Using Avenue Summarization to Actualize Absence Routes
Generally speaking, avenue summarization combines abate abode ranges into a baby cardinal of
larger abode ranges. From that perspective, 0.0.0.0/0 is the better accessible summary, because it
includes all accessible IPv4 addresses. And, as it turns out, EIGRP avenue summarization supports
summarizing the 0.0.0.0/0 supernet, finer creating a absence route.
Because avenue summarization causes a absent avenue to be created for the summary, some Cisco
documentation advises adjoin application avenue summarization to actualize a absence route. For example,
in Figure 10-8, brainstorm that R9 is endemic by this network’s ISP, and R1 learns a absence route
(0.0.0.0/0) via EBGP from R9. However, back R1 configures an EIGRP absence avenue application route
summarization, R1 will additionally actualize a bounded avenue to 0.0.0.0/0 as well, but with destination null0.
The EBGP avenue has a college AD (20) than the EIGRP arbitrary avenue to null0 (AD 5), so R1 will
now alter its BGP-learned absence avenue with the arbitrary avenue to null0—preventing R1 from
being able to accelerate packets to the Internet.
Route summarization can still be acclimated to actualize absence routes with the able precautions. The
following account capacity a few of the requirements and options:
■ The bounded router creates a bounded arbitrary route, destination null0, application AD 5 (EIGRP), when
deciding if its avenue is the best one to add to the bounded acquisition table.
■ EIGRP advertises the arbitrary to added routers as AD 90 (internal).
■ This adjustment is not accurate by RIP and OSPF.
■ To affected the admonition of EIGRP’s absence avenue actuality set to absent by accepting a low AD, set
the AD college (as needed) with the ip summary-address command.
Example 10-11 lists a sample agreement on R1 again, this time creating arbitrary routes to
0.0.0.0/0 for EIGRP.
Example 10-11 EIGRP and IS-IS Agreement for Creating Absence Arbitrary Routes
! EIGRP avenue summarization is done beneath s0/0/0.4, the subnet affiliated to R4. In this
! example, the AD was afflicted to 7 (default 5) aloof to appearance how to change the AD. To
! abstain the botheration with the absence avenue to null0 on R1, the AD should accept been set
! college than the absence abstruse via BGP.
interface Serial0/0/0.4 point-to-point
ip abode 14.1.1.1 255.255.255.0
ip summary-address eigrp 1 0.0.0.0 0.0.0.0 7
! In this example, R1 has two sources for a bounded avenue to 0.0.0.0/0: EIGRP
! (AD 7, per the ip summary-address command), and BGP from R9
! (AD 20). R1 installs the EIGRP avenue based on the everyman AD.
R1# appearance ip avenue eigrp
14.0.0.0/8 is variably subnetted, 3 subnets, 2 masks
D 14.1.2.0/24 [90/2172416] via 14.1.1.4, 00:01:03, Serial0/0/0.4
D 14.0.0.0/8 is a summary, 05:53:19, Null0
D* 0.0.0.0/0 is a summary, 00:01:08, Null0
! Next, R4’s EIGRP avenue shows AD 90, instead of the AD 7 configured at R1. AD is
! a bounded parameter—R4 uses its absence AD of 90 for centralized routes.
R4# appearance ip avenue eigrp
D* 0.0.0.0/0 [90/2172416] via 14.1.1.1, 00:01:14, Serial0/0/0.1