OSPF Butt Router Configuration
Defined in RFC 3137, and aboriginal accurate in Cisco IOS Software Release 12.2(4)T, the OSPF stub
router feature—not to be abashed with chubby areas—allows a router to either briefly or
permanently be prevented from acceptable a alteration router. In this context, a alteration router is simply
one to which packets are forwarded, with the apprehension that the alteration router will advanced the
packet to yet addition router. Conversely, non-transit routers alone advanced packets to and from
locally absorbed subnets.
Figure 9-13 shows one archetypal case in which a butt router ability be useful.
Figure 9-13 OSPF Butt Router
Both ASBR1 and ASBR2 acquaint defaults into the network, assured to accept the capability
to avenue to the Internet through BGP-learned routes. In this case, ASBR2 is already up, fully
converged. However, if ASBR1 reloads, aback it comes aback up, OSPF is acceptable to assemble faster
than BGP. As a result, ASBR1 will acquaint its absence route, and OSPF routers may accelerate packets
to ASBR1, but ASBR1 will end up auctioning the packets until BGP converges.
Using the butt router affection on the ASBRs solves the botheration by authoritative them acquaint infinite
metric routes (cost 16,777,215) for any alteration routes—either for a configured time aeon or until
BGP aggregation is complete. To do so, beneath router ospf, the ASBRs would use either the maxmetric
router-lsa on-startup announce-time command or the max-metric router-lsa on-startup
wait-for-bgp command. With the aboriginal version, the absolute time aeon (in seconds) can be set. With
the second, OSPF waits until BGP signals that aggregation is complete or until 10 account pass,
whichever comes first.
NOTE OSPF affidavit is a acceptable abode for catchy CCIE lab questions—ones that can be
solved in a few account if you apperceive all the intricacies.
Default Routes Internet
BGP
BGP
ASBR1 Come Up:
OSPF Convergence: 1 minute
BGP Convergence: 5 minutes
2
OSPF and BGP
Already Converged
1
ABR1 ASBR1
ABR2 ASBR2
This breadth lists added capacity and facts to annular out the advantage of the capacity in this chapter.
Unlike best of the Cisco Press Assay Certification Guides, this “Foundation Summary” does not
repeat advice presented in the “Foundation Topics” breadth of the chapter. Please booty the
time to apprehend and abstraction the capacity in the “Foundation Topics” breadth of the chapter, as able-bodied as
review items acclaimed with a Key Topic icon.
Table 9-10 lists some of the key protocols apropos OSPF.
Table 9-11 lists some of the best accepted IOS commands accompanying to the capacity in this chapter. Also,
refer to Tables 9-7 through 9-9 for references to OSPF affidavit commands.
Table 9-10 Protocols and Corresponding Standards for Affiliate 9
Name Standard
OSPF Adaptation 2 RFC 2328
The OSPF Opaque LSA Advantage RFC 2370
The OSPF Not-So-Stubby Breadth (NSSA) Advantage RFC 3101
OSPF Butt Router Advertisement RFC 3137
Traffic Engineering (TE) Extensions to OSPF Adaptation 2 RFC 3630
Graceful OSPF Restart RFC 3623
Table 9-11 Command Reference for Affiliate 9
Command Command Approach and Description
router ospf process-id Global config; puts user in OSPF agreement approach for that
PID.
network ip-address [wildcard-mask]
area area
OSPF config mode; defines analogous parameters, compared
to interface IP addresses, to aces interfaces on which to
enable OSPF.
ip ospf process-id breadth area-id
[secondaries none]
Interface config mode; another to the arrangement command
for enabling OSPF on an interface.
neighbor ip-address [priority number]
[poll-interval seconds]
[cost number] [database-filter all]
OSPF config mode; acclimated aback neighbors charge be defined, it
identifies the neighbor’s IP address, priority, cost, and poll
interval.
auto-cost reference-bandwidth ref-bw OSPF config mode; changes the numerator in the blueprint to
calculate interface amount for all OSPF interfaces on that router.
Foundation Arbitrary 291
Command Command Approach and Description
router-id ip-address OSPF config mode; statically sets the router ID.
ospf log-neighbor-changes [detail] EIGRP subcommand; displays log letters aback neighbor
status changes. On by default.
passive-interface [default]
{interface-type interface-number}
OSPF config mode; causes OSPF to stop sending Hellos on the
specified interface. OSPF will still acquaint the subnet as a
stub network.
area area-id butt [no-summary] OSPF config mode; sets the breadth blazon to butt or totally
stubby.
area area-id nssa [no-redistribution]
[default-information-originate
[metric] [metric-type]] [no-summary]
OSPF config mode; sets the breadth blazon to NSSA or totally
NSSA.
area area-id default-cost amount OSPF config mode; sets the amount of absence avenue created by
ABRs and beatific into chubby areas.
area area-id nssa construe type7
suppress-fa
OSPF config mode; sets an NSSA ABR to set the forwarding
address to 0.0.0.0 for the blazon 5 LSAs it translates from type
7.
area area-id ambit ip-address mask
[advertise | not-advertise] [cost cost]
OSPF config mode; summarizes routes into a beyond prefix at
ABRs. Optionally filters blazon 3 LSAs (not-advertise option).
area {area-id} filter-list prefix {prefixlist-
name in | out}
OSPF config mode; filters blazon 3 LSA conception at ABR.
distribute-list [ACL] | [route-map
map-tag] in [int-type | int-number]
OSPF config mode; defines ACL or prefix account to clarify what
OSPF puts into the acquisition table.
area area-id virtual-link router-id
[authentication [message-digest |
null]] [hello-interval seconds]
[retransmit-interval seconds]
[transmit-delay seconds] [deadinterval
seconds] [[authenticationkey
key] | [message-digest-key key-id
md5 key]]
OSPF config mode; creates a basal link, with typical
interface agreement settings to affected actuality that the link
is virtual.
ip ospf hello-interval abnormal Interface subcommand; sets the breach for alternate Hellos.
ip ospf dead-interval {seconds |
minimal hello-multiplier multiplier}
Interface subcommand; defines the asleep interval, or
optionally the basal asleep breach of 1 second.
ip ospf name-lookup Global command; causes the router to use DNS to correlate
RIDs to host names for appearance command output.
Table 9-11 Command Reference for Affiliate 9
292 Affiliate 9: OSPF
Command Command Approach and Description
ip ospf amount interface-cost Interface subcommand; sets the cost.
ip ospf mtu-ignore Interface subcommand; tells the router to avoid the analysis for
equal MTUs that occurs aback sending DD packets.
ip ospf arrangement {broadcast | nonbroadcast
| {point-to-multipoint
[non-broadcast] | point-to-point}}
Interface subcommand; sets the OSPF arrangement blazon on an
interface.
ip ospf antecedence number-value Interface subcommand; sets the OSPF antecedence on an
interface.
ip ospf retransmit-interval abnormal Interface subcommand; sets the time amid LSA
transmissions for adjacencies acceptance to an interface.
ip ospf transmit-delay abnormal Interface subcommand; defines the estimated time expected
for the manual of an LSU.
max-metric router-lsa [on-startup
{announce-time | wait-for-bgp}]
OSPF config mode; configures a butt router, dabbling the
point at which it can become a alteration router.
show ip ospf border-routers User mode; displays hidden LSAs for ABRs and ASBRs.
show ip ospf [process-id [area-id]]
database
User mode; has abounding options not apparent here. Displays the
OSPF LSDB.
show ip ospf acquaintance [interface-type
interface-number] [neighbor-id] [detail]
User mode; lists advice about OSPF neighbors.
show ip ospf [process-id] summaryaddress
User mode; lists advice about avenue summaries in OSPF.
show ip ospf virtual-links User mode; displays cachet and advice about basal links.
show ip avenue ospf User mode; displays all OSPF routes in the IP acquisition table.
show ip ospf interface [interface-type
interface-number] [brief]
User mode; lists OSPF agreement timers and statistics per
interface.
show ip ospf statistics [detail] User mode; displays OSPF SPF adding statistics.
clear ip ospf [pid] {process |
redistribution | counters [neighbor
[neighbor-interface] [neighbor-id]]}
Enable mode; restarts the OSPF process, clears redistributed
routes, or clears OSPF counters.
debug ip ospf accost Enable mode; displays letters apropos Hellos, including
Hello constant mismatches.
debug ip ospf adj Enable mode; displays letters apropos adjacency
changes.
show ip ospf interface [type number]
[brief]
User mode; lists abounding interface settings.
Table 9-11 Command Reference for Affiliate 9
Foundation Arbitrary 293
Table 9-12 summarizes abounding OSPF timers and their meaning.
Table 9-12 OSPF Timer Summary
Timer Meaning
MaxAge The best time an LSA can be in a router’s LSDB, afterwards accepting a newer
copy of the LSA, afore the LSA is removed. Absence is 3600 seconds.
LSRefresh The timer breach per LSA on which a router refloods an identical LSA, except for a
1-larger arrangement number, to anticipate the cessation of MaxAge. Absence is 1800
seconds.
Hello Per interface; time breach amid Hellos. Absence is 10 or 30 seconds, depending on
interface type.
Dead Per interface; time breach in which a Accost should be accustomed from a neighbor. If
not received, the acquaintance is advised to accept failed. Absence is four times Hello.
Wait Per interface; set to the aforementioned cardinal as the asleep interval. Defines the time a router
will delay to get a Accost asserting a DR afterwards extensive a 2WAY accompaniment with that
neighbor.
Retransmission Per interface; the time amid sending an LSU, not accepting an acknowledgement,
and again resending the LSU. Absence is 5 seconds.
Inactivity Countdown timer, per neighbor, acclimated to ascertain aback a acquaintance has not been heard
from for a complete asleep interval. It starts according to the asleep interval, counts down,
and is displace to be according to the asleep breach aback anniversary Accost is received.
Poll Breach On NBMA networks, the aeon at which Hellos are beatific to a acquaintance aback the
neighbor is down. Absence is 60 seconds.
Flood (Pacing) Per interface; defines the breach amid alternating LSUs aback calamity LSAs.
Default is 33 ms.
Retransmission
(Pacing)
Per interface; defines the breach amid retransmitted packets as allotment of a single
retransmission event. Absence is 66 ms.
Lsa-group
(Pacing)
Per OSPF process. LSA’s LSRefresh intervals time out independently. This timer
improves LSU reflooding ability by waiting, accession several LSAs whose
LSRefresh timers expire, and calamity all these LSAs together. Absence is 240
seconds.
294 Affiliate 9: OSPF
Table 9-13 lists OSPF acquaintance states and their meaning.
Table 9-14 lists several key OSPF numeric values.
Memory Builders
The CCIE Acquisition and Switching accounting exam, like all Cisco CCIE accounting exams, covers a fairly
broad set of topics. This breadth provides some basal accoutrement to advice you exercise your anamnesis about
some of the broader capacity covered in this chapter.
Table 9-13 OSPF Acquaintance States
State Meaning
Down No Hellos accept been accustomed from this acquaintance for added than the asleep interval.
Attempt This router is sending Hellos to a manually configured neighbor.
Init A Accost has been accustomed from the neighbor, but it did not accept the router’s RID in
it. This is a abiding accompaniment aback Accost ambit do not match.
2WAY A Accost has been accustomed from the neighbor, and it has the router’s RID in it. This is
a abiding accompaniment for pairs of DROther neighbors.
ExStart Currently negotiating the DD arrangement numbers and master/slave argumentation acclimated for DD
packets.
Exchange Finished negotiating, and currently exchanging DD packets.
Loading All DD packets exchanged, and currently affairs the complete LSDB entries with
LSU packets.
Full Neighbors are adjoining (fully adjacent), and should accept identical LSDB entries for
the breadth in which the articulation resides. Acquisition table calculations begin.
Table 9-14 OSPF Numeric Ranges
Setting Ambit of Values
Single interface amount 1 to 65,535 (216 – 1)
Complete avenue amount 1 to 16,777,215 (224 – 1)
Infinite avenue amount 16,777,215 (224 – 1)
Reference bandwidth (units: Mbps) 1 to 4,294,967
OSPF PID 1 to 65,535 (216 – 1)
Further Reading 295
Fill in Key Tables from Memory
Appendix E, “Key Tables for CCIE Study,” on the CD in the aback of this book contains abandoned sets
of some of the key arbitrary tables in anniversary chapter. Print Appendix E, accredit to this chapter’s tables
in it, and ample in the tables from memory. Accredit to Appendix F, “Solutions for Key Tables for CCIE
Study,” on the CD to analysis your answers.
Definitions
Next, booty a few moments to address bottomward the definitions for the afterward terms:
LSDB, Dijkstra, link-state acquisition protocol, LSA, LSU, DD, Hello, LSAck, RID,
neighbor state, neighbor, adjacent, absolutely adjacent, 2-Way, 224.0.0.5, 224.0.0.6, area,
stub breadth type, arrangement type, alien route, E1 route, E2 route, Accost timer, asleep time/
interval, arrangement number, DR, BDR, DROther, priority, LSA flooding, DR election,
SPF calculation, fractional SPF calculation, abounding SPF calculation, LSRefresh, accost time/
interval, Maxage, ABR, ASBR, centralized router, courage area, alteration network, stub
network, LSA type, butt area, NSSA, absolutely chubby area, absolutely NSSA area, virtual
link, butt router, alteration router, SPF algorithm, All OSPF DR Routers, All OSPF
Routers, adroit restart
Refer to the comment to analysis your answers.
Further Reading
Jeff Doyle’s Acquisition TCP/IP, Volume I, Added Edition—every chat a charge for CCIE Routing
and Switching.
Cisco OSPF Command and Agreement Handbook, by Dr. William Parkhurst, covers every
OSPF-related command accessible in Cisco IOS at the time of that book’s publication, with
examples of anniversary one.