Cisco ENARSI: Enterprise Advanced Routing and Services: key terms

Flashcards for this exam Cisco ENARSI: Enterprise Advanced Routing and Services guide Class dates

• Address FamilyA BGP (or other protocol) configuration context identifying a type of network layer information, such as IPv4 unicast or L2VPN EVPN.Advanced
• Adj-RIB-InThe conceptual BGP table holding unprocessed routes received from a specific peer before inbound policy.Advanced
• Adjacency TableThe CEF table holding Layer 2 rewrite information for each next hop, built from ARP and other resolution protocols.Advanced
• Administrative DistanceA Cisco value (0-255) ranking the trustworthiness of route sources; the lowest AD wins when protocols offer the same prefix.Foundational
• Allowas-InA BGP option permitting a router to accept routes containing its own ASN in the AS_PATH.Advanced
• Anycast RPConfiguring the same RP address on multiple routers for redundancy and load sharing, synchronized by MSDP or PIM.Advanced
• Area Border Router (ABR)An OSPF router with interfaces in area 0 and at least one other area, which summarizes routes between them.Intermediate
• AS Path PrependingAdding extra copies of an ASN to the AS_PATH of advertised routes to make a path less attractive to other ASes.Intermediate
• AS_PATH AttributeA well-known mandatory BGP attribute listing the ASNs a route has traversed; used for loop prevention and path length comparison.Intermediate
• Auto-RPCisco-proprietary method for distributing RP information using candidate RP announcements to 224.0.1.39 and mapping agent discovery to 224.0.1.40.Advanced
• Autonomous System (AS)A set of networks under one administrative control with a single routing policy, identified in BGP by an AS number.Foundational
• Autonomous System Boundary Router (ASBR)An OSPF router that redistributes external routes into OSPF, originating Type 5 or Type 7 LSAs.Intermediate
• Autonomous System Number (ASN)A number identifying a BGP autonomous system: 2-byte (1-65535) or 4-byte (up to 4,294,967,295).Intermediate
• BGP Aggregate AddressA BGP command that creates a summary route when at least one component exists in the BGP table.Intermediate
• BGP Best Path SelectionBGP's ordered decision process for choosing one best path per prefix.Intermediate
• BGP CommunityAn optional transitive 32-bit BGP attribute (often written AS:value) used to tag routes for policy decisions.Intermediate
• BGP Conditional AdvertisementAdvertising a prefix to a peer only when another prefix is present (exist-map) or absent (non-exist-map) in the BGP table.Advanced
• BGP ConfederationSplitting one AS into multiple internal sub-ASes that run eBGP-like sessions among themselves, reducing iBGP mesh size.Advanced
• BGP Default-OriginateA BGP neighbor option that sends a default route to a specific peer, even if none exists in the BGP table.Intermediate
• BGP Full MeshThe requirement that every iBGP speaker peer with every other iBGP speaker unless route reflectors or confederations are used.Intermediate
• BGP Message TypesBGP's message types: OPEN, UPDATE, KEEPALIVE, NOTIFICATION and ROUTE-REFRESH.Intermediate
• BGP MultihomingConnecting an AS to two or more upstream links or providers using BGP for redundancy and traffic control.Intermediate
• BGP MultipathAllowing BGP to install multiple equal paths for a prefix for load sharing.Advanced
• BGP Neighbor StatesThe BGP finite state machine: Idle, Connect, Active, OpenSent, OpenConfirm and Established.Intermediate
• BGP Network StatementThe BGP command that advertises a prefix only if an exact match (prefix and mask) exists in the routing table.Intermediate
• BGP Path AttributesProperties attached to BGP routes, categorized as well-known mandatory, well-known discretionary, optional transitive and optional non-transitive.Intermediate
• BGP Peer GroupA Cisco IOS construct that applies the same outbound policy to many neighbors and generates updates once per group.Intermediate
• BGP Regular ExpressionPattern matching used in AS-path access lists to filter or select routes by AS_PATH contents.Intermediate
• BGP Route Reflector (RR)An iBGP router allowed to re-advertise iBGP-learned routes to its clients, removing the full-mesh requirement.Intermediate
• Bidirectional Forwarding Detection (BFD)A lightweight hello protocol that detects forwarding path failures in milliseconds and notifies routing protocols.Intermediate
• Bidirectional PIM (Bidir-PIM)A PIM variant with only shared trees where traffic flows up and down the tree to the RP, suited to many-to-many applications.Advanced
• Bootstrap Router (BSR)The standards-based PIMv2 mechanism for distributing RP-to-group mappings hop by hop.Advanced
• Border Gateway Protocol (BGP)The path-vector EGP (BGP-4) that exchanges reachability between autonomous systems and is the routing protocol of the Internet.Foundational
• Cisco Express Forwarding (CEF)Cisco's default topology-based switching mechanism using a precomputed FIB and adjacency table for fast forwarding.Intermediate
• Cisco IOS XECisco's Linux-based modular operating system for enterprise routers and switches such as Catalyst 8000 and 9000, running IOS as a daemon.Foundational
• Cluster ListAn optional non-transitive BGP attribute listing the cluster IDs of route reflectors a route has passed through.Advanced
• ConvergenceThe state in which all routers have consistent, accurate routing information after a topology change.Foundational
• Default-Information OriginateA command that makes OSPF (or other protocols) advertise a default route into the routing domain.Intermediate
• Designated Router (DR)The OSPF router elected on a multi-access segment to form full adjacencies with all others and originate the Type 2 LSA.Intermediate
• Diffusing Update Algorithm (DUAL)The EIGRP algorithm that guarantees loop-free paths and selects successors and feasible successors.Intermediate
• Discard RouteA route that silently drops matching packets, used to anchor aggregates and prevent loops (Junos discard next hop).Intermediate
• Distribute ListA Cisco command that filters routing updates in or out of a routing protocol using an ACL, prefix list or route map.Intermediate
• DMVPN Phase 3The DMVPN mode where the hub sends NHRP redirects and spokes install NHRP shortcut routes, allowing hub summarization.Advanced
• Dynamic Multipoint VPN (DMVPN)A Cisco hub-and-spoke VPN design using mGRE, NHRP and usually IPsec that can build dynamic spoke-to-spoke tunnels.Advanced
• eBGP MultihopA BGP option that raises the TTL of eBGP packets so peers need not be directly connected, such as loopback peering.Intermediate
• EIGRP AuthenticationMD5 or HMAC-SHA-256 authentication of EIGRP packets using key chains (MD5) or named-mode passwords.Intermediate
• EIGRP Composite MetricEIGRP's metric calculated from bandwidth and delay by default, with optional load and reliability via K values.Intermediate
• EIGRP Hello and Hold TimersEIGRP sends hellos every 5 seconds (60 on slow NBMA) and declares a neighbor down after the hold time (15 or 180 seconds).Intermediate
• EIGRP Named ModeHierarchical EIGRP configuration that groups all IPv4 and IPv6 settings under address families in one process.Intermediate
• EIGRP Packet TypesThe five EIGRP packet types: Hello, Update, Query, Reply and Acknowledgment (plus SIA-Query and SIA-Reply).Intermediate
• EIGRP StubAn EIGRP router configured to advertise only selected routes and to be excluded from query propagation.Intermediate
• EIGRP Summary RouteA manually configured EIGRP summary advertised out an interface, which installs a Null0 discard route with AD 5.Intermediate
• EIGRP Topology TableThe EIGRP table storing all learned routes with their FD and RD, including successors and feasible successors.Intermediate
• EIGRP VarianceAn EIGRP multiplier that allows feasible successors with an FD up to variance times the best FD to be used for load sharing.Intermediate
• EIGRP Wide Metrics64-bit EIGRP metrics used in named mode to differentiate high-speed links, with a RIB scaling factor.Advanced
• Enhanced Interior Gateway Routing Protocol (EIGRP)Cisco-developed advanced distance-vector IGP using DUAL for loop-free fast convergence; published as informational RFC 7868.Foundational
• Equal-Cost Multipath (ECMP)Installing and load sharing across multiple routes to the same prefix that have identical cost.Intermediate
• External BGP (eBGP)A BGP session between routers in different autonomous systems.Foundational
• Feasibility Condition (FC)EIGRP's loop-free test: a neighbor qualifies as a feasible successor only if its reported distance is less than the current feasible distance.Intermediate
• Feasible Distance (FD)The lowest total EIGRP metric from the local router to a destination, through the successor.Intermediate
• Feasible Successor (FS)An EIGRP backup neighbor whose reported distance is lower than the current feasible distance, guaranteeing a loop-free path.Intermediate
• Forwarding Information Base (FIB)The data-plane table, derived from the RIB, used to forward packets at high speed.Intermediate
• Generalized TTL Security Mechanism (GTSM)Protection that accepts BGP packets only if their TTL is at or near 255, blocking spoofed sessions from distant attackers.Advanced
• Graceful RestartA mechanism letting neighbors keep forwarding through a restarting router's control plane using preserved forwarding state.Advanced
• Incremental SPF (iSPF)An optimization where only the affected portion of the SPF tree is recalculated after a change.Advanced
• Internal BGP (iBGP)A BGP session between routers in the same autonomous system.Intermediate
• IP MulticastOne-to-many or many-to-many delivery where a source sends a single stream to a group address and routers replicate it only where receivers exist.Foundational
• IP SLACisco IOS feature that generates synthetic probes (ICMP, UDP jitter, HTTP) to measure reachability and performance.Intermediate
• K ValuesThe EIGRP metric weights K1-K5 (plus K6 in wide metrics) that determine which components are used in the metric.Intermediate
• Key ChainA Cisco construct holding one or more numbered keys with optional lifetimes, used for routing protocol authentication.Intermediate
• Link-State Advertisement (LSA)An OSPF data unit describing a router's links, a network, or external and summary prefixes, flooded to build the LSDB.Intermediate
• Link-State Database (LSDB)The collection of LSAs a link-state router holds, identical among all routers in the same area.Intermediate
• Local Preference (LOCAL_PREF)A well-known discretionary BGP attribute shared among iBGP peers; the highest value is preferred for outbound traffic from the AS.Intermediate
• Locator/ID Separation Protocol (LISP)A protocol that separates endpoint identity (EID) from location (RLOC) using a mapping system, used as the SD-Access control plane.Advanced
• Loop-Free Alternate (LFA)A precomputed backup next hop that is guaranteed loop-free, enabling sub-50 ms IGP fast reroute.Advanced
• Maximum-PrefixA BGP neighbor limit on the number of prefixes accepted, protecting against route leaks and table overflows.Advanced
• MPLS Layer 3 VPN (L3VPN)A provider service (RFC 4364) that carries customer IP routes in separate VRFs across an MPLS core using MP-BGP.Advanced
• Multi-Exit Discriminator (MED)An optional non-transitive BGP attribute that suggests to a neighboring AS which entry point to use; lower is preferred.Intermediate
• Multicast Listener Discovery (MLD)The IPv6 equivalent of IGMP, carried in ICMPv6, for hosts to signal multicast group membership.Intermediate
• Multicast Source Discovery Protocol (MSDP)A TCP-based protocol (port 639) that shares active source information between RPs in different PIM domains or anycast RP sets.Advanced
• Multipoint GRE (mGRE)A GRE tunnel interface with no fixed destination that can reach many endpoints, used as the DMVPN overlay.Advanced
• Multiprotocol BGP (MP-BGP)Extensions to BGP (RFC 4760) that carry routing information for multiple address families such as IPv6, VPNv4, EVPN and multicast.Advanced
• Mutual RedistributionRedistributing routes in both directions between two routing protocols, a common source of loops when done at multiple points.Advanced
• Next Hop Resolution Protocol (NHRP)A protocol that maps overlay tunnel addresses to underlay NBMA addresses, acting like ARP for DMVPN.Advanced
• Next-Hop-SelfA BGP neighbor option that rewrites the NEXT_HOP to the router's own address when advertising to iBGP peers.Intermediate
• NEXT_HOP AttributeA well-known mandatory BGP attribute giving the IP address to forward toward for a prefix.Intermediate
• Not-So-Stubby Area (NSSA)An OSPF stub-like area that blocks Type 5 LSAs but allows a local ASBR to inject externals as Type 7 LSAs.Advanced
• Null0 InterfaceA virtual Cisco interface that discards traffic routed to it, used for summary and blackhole routes.Intermediate
• Object TrackingCisco feature that monitors the state of an interface, route or IP SLA and lets other features react to it.Intermediate
• Open Shortest Path First (OSPF)An open-standard link-state IGP that uses areas, LSAs and Dijkstra's SPF algorithm with a cost metric.Foundational
• Origin AttributeA well-known mandatory BGP attribute indicating how a route entered BGP: IGP (i), EGP (e) or incomplete (?).Intermediate
• Originator IDAn optional non-transitive BGP attribute set by a route reflector to the router ID of the route's original iBGP source.Advanced
• OSPF Area RangeInter-area summarization configured on an ABR to advertise one Type 3 LSA for a block of intra-area routes.Intermediate
• OSPF AuthenticationMethods for authenticating OSPF packets: null, plaintext, MD5 and HMAC-SHA with key chains.Intermediate
• OSPF External Route TypeOSPF E1 and E2 routes: E2 (default) keeps only the redistributed metric, while E1 adds internal cost to reach the ASBR.Intermediate
• OSPF Forwarding AddressA field in Type 5 and Type 7 LSAs that tells routers which next-hop address to use for an external route.Advanced
• OSPF Hello and Dead IntervalsOSPF timers: how often hellos are sent and how long to wait without hellos before declaring a neighbor down.Foundational
• OSPF LSA FilteringTechniques that block specific LSAs from entering an area, such as filter-list on an ABR or database-filter on an interface.Advanced
• OSPF MTU MismatchA condition where OSPF neighbors have different interface MTUs, leaving the adjacency stuck in ExStart or Exchange.Intermediate
• OSPF Neighbor StatesThe progression an OSPF adjacency follows: Down, Attempt, Init, 2-Way, ExStart, Exchange, Loading, Full.Intermediate
• OSPF Network TypesOSPF interface modes - broadcast, non-broadcast, point-to-point, point-to-multipoint and loopback - controlling DR election, timers and neighbor discovery.Intermediate
• OSPF Packet TypesThe five OSPF packet types: Hello, Database Description (DBD), Link-State Request (LSR), Link-State Update (LSU) and Link-State Acknowledgment.Intermediate
• OSPF Path Selection OrderOSPF prefers intra-area over inter-area over E1/N1 over E2/N2 routes for the same prefix, before comparing cost.Intermediate
• OSPF Stub AreaAn OSPF area that blocks Type 4 and Type 5 external LSAs and receives a default route from the ABR instead.Intermediate
• OSPF Stub Router AdvertisementOSPF max-metric router-lsa: advertising maximum link cost so other routers avoid transiting this router.Advanced
• OSPF Summary AddressAn ASBR command that summarizes external (Type 5 or Type 7) routes redistributed into OSPF.Intermediate
• OSPF Totally Stubby AreaA Cisco OSPF area type that blocks Type 3, 4 and 5 LSAs, leaving only a default route from the ABR.Intermediate
• OSPF Type 1 Router LSAThe LSA every OSPF router originates in each area, describing its links, costs and neighbors; flooded within the area only.Intermediate
• OSPF Type 2 Network LSAThe LSA originated by the DR on a multi-access segment, listing all routers attached to that transit network.Intermediate
• OSPF Type 3 Summary LSAThe LSA an ABR originates to advertise prefixes from one area into another.Intermediate
• OSPF Type 4 ASBR Summary LSAAn LSA originated by an ABR to tell routers in another area how to reach an ASBR.Advanced
• OSPF Type 5 External LSAAn LSA originated by an ASBR to advertise routes redistributed from outside OSPF, flooded throughout all normal areas.Intermediate
• OSPF Type 7 NSSA LSAAn external LSA originated by an ASBR inside an NSSA, converted to Type 5 by the NSSA ABR.Advanced
• OSPF Virtual LinkA logical link through a transit area that connects a remote area or partitioned backbone to area 0.Advanced
• OSPFv3The version of OSPF for IPv6 (and, with address families, IPv4) defined in RFC 5340.Intermediate
• Passive InterfaceAn interface on which a routing protocol advertises the connected network but does not send hellos or form adjacencies.Foundational
• Path-Vector Routing ProtocolA routing protocol class that advertises the full sequence of autonomous systems a route has traversed, used for loop prevention.Intermediate
• PIM Dense Mode (PIM-DM)A push-model multicast protocol that floods traffic everywhere and prunes branches without receivers.Intermediate
• PIM Designated RouterThe PIM router elected on a multiaccess segment (highest DR priority, then highest IP) to send joins and registers for local hosts.Advanced
• PIM Sparse Mode (PIM-SM)A pull-model multicast protocol where routers explicitly join a shared tree rooted at a rendezvous point and may switch to a source tree.Intermediate
• Policy-Based Routing (PBR)Forwarding packets based on criteria other than destination, such as source address or application, using a route map.Intermediate
• Prefix ListA Cisco filter that matches routes by prefix and prefix length range, preferred over ACLs for route filtering.Intermediate
• Private AS NumberASN ranges 64512-65534 (2-byte) and 4200000000-4294967294 (4-byte) reserved for private use and not routed on the Internet.Intermediate
• Protocol Independent Multicast (PIM)The family of multicast routing protocols that build distribution trees using the existing unicast routing table for RPF checks.Intermediate
• Recursive Route LookupResolving a route's next-hop address by performing another routing table lookup to find the outgoing interface.Intermediate
• Remote-Triggered Black Hole (RTBH)A DDoS mitigation technique where a BGP-signaled route sends attack traffic to a discard (Null0) next hop at the network edge.Advanced
• Rendezvous Point (RP)The PIM-SM router where sources and receivers meet; it roots the shared tree for a group.Intermediate
• Reported Distance (RD)The EIGRP metric a neighbor advertises for its own path to a destination; also called advertised distance.Intermediate
• Reverse Path Forwarding Check (RPF)The multicast loop-prevention check that accepts a packet only if it arrives on the interface the router would use to reach the source (or RP).Intermediate
• Route Flap DampeningA BGP mechanism that suppresses routes that repeatedly flap by assigning penalties that decay over time.Advanced
• Route LeakingSelectively copying routes between routing tables such as VRFs or IS-IS levels to allow controlled reachability.Advanced
• Route MapAn ordered Cisco IOS policy construct of permit/deny sequences with match and set clauses, used for filtering, redistribution, PBR and BGP policy.Intermediate
• Route RedistributionInjecting routes learned from one routing source into another routing protocol.Intermediate
• Route RefreshA BGP capability that lets a router ask a peer to resend its routes so new inbound policy can be applied without resetting the session.Intermediate
• Route SummarizationAdvertising a single aggregate prefix that covers multiple more specific routes to reduce table size and contain instability.Intermediate
• Route TagA numeric label attached to a route that carries through redistribution and can be matched by policy.Intermediate
• Router ID (RID)A 32-bit identifier, written as an IPv4 address, that uniquely identifies a router within OSPF, EIGRP or BGP.Foundational
• Routing Information Base (RIB)The control-plane table holding the best routes chosen from all routing sources, used to build the forwarding table.Intermediate
• Seed MetricThe initial metric assigned to routes redistributed into a routing protocol.Intermediate
• Shared TreeA multicast distribution tree rooted at the RP and used by all sources for a group, written (*,G).Intermediate
• Shortest Path First Algorithm (SPF)Dijkstra's algorithm, used by OSPF and IS-IS to compute a loop-free shortest path tree from the LSDB.Intermediate
• Shortest Path Tree (SPT)A multicast source tree rooted at the source, written (S,G), giving the optimal path from source to receivers.Intermediate
• Soft ReconfigurationA BGP option that stores unmodified inbound routes from a neighbor so policy can be re-applied locally.Advanced
• Source-Specific Multicast (SSM)A PIM mode in which receivers join a specific (S,G) channel directly, eliminating the RP and shared tree.Intermediate
• Split HorizonA loop-prevention rule that stops a router from advertising a route out the interface on which it was learned.Intermediate
• Stuck in Active (SIA)An EIGRP condition where a router waits too long for replies to queries, eventually resetting neighbor adjacencies.Advanced
• SuccessorThe EIGRP neighbor offering the lowest-metric loop-free path to a destination; its route is installed in the routing table.Intermediate
• Transit ASAn autonomous system that carries traffic between two other autonomous systems.Intermediate
• Unequal-Cost Load BalancingDistributing traffic across paths with different metrics, natively supported by EIGRP through the variance command.Intermediate
• Unicast Reverse Path Forwarding (uRPF)A feature that drops packets whose source address is not reachable via the routing table, mitigating spoofing.Advanced
• Update-SourceA BGP neighbor option that sets the source interface (usually a loopback) for the TCP session.Intermediate
• Virtual Routing and Forwarding (VRF)A technology that creates multiple independent routing tables on one router so overlapping or separated networks coexist.Intermediate
• VRF-LiteUsing VRFs for Layer 3 segmentation without MPLS or MP-BGP, by carrying each VRF on separate interfaces or subinterfaces hop by hop.Intermediate
• WeightA Cisco-proprietary BGP value local to one router; the highest weight is preferred first in best-path selection.Intermediate
• Well-Known BGP CommunitiesReserved communities with standard meaning: NO_EXPORT, NO_ADVERTISE, NO_EXPORT_SUBCONFED (local-AS) and others.Intermediate