CCNA Complete Guide 2nd Edition.pdf - Cisco Learning Home

- Static routes can be said to support VLSM, as they can be used to define summary routes for
routing protocols that do not support VLSM, eg: RIP and IGRP.
- Below shows the routing table on RT2 after route summarization:
RT2#sh ip route
Gateway of last resort is not set
10.0.0.0/8 is variably subnetted, 5 subnets, 2 masks
D 10.2.0.0/16 [90/2195456] via 10.1.2.1, 00:03:34, Serial0/0
D 10.3.0.0/16 [90/2195456] via 10.1.3.2, 00:02:06, Serial0/1
C 10.1.1.0/24 is directly connected, Ethernet1/0
C 10.1.2.0/30 is directly connected, Serial0/0
C 10.1.3.0/30 is directly connected, Serial0/1
RT2#
- RT1 no longer seeing the 4 10.3.x.x routes, and RT3 no longer seeing the 4 10.2.x.x routes.
With route summarization, both routers advertise only the summary routes.
Route summarization conserve great amount of bandwidth (smaller routing update packets)
and minimizes processing for routing updates.
- EIGRP route summarization configuration introduces a route destined to the Null0 interface,
where packets matching the route will be discarded. Whenever the summarizing router receives a
packet destined for the summary route but the specific route does not exist in the routing table,
the packet will be matched to the null route and being discarded. This prevents the summarizing
router from forwarding the packet to its default route which would possibly create a routing loop.
The null route will only be seen on the summarizing router.
- To find the best summary route, locate the bits of the subnet numbers that have the common bit
pattern, from left to right (highest-order bits) – this is the subnet mask for the summary route.
- Best summary route for RT1:
00001010 00000010 00000001 00000000 – 10.2.1.0
00001010 00000010 00000010 00000000 – 10.2.2.0
00001010 00000010 00000011 00000000 – 10.2.3.0
00001010 00000010 00000100 00000000 – 10.2.4.0
The best summary route for RT1 is 10.2.0.0, subnet mask 255.255.248.0.
- Best summary route for RT3:
00001010 00000011 00000100 00000000 – 10.3.4.0
00001010 00000011 00000101 00000000 – 10.3.5.0
00001010 00000011 00000110 00000000 – 10.3.6.0
00001010 00000011 00000111 00000000 – 10.3.7.0
The best summary route for RT3 is 10.3.4.0, subnet mask 255.255.252.0.
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