OSPF within what area is an OSPF router considered to be

OSPF


[OSPF](/ospf), by design, prevents [routing](/routing) loops when exchanging summary (Type 3) LSAs.  This is done using several principles when exchanging inter-area routes between the [backbone area](/ospf-backbone-area-0) and other areas.

In OSPF, the backbone area is used for exchanging inter-area routes between all other areas.  Since there is no common topology shared among different areas, loop prevention is  based on [distance-vector](/routing-distance-vector-routing-protocols) principles.

There are three main rules of generating and receiving inter-area routes (type-3 LSAs) in OSPF that prevent control-plane routing loops:

1) Area Border Router (ABR) is a router that has at least one interface in Area 0 and this interface is not in a down state. ABRs will set the B (border) bit in their router LSAs to signal other routers in the same area of their ABR status. Only ABRs are allowed to generate summary LSAs and inject them in the attached areas.
2) ABRs expect summary LSAs from Area 0 only. This means there should be at least one adjacency in a FULL state built over an Area 0 interface. If an ABR has such an adjacency, it will ignore summary-LSAs received over non-backbone areas. These LSAs will be installed in the database, but will not be used for SPF calculations.
3) ABRs will accept and use summary-LSAs learned over non-backbone area they do not have a FULL adjacency built over an Area 0 interface. It is safe to do so, since the ABR will not be able to flood the summary back into Area 0 creating routing loops.


Links:

https://cdn2.hubspot.net/hubfs/3985396/Blog/Loop-Prevention-in-OSPF.pdf

https://community.cisco.com/t5/switching/ospf-backbone-area-rule/td-p/2002048

OSPF ABR and Loop Prevention

Network - Monitoring multiple IPs of a single device with Observium

Network - Observium vs LibreNMS

Network - broadcast domain

Network - control plane

Network - data plane

Network Address Translation (NAT)

Network Collisions

Network Design - Spine and leaf architecture

Network Design - Switch oversubscription ratios

Network Design - Traffic Engineering

Network Design - on-premises and off-premises

Network Time Protocol (NTP)

Network design - confine VLANs to as few access switches as possible

Network planes

Networking - what is a socket

Networks - Overlay Network

Networks - Overlay Transport Virtualization (OTV)

Networks - Spanning Layer 2 networks across remote sites

Networks - Underlay Network

Next Generation Multicast Virtual Private Network (NG-MVPN)

Nexus - using vPC with HSRP

Nexus - what is vPC

Nexus lacp rate fast

Nexus

OSI Model

OSPF - Advertising point to multipoint networks

OSPF - Cost of summarized routes

OSPF - DR-BDR election per broadcast domain

OSPF - DR-BDR election priority of zero

OSPF - ECMP across two different areas

OSPF - Enabling authentication

OSPF - HMAC-SHA Extended Authentication

OSPF - Link State Updates (LSUs)

OSPF - MPLS SuperBackbone

OSPF - NBMA neighbors listen on 224.0.0.5

OSPF - NSSA P-bit

OSPF - Propagation of routes between areas

OSPF - Stub areas and allowed LSAs

OSPF - Type1 LSA

OSPF - Update pacing timers

OSPF - default-information originate always command

OSPF - distribute-list filtering

OSPF - manual neighbor configuration

OSPF - point-to-multipoint network type

OSPF - route filtering

OSPF - route summarization

OSPF - summary-address nssa-only on an ABR

OSPF - what triggers a DR-BDR election

OSPF ABR Type 3 LSA filtering using access lists

OSPF Authentication for v2 and v3

OSPF DR BDR election and neighbor adjacencies

OSPF DR BDR election criteria

OSPF DR BDR election process

OSPF Design - When to create new instance

OSPF Determining which LSA is newer

OSPF Discontiguous Area 0

OSPF E1 E2 N1 N2 routes

OSPF Forwarding Address

OSPF Hello packets unicast and multicast

OSPF IP Unnumbered

OSPF IS-IS comparison

OSPF Incremental SPF and receipt of LSAs

OSPF LSA Recursion

OSPF LSA Sequence Numbers

OSPF LSA Types

OSPF NSSA ABR advertises default route

OSPF Not So Stubby Area NSSA

OSPF Path selection

OSPF Routing Bit

OSPF SPF Scheduling and throttling timers

OSPF TTL Security Check

OSPF Troubleshooting

OSPF Type 4 LSA

OSPF Type 5 LSA

OSPF Type 7 LSA

OSPF Why it is not suitable for use on the Internet

OSPF advertising loopback network

OSPF area inactive

OSPF area notation

OSPF area type mismatch

OSPF backbone area 0

OSPF default metric values

OSPF design - when to create a new area

OSPF does not redistribute default route

OSPF downheap LSA

OSPF duplicate router IDs

OSPF extent of reconvergence process

OSPF immediate hello packet

OSPF in a GRE multipoint environment

OSPF loop prevention

OSPF master slave election

OSPF message options field

OSPF metric sum of outgoing interface costs

OSPF modifying the router ID

OSPF network command

OSPF network types

OSPF non directly connected neighbor adjacencies

OSPF packet types

OSPF point to point network type for Ethernet

OSPF ABR and Loop Prevention

Links to this page: