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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.
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
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Multiprotocol Label Switching (MPLS) Related Tutorials
|Internet Protocol version 4 (IPv4) Tutorial|
Multiprotocol Label Switching (MPLS) Related Interview Questions
|Switching Interview Questions||Broadcast Interview Questions|
|Firewall Support Interview Questions||Veritas Volume Manager (VVM or VxVM) Interview Questions|
|Internet Protocol version 4 (IPv4) Interview Questions||Firewall (computing) Interview Questions|
|Border Gateway Protocol (BGP) Interview Questions||Enhanced Interior Gateway Routing Protocol (EIGRP) Interview Questions|
|Virtual Private Network (VPN) Interview Questions||OSPF Interview Questions|
|Protocol Testing Interview Questions|
Multiprotocol Label Switching (MPLS) Related Practice Tests
|Switching Practice Tests||Broadcast Practice Tests|
|Firewall Support Practice Tests||Internet Protocol version 4 (IPv4) Practice Tests|
|Firewall (computing) Practice Tests||Border Gateway Protocol (BGP) Practice Tests|
|Enhanced Interior Gateway Routing Protocol (EIGRP) Practice Tests|
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