EIGRP and PPP

I have two routers directly connected with both Ethernet and Serial links. The serial links connecting Router D and Router C are using PPP encapsulation with a \30 mask.

The other Serial Link (S0/1) is using on RTC is connecting to another router RTB using HDLC encapsulation and a \30 mask

My routing protocol is EIGRP and it has been enabled on all interfaces. Here are the ip int configs on both routers

Router

RTD#sh ip int brief
Interface                  IP-Address      OK? Method Status                Link

FastEthernet0/0            10.8.0.5        YES manual up                    up

Serial0/0                  10.200.1.2      YES NVRAM  up                    up

FastEthernet0/1            unassigned      YES manual administratively down down

Serial0/1                  unassigned      YES NVRAM  administratively down down

Serial0/2                  unassigned      YES NVRAM  administratively down down

Loopback0                  10.10.10.10     YES NVRAM  up                    up

Loopback1                  20.20.20.20     YES NVRAM  up                    up

Router C:

RTC#sh ip int brief
Interface                  IP-Address      OK? Method Status                Link

FastEthernet0/0            10.8.0.4        YES manual up                    up

Serial0/0                  10.200.1.1      YES manual up                    up

FastEthernet0/1            unassigned      YES manual administratively down down

Serial0/1                  10.200.2.1      YES manual up                    up

Loopback0                  30.30.30.30     YES NVRAM  up                    up

Both connected interfaces have become neighbors and routing information is being shared and added to the respected topologies. Here is the topology for Router C

RTC#sh ip eigrp top
IP-EIGRP Topology Table for AS(1)/ID(30.30.30.30)

Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
       r - reply Status, s - sia Status

P 30.30.30.30/32, 1 successors, FD is 128256
        via Connected, Loopback0
P 20.20.20.20/32, 1 successors, FD is 156160
        via 10.8.0.5 (156160/128256), FastEthernet0/0
        via 10.200.1.2 (2297856/128256), Serial0/0
P 10.10.10.10/32, 1 successors, FD is 156160
        via 10.8.0.5 (156160/128256), FastEthernet0/0
        via 10.200.1.2 (2297856/128256), Serial0/0
P 10.8.0.0/24, 1 successors, FD is 28160
        via Connected, FastEthernet0/0
P 10.200.2.0/30, 1 successors, FD is 2169856
        via Connected, Serial0/1
P 10.200.1.2/32, 1 successors, FD is 2169856
        via Rconnected (2169856/0)
P 10.200.1.1/32, 0 successors, FD is Inaccessible
        via 10.8.0.5 (2172416/2169856), FastEthernet0/0
        via 10.200.1.2 (2681856/2169856), Serial0/0
P 10.200.1.0/30, 1 successors, FD is 2169856
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
       r - reply Status, s - sia Status

        via Connected, Serial0/0

My questions are

Why is PPP creating 32 bit masks for the serial interfaces?

Why does HDLC not do this as well since it establishes a point to point link?

Why is the physical serial interface of RTC (10.200.1.1) listed in the topology table?

Why is 10.200.1.1 have an FD that is Inaccessible, it is running EIGRP?

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Comments

gojericho0

My questions are

Why is PPP creating 32 bit masks for the serial interfaces?

Why does HDLC not do this as well since it establishes a point to point link?

Why is the physical serial interface of RTC (10.200.1.1) listed in the topology table?

Why is 10.200.1.1 have an FD that is Inaccessible, it is running EIGRP?

Well I think I figured out my first question and am using this answer to derive theories for the other questions. Someone please correct me if I am wrong:

1) There is an IPCP option that allows PPP to auto-negotiate an IP for the other end of the link. If an IP is already assigned or this option is disabled, a NACK will be sent from the remote host. The local host will see this reply and put a host route in the routing table.

2) HDLC does not do this since it is not designed to have CPs

3) Since 10.200.1.1 is derived from the IPCP function of PPP and not EIGRP, EIGRP has doesn't recognize it and considers in inaccessible

EdTheLad

Great question,this is something i never noticed and could appear in the ccie lab.
This is a feature of PPP called "peer neighbor-route" , so if you want to stop ppp advertising the /32 addresses you must disbale this feature on the interface and then toggle the port.
router(config-if)#no peer neighbor-route

gojericho0

EdTheLad wrote:

Great question,this is something i never noticed and could appear in the ccie lab.
This is a feature of PPP called "peer neighbor-route" , so if you want to stop ppp advertising the /32 addresses you must disbale this feature on the interface and then toggle the port.
router(config-if)#no peer neighbor-route

Thanks Ed,

I can see this would be useful in a dynamic environment like a lab, but in my opinion this seems way to aggressive of a feature to be enabled automatically especially since it happens on a leased line with both connections in the same subnet!