Term
| In an MPLS network, where is the only place where a routing decision is based on a routing table lookup? |
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Definition
| In MPLS networks, only the edge labal-switching routers (LSRs) perform the routing table lookup. All non-edge LSRs perform their forwarding processes based on the label only, not on the L3 header information. |
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Term
| In an MPLS network, when is the examination and labeling process performed? |
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Definition
| There is only one examination and assignment of a packet, which is done at the MPLS ingress node. The ingress label is used as an index to allow the choice of an egress label indentifying the next-hop device. The ingress label is then discarded by the device and replaced with an approproiate new label that will get it to the next hop. |
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Term
| Why are MPLS networks also called MPLS VPNs? |
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Definition
| MPLS technologies allow each customer's routing information to be isolated from every other customer's routing information within the provider cloud, implying a secure and reliable transport. |
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Term
| Describe how a penultimate hop pop works. |
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Definition
| When a labeled packet traverses an MPLS network and reaches the final edge router, it pops (removes) the label from the packet and performs a new routing table lookup. A penultimate hop pop is when the LSR prior to the destination edge router pops the label before sending the packet to the final edge LSR or node. The advantage is that the final device does not need to perform both a label lookup and a network layer lookup once it figures out that it is the last hop prior to the destination. |
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Term
| How long does an entry in the fast-switching cache remain before being flushed? |
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Definition
| Entries in the fast-switching cache age out after 60 seconds. |
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Term
| Describe the concept of Cisco Express Forwarding. |
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Definition
| Cisco Express Forwarding (CEF) is a topology-driven technology that makes use of a forwarding information base (FIB). The FIB is basically a mirror image of the IP routing table, and it only changes when a L3 routing protocol changes the IP routing table. |
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Term
| How does the CEF FIB differ from fast-switching cache in regards to what information it keeps? |
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Definition
| The FIB does not contain information regarding the egress interface and corresponding L2 encapsulation information. CEF maintains an adjacency table for this purpose. |
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Term
| What criteria needs to be met for a node to be considered adjacent in the CEF FIB adjacency table? |
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Definition
| Nodes are considered adjacent if they are able to make contact across a single L2 connection. Adjacencies are built at L2 and linked to the FIB, thereby eliminating any need for ARP requests. |
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Term
| Why shouldn't CEF be enabled on Internet-facing devices? |
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Definition
| Because the Internet routing table is to be redistributed into that router, this is not recommended due to the sheer size of the job. |
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Term
| What are the pros and cons of process switching? |
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Definition
| Process switching performs recursive routing lookup, so you get up-to-date information all the time. But it results in slow and inefficient repetition of lookups |
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Term
| What are the pros and cons of fast-switching, AKA cache-driven switching? |
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Definition
| Fast switching is interrupt code driven (packets are handles immidiately) and significantly fast than process switching. But because only the first packet is process switched, load balancing can be difficult to obtain. |
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Term
| What are the pros and cons of CEF switching, AKA topology-driven switching? |
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Definition
| With CEF switching, full load balancing is capable on a per-packet basis, or based on source or destination address, or other characteristics. The trade-off is that it can have high memory and CPU utilization. |
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Term
| What are the 2 major components that make up the underlying architecture of MPLS? |
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Definition
- Control plane - Maintains routing and label information exchange between adjacent devices. - Data plane - Forwards traffic based on destination addresses or labels (AKA the forwarding plane). |
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Term
| What are the label-based routing protocols use by MPLS? |
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Definition
| The 2 label-based routing protocols used are Tag Distribution Protocol (TDP) and Label Distribution Protocol (LDP). TDP is the earlier predecessor of LDP developed by Cisco before there was an accepted standard. |
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Term
| What does MPLS use for traffic enginerring, such as bandwidth reservation? |
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Definition
| MPLS used Resource Reservation Protocol (RSVP) for traffic engineering. RSVP allocates bandwidth on demand, if available, for a requesting entity. |
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Term
| What is the main purpose of the MPLS control plane? |
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Definition
| The control plane deals with the complexities of routing in general. It includes regular routing protocols like OSPF, EIGRP, and BGP. It also includes label-based routing protocols, such as TDP and LDP. |
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Term
| What is the main purpose of the MPLS data plane. |
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Definition
| The data plane exists for the sole purpose of forwarding traffic based on information gleaned from the routing protocols or LDP. |
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Term
| What does the MPLS data plane use to determine how to dispatch packets toward their destinations? |
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Definition
| A Label Forwarding Information Base (LFIB) is created to store label information for use by the forwarding engine to send packets to their destinations. The LFIB is built by information from sources like LDP, BGP, or RSVP. |
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Term
| What is the general purpose of an MPLS label? |
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Definition
| The MPLS label provides a mechanism by which packets can be sorted into their various FECs without the need for examination of the L3 header. |
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Term
| Describe the concept of frame mode MPLS. |
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Definition
| Frame mode MPLS denotes the use of MPLS with Ethernet-encapsulation or other frame-based encapsulated interfaces. It inserts a label between the L2 header and the L3 header, so that examination of the L3 header is not required for making forwarding decisions. |
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Term
| How does cell mode MPLS work with ATM? |
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Definition
| Because the cell structure cannot be altered, MPLS uses the Virtual Path Identifier and Virtual Channel Identifier (VPI/VCI) fields in the ATM header to carry the label information. |
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Term
| Describe the basic use of label stacks. |
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Definition
| Label stacks function in much the same manner as IP over IP packets. An addition label is added for a specific application purpose. The additional label(s) underneath are not examined until the preceeding label has outlived its usefulness and been stripped away. |
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Term
| What does a PE router do when it receives a packet that it's outbound interface is an MPLS-enabled interface? |
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Definition
| The router imposes the proper label and encapstulates the packet with the proper L2 framing structure. The router will also take the opportunity to alter the Ethertype field in the frame header, in the case of an Ethernet interface, to specify the proper L3 protocol. |
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Term
| Describe the basic use of label switching routers. |
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Definition
| An LSR have the capability to function on both the control and data plane. They need to be able to exchange routing information, and also perform packet forwarding based on labels. |
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Term
| What additional tasks much edge LSRs perform other than label switching? |
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Definition
| Edge LSRs also have the added job of adding or removing labels, with the normal task of label switching. |
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Term
| How is the distribution of labels performed on a label switching router? |
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Definition
| The distribution of labels is performed by a label distribution protocol such as LDP. LDP populates the LFIB in the data plane to maintain label switching information. |
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Term
| What happens to a labeled packet when there is a lack of an LFIB entry for it? |
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Definition
| The labeled packet is dropped, even if the destination exists in the routing table. Similarly, a received IP packet might be dropped if there is no routing entry in the routing table even if the entry does exist inthe LFIB for the destination. |
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Term
| Describe the function of the label information base (LIB) in a label-switching router (LSR). |
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Definition
| A LIB contains labels assigned to each network designation represented in the routing table. These locally significant labels are then anounced to an LSR's adjacent peers, who, in turn, propagate the information to their peers. |
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Term
| What does a label-switching router do with the received labels that it gets from its peers? |
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Definition
| This information is stored in the forwarding information base (FIB) and label forwarding information base (LFIB). Peers use received label information to associate next-hop label information with network destinations. |
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Term
| Where are IP prefixes that are associated with their local and next-hop label entries learned from peers stored. |
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Definition
| These are stored in the label information base (LIB). The LIB is part of the control plane and provides the database used by LDP for label distribution. |
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