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Configuration of RIP and IGRP 383
RIP-1 and IGRP--No Subnet Masks
RIP-1 and IGRP do not transmit the subnet mask in the routing updates, as seen in the debug
output examples in this section. As a CCNA, Cisco expects you to be able to articulate the
implications of the missing mask to the function of the routing protocol. Several subtle actions
are taken in light of the lack of mask information in the update:
·
Updates sent out an interface in network X, when containing routes about subnets of
network X, contain the subnet numbers of the subnets of network X but not the
corresponding masks.
·
Updates sent out an interface in network X, when containing routes about subnets of
network Y, are summarized into one route about the entire network Y.
·
When receiving a routing update containing routes referencing subnets of network X, the
receiving router assumes that the mask in use is the same mask it uses on an interface with
an address in network X.
·
When receiving an update about network X, if the receiving router has no interfaces in
network X, it treats the route as a route to the entire Class A, B, or C network X.
Example 6-6, Example 6-7, and Example 6-8 contain show and debug command output on
Albuquerque, Yosemite, and Seville with the effects described in the preceding list. The
network of Figure 6-10 is still in use, but the subnet on Seville's Ethernet has been changed
from 10.1.3.0/24 to 10.1.3.192/26. Because RIP-1 does not send the mask in the update, Seville
chooses not to address 10.1.3.192/26 onto its serial links (which use mask 255.255.255.0),
because the update would be ambiguous.
Example 6-6
Configuration and Debug IP RIP on Albuquerque
interface ethernet 0
ip addr 10.1.1.251 255.255.255.0
interface serial 0
ip addr 10.1.4.251 255.255.255.0
interface serial 1
ip addr 10.1.6.251 255.255.255.0
!
router rip
network 10.0.0.0
_______________________________________________________________________
Albuquerque#debug ip rip
RIP protocol debugging is on
Albuquerque#
00:38:23: RIP: received v1 update from 10.1.4.252 on Serial0
00:38:23: 10.1.2.0 in 1 hops
00:38:23: 10.1.5.0 in 1 hops
00:38:33: RIP: sending v1 update to 255.255.255.255 via Serial0 (10.1.4.251)
00:38:33: subnet 10.1.1.0, metric 1
00:38:33: subnet 10.1.6.0, metric 1
00:38:33: RIP: sending v1 update to 255.255.255.255 via Serial1 (10.1.6.251)
00:38:33: subnet 10.1.2.0, metric 2
continues
ch06.fm Page 383 Monday, March 20, 2000 5:11 PM