Frame Relay interface types

That's an awesome piece of explanation i borrowed from Marco Rizzi Blog and really it's very helpful.

There are several interface types on frame-relay, according on how you configure your serial interface and on what is the purpose you will archive:

1) physical interface
2) subinterface multipoint
3) subinterface point-to-point

Let's talk about each type..

1) Physical interfaces
-are threated as multipoint
-all DLCIs declared by lmi are assigned to physical interfaces
From a L3 point of view, all the neighbors are expected in the same subnet and you can use a static L3 to L2 mapping or inverse arp, as previously seen.
Note also that by default, on frame-relay physical interfaces the SPLIT ORIZON is disabled, useful to solve distance-vector routing protocol issues.



Example:

R4#sh run int ser 0/3/0 | beg int
interface Serial0/3/0
 description R4 - (R1|R2) without intverse-arp
 ip address 10.0.0.4 255.255.255.0
 encapsulation frame-relay
 frame-relay map ip 10.0.0.2 402 broadcast
 frame-relay map ip 10.0.0.1 401 broadcast
 no frame-relay inverse-arp
end

R4#sh frame-relay map
Serial0/3/0 (up): ip 10.0.0.1 dlci 401(0x191,0x6410), static,
              broadcast,
              CISCO, status defined, active
Serial0/3/0 (up): ip 10.0.0.2 dlci 402(0x192,0x6420), static,
              broadcast,
              CISCO, status defined, active

R4#show ip interface ser 0/3/0
Serial0/3/0 is up, line protocol is up
  Internet address is 10.0.0.4/24
  Broadcast address is 255.255.255.255
  Address determined by setup command
  MTU is 1500 bytes
  Helper address is not set
  Directed broadcast forwarding is disabled
  Outgoing access list is not set
  Inbound  access list is not set
  Proxy ARP is enabled
  Local Proxy ARP is disabled
  Security level is default
  Split horizon is disabled
  ICMP redirects are always sent
  ICMP unreachables are always sent
  ICMP mask replies are never sent
  IP fast switching is enabled
  IP fast switching on the same interface is enabled
  IP Flow switching is disabled
  IP CEF switching is enabled
  IP CEF switching turbo vector
  IP multicast fast switching is enabled
  IP multicast distributed fast switching is disabled
  IP route-cache flags are Fast, CEF
  Router Discovery is disabled
  IP output packet accounting is disabled
  IP access violation accounting is disabled
  TCP/IP header compression is disabled
  RTP/IP header compression is disabled
  Policy routing is disabled
  Network address translation is disabled
  BGP Policy Mapping is disabled
  Input features: MCI Check
  WCCP Redirect outbound is disabled
  WCCP Redirect inbound is disabled
  WCCP Redirect exclude is disabled

Subinterfaces Multipoint
-DLCIs must manually assigned to the subinterface if inverse arp is used
-ip/ipv6 must be manually mapped if inverse arp is disabled
With subinterfaces multipoint, split orizon is enabled by default, as shown in the following example:

R4#sh run int ser 0/3/0.1  | beg int
interface Serial0/3/0.1 multipoint
 description R4 - (R1|R2) with inverse-arp
 ip address 10.0.0.4 255.255.255.0
 snmp trap link-status
 frame-relay interface-dlci 401
 frame-relay interface-dlci 402
end

R4#sh frame-relay pvc | inc DLCI
DLCI = 401, DLCI USAGE = LOCAL, PVC STATUS = ACTIVE, INTERFACE = Serial0/3/0.1
DLCI = 402, DLCI USAGE = LOCAL, PVC STATUS = ACTIVE, INTERFACE = Serial0/3/0.1
DLCI = 403, DLCI USAGE = UNUSED, PVC STATUS = ACTIVE, INTERFACE = Serial0/3/0

R4#sh ip interface serial 0/3/0.1
Serial0/3/0.1 is up, line protocol is up
  Internet address is 10.0.0.4/24
  Broadcast address is 255.255.255.255
  Address determined by setup command
  MTU is 1500 bytes
  Helper address is not set
  Directed broadcast forwarding is disabled
  Outgoing access list is not set
  Inbound  access list is not set
  Proxy ARP is enabled
  Local Proxy ARP is disabled
  Security level is default
  Split horizon is enabled
  ICMP redirects are always sent
  ICMP unreachables are always sent
  ICMP mask replies are never sent
  IP fast switching is enabled
  IP fast switching on the same interface is enabled
  IP Flow switching is disabled
  IP CEF switching is enabled
  IP CEF switching turbo vector
  IP multicast fast switching is enabled
  IP multicast distributed fast switching is disabled
  IP route-cache flags are Fast, CEF
  Router Discovery is disabled
  IP output packet accounting is disabled
  IP access violation accounting is disabled
  TCP/IP header compression is disabled
  RTP/IP header compression is disabled
  Policy routing is disabled
  Network address translation is disabled
  BGP Policy Mapping is disabled
  Input features: MCI Check
  WCCP Redirect outbound is disabled
  WCCP Redirect inbound is disabled
  WCCP Redirect exclude is disabled
R4#

Subinterfaces Point-to-point
-a single DLCI can be specified with the "frame-relay interface-dlci " command
-don't send out inverse-arp queries, but still respond to the received inarp queries.
This type of interface assumes that the whole subnet is reachable through the only DLCI, so no need of L3 to L2 resolution and it's nice to have Split Orizon enabled for distance-vector routing protocols (otherwise you will maybe have routing l00ps)

Example:

R2#sh run | beg Serial0/2/0
interface Serial0/2/0:1
 no ip address
 encapsulation frame-relay
!
interface Serial0/2/0:1.1 point-to-point
 description R2 - R4
 ip address 10.0.0.2 255.255.255.0
 snmp trap link-status
 frame-relay interface-dlci 204
!
interface Serial0/2/0:1.2 point-to-point
 description R2 - R3
 ip address 10.0.1.2 255.255.255.0
 snmp trap link-status
 frame-relay interface-dlci 203

R2#sh frame-relay map
Serial0/2/0:1.2 (up): point-to-point dlci, dlci 203(0xCB,0x30B0), broadcast
          status defined, active
Serial0/2/0:1.1 (up): point-to-point dlci, dlci 204(0xCC,0x30C0), broadcast
          status defined, active

R2#sh ip interface ser 0/2/0:1.2
Serial0/2/0:1.2 is up, line protocol is up
  Internet address is 10.0.1.2/24
  Broadcast address is 255.255.255.255
  Address determined by setup command
  MTU is 1500 bytes
  Helper address is not set
  Directed broadcast forwarding is disabled
  Outgoing access list is not set
  Inbound  access list is not set
  Proxy ARP is enabled
  Local Proxy ARP is disabled
  Security level is default
  Split horizon is enabled
  ICMP redirects are always sent
  ICMP unreachables are always sent
  ICMP mask replies are never sent
  IP fast switching is enabled
  IP fast switching on the same interface is enabled
  IP Flow switching is disabled
  IP CEF switching is enabled
  IP CEF switching turbo vector
  IP Null turbo vector
  IP multicast fast switching is enabled
  IP multicast distributed fast switching is disabled
  IP route-cache flags are Fast, CEF
  Router Discovery is disabled
  IP output packet accounting is disabled
  IP access violation accounting is disabled
  TCP/IP header compression is disabled
  RTP/IP header compression is disabled
  Policy routing is disabled
  Network address translation is disabled
  BGP Policy Mapping is disabled
  Input features: MCI Check
  WCCP Redirect outbound is disabled
  WCCP Redirect inbound is disabled
  WCCP Redirect exclude is disabled

!-- END of part 3: next part will hopefully follow soon, with dirty tricks and routing examples