Cisco CCNP / BSCI Exam Tutorial: EIGRP Route Summarization

Summarizing routes is a vital skill to learn to passaddress is 16.0.0.0 252.0.0.0.In EIGRP, the
the BSCI exam and get one step closer tosummary address is actually configured on an
earning your CCNP. The actual binary conversionsinterface, not under the routing
are only part of the test, though! You've got toprocess.R1(config)#interface serial0R1(config-if)#ip
know how to correctly apply the summarysummary-address eigrp 100 16.0.0.0
routes, and that differs from one protocol to the252.0.0.002:39:50: %DUAL-5-NBRCHANGE:
next. In the last few CCNP / BSCI tutorials, we'veIP-EIGRP 100: Neighbor
looked at using the "area range" and172.12.123.3 (Serial0) is down: summary
"summary-address" commands to perform OSPFconfigured02:39:50: %DUAL-5-NBRCHANGE:
route summarization. Today, we'll take a look atIP-EIGRP 100: Neighbor
summarizing routes in EIGRP.We'll use the following172.12.123.2 (Serial0) is down: summary
four loopback addresses in this example:Loopbackconfigured02:40:16: %DUAL-5-NBRCHANGE:
16, 16.16.16.16 /32Loopback 17, 17.17.17.17IP-EIGRP 100: Neighbor
32Loopback 18, 18.18.18.18 /32Loopback 19.172.12.123.2 (Serial0) is up : new
19.19.19.19 /32On R1, we'll place these fouradjacency02:40:17: %DUAL-5-NBRCHANGE:
addresses into EIGRP AS 100.R1(config-if)#routerIP-EIGRP 100: Neighbor
eigrp 100R1(config-router)#network 16.16.16.16172.12.123.3 (Serial0) is up: new adjacencyThere's
0.0.0.0R1(config-router)#network 17.17.17.17an immediate side effect here that most books
0.0.0.0R1(config-router)#network 18.18.18.18leave out. Your EIGRP adjacencies are going to
0.0.0.0R1(config-router)#network 19.19.19.19come down after you configure this summary,
0.0.0.0R3 is an EIGRP neighbor of R1, and thatbut they should come back up quickly. The key
router's EIGRP routing table now looks likeword there is "should". If you configure EIGRP
this:R3#show ip route eigrp17.0.0.0/32 issummary addresses on a production network,
subnetted, 1 subnetsD 17.17.17.17 [90/2297856]you may want to do this during non-peak hours.
via 172.12.123.1, 00:00:29, Serial016.0.0.0/32 isThe timestamps on the above commands indicate
subnetted, 1 subnetsD 16.16.16.16 [90/2297856]that the adjacencies were down for about 27
via 172.12.123.1, 00:00:36, Serial019.0.0.0/32 isseconds over the NBMA network. That's about 30
subnetted, 1 subnetsD 19.19.19.19 [90/2297856]minutes in end-user time. ;)Check R3's EIGRP
via 172.12.123.1, 00:00:08, Serial018.0.0.0/32 isrouting table.R3#show ip route eigrpD 16.0.0.0/6
subnetted, 1 subnetsD 18.18.18.18 [90/2297856][90/2297856] via 172.12.123.1, 00:01:46, Serial0The
via 172.12.123.1, 00:00:22, Serial0To performfour summarized routes are no longer in the
manual route summarization, write out therouting table, and they have been replaced by the
network addresses in binary and then determinesummary route shown at the bottom of the
the point at which the addresses no longer have arouting table. Notice the mask is /5, which is
bit in common. For these four addresses, it will beprefix notation for 248.0.0.0.Knowing how and
enough to write out the first octet in binary:16why to summarize routes is a valuable skill,
0001000017 0001000118 0001001019regardless of the protocol in use. But before you
00010011Working from left to right, the commontake the BSCI exam on your way to the CCNP,
bits are the first six bits - 000100xx. In decimal,make sure you know how to perform
this value is 16. The summary mask must besummarization with all of the core protocols!Chris
determined as well, and that value is derived fromBryant, CCIE #12933, is the owner of The Bryant
putting a "1" in the mask for each common bit.Advantage, home of free CCNP and CCNA
With the first six bits all set to one - 11111100 -tutorials, The Ultimate CCNA Study Package, and
the resulting mask is 252.0.0.0. The full summaryUltimate CCNP Study Packages.