PPP 101

PPP is an open-standard, media-independent Layer 2 protocol that can provide such features as authentication, multilink, compression, reliability.

PPP uses 2 negotiation phases. First, LCP (Link Control Protocol) is used to set up the link and to negotiate authentication, compression mechanisms, available MTU size, and so on, and then there’s a NCP phase (Network Control Protocol) specific for each network layer protocol. There’s IPCP – for IPv4, IPv6CP – for IPv6 or CDPCP – for CDP (CDP is a actually a Layer 2 protocol that works over PPP)

For example, after LCP phase is over, the IPCP protocol will be used to exchange IP related information, like getting the IP address of the interface.

1. Physical interface vs Dialer Interface

1.1 Physical interface

For back to back connections between 2 routers, the easiest way is to use PPP encapsulation on the physical interfaces connecting them:

R(config-if)# encapsulation ppp
R(config-if)# no shut

1.2 Dialer Interface

In other environments, you can define the server’s physical interface to use PPP encapsulation as above, and use a Dialer Interface on the client.

R(config)# interface DIALER-INT
R(config-if)# encapsulation ppp
! By default, dialer interfaces use HDLC encapsulation
R(config-if)# dialer-pool POOL-ID

The connection between the Dialer interface and physical interfaces is done via a Dialer Pool:

PPP Dialer

To configure the physical interface to be a member of a Dialer Pool, use:

R(config)# interface SERIAL-INT
R(config)# encapsulation ppp
R(config)# dialer in-line
R(config-if)# dialer pool-memeber POOL-ID

From now on, all PPP configuration is done on the Dialer interface, which acts as a PPP Profile that is applied on the physical interfaces.

A Dial interfaces is activated whenever interesting traffic is sent over it. To define the interesting traffic you will have to configure the Dial interface to be part of a Dial Group.

R(config-if)# dialer-group DIALER-LIST

Then, the Dial Group will reference a Dial List, which will define interesting traffic. This configuration is done globally:

R(config)# dialer-list DIALER-LIST protocol PROTOCOL {list ACL|permit|deny}
! PROTOCOL - ip, ipv6, bridge, etc
! permit - will define all PROTOCOL traffic as interesting traffic
! deny - will remove all PROTOCOL traffic from the interesting traffic
! list ACL - will define interesting traffic based on ACL

Whenever there is interesting traffic to be sent over the Dial interface, it will generate a Dial Call.
You can configure the Dial interface to be always up if you use:

R(config-if)# dialer persistent
! To keep the interface up/up even if there is no interesting traffic
! This command will auto-generate:
R(config-if)# dialer idle-timeout 0

You can verify the status of a Dialer interface using:

R# show dialer

2. Keepalives

PPP uses keepalives to monitor the link state. By default, keepalives are sent and expected every 10 seconds. Unlike most protocols, five missed keepalives will move the interface protocol to a “down” status.

R(config-if)#keepalive ?
< 0-32767> Keepalive period (default 10 seconds)
< cr>

Hitting enter will set the default value of 10 seconds. To disable keepalives completly use:

R1(config-if)# no keepalive

To verify keepalives, use:

R1# show interface serial0/0 | i alive
Keepalive set (10 sec)

3. IP address Pooling

3.1 Client Config

A PPP client can manually set its IP address with

R(config-if)# ip address IP-ADDR NETMASK [secondary]

But it can also use IPCP to discover the IP address it should use. To enable the use of IPCP for address assginement, use:

R(config-if)# ip address negotiated [previous]

If the previous keyword is used, then the client will attempt to get the previously assigned address from the server. The server can ignore this request if it uses:

R(config-if)# peer ip address forced

Addresses assigned using IPCP are always /32 addresses. Additional information may be requested by the client if it is explicitly configured:

R(config-if)# ppp ipcp dns request
R(config-if)# ppp ipcp mask request
! While this is not used by PPP, it may be used by other functions like DHCP ODAP
R(config-if)# ppp ipcp wins request

In order to reply to these requests, the server must be explicitly configured to send them. See next section.

3.2 Server Config

When a PPP client requests its IP Address from the PPP Dial-in server, the latter will send by default an IP address from a local pool. This default mechanism can be overridden globally or on the interface,

R(config)# ip address-pool {local|dhcp-pool|dhcp-proxy-client}
! local - Uses the default local pool (Default)
! dhcp-pool - Uses a local DHCP pool
! dhcp-proxy-client - Proxies the request to a DHCP server 

Per interface:

R(config-if)# peer default ip address {CLIENT-IP | pool [IP-POOL]| dhcp-pool [DHCP-POOL] |dhcp}
! CLIENT-IP: assigns the configured address to clients on this interface
! pool [IP-POOL]: Sets a local-pool as default mechanism on this interface.
! dhcp-pool [DHCP-POOL]: Uses a local DHCP Pool as the default mechanism on this interface
! dhcp: Acts a a DHCP Proxy Client and forwards requests to a remote DHCP Server

3.2.1 Local Address Pools

To define a local IP-POOL, then use:

R(config)# ip local pool {IP-POOL|default} START-IP [END-IP] ...

3.2.2 Local DHCP Pools

To define a DHCP-POOL and enter DHCP Pool Configuration Mode, use:

R(config)# ip dhcp pool DHCP-POOL

3.2.3 DHCP Proxy

For DHCP Proxy client, configure the remote DHCP Server, using:

 R(config)# ip dhcp-server IP-ADDR
! Optionally, allow DNS and Netbios discovery:
R(config)# ip dhcp-client network-discovery informs INFORMS discovers DISCOVERS period SEC

There is a problem when using the DHCP client. The router will forward the request sourced from the PPP interface. The DHCP server will send the reply unicast to this address. But since the negotiation is not completed until the other side receives an address (or renounces after too many fails – which will be too late anyway), the interface will have an up/down status. This means that it won’t be advertised by routing protocols, so probably the DHCP server doesn’t have a route back to it. This can be resolved by using static routes on the path from the server to our router, or in a more dynamic way, by using a loopback address and setting the address on the PPP interface as unnumbered from that loopback. This will let the prefix to be advertised by routing protocols.

3.2.4 Responding to IPCP requests

When an IPCP client requests additional information, the router has to explicitly define them with:

R(config-if)# ppp ipcp dns SERVER1 [SERVER2...]
R(config-if)# ppp ipcp netmask MASK
R(config-if)# ppp ipcp wins SERVER1 [SERVER2...]

4. PPP host route

When a PPP link is up, it will add a /32 route to that points to the IP address on the other end of the PPP link. This makes it possible for hosts with IP addresses in different subnets to be directly connected over a PPP link.

R2#sh ip route
Gateway of last resort is not set is subnetted, 1 subnets
C is directly connected, Serial1/1 is subnetted, 1 subnets
C is directly connected, Serial1/1

This feature can be disabled using the command:

R2(config-if)# no peer neighbor-route

5. Authentication

PPP supports multiple authentication protocols, like PAP, CHAP, MS-CHAP, MS-CHAPv2 and EAP.

6. Compression

PPP can be configured to use a software compression algortithm. Compared to HDLC, PPP supports more compression algorithms and can be configured with different algorithms on each end. PPP LCP phase will take care of the negotiation.

To see the available compression algorithms use:

R(config-if)#compress ?
  lzs        lzs compression type
  mppc       MPPC compression type
  predictor  predictor compression type
  stac       stac compression algorithm
  < cr>

To see compression statistics use:

R1#sh compress details
	 Software compression enabled
	 uncompressed bytes xmt/rcv 15627/15729
	 compressed bytes   xmt/rcv 3868/4258
	 Compressed bytes sent:      3868 bytes   0 Kbits/sec  ratio: 4.040
	 Compressed bytes recv:      4258 bytes   0 Kbits/sec  ratio: 3.693
	 1  min avg ratio xmt/rcv 0.130/0.131
	 5  min avg ratio xmt/rcv 0.061/0.062
	 10 min avg ratio xmt/rcv 0.061/0.062
	 no bufs xmt 0 no bufs rcv 0
	 resyncs 0
	 Compression   Protocol used: PPP Stac LZS
	 Check Mode = 3
	 Decompression Protocol used: Predictor 1

Notice the compression ratio and that different protocols are used for compression (TX) and decompression (RX)

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