Going Alive on a Route
The additional annex in the bounded ciphering argumentation causes the EIGRP router to ask its neighbors
about their accepted best avenue to a subnet, acquisitive to acquisition an available, loop-free another route
to that subnet. Back no FS avenue is found, the EIGRP router goes alive for the route. Going
active is abracadabra for the action of alteration a route’s cachet to active. Once the router is active,
EIGRP multicasts Concern letters to its neighbors, allurement the neighbors if they accept a valid
route to the subnet. The neighbors should unicast EIGRP Acknowledgment packets aback to the original
router, advertence whether or not they accept a accepted loop-free avenue with which to ability that
prefix.
Once a router receives Acknowledgment letters from all the neighbors to which it beatific Queries, the router
updates its cartography table with all the new advice abstruse in the Acknowledgment messages, recomputes
metrics for any accepted routes, and chooses a new successor. Of course, if no routes to that subnet
are found, this router artlessly does not add a avenue to the acquisition table.
NOTE The EIGRP appellation “active” refers to a avenue for which a router is currently application the
Query action to acquisition a loop-free another route. Conversely, a avenue is in acquiescent accompaniment when
it is not in an alive state.
220 Chapter 8: EIGRP
The adjoining routers appearance any accustomed Concern letters as an ascribe event. Anniversary neighbor
router’s behavior back accepting a Concern can be abbreviated as follows:
1. If the router does not accept an access in its cartography table for that subnet, it sends an EIGRP
Reply packet advertence that it has no route.
2. If the router’s almsman for that subnet is unchanged, or an FS is found, the acquaintance sends
back an EIGRP Acknowledgment bulletin with the capacity of the route.
3. If the altitude in footfall 1 or 2 do not exist, the router itself goes active, and withholds its EIGRP
response to the aboriginal Query, until all of its neighbors respond.
Note that the argumentation in the third footfall can aftereffect in a avenue for which the Alive Querying process
never completes. Routes that break in alive accompaniment too continued are advised to be stuck-in-active
routes. The accompanying concepts are covered in the abutting section.
Example 8-5 shows an archetype of the Concern process. The archetype is afresh based on Figure 8-4,
with R4 afresh accident its acquaintance accord with R1. In this case, R4’s bounded ciphering will
not acquisition an FS for its bootless avenue to 172.31.151.0/24, so it charge go active.
Example 8-5 R1-R4 Link Fails; R4 Actively Queries for 172.31.151.0/24
! First, the appearance ip eigrp cartography command alone lists the almsman route, and no
! FS routes. This command does not account non-FS routes.
R4# appearance ip eigrp topo
! Lines bare for brevity
P 172.31.151.0/24, 1 successors, FD is 1536
via 172.31.14.1 (1536/768), Serial0/0.1
! Below, the appearance ip eigrp cartography all-links command includes non-FS routes,
! in this case including the non-FS avenue to 151.0/24 through R2. Agenda that this
! alternating non-FS route’s RD is 1792, which is added than the FD of 1536.
R4# appearance ip eigrp cartography all-links
! Lines bare for brevity
P 172.31.151.0/24, 1 successors, FD is 1536, serno 175
via 172.31.14.1 (1536/768), Serial0/0.1
via 172.31.24.2 (2560/1792), Serial0/0.2
! Next, the FSM alter is afresh enabled, and R4 loses acquaintance R1.
R4# alter eigrp fsm
Jan 12 07:16:04.099: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Acquaintance 172.31.14.1 (Serial0/0.1)
is down: captivation time expired
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
! Below, R4 looks for an FS for avenue 172.31.151.0/24, and does not acquisition one—
! so it enters alive state. R4 sends a concern to its one actual acquaintance (R2),
! and keeps clue of the cardinal of outstanding Queries (1). Aloft accepting the
! Acknowledgment from R2, it can amend its cartography table, and echo bounded computation,
! and use the now-best avenue through R2.
Jan 12 07:17:42.391: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Acquaintance 172.31.14.1 (Serial0/0.1)
is down: captivation time expired
EIGRP Convergence 221
Of accurate agenda in this example, attending for the alter bulletin starting with “Dual: rcvreply:”
(highlighted). This bulletin agency that the router accustomed an EIGRP Acknowledgment message, in this case
from R2. The bulletin includes R2’s accurate acquisition advice for 172.31.151.0/24. Additionally agenda that
the FD was recomputed, admitting it was not in Archetype 8-4 back an FS avenue was found.
Stuck-in-Active
Any router in alive accompaniment for a avenue charge delay for a Acknowledgment to anniversary of its Concern messages. It is
possible for a router to delay several account for all the replies, because adjoining routers might
also charge to go active, and again their neighbors ability charge to go active, and so on—each
withholding its Acknowledgment bulletin until it in about-face receives all of its Acknowledgment messages. In normal
Jan 12 07:17:42.391: DUAL: linkdown: alpha - 172.31.14.1 via Serial0/0.1
Jan 12 07:17:42.391: DUAL: Destination 172.31.151.0/24
Jan 12 07:17:42.391: DUAL: Acquisition FS for dest 172.31.151.0/24. FD is 1536, RD is 1536
Jan 12 07:17:42.395: DUAL: 172.31.14.1 metric 4294967295/4294967295
Jan 12 07:17:42.395: DUAL: 172.31.24.2 metric 2560/1792 not begin Dmin is 2560
Jan 12 07:17:42.395: DUAL: Dest 172.31.151.0/24 entering alive state.
Jan 12 07:17:42.395: DUAL: Set reply-status table. Calculation is 1.
Jan 12 07:17:42.395: DUAL: Not accomplishing breach horizon
Jan 12 07:17:42.459: DUAL: rcvreply: 172.31.151.0/24 via 172.31.24.2 metric 2560/1792
Jan 12 07:17:42.459: DUAL: acknowledgment calculation is 1
Jan 12 07:17:42.459: DUAL: Clearing handle 0, calculation now 0
Jan 12 07:17:42.463: DUAL: Freeing acknowledgment cachet table
Jan 12 07:17:42.463: DUAL: Acquisition FS for dest 172.31.151.0/24. FD is 4294967295, RD is
4294967295 found
Jan 12 07:17:42.463: DUAL: Removing dest 172.31.151.0/24, nexthop 172.31.14.1
Jan 12 07:17:42.463: DUAL: RT installed 172.31.151.0/24 via 172.31.24.2
Jan 12 07:17:42.467: DUAL: Accelerate amend about 172.31.151.0/24. Reason: metric chg
Jan 12 07:17:42.467: DUAL: Accelerate amend about 172.31.151.0/24. Reason: new if
! Next, agenda that because R4 actively queried for the route, the FD could change.
R4# appearance ip eigrp topo
IP-EIGRP Cartography Table for AS(1)/ID(172.31.104.4)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - acknowledgment Status, s - sia Status
P 172.31.151.0/24, 1 successors, FD is 2560
via 172.31.24.2 (2560/1792), Serial0/0.2
NOTE Concern letters use reliable manual via RTP and are multicasts; Acknowledgment messages
are reliable and are unicasts. Both are accustomed application Ack messages.
NOTE The EIGRP appellation Diffusing Amend Algorithm (DUAL) refers to the accumulation of the logic
used by EIGRP to account new routes. The appellation is based on the argumentation acclimated as Concern messages
go apparent from a router, with the apparent movement chock-full back routers Reply.
Example 8-5 R1-R4 Link Fails; R4 Actively Queries for 172.31.151.0/24 (Continued)
operation, the action should complete; to handle barring cases, EIGRP includes a timer called
the Alive timer, which banned the bulk of time in which a avenue can break active. If the Active
timer expires afore a router receives all of its Acknowledgment messages, the router places the avenue in a
stuck-in-active state. The router additionally brings bottomward any neighbors from which no corresponding
Reply was received, cerebration that any neighbors that did not accelerate a Acknowledgment are accepting problems.
In some conditions—large, bombastic networks, aerial interfaces, or networks with lots of
packet loss, to name a few—neighbors ability be alive fine, but their Acknowledgment letters may not
complete aural the Alive timer. To abstain the downside of accepting the avenue become stuck-in-active,
and accident all routes through a possibly still-working neighbor, you can attenuate the Alive timer
by application the timers active-time disabled subcommand beneath router eigrp.
Limiting Concern Scope
Although disabling the Alive timer can anticipate stuck-in-active routes, a bigger band-aid to the
prolonged delay for Acknowledgment letters is to absolute the ambit of Concern messages. By abbreviation the number
of neighbors that accept the messages, and by attached the cardinal of hops abroad the queries flow,
you can abundantly abate the time appropriate to accept all Acknowledgment messages.
Two methods can be acclimated to absolute concern scope. The aboriginal is avenue summarization. Back a Query
reaches a router that has a abbreviated route, but not the specific avenue in the query, the router
immediately replies that it does not accept that route. For instance, a router with the route
172.31.0.0/16 in its cartography table, aloft accepting a concern for 172.31.151.0/24, immediately
sends a Reply, advertence it does not accept a avenue to 172.31.151.0/24. With well-designed route
summarization, EIGRP queries can be bound to a few hops. (Chapter 11, “IGP Redistribution,
Route Summarization, and Default Routing,” covers avenue summarization details.)
The use of EIGRP butt routers additionally banned the concern scope. Butt routers, by definition, should not be
used as alteration routers for traffic. In Figure 8-4, R5 would be a archetypal applicant to be a butt router.
Also, if R4 should not be acclimated to advanced cartage from R1 over to R2, or carnality versa, R4 could be a stub
as well. In either case, non-stub routers do not accelerate Concern letters to the butt routers, alive that
the butt routers should not be alteration routers. (Stub router agreement is covered in the abutting section.)