RIP Adaptation 2 Basics
CCIE candidates may already apperceive abounding of the appearance and agreement options of RIP.
Although RIPv2 is no best on the CCIE Acquisition and Switching accomplishment assay blueprint, it
is acutely accessible to accept its operations to strengthen your butt on IGPs in accepted and the
differences amid ambit agent and link-state protocols. This affiliate summarizes RIPv2’s
protocol appearance and concepts. Table 7-2 provides a high-level overview of RIPv2’s operation.
Table 7-2 RIP Affection Summary
Function Description
Transport UDP, anchorage 520.
Metric Hop count, with 15 as the best accessible metric, and 16 advised to be
infinite.
Hello breach None; RIP relies on the approved abounding acquisition updates instead.
Update destination Local subnet advertisement (255.255.255.255) for RIPv1; 224.0.0.9 multicast for RIPv2.
Update breach 30 seconds.
Full or fractional updates Abounding updates anniversary interval. For on-demand circuits, allows RIP to accelerate full
updates once, and afresh abide bashful until changes occur, per RFC 2091. Full
updates anniversary interval.
Triggered updates Yes, aback routes change.
Multiple routes to the
same subnet
Allows installing 1 to 6 (default 4) equal-metric routes to the aforementioned subnet in a
single acquisition table.
Authentication* Allows both plain-text and MD5 authentication.
Subnet affectation in
updates*
RIPv2 transmits the subnet affectation with anniversary route, thereby acknowledging VLSM,
making RIPv2 classless. This affection additionally allows RIPv2 to support
discontiguous networks.
VLSM* Supported as a aftereffect of the admittance of subnet masks in the acquisition updates.
RIP Aggregation and Loop Prevention 187
RIP exchanges routes by sending RIP updates on anniversary interface based on an Amend timer (update
interval). A RIP router advertises its affiliated routes, as able-bodied as added RIP-learned routes that are
in the router’s IP acquisition table. Agenda that RIP does not accumulate a abstracted cartography table. RIP routers
do not anatomy acquaintance relationships, nor do they use a Hello protocol—each router artlessly sends
updates, with destination abode 224.0.0.9. (Note: RIPv1 uses advertisement abode 255.255.255.255.)
RIPv2 uses the aforementioned hop-count metric as RIPv1, with 15 actuality the better accurate metric, and 16
considered to be infinity. Interestingly, a RIP router does not put its own metric in the avenue of a
sent acquisition update; rather, it aboriginal adds 1 to anniversary metric aback architecture the update. For instance,
if RouterA has a avenue with metric 2, it advertises that avenue with metric 3—in effect, cogent the
receiving router what its metric should be.
When Cisco RIP routers apprentice assorted routes to the aforementioned subnet, the lowest-metric avenue is
chosen, of course. If assorted equal-hop routes exist, the router (by default) installs up to 4 such
routes in its acquisition table, or amid 1 and 6 of such routes, based on the ip maximum-paths
number command beneath the router rip command.
RIP Aggregation and Loop Prevention
The best absorbing and complicated allotment of RIP relates to loop-prevention methods acclimated during
convergence afterwards a avenue has failed. Some protocols, like OSPF, IS-IS, and EIGRP, accommodate loop
prevention as a ancillary aftereffect of their basal avenue computations. However, RIP, like other
distance agent protocols, uses several loop-prevention tools. Unfortunately, these loop-prevention
tools additionally decidedly access aggregation time—a actuality that is absolutely the better negative
feature of RIP, alike for RIPv2. Table 7-3 summarizes some of the key appearance and agreement related
to RIP convergence, with added explanations afterward the table.
Function Description
Route Tags* Allows RIP to tag routes as they are redistributed into RIP.
Next Hop field* Supports the appointment of a next-hop IP abode for a route, acceptance a router
to acquaint a next-hop router that is altered from itself.
* RIPv2-only features
Table 7-3 RIP Appearance Related to Aggregation and Loop Prevention
Function Description
Split border Instead of announcement all routes out a accurate interface, RIP omits the routes whose
outgoing interface acreage matches the interface out which the amend would be sent.
Triggered
update
The absolute sending of a new amend aback acquisition advice changes, instead of
waiting for the Amend timer to expire.
continues
Table 7-2 RIP Affection Arbitrary (Continued)
188 Affiliate 7: RIP Adaptation 2
The blow of this breadth shows examples of the aggregation features, application RIP appearance and debug
command achievement to appearance examples of their use. Figure 7-1 shows the sample internetwork that
is acclimated in these examples of the assorted loop-prevention tools.
Figure 7-1 Sample Internetwork Acclimated for Loop-Prevention Examples
Converged Steady-State Operation
Example 7-1 shows a few capacity of R1’s operation while all interfaces in Figure 7-1 are up and
working. The archetype lists the basal (and identical) RIP agreement on all four routers;
configuration will be covered in added detail afterwards in the chapter. As configured, all four routers are
Function Description
Route
poisoning
The action of sending an infinite-metric (hop calculation 16) avenue in acquisition updates when
that avenue fails.
Poison about-face The act of announcement a berserk avenue (metric 16) out an interface, but in acknowledgment to
receiving that aforementioned berserk avenue in an amend accustomed on that aforementioned interface.
Update timer The timer that specifies the time breach over which updates are sent. Anniversary interface
uses an absolute timer, abaft to 30 seconds.
Holddown
timer
A per-route timer (default 180 seconds) that begins aback a route’s metric changes to a
larger value. The router does not add an another avenue for this subnet to its routing
table until the Holddown timer for that avenue expires.
Invalid timer A per-route timer that increases until it receives a acquisition amend that confirms the
route is still valid, aloft which the timer is displace to 0. If the updates cease, the Invalid
timer will abound until it alcove the timer ambience (default 180 seconds), afterwards which the
route is advised invalid.
Flush
(Garbage)
timer
A per-route timer that is displace and grows with the Invalid timer. Aback the Even timer
mark is accomplished (default 240 seconds), the router removes the avenue from the routing
table and accepts new routes to the bootless subnet.
Table 7-3 RIP Appearance Related to Aggregation and Loop Prevention (Continued)
R3
Failing interfaces
E0/0 R1
103.3/24
E0/0 R4 R2
103.4/24
S0/0.1
13.2/30
S0/0.2
24.1/30
S0/0.3
13.1/30
S0/0.4
24.2
Fa0/0
11.1/24
Fa0/0
11.2/24
Network 172.31.0.0
RIP Aggregation and Loop Prevention 189
using alone RIPv2, on all interfaces apparent in Figure 7-1. Read the comments in Archetype 7-1 for
explanations of the output.
Example 7-1 Steady-State RIP Operation in Figure 7-1
! All routers use the aforementioned three curve of RIP configuration.
router rip
network 172.31.0.0
version 2
! Below, the appearance ip agreement command lists abounding of RIP’s operational settings,
! including RIP timers, adaptation used, and neighbors from which RIP updates have
! been accustomed (listed as “Routing Advice Sources”).
R1# appearance ip protocol
Routing Agreement is "rip"
Sending updates every 30 seconds, abutting due in 24 seconds
Invalid afterwards 180 seconds, authority bottomward 180, ablaze afterwards 240
Outgoing amend clarify account for all interfaces is not set
Incoming amend clarify account for all interfaces is not set
Redistributing: rip
Default adaptation control: accelerate adaptation 2, accept adaptation 2
Interface Accelerate Recv Triggered RIP Key-chain
FastEthernet0/0 2 2
Serial0/0.3 2 2
Automatic arrangement summarization is in effect
Maximum path: 4
Routing for Networks:
172.31.0.0
Routing Advice Sources:
Gateway Ambit Aftermost Update
172.31.11.2 120 00:00:15
172.31.13.2 120 00:00:08
Distance: (default is 120)
! Below, the accepted Invalid timer is listed by anniversary RIP route. Agenda that it took
! about 3 abnormal amid the aloft appearance ip protocols command and the upcoming
! appearance ip avenue command, so the aftermost amend from 172.31.13.2 (above)
! was 8 seconds; 3 abnormal later, the Invalid timer for a avenue abstruse from
! 172.31.13.2 is now 11 seconds.
R1# appearance ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA alien blazon 1, N2 - OSPF NSSA alien blazon 2
E1 - OSPF alien blazon 1, E2 - OSPF alien blazon 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - applicant default, U - per-user changeless route
o - ODR, P - alternate downloaded changeless route
continues
190 Affiliate 7: RIP Adaptation 2
Triggered (Flash) Updates and Berserk Routes
When RIP knows for abiding that a avenue to a subnet has failed, RIPv2 can assemble to an alternate
route about in beneath than a minute. Archetype 7-2 capacity the accomplish abaft one such example, using
Figure 7-1, with the accomplish categorical in the afterward account (the comments in Archetype 7-2 accredit to
these accomplish by number):
1. RIP alter letters appearance R1’s RIP updates, including R1’s use of breach horizon.
2. R3’s E0/0 interface is shut down, assuming a failure.
3. R3 anon sends a triggered amend (also alleged a beam update), because R3 knows for
sure that the avenue has failed. R3’s advertised avenue is a berserk avenue to 172.31.103.0/24.
4. R1 anon (due to triggered updates) advertises a adulteration about-face avenue for 172.31.103.0/24,
back to R3, and sends a triggered amend out its fa0/0 interface.
5. R1 removes its avenue to 172.31.103.0/24 from its acquisition table.
6. R1 waits for R2’s abutting update, beatific based on R2’s Amend timer on its fa0/0 interface. That
update includes a avenue to 172.31.103.0/24. R1 adds that avenue to its acquisition table.
Gateway of aftermost resort is not set
172.31.0.0/16 is variably subnetted, 4 subnets, 2 masks
R 172.31.24.0/30 [120/1] via 172.31.11.2, 00:00:18, FastEthernet0/0
C 172.31.11.0/24 is anon connected, FastEthernet0/0
C 172.31.13.0/30 is anon connected, Serial0/0.3
R 172.31.103.0/24 [120/1] via 172.31.13.2, 00:00:11, Serial0/0.3
! Below, the appearance ip rip database command lists advice for anniversary route
! advised by RIP.
R1# appearance ip rip database
172.31.0.0/16 auto-summary
172.31.11.0/24 anon connected, FastEthernet0/0
172.31.13.0/30 anon connected, Serial0/0.3
172.31.24.0/30
[1] via 172.31.11.2, 00:00:01, FastEthernet0/0
172.31.103.0/24
[1] via 172.31.13.2, 00:00:23, Serial0/0.3
NOTE The appearance ip rip database command lists all RIP abstruse routes, and all connected
routes that RIP is advertising.
Example 7-1 Steady-State RIP Operation in Figure 7-1 (Continued)
RIP Aggregation and Loop Prevention 191
Example 7-2 R1’s Aggregation for 172.31.103.0/24 aloft R3’s E0/0 Interface Failure
! First, the alter ip rip command enables RIP debugging. This command will show
! letters that appearance every avenue in the beatific and accustomed updates.
R1# alter ip rip
RIP agreement debugging is on
! (Step 1) Below, the achievement exhibits breach horizon—for example, 172.31.103.0/24
! is not advertised out s0/0.3, but it is advertised out fa0/0.
*Mar 3 22:44:08.176: RIP: sending v2 amend to 224.0.0.9 via Serial0/0.3 (172.31.13.1)
*Mar 3 22:44:08.176: RIP: body amend entries
*Mar 3 22:44:08.176: 172.31.11.0/24 via 0.0.0.0, metric 1, tag 0
*Mar 3 22:44:08.176: 172.31.24.0/30 via 0.0.0.0, metric 2, tag 0
*Mar 3 22:44:12.575: RIP: sending v2 amend to 224.0.0.9 via FastEthernet0/0 (172.31.11.1)
*Mar 3 22:44:12.575: RIP: body amend entries
*Mar 3 22:44:12.575: 172.31.13.0/30 via 0.0.0.0, metric 1, tag 0
*Mar 3 22:44:12.575: 172.31.103.0/24 via 0.0.0.0, metric 2, tag 0
! Next, R1 receives a RIP amend from R3. The metric 1 avenue in the amend below
! is R1’s best route, and is placed into R1’s acquisition table. Agenda that the metric
! in the accustomed amend is R1’s absolute metric to ability the route.
*Mar 3 22:44:21.265: RIP: accustomed v2 amend from 172.31.13.2 on Serial0/0.3
*Mar 3 22:44:21.269: 172.31.24.0/30 via 0.0.0.0 in 2 hops
*Mar 3 22:44:21.269: 172.31.103.0/24 via 0.0.0.0 in 1 hops
! (Step 2) R3’s E0/0 interface is shut bottomward at this point. (Not shown).
! (Step 3) Below, R1 receives a triggered update, with two adulteration routes from R3—
! the aforementioned two routes that R3 advertised in the antecedent acquisition amend above.
! Agenda that the triggered amend alone includes afflicted routes, with abounding updates
! continuing on the aforementioned amend interval.
*Mar 3 22:44:46.338: RIP: accustomed v2 amend from 172.31.13.2 on Serial0/0.3
*Mar 3 22:44:46.338: 172.31.24.0/30 via 0.0.0.0 in 16 hops (inaccessible)
*Mar 3 22:44:46.338: 172.31.103.0/24 via 0.0.0.0 in 16 hops (inaccessible)
! (Step 4) Above, R1 reacts to its cancellation of berserk routes, sending a triggered
! amend out its fa0/0 interface. Agenda that the alter refers to the triggered
! amend as a beam update.
*Mar 3 22:44:48.341: RIP: sending v2 beam amend to 224.0.0.9 via FastEthernet 0/0
(172.31.11.1)
*Mar 3 22:44:48.341: RIP: body beam amend entries
*Mar 3 22:44:48.341: 172.31.103.0/24 via 0.0.0.0, metric 16, tag 0
! (Step 4) R1 additionally sends a triggered amend out s0/0.3 to R3, which includes
! a adulteration about-face avenue to 172.31.103.0/24, aback to R3. R1 does not accelerate aback a
! adulteration avenue to 172.31.24.0, because R1’s avenue to 172.31.24.0 was
! pointing appear R2, not R3—so R1’s avenue to 172.31.24.0/24 did not fail.
*Mar 3 22:44:48.341: RIP: sending v2 beam amend to 224.0.0.9 via Serial0/0.3
(172.31.13.1)
! (Step 5) Below, agenda the absence of a avenue to 103.0/24 in R1’s acquisition table.
R1# appearance ip avenue 172.31.103.0
% Subnet not in table
! (Step 6) Below, 23 abnormal back the antecedent alter message, R2’s abutting routing
! amend arrives at R1, announcement 172.31.103.0/24. Afterward that, R1 now has
! a 2-hop route, through R2, to 172.31.103.0/24.
continues
192 Affiliate 7: RIP Adaptation 2
If you appraise the alter bulletin time stamps in Archetype 7-2, you will see that amid 25 and
45 abnormal anesthetized from aback R1 heard the berserk routes until R1 heard R2’s new routing
update with a now-best avenue to 172.31.103.0/24. While not on par with EIGRP or OSPF, this
convergence is analytic fast for RIP.
RIP Aggregation Aback Acquisition Updates Cease
When a router ceases to accept acquisition updates, RIP charge delay for some timers to expire before
it decides that routes ahead abstruse from the now-silent router can be advised to be failed
routes. To accord with such cases, RIP uses its Invalid, Flush, and Holddown timers to anticipate loops.
Coincidentally, RIP’s aggregation time increases to several account as a result.
Example 7-3 capacity aloof such a case, breadth R1 artlessly ceases to apprehend RIP updates from R3. (To
create the failure, R3’s s0/0.1 subinterface was shut down, assuming abortion of a Frame Relay
PVC.) The archetype uses the internetwork illustrated in Figure 7-1 again, and begins with all
interfaces up, and all four routes accepted in anniversary of the four routers. The archetype follows this
sequence (the comments in Archetype 7-3 accredit to these accomplish by number):
1. R3’s s0/0.1 subinterface fails, but R1’s Frame Relay subinterface stays up—so R1 charge use
its timers to ascertain avenue failures.
2. R1’s Invalid and Even timers for avenue 172.31.103.0/24 abound because it does not apprehend any
further updates from R3.
3. Afterwards the Invalid timer expires (180 seconds) for R1’s avenue to 172.31.103.0/24, R1 begins a
Holddown timer for the route. Holddown starts at (default) 180 seconds, and counts down.
*Mar 3 22:45:11.271: RIP: accustomed v2 amend from 172.31.11.2 on FastEthernet0/0
*Mar 3 22:45:11.271: 172.31.24.0/30 via 0.0.0.0 in 1 hops
*Mar 3 22:45:11.271: 172.31.103.0/24 via 0.0.0.0 in 2 hops
R1# appearance ip avenue 172.31.103.0
Routing access for 172.31.103.0/24
Known via "rip", ambit 120, metric 2
Redistributing via rip
Last amend from 172.31.11.2 on FastEthernet0/0, 00:00:01 ago
Routing Descriptor Blocks:
* 172.31.11.2, from 172.31.11.2, 00:00:01 ago, via FastEthernet0/0
Route metric is 2, cartage allotment calculation is 1
NOTE Do not abash the appellation triggered amend with the appellation triggered extensions to RIP.
RFC 2091 defines how RIP can accept to accelerate abounding updates alone once, and afresh be silent, to
support appeal circuits. The affection is enabled per interface by the ip rip triggered interface
subcommand.
Example 7-2 R1’s Aggregation for 172.31.103.0/24 aloft R3’s E0/0 Interface Abortion (Continued)
RIP Aggregation and Loop Prevention 193
4. The Even timer expires afterwards a absolute 240 seconds, or 60 abnormal accomplished the Invalid timer. As a
result, R1 flushes the avenue to 172.31.103.0/24 from its acquisition table, which additionally removes the
Holddown timer for the route.
Example 7-3 R1 Ceases to Apprehend R3’s Updates: Invalid, Flush, and Holddown Timers Required
! First, the alter ip rip accident command is used, which displays letters when
! updates are beatific and received, but does not affectation the capacity of the updates.
R1# alter ip rip event
RIP accident debugging is on
! (Step 1) Not Shown: R3’s S0/0.1 subinterface is shut down.
! (Step 2) Below, the Invalid timer for 172.31.103.0/24 has accomplished 35, meaning
! that 35 abnormal accept anesthetized back the aftermost accustomed amend from which this route
! was learned. An Invalid timer over 30 abnormal agency that at atomic one RIP
! amend was not received.
R1# appearance ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA alien blazon 1, N2 - OSPF NSSA alien blazon 2
E1 - OSPF alien blazon 1, E2 - OSPF alien blazon 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - applicant default, U - per-user changeless route
o - ODR, P - alternate downloaded changeless route
Gateway of aftermost resort is not set
172.31.0.0/16 is variably subnetted, 4 subnets, 2 masks
R 172.31.24.0/30 [120/1] via 172.31.11.2, 00:00:09, FastEthernet0/0
C 172.31.11.0/24 is anon connected, FastEthernet0/0
C 172.31.13.0/30 is anon connected, Serial0/0.3
R 172.31.103.0/24 [120/1] via 172.31.13.2, 00:00:35, Serial0/0.3
! Below, one archetype set of alter letters are shown. (Many added alter messages
! occurred while cat-and-mouse for convergence, but those were omitted.) The messages
! about R1’s accustomed updates from R2 action every 30 abnormal or so. The contents
! accommodate a 2-hop avenue to 172.31.103.0/24, which R1 ignores until the Even timer
! expires.
*Mar 3 21:59:58.921: RIP: accustomed v2 amend from 172.31.11.2 on FastEthernet0/0
*Mar 3 21:59:58.921: RIP: Amend contains 2 routes
! (Step 3) Below, the Invalid timer expires, almost 3 account afterwards the failure.
! Agenda that the avenue is listed as “possibly down,” which occurs aback the
! Invalid timer has asleep but the Even timer has not. Agenda that the appearance ip
! avenue command does not account the Even timer settings, but the accessible show
! ip avenue 172.31.103.0 command does.
R1# appearance ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA alien blazon 1, N2 - OSPF NSSA alien blazon 2
E1 - OSPF alien blazon 1, E2 - OSPF alien blazon 2
continues
194 Affiliate 7: RIP Adaptation 2
At the end of the example, the alone actual footfall for aggregation is for R1 to accept R2’s next
regular abounding acquisition update, which includes a two-hop avenue to 172.31.103.0/24. R2 will accelerate that
update based on R2’s approved amend interval. R1 would abode that avenue in its acquisition table,
completing convergence.
Note that either the Even timer or the Holddown timer charge expire afore new acquisition information
would be acclimated in this case. Here, the Even timer for avenue 172.31.103.0/24 asleep first, resulting
in the avenue actuality removed from R1’s acquisition table. Aback the avenue is ablaze (removed), any
associated timers are additionally removed, including the Holddown timer. Had the Holddown timer been
smaller, and had it asleep afore the Even timer, R1 would accept been able to use the route
advertised by R2 at that point in time.
Convergence Extras
Convergence in Archetype 7-3 took a little over 4 minutes, but it could be bigger in some cases.
The RIP timers can be acquainted with the timers basal amend invalid hold-down even subcommand
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - applicant default, U - per-user changeless route
o - ODR, P - alternate downloaded changeless route
Gateway of aftermost resort is not set
172.31.0.0/16 is variably subnetted, 4 subnets, 2 masks
R 172.31.24.0/30 [120/1] via 172.31.11.2, 00:00:20, FastEthernet0/0
C 172.31.11.0/24 is anon connected, FastEthernet0/0
C 172.31.13.0/30 is anon connected, Serial0/0.3
R 172.31.103.0/24 is possibly down,
routing via 172.31.13.2, Serial0/0.3
! (Step 3) Next, the command shows the metric as inaccessible, acceptation an
! absolute metric, as able-bodied as the accepted Even timer (3:23), which counts up.
! Also, the Holddown timer for this avenue has started (at 180 seconds), with 159
! abnormal in its countdown. The Holddown timer prevents R1 from application the route
! heard from R2.
R1# appearance ip avenue 172.31.103.0
Routing access for 172.31.103.0/24
Known via "rip", ambit 120, metric 4294967295 (inaccessible)
Redistributing via rip
Last amend from 172.31.13.2 on Serial0/0.3, 00:03:23 ago
Hold bottomward timer expires in 159 secs
! (Step 4) Below, aloof afterwards 4 account has passed, the Even timer has expired,
! and the avenue to 172.31.103.0/24 has been ablaze from the acquisition table.
R1# appearance ip avenue 172.31.103.0
% Subnet not in table
Example 7-3 R1 Ceases to Apprehend R3’s Updates: Invalid, Flush, and Holddown Timers Required (Continued)
under router rip, although affliction should be taken aback alteration these timers. The timers should
be constant beyond routers, and abate ethics access the adventitious of acquisition loops actuality formed
during convergence.
The bright ip avenue * command additionally speeds aggregation by removing all routes from the routing
table, forth with any per-route timers. In Archetype 7-3, the bright ip avenue 172.31.103.0 command
would accept formed as well, aloof deleting that one route. Because the bright command bypasses
loop-prevention appearance by deleting the avenue and timers, it can be risky, but it absolutely speeds
convergence. Also, afterwards the bright command, R1 would anon affair RIP appeal packets,
which account the adjoining routers to accelerate abounding acquisition updates to R1, instead of cat-and-mouse on their
next amend time.