Re: Frame Relay Sub-Interfaces

kfarrington · ‎11-21-2005

Guys,

I have just read an extract from the Cisco Frame-relay Solutions guide and I see the following :-

On a Cisco router, by default, physical interfaces are multipoint interfaces

So why is a physical interface with FR encap have split-horizon disabled by default and a "multipoint" sub-interface have Split-horizon enabled by default

I have the following

Physical - SH disabled

Sub-int Multipoint - SH enabled

Sub-int P2P - SH enabled

This seems a bit strange to me, as I would expect the first two to emulate each other and have the same interface characteristics as they are basically the same type of interface?

Am I missing sommat here and is there a matrix for the FR configurations and SH rules.

Many kind regards,

Ken

output below :-

!

interface Serial0/1

ip address 33.33.33.33 255.255.255.0

encapsulation frame-relay

!

interface Serial0/1.100 multipoint

ip address 44.44.44.44 255.255.255.0

!

interface Serial0/1.101 point-to-point

ip address 55.55.55.55 255.255.255.0

!

Dual3#sh ip int s0/1

Serial0/1 is down, line protocol is down

Internet address is 33.33.33.33/24

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

.....text omiited

!

Dual3#sh ip int s0/1.100

Serial0/1.100 is down, line protocol is down

Internet address is 44.44.44.44/24

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

.....text omiited

!

Dual3#sh ip int s0/1.101

Serial0/1.101 is down, line protocol is down

Internet address is 55.55.55.55/24

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

.....text omiited

a-vazquez · ‎11-25-2005

Framerelay interfaces are normally NBMA(Non-Broadcast Multiaccess) in nature.

Once a router interface/subinterface is configured as a Multipoint, automatically Split horizon is enabled.The reason is, since in multipoint the same intf/subinterface cannot send the data back through itself to reach another destination of this multipoint network.

If we treat a subinterface a P2P, then no problem, each subintf is treated as separate p2p link.That is data received on that subinterface p2p can be transmitted via the same sub intf to another destination which is also treated as p2p.

In short,a p2p int/subint -> split horizon is disabled (means split horizon issue is solved)

a p2MP int/subint - > splithorizon still holds true(the pblm is not solved and its enabled).

kfarrington · ‎12-12-2005

Sorry, I am confused. I have configured the following ?

!

interface Serial0/2

ip address 4.4.4.4 255.255.255.0

encapsulation frame-relay

!

interface Serial0/2.1 multipoint

ip address 5.5.5.5 255.255.255.0

!

interface Serial0/2.2 point-to-point

ip address 5.5.6.5 255.255.255.0

!

one major, one multipoint subint and one p2p subint

IS this correct, that both subints have split horizon enabled and the major interface disabled?

they all run frame encap

Kind regards,

Ken

Dual3#

Dual3#sh ip int s0/2

Serial0/2 is down, line protocol is down

Internet address is 4.4.4.4/24

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

Dual3#sh ip int s0/2.1

Serial0/2.1 is down, line protocol is down

Internet address is 5.5.5.5/24

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

Dual3#sh ip int s0/2.2

Serial0/2.2 is down, line protocol is down

Internet address is 5.5.6.5/24

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

Dual3#

Dual3#sh int s0/2

Serial0/2 is down, line protocol is down

Hardware is PowerQUICC Serial

Internet address is 4.4.4.4/24

MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,

reliability 255/255, txload 1/255, rxload 1/255

Encapsulation FRAME-RELAY, loopback not set

Keepalive set (10 sec)

LMI enq sent 0, LMI stat recvd 0, LMI upd recvd 0, DTE LMI down

LMI enq recvd 0, LMI stat sent 0, LMI upd sent 0

LMI DLCI 1023 LMI type is CISCO frame relay DTE

Broadcast queue 0/64, broadcasts sent/dropped 0/0, interface broadcasts 0

Last input never, output never, output hang never

Last clearing of "show interface" counters 00:01:55

Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0

Queueing strategy: weighted fair

Output queue: 0/1000/64/0 (size/max total/threshold/drops)

Conversations 0/0/256 (active/max active/max total)

Reserved Conversations 0/0 (allocated/max allocated)

Available Bandwidth 1158 kilobits/sec

5 minute input rate 0 bits/sec, 0 packets/sec

5 minute output rate 0 bits/sec, 0 packets/sec

0 packets input, 0 bytes, 0 no buffer

DCD=up DSR=up DTR=down RTS=down CTS=up

Dual3#sh int s0/2.1

Serial0/2.1 is down, line protocol is down

Hardware is PowerQUICC Serial

Internet address is 5.5.5.5/24

MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,

reliability 255/255, txload 1/255, rxload 1/255

Encapsulation FRAME-RELAY

Last clearing of "show interface" counters never

Dual3#sh int s0/2.2

Serial0/2.2 is down, line protocol is down

Hardware is PowerQUICC Serial

Internet address is 5.5.6.5/24

MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,

reliability 255/255, txload 1/255, rxload 1/255

Encapsulation FRAME-RELAY

Last clearing of "show interface" counters never

Dual3#

Richard Burts · ‎12-12-2005

Ken

Your question appears pretty simple:

IS this correct, that both subints have split horizon enabled and the major interface disabled?

And the output of show ip interface provices a pretty direct answer that yes you are correct.

Looking at the original post I believe that this is about your discomfort that there is inconsistency between the behavior of the physical interface (multipoint with split horizon disabled) and the behavior of the multipoint subinterface (multipoint with split horizon enabled). Is that correct?

I do not know the authoritative explanation for this (perhaps some of the Cisco folk might fill it in) but have thought about it this way: If you just take simple defaults and use the physical interface, the DLCIs will default to being associated with the physical interface, it defaults to multipoint since there is the possibility of multiple DLCIs, and to protect those who are not doing much configuration Cisco turns off split horizon. When you are doing more configuration and are placing DLCIs where you want them (both for point to point and for multipoint) Cisco assumes that if you want split horizon disabled that you will do it yourself. They have provided an exception to the normal behavior of interfaces (by disabling split horizon) on the physical interface and have left the subinterfaces to conform with more normal behavior.

HTH

Rick

HTH

Rick