Apr 12, 2008

Remove unwanted PPP peer route

When IOS started supporting dynamic allocation of IPCP (IP over PPP) addresses, it also got the mechanism to insert a dynamic host route toward the neighbor's IP address once the IP addresses were negotiated. This mechanism makes perfect sense in dynamic address allocation environments, as the subnet mask is not negotiated with IPCP. Without a host route toward the other end of the PPP link, there would be no easy way to reach the IP prefixes reachable via the PPP link.

However, the PPP code got way too aggressive in the recent IOS releases. For example, in the 12.4T train, you get a connected host route toward the IP address of the PPP peer even on a leased line where both addresses are in the same IP subnet. Here's a sample printout from my lab router that illustrates this behavior:

R9#sh 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 external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

200.0.0.0/32 is subnetted, 1 subnets
C 200.0.0.9 is directly connected, Loopback0
150.50.0.0/16 is variably subnetted, 2 subnets, 2 masks
C 150.50.69.1/32 is directly connected, Serial1/1
C 150.50.69.0/30 is directly connected, Serial1/1


To disable the automatic insertion of the connected host route, use the no peer neighbor-route interface configuration command.

R9#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R9(config)#int s1/1
R9(config)#no peer neighbor-route
R9(config-if)#shutdown
R9(config-if)#no shutdown
*Mar 1 00:51:11.571: %LINK-5-CHANGED: Interface Serial1/1, changed state to administratively down
*Mar 1 00:51:12.571: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial1/1, changed state to down
*Mar 1 00:51:15.591: %LINK-3-UPDOWN: Interface Serial1/1, changed state to up
*Mar 1 00:51:16.731: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial1/1, changed state to up
R9(config-if)#^Z
R9#sh 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 external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

200.0.0.0/32 is subnetted, 1 subnets
C 200.0.0.9 is directly connected, Loopback0
150.50.0.0/30 is subnetted, 1 subnets
C 150.50.69.0 is directly connected, Serial1/1
R9#
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