure AToM; to form AToM connectivity we do use xconnect (cross connect) peering with unique VC-ID to distinguish multiple clients.
For more information on AToM, visit the following web page:
Q. Describe the roles of RD and RT in MPLS L3VPN? Is it possible to assign a same RD to multiple customers?
A. Route distinguisher (RD) is used to distinguish identical routes in different VRFs, whereas route target (RT) is used to share routes between them. Route target also specifies which prefixes get imported or exported on the PE router.
No, we can’t assign same RD to multiple customers on a PE router because the main purpose of defining RD is limited to provide uniqueness in different VRFs.
To know the structural detail of RD, refer to the following link:
Q. How can you differentiate VPNv4 and IPv4 address-family?
A. Address-family VPNv4 is used to transport VRF traffic, which is quite heavier (96 bits) than normal IPv4 packets, and address-family IPv4 is used to transport traditional 32-bit IPv4 packets. When we deploy L3VPN, we need VPNv4 peering between the PEs to take the VRF routes (labeled with RD, RT) to the corresponding destination PE router.
Q. What are LIB and LFIB? If we disable CEF (on a Cisco device) will MPLS work?
A. The difference between two is almost like RIB and FIB, as RIB uses FIB to forward traffic. Label Information Base (LIB) is a table where prefix to label bindings are built and stored in the control plane and can be seen with “show mpls ldp bindings.” Label Forwarding Information Base (LFIB) is another MPLS table used to forward label packets throughout the MPLS network and build in the data plane. To check LFIB, run “show mpls forwarding-table.” The lookups are in fact done in the LFIB, not the LIB.
If CEF is disabled, MPLS will not work, as it uses LFIB to forward labeled packets and LFIB built from the FIB table.
Q. Name BGP path attributes to control incoming and outgoing traffic.
A. BGP has multiple path attributes to influencing traffic flow. In the most common practice we do manipulate incoming traffic using AS-Path prepending and MED (multi-exit discriminator). And, to influence outgoing traffic, weight and local preference are the best choices.
Q. My BGP is showing 0.0.0.0 as router-id; what could be the possible reason?
A. This happens when we configure BGP on a router that has non-IP interfaces, meaning that if a router’s interfaces are not configured with any IP address, then BGP cannot get the router-id. In that case, BGP assumes 0.0.0.0 as router-id.
Q. If my BGP neighbor is stuck in idle or active state, what should I do?
A. If BGP peer is in idle state, then it could be due to physical connectivity failure or the neighbor is not defined properly with respective AS. In connect state, BGP tries to establish a TCP session over port number 179; if it fails to establish the connection, then it goes to active state, where it tries again to establish a TCP connection. By using “debug ip bgp” and “debug ip tcp transactions” commands, we can learn the exact cause of TCP connection failure. Lack of the “update-source” or “ebgp-multihop” command can also be a reason of neighbor active state; to know more, visit the following link:
Q. Explain BGP recursive lookup and site of origin (SoO).
A. BGP recursive lookup involves the router looking up the BGP route and BGP next hop to reach a destination in the remote AS. Then the router will look up the route into IGP to reach the BGP next hop. This mechanism is known as BGP recursive lookup.
BGP site of origin (SoO) is a loop prevention technique in the MPLS backbone environment. BGP SoO is associated with a CE-neighbor; that’s why whenever the PE router advertise a prefix to a CE neighbor, it check the SoO of the prefix with the SoO of the BGP neighbor to avoid loops.
Q. What do you understand by BGP split-horizon rule?
A. This rule states, “Network prefixes learned from an iBGP peer will not be shared with another iBGP peer” to prevent routing loops. BGP uses the AS_PATH attribute to avoid routing loops, which means a route will not be accepted if the AS_PATH contains the same AS number of the router where it is receiving. Route-reflector and confederation techniques are used to share iBGP learned networks with another iBGP neighbor.
Q. Describe BGP communities. Name well-known communities.
A. BGP communities are commonly used to control route policies in the BGP environment and used as flags in order to mark it over the set of network prefixes. BGP communities’ information can be shared with a neighbor by using the command “neighbor x.x.x.x send-community standard | extended | both” and can be applied using route-maps to a defined neighbor. Some well-known BGP communities are:
Local-AS: Share the prefixes with iBGP-confederation peers only.
No-Advertise: Do not share the prefixes with anyone.
No-Export: Do not share the prefixes with another Global AS, share within the Public-AS only.
Internet: Share the prefixes with all.
The questions above are very collective and important from the standpoint of clearing an interview in BGP/MPLS network environment. In this article, I have tried to explain many of the frequently asked questions, and you can get more frequently asked interview questions for BGP/MPLS’s Jobs from the download link posted here. If you find any difficulty in answering any question, please write me @ Comment section.
Before facing any Interview, a candidate should take care of the following points:
A quick recap of relevant technologies
Read your resume throughout; and detail your strengths/weaknesses, achievements and prior job roles and responsibilities
Professional certifications allow the interviewer to understand your knowledge level, so point to your certifications (if you have any)
An updated LinkedIn profile helps you a lot in getting job recommendation and referrals for your desired job roles
This article is the output of my extensive research and work experience, and my team, corporate clients, and colleagues helped me a lot in developing this article. With this article, I am trying to help/guide candidates in interview preparation for getting an engineering or administrative position in BGP/MPLS network environment.
I hope this article will be appreciated so that I will be able to offer more in this segment. You can write me @comment section below for providing any query/feedback; I will try my best to resolve your queries at the earliest. And don’t forget to spread the link of this article on your Facebook, Twitter, and LinkedIn accounts so the maximum of people can get this exclusive piece of information. Keep reading @ Instanseschool.com and consider joining our Facebook group, http://www.facebook.com/intenseschool, to get updates on new posts and technologies.