RFC 7868
Cisco's Enhanced Interior Gateway Routing Protocol (EIGRP)
Independent Submission D. Savage Request for Comments: 7868 J. Ng Category: Informational S. Moore ISSN: 2070-1721 Cisco Systems D. Slice Cumulus Networks P. Paluch University of Zilina R. White LinkedIn May 2016 Cisco's Enhanced Interior Gateway Routing Protocol (EIGRP)Abstract
This document describes the protocol design and architecture for Enhanced Interior Gateway Routing Protocol (EIGRP). EIGRP is a routing protocol based on Distance Vector technology. The specific algorithm used is called "DUAL", a Diffusing Update Algorithm as referenced in "Loop-Free Routing Using Diffusing Computations" (Garcia-Luna-Aceves 1993). The algorithm and procedures were researched, developed, and simulated by SRI International. Status of This Memo This document is not an Internet Standards Track specification; it is published for informational purposes. This is a contribution to the RFC Series, independently of any other RFC stream. The RFC Editor has chosen to publish this document at its discretion and makes no statement about its value for implementation or deployment. Documents approved for publication by the RFC Editor are not a candidate for any level of Internet Standard; see Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7868.
Copyright Notice Copyright (c) 2016 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. This document may not be modified, and derivative works of it may not be created, except to format it for publication as an RFC or to translate it into languages other than English.Table of Contents
1. Introduction ....................................................5 2. Conventions .....................................................5 2.1. Requirements Language ......................................5 2.2. Terminology ................................................5 3. The Diffusing Update Algorithm (DUAL) ...........................9 3.1. Algorithm Description ......................................9 3.2. Route States ..............................................10 3.3. Feasibility Condition .....................................11 3.4. DUAL Message Types ........................................13 3.5. DUAL Finite State Machine (FSM) ...........................13 3.6. DUAL Operation -- Example Topology ........................18 4. EIGRP Packets ..................................................20 4.1. UPDATE Packets ............................................21 4.2. QUERY Packets .............................................21 4.3. REPLY Packets .............................................22 4.4. Exception Handling ........................................22 4.4.1. Active Duration (SIA) ..............................22 4.4.1.1. SIA-QUERY .................................23 4.4.1.2. SIA-REPLY .................................24 5. EIGRP Operation ................................................25 5.1. Finite State Machine ......................................25 5.2. Reliable Transport Protocol ...............................25 5.2.1. Bandwidth on Low-Speed Links .......................32 5.3. Neighbor Discovery/Recovery ...............................32 5.3.1. Neighbor Hold Time .................................32 5.3.2. HELLO Packets ......................................33 5.3.3. UPDATE Packets .....................................33 5.3.4. Initialization Sequence ............................34 5.3.5. Neighbor Formation .................................35 5.3.6. QUERY Packets during Neighbor Formation ............35
5.4. Topology Table ............................................36
5.4.1. Route Management ...................................36
5.4.1.1. Internal Routes ...........................37
5.4.1.2. External Routes ...........................37
5.4.2. Split Horizon and Poison Reverse ...................38
5.4.2.1. Startup Mode ..............................38
5.4.2.2. Advertising Topology Table Change .........39
5.4.2.3. Sending a QUERY/UPDATE ....................39
5.5. EIGRP Metric Coefficients .................................39
5.5.1. Coefficients K1 and K2 .............................40
5.5.2. Coefficient K3 .....................................40
5.5.3. Coefficients K4 and K5 .............................40
5.5.4. Coefficient K6 .....................................41
5.5.4.1. Jitter ....................................41
5.5.4.2. Energy ....................................41
5.6. EIGRP Metric Calculations .................................41
5.6.1. Classic Metrics ....................................41
5.6.1.1. Classic Composite Formulation .............42
5.6.1.2. Cisco Interface Delay Compatibility .......43
5.6.2. Wide Metrics .......................................43
5.6.2.1. Wide Metric Vectors .......................44
5.6.2.2. Wide Metric Conversion Constants ..........45
5.6.2.3. Throughput Calculation ....................45
5.6.2.4. Latency Calculation .......................46
5.6.2.5. Composite Calculation .....................46
6. EIGRP Packet Formats ...........................................46
6.1. Protocol Number ...........................................46
6.2. Protocol Assignment Encoding ..............................47
6.3. Destination Assignment Encoding ...........................47
6.4. EIGRP Communities Attribute ...............................48
6.5. EIGRP Packet Header .......................................49
6.6. EIGRP TLV Encoding Format .................................51
6.6.1. Type Field Encoding ................................52
6.6.2. Length Field Encoding ..............................52
6.6.3. Value Field Encoding ...............................52
6.7. EIGRP Generic TLV Definitions .............................52
6.7.1. 0x0001 - PARAMETER_TYPE ............................53
6.7.2. 0x0002 - AUTHENTICATION_TYPE .......................53
6.7.2.1. 0x02 - MD5 Authentication Type ............54
6.7.2.2. 0x03 - SHA2 Authentication Type ...........54
6.7.3. 0x0003 - SEQUENCE_TYPE .............................54
6.7.4. 0x0004 - SOFTWARE_VERSION_TYPE .....................55
6.7.5. 0x0005 - MULTICAST_SEQUENCE_TYPE ...................55
6.7.6. 0x0006 - PEER_INFORMATION_TYPE .....................55
6.7.7. 0x0007 - PEER_ TERMINATION_TYPE ....................56
6.7.8. 0x0008 - TID_LIST_TYPE .............................56
6.8. Classic Route Information TLV Types .......................57
6.8.1. Classic Flag Field Encoding ........................57
6.8.2. Classic Metric Encoding ............................57
6.8.3. Classic Exterior Encoding ..........................58
6.8.4. Classic Destination Encoding .......................59
6.8.5. IPv4-Specific TLVs .................................59
6.8.5.1. IPv4 INTERNAL_TYPE ........................60
6.8.5.2. IPv4 EXTERNAL_TYPE ........................60
6.8.5.3. IPv4 COMMUNITY_TYPE .......................62
6.8.6. IPv6-Specific TLVs .................................62
6.8.6.1. IPv6 INTERNAL_TYPE ........................63
6.8.6.2. IPv6 EXTERNAL_TYPE ........................63
6.8.6.3. IPv6 COMMUNITY_TYPE .......................65
6.9. Multiprotocol Route Information TLV Types .................66
6.9.1. TLV Header Encoding ................................66
6.9.2. Wide Metric Encoding ...............................67
6.9.3. Extended Metrics ...................................68
6.9.3.1. 0x00 - NoOp ...............................69
6.9.3.2. 0x01 - Scaled Metric ......................70
6.9.3.3. 0x02 - Administrator Tag ..................70
6.9.3.4. 0x03 - Community List .....................71
6.9.3.5. 0x04 - Jitter .............................71
6.9.3.6. 0x05 - Quiescent Energy ...................71
6.9.3.7. 0x06 - Energy .............................72
6.9.3.8. 0x07 - AddPath ............................72
6.9.3.8.1. AddPath with IPv4 Next Hop .....73
6.9.3.8.2. AddPath with IPv6 Next Hop .....74
6.9.4. Exterior Encoding ..................................75
6.9.5. Destination Encoding ...............................76
6.9.6. Route Information ..................................76
6.9.6.1. INTERNAL TYPE .............................76
6.9.6.2. EXTERNAL TYPE .............................76
7. Security Considerations ........................................77
8. IANA Considerations ............................................77
9. References .....................................................77
9.1. Normative References ......................................77
9.2. Informative References ....................................78
Acknowledgments ...................................................79
Authors' Addresses ................................................80
1. Introduction
This document describes the Enhanced Interior Gateway Routing Protocol (EIGRP), a routing protocol designed and developed by Cisco Systems, Inc. DUAL, the algorithm used to converge the control plane to a single set of loop-free paths is based on research conducted at SRI International [3]. The Diffusing Update Algorithm (DUAL) is the algorithm used to obtain loop freedom at every instant throughout a route computation [2]. This allows all routers involved in a topology change to synchronize at the same time; the routers not affected by topology changes are not involved in the recalculation. This document describes the protocol that implements these functions.2. Conventions
2.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [1].2.2. Terminology
The following is a list of abbreviations and terms used throughout this document: ACTIVE State: The local state of a route on a router triggered by any event that causes all neighbors providing the current least-cost path to fail the Feasibility Condition check. A route in Active state is considered unusable. During Active state, the router is actively attempting to compute the least-cost loop-free path by explicit coordination with its neighbors using Query and Reply messages. Address Family Identifier (AFI): Identity of the network-layer protocol reachability information being advertised [12]. Autonomous System (AS): A collection of routers exchanging routes under the control of one or more network administrators on behalf of a single administrative entity.
Base Topology:
A routing domain representing a physical (non-virtual) view of the
network topology consisting of attached devices and network
segments EIGRP uses to form neighbor relationships. Destinations
exchanged within the Base Topology are identified with a Topology
Identifier value of zero (0).
Computed Distance (CD):
Total distance (metric) along a path from the current router to a
destination network through a particular neighbor computed using
that neighbor's Reported Distance (RD) and the cost of the link
between the two routers. Exactly one CD is computed and
maintained per the [Destination, Advertising Neighbor] pair.
CR-Mode
Conditionally Received Mode
Diffusing Computation:
A distributed computation in which a single starting node
commences the computation by delegating subtasks of the
computation to its neighbors that may, in turn, recursively
delegate sub-subtasks further, including a signaling scheme
allowing the starting node to detect that the computation has
finished while avoiding false terminations. In DUAL, the task of
coordinated updates of routing tables and resulting best path
computation is performed as a diffusing computation.
Diffusing Update Algorithm (DUAL):
A loop-free routing algorithm used with distance vectors or link
states that provides a diffused computation of a routing table.
It works very well in the presence of multiple topology changes
with low overhead. The technology was researched and developed at
SRI International [3].
Downstream Router:
A router that is one or more hops away from the router in question
in the direction of the destination.
EIGRP:
Enhanced Interior Gateway Routing Protocol.
Feasibility Condition:
The Feasibility Condition is a sufficient condition used by a
router to verify whether a neighboring router provides a loop-free
path to a destination. EIGRP uses the Source Node Condition
stating that a neighboring router meets the Feasibility Condition
if the neighbor's RD is less than this router's Feasible Distance.
Feasible Distance (FD):
Defined as the least-known total metric to a destination from the
current router since the last transition from ACTIVE to PASSIVE
state. Being effectively a record of the smallest known metric
since the last time the network entered the PASSIVE state, the FD
is not necessarily a metric of the current best path. Exactly one
FD is computed per destination network.
Feasible Successor:
A neighboring router that meets the Feasibility Condition for a
particular destination, hence, providing a guaranteed loop-free
path.
Neighbor/Peer:
For a particular router, another router toward which an EIGRP
session, also known as an "adjacency", is established. The
ability of two routers to become neighbors depends on their mutual
connectivity and compatibility of selected EIGRP configuration
parameters. Two neighbors with interfaces connected to a common
subnet are known as adjacent neighbors. Two neighbors that are
multiple hops apart are known as remote neighbors.
PASSIVE state:
The local state of a route in which at least one neighbor
providing the current least-cost path passes the Feasibility
Condition check. A route in PASSIVE state is considered usable
and not in need of a coordinated re-computation.
Network Layer Reachability Information (NLRI):
Information a router uses to calculate the global routing table to
make routing and forwarding decisions.
Reported Distance (RD):
For a particular destination, the value representing the router's
distance to the destination as advertised in all messages carrying
routing information. RD is not equivalent to the current distance
of the router to the destination and may be different from it
during the process of path re-computation. Exactly one RD is
computed and maintained per destination network.
Sub-Topology:
For a given Base Topology, a sub-topology is characterized by an
independent set of routers and links in a network for which EIGRP
performs an independent path calculation. This allows each sub-
topology to implement class-specific topologies to carry class-
specific traffic.
Successor:
For a particular destination, a neighboring router that meets the
Feasibility Condition and, at the same time, provides the least-
cost path.
Stuck In Active (SIA):
A destination that has remained in the ACTIVE State in excess of a
predefined time period at the local router (Cisco implements this
as 3 minutes).
Successor-Directed Acyclic Graph (SDAG):
For a particular destination, a graph defined by routing table
contents of individual routers in the topology, such that nodes of
this graph are the routers themselves and a directed edge from
router X to router Y exists if and only if router Y is router X's
successor. After the network has converged, in the absence of
topological changes, SDAG is a tree.
Topology Change / Topology-Change Event:
Any event that causes the CD for a destination through a neighbor
to be added, modified, or removed. As an example, detecting a
link-cost change, receiving any EIGRP message from a neighbor
advertising an updated neighbor's RD.
Topology Identifier (TID):
A number that is used to mark prefixes as belonging to a specific
sub-topology.
Topology Table:
A data structure used by EIGRP to store information about every
known destination including, but not limited to, network prefix /
prefix length, FD, RD of each neighbor advertising the
destination, CD over the corresponding neighbor, and route state.
Type, Length, Value (TLV):
An encoding format for information elements used in EIGRP messages
to exchange information. Each TLV-formatted information element
consists of three generic fields: Type identifying the nature of
information carried in this element, Length describing the length
of the entire TLV triplet, and Value carrying the actual
information. The Value field may, itself, be internally
structured; this depends on the actual type of the information
element. This format allows for extensibility and backward
compatibility.
Upstream Router:
A router that is one or more hops away from the router in
question, in the direction of the source of the information.
VID:
VLAN Identifier
Virtual Routing and Forwarding (VRF):
Independent Virtual Private Network (VPN) routing/forwarding
tables that coexist within the same router at the same time.
3. The Diffusing Update Algorithm (DUAL)
The Diffusing Update Algorithm (DUAL) constructs least-cost paths to
all reachable destinations in a network consisting of nodes and edges
(routers and links). DUAL guarantees that each constructed path is
loop free at every instant including periods of topology changes and
network reconvergence. This is accomplished by all routers, which
are affected by a topology change, computing the new best path in a
coordinated (diffusing) way and using the Feasibility Condition to
verify prospective paths for loop freedom. Routers that are not
affected by topology changes are not involved in the recalculation.
The convergence time with DUAL rivals that of any other existing
routing protocol.
3.1. Algorithm Description
DUAL is used by EIGRP to achieve fast loop-free convergence with
little overhead, allowing EIGRP to provide convergence rates
comparable, and in some cases better than, most common link state
protocols [10]. Only nodes that are affected by a topology change
need to propagate and act on information about the topology change,
allowing EIGRP to have good scaling properties, reduced overhead, and
lower complexity than many other interior gateway protocols.
Distributed routing algorithms are required to propagate information
as well as coordinate information among all nodes in the network.
Unlike basic Bellman-Ford distance vector protocols that rely on
uncoordinated updates when a topology change occurs, DUAL uses a
coordinated procedure to involve the affected part of the network
into computing a new least-cost path, known as a "diffusing
computation". A diffusing computation grows by querying additional
routers for their current RD to the affected destination, and it
shrinks by receiving replies from them. Unaffected routers send
replies immediately, terminating the growth of the diffusing
computation over them. These intrinsic properties cause the
diffusing computation to self-adjust in scope and terminate as soon
as possible.
One attribute of DUAL is its ability to control the point at which
the diffusion of a route calculation terminates by managing the
distribution of reachability information through the network.
Controlling the scope of the diffusing process is accomplished by hiding reachability information through aggregation (summarization), filtering, or other means. This provides the ability to create effective failure domains within a single AS, and allows the network administrator to manage the convergence and processing characteristics of the network.3.2. Route States
A route to a destination can be in one of two states: PASSIVE or ACTIVE. These states describe whether the route is guaranteed to be both loop free and the shortest available (the PASSIVE state) or whether such a guarantee cannot be given (the ACTIVE state). Consequently, in PASSIVE state, the router does not perform any route recalculation in coordination with its neighbors because no such recalculation is needed. In ACTIVE state, the router is actively involved in re-computing the least-cost loop-free path in coordination with its neighbors. The state is reevaluated and possibly changed every time a topology change is detected. A topology change is any event that causes the CD to the destination over any neighbor to be added, changed, or removed from EIGRP's topology table. More exactly, the two states are defined as follows: o Passive A route is considered to be in the Passive state when at least one neighbor that provides the current least-total-cost path passes the Feasibility Condition check that guarantees loop freedom. A route in the PASSIVE state is usable and its next hop is perceived to be a downstream router. o Active A route is considered to be in the ACTIVE state if neighbors that do not pass the Feasibility Condition check provide lowest-cost path, and therefore the path cannot be guaranteed loop free. A route in the ACTIVE state is considered unusable and this router must coordinate with its neighbors in the search for the new loop- free least-total-cost path. In other words, for a route to be in PASSIVE state, at least one neighbor that provides the least-total-cost path must be a Feasible Successor. Feasible Successors providing the least-total-cost path are also called "successors". For a route to be in PASSIVE state, at least one successor must exist.
Conversely, if the path with the least total cost is provided by routers that are not Feasible Successors (and thus not successors), the route is in the ACTIVE state, requiring re-computation. Notably, for the definition of PASSIVE and ACTIVE states, it does not matter if there are Feasible Successors providing a worse-than-least- total-cost path. While these neighbors are guaranteed to provide a loop-free path, that path is potentially not the shortest available. The fact that the least-total-cost path can be provided by a neighbor that fails the Feasibility Condition check may not be intuitive. However, such a situation can occur during topology changes when the current least-total-cost path fails and the next-least-total-cost path traverses a neighbor that is not a Feasible Successor. While a router has a route in the ACTIVE state, it must not change its successor (i.e., modify the current SDAG) nor modify its own Feasible Distance or RD until the route enters the PASSIVE state again. Any updated information about this route received during ACTIVE state is reflected only in CDs. Any updates to the successor, FD, and RD are postponed until the route returns to PASSIVE state. The state transitions from PASSIVE to ACTIVE and from ACTIVE to PASSIVE are controlled by the DUAL FSM and are described in detail in Section 3.5.3.3. Feasibility Condition
The Feasibility Condition is a criterion used to verify loop freedom of a particular path. The Feasibility Condition is a sufficient but not a necessary condition, meaning that every path meeting the Feasibility Condition is guaranteed to be loop free; however, not all loop-free paths meet the Feasibility Condition. The Feasibility Condition is used as an integral part of DUAL operation: every path selection in DUAL is subject to the Feasibility Condition check. Based on the result of the Feasibility Condition check after a topology change is detected, the route may either remain PASSIVE (if, after the topology change, the neighbor providing the least cost path meets the Feasibility Condition) or it needs to enter the ACTIVE state (if the topology change resulted in none of the neighbors providing the least cost path to meet the Feasibility Condition). The Feasibility Condition is a part of DUAL that allows the diffused computation to terminate as early as possible. Nodes that are not affected by the topology change are not required to perform a DUAL computation and may not be aware a topology change occurred. This can occur in two cases:
First, if informed about a topology change, a router may keep a route
in PASSIVE state if it is aware of other paths that are downstream
towards the destination (routes meeting the Feasibility Condition).
A route that meets the Feasibility Condition is determined to be loop
free and downstream along the path between the router and the
destination.
Second, if informed about a topology change for which it does not
currently have reachability information, a router is not required to
enter into the ACTIVE state, nor is it required to participate in the
DUAL process.
In order to facilitate describing the Feasibility Condition, a few
definitions are in order.
o A successor for a given route is the next hop used to forward data
traffic for a destination. Typically, the successor is chosen
based on the least-cost path to reach the destination.
o A Feasible Successor is a neighbor that meets the Feasibility
Condition. A Feasible Successor is regarded as a downstream
neighbor towards the destination, but it may not be the least-cost
path but could still be used for forwarding data packets in the
event equal or unequal cost load sharing was active. A Feasible
Successor can become a successor when the current successor
becomes unreachable.
o The Feasibility Condition is met when a neighbor's advertised
cost, (RD) to a destination is less than the FD for that
destination, or in other words, the Feasibility Condition is met
when the neighbor is closer to the destination than the router
itself has ever been since the destination has entered the PASSIVE
state for the last time.
o The FD is the lowest distance to the destination since the last
time the route went from ACTIVE to PASSIVE state. It should be
noted it is not necessarily the current best distance; rather, it
is a historical record of the best distance known since the last
diffusing computation for the destination has finished. Thus, the
value of the FD can either be the same as the current best
distance, or it can be lower.
A neighbor that advertises a route with a cost that does not meet the
Feasibility Condition may be upstream and thus cannot be guaranteed
to be the next hop for a loop-free path. Routes advertised by
upstream neighbors are not recorded in the routing table but saved in
the topology table.
3.4. DUAL Message Types
DUAL operates with three basic message types: QUERY, UPDATE, and REPLY. o UPDATE - sent to indicate a change in metric or an addition of a destination. o QUERY - sent when the Feasibility Condition fails, which can happen for reasons like a destination becoming unreachable or the metric increasing to a value greater than its current FD. o REPLY - sent in response to a QUERY or SIA-QUERY In addition to these three basic types, two additional sub-types have been added to EIGRP: o SIA-QUERY - sent when a REPLY has not been received within one- half of the SIA interval (90 seconds as implemented by Cisco). o SIA-REPLY - sent in response to an SIA-QUERY indicating the route is still in ACTIVE state. This response does not stratify the original QUERY; it is only used to indicate that the sending neighbor is still in the ACTIVE state for the given destination. When in the PASSIVE state, a received QUERY may be propagated if there is no Feasible Successor found. If a Feasible Successor is found, the QUERY is not propagated and a REPLY is sent for the destination with a metric equal to the current routing table metric. When a QUERY is received from a non-successor in ACTIVE state, a REPLY is sent and the QUERY is not propagated. The REPLY for the destination contains a metric equal to the current routing table metric.3.5. DUAL Finite State Machine (FSM)
The DUAL FSM embodies the decision process for all route computations. It tracks all routes advertised by all neighbors. The distance information, known as a metric, is used by DUAL to select efficient loop-free paths. DUAL selects routes to be inserted into a routing table based on Feasible Successors. A successor is a neighboring router used for packet forwarding that has a least-cost path to a destination that is guaranteed not to be part of a routing loop. When there are no Feasible Successors but there are neighbors advertising the destination, a recalculation must occur to determine a new successor.
The amount of time it takes to calculate the route impacts the convergence time. Even though the recalculation is not processor intensive, it is advantageous to avoid recalculation if it is not necessary. When a topology change occurs, DUAL will test for Feasible Successors. If there are Feasible Successors, it will use any it finds in order to avoid any unnecessary recalculation. The FSM, which applies per destination in the topology table, operates independently for each destination. It is true that if a single link goes down, multiple routes may go into ACTIVE state. However, a separate SDAG is computed for each destination, so loop- free topologies can be maintained for each reachable destination.
+------------+ +-----------+ | \ / | | \ / | | +=================================+ | | | | | |(1)| Passive |(2)| +-->| |<--+ +=================================+ ^ | ^ ^ ^ | (14)| |(15)| |(13)| | | (4)| |(16)| | (3)| | | | | | +------------+ | | | | | \ +-------+ + + | +-------------+ \ / / / | \ \ / / / +----+ \ \ | | | | | | | v | | | v +==========+(11) +==========+ +==========+(12) +==========+ | Active |---->| Active |(5) | Active |---->| Active | | | (9)| |---->| | (10)| | | oij=0 |<----| oij=1 | | oij=2 |<----| oij=3 | +--| | +--| | +--| | +--| | | +==========+ | +==========+ | +==========+ | +==========+ | ^ |(5) | ^ | ^ ^ | ^ | | +-----|------|---------|----+ | | | +------+ +------+ +---------+ +---------+ (6,7,8) (6,7,8) (6,7,8) (6,7,8) Figure 1: DUAL Finite State Machine Legend: i Node that is computing route j Destination node or network k Any neighbor of node i oij QUERY origin flag 0 = metric increase during ACTIVE state 1 = node i originated 2 = QUERY from, or link increase to, successor during ACTIVE state 3 = QUERY originated from successor rijk REPLY status flag for each neighbor k for destination j 1 = awaiting REPLY 0 = received REPLY lik = the link connecting node i to neighbor k
The following describes in detail the state/event/action transitions
of the DUAL FSM. For all steps, the topology table is updated with
the new metric information from either QUERY, REPLY, or UPDATE
received.
(1) A QUERY is received from a neighbor that is not the current
successor. The route is currently in PASSIVE state. As the
successor is not affected by the QUERY, and a Feasible Successor
exists, the route remains in PASSIVE state. Since a Feasible
Successor exists, a REPLY MUST be sent back to the originator of
the QUERY. Any metric received in the QUERY from that neighbor
is recorded in the topology table and the Feasibility Check (FC)
is run to check for any change to current successor.
(2) A directly connected interface changes state (connects,
disconnects, or changes metric), or similarly an UPDATE or QUERY
has been received with a metric change for an existing
destination, the route will stay in the PASSIVE state if the
current successor is not affected by the change, or it is no
longer reachable and there is a Feasible Successor. In either
case, an UPDATE is sent with the new metric information if it
has changed.
(3) A QUERY was received from a neighbor who is the current
successor and no Feasible Successors exist. The route for the
destination goes into ACTIVE state. A QUERY is sent to all
neighbors on all interfaces that are not split horizon. Split
horizon takes effect for a query or update from the successor it
is using for the destination in the query. The QUERY origin
flag is set to indicate the QUERY originated from a neighbor
marked as successor for route. The REPLY status flag is set for
all neighbors to indicate outstanding replies.
(4) A directly connected link has gone down or its cost has
increased, or an UPDATE has been received with a metric
increase. The route to the destination goes to ACTIVE state if
there are no Feasible Successors found. A QUERY is sent to all
neighbors on all interfaces. The QUERY origin flag is to
indicate that the router originated the QUERY. The REPLY status
flag is set to 1 for all neighbors to indicate outstanding
replies.
(5) While a route for a destination is in ACTIVE state, and a QUERY
is received from the current successor, the route remains in
ACTIVE state. The QUERY origin flag is set to indicate that
there was another topology change while in ACTIVE state. This
indication is used so new Feasible Successors are compared to
the metric that made the route go to ACTIVE state with the
current successor.
(6) While a route for a destination is in ACTIVE state and a QUERY
is received from a neighbor that is not the current successor, a
REPLY should be sent to the neighbor. The metric received in
the QUERY should be recorded.
(7) If a link cost changes, or an UPDATE with a metric change is
received in ACTIVE state from a non-successor, the router stays
in ACTIVE state for the destination. The metric information in
the UPDATE is recorded. When a route is in the ACTIVE state,
neither a QUERY nor UPDATE are ever sent.
(8) If a REPLY for a destination, in ACTIVE state, is received from
a neighbor or the link between a router and the neighbor fails,
the router records that the neighbor replied to the QUERY. The
REPLY status flag is set to 0 to indicate this. The route stays
in ACTIVE state if there are more replies pending because the
router has not heard from all neighbors.
(9) If a route for a destination is in ACTIVE state, and a link
fails or a cost increase occurred between a router and its
successor, the router treats this case like it has received a
REPLY from its successor. When this occurs after the router
originates a QUERY, it sets the QUERY origin flag to indicate
that another topology change occurred in ACTIVE state.
(10) If a route for a destination is in ACTIVE state, and a link
fails or a cost increase occurred between a router and its
successor, the router treats this case like it has received a
REPLY from its successor. When this occurs after a successor
originated a QUERY, the router sets the QUERY origin flag to
indicate that another topology change occurred in ACTIVE state.
(11) If a route for a destination is in ACTIVE state, the cost of the
link through which the successor increases, and the last REPLY
was received from all neighbors, but there is no Feasible
Successor, the route should stay in ACTIVE state. A QUERY is
sent to all neighbors. The QUERY origin flag is set to 1.
(12) If a route for a destination is in ACTIVE state because of a
QUERY received from the current successor, and the last REPLY
was received from all neighbors, but there is no Feasible
Successor, the route should stay in ACTIVE state. A QUERY is
sent to all neighbors. The QUERY origin flag is set to 3.
(13) Received replies from all neighbors. Since the QUERY origin
flag indicates the successor originated the QUERY, it
transitions to PASSIVE state and sends a REPLY to the old
successor.
(14) Received replies from all neighbors. Since the QUERY origin
flag indicates a topology change to the successor while in
ACTIVE state, it need not send a REPLY to the old successor.
When the Feasibility Condition is met, the route state
transitions to PASSIVE.
(15) Received replies from all neighbors. Since the QUERY origin
flag indicates either the router itself originated the QUERY or
FC was not satisfied with the replies received in ACTIVE state,
FD is reset to infinite value and the minimum of all the
reported metrics is chosen as FD and route transitions back to
PASSIVE state. A REPLY is sent to the old-successor if oij
flags indicate that there was a QUERY from successor.
(16) If a route for a destination is in ACTIVE state because of a
QUERY received from the current successor or there was an
increase in distance while in ACTIVE state, the last REPLY was
received from all neighbors, and a Feasible Successor exists for
the destination, the route can go into PASSIVE state and a REPLY
is sent to the successor if oij indicates that QUERY was
received from the successor.
3.6. DUAL Operation -- Example Topology
The following topology (Figure 2) will be used to provide an example
of how DUAL is used to reroute after a link failure. Each node is
labeled with its costs to destination N. The arrows indicate the
successor (next hop) used to reach destination N. The least-cost
path is selected.
N | (1)A ---<--- B(2) | | ^ | | | (2)D ---<--- C(3) Figure 2: Stable Topology In the case where the link between A and D fails (Figure 3); N N | | A ---<--- B A ---<--- B | | | | X | ^ | | | | | D ---<--- C D ---<--- C Q-> <-R N | (1)A ---<--- B(2) | ^ | (4)D --->--- C(3) Figure 3: Link between A and D Fails Only observing the destination provided by node N, D enters the ACTIVE state and sends a QUERY to all its neighbors, in this case node C. C determines that it has a Feasible Successor and replies immediately with metric 3. C changes its old successor of D to its new single successor B and the route to N stays in PASSIVE state. D receives the REPLY and can transition out of ACTIVE state since it received replies from all its neighbors. D now has a viable path to N through C. D selects C as its successor to reach node N with a cost of 4. Notice that nodes A and B were not involved in the recalculation since they were not affected by the change.
Let's consider the situation in Figure 4, where Feasible Successors may not exist. If the link between node A and B fails, B goes into ACTIVE state for destination N since it has no Feasible Successors. Node B sends a QUERY to node C. C has no Feasible Successors, so it goes active for destination N; and since C has no neighbors, it replies to the QUERY, deletes the destination, and returns to the PASSIVE state for the unreachable route. As C removes the (now unreachable) destination from its table, C sends REPLY to its old successor. B receives this REPLY from C, and determines this is the last REPLY it is waiting on before determining what the new state of the route should be; on receiving this REPLY, B deletes the route to N from its routing table. Since B was the originator of the initial QUERY, it does not have to send a REPLY to its old successor (it would not be able to any ways, because the link to its old successor is down). Note that nodes A and D were not involved in the recalculation since their successors were not affected. N N | | (1)A ---<--- B(2) A ------- B Q | | | | |^ ^ ^ ^ ^ | v| | | | | | | | (2)D C(3) D C ACK R Figure 4: No Feasible Successors When Link between A and B Fails
(page 20 continued on part 2)
