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/31 on Catalyst 3750 routed port

I've discovered the feature 'Using 31-Bit Prefixes on IPv4 Point-to-Point Links' and I'm interested in implementing it on a point-to-point uplink network that terminates on one end to a Catalyst 3750 routed FastEthernet port ("no switchport").

I can enter the IP address in the interface config (172.16.100.0 255.255.255.254), and I get the warning "% Warning: use /31 mask on non point-to-point interface cautiously". Since I can guarantee that this is in fact a point-to-point uplink to a Layer 3 device, can I safely ignore the warning? Is there any way to configure the interface explicitly as a Point-to-Point interface instead of a broadcast interface?

Thanks!

-Mason

1 ACCEPTED SOLUTION

Accepted Solutions

Re: /31 on Catalyst 3750 routed port

Hi,

Forgot to mention a couple of things in my previous post. Though you are using a /31 bit mask on the ethernet it still supports local-link (limited) broadcast and broadcast (all 1's). However, directed broadcast isn't supported with this configuration. Local-link is needed for the interface to resolve the IP-ARP of other devices on the same multiaccess segment.

For more info on this check out the RFC 3021 at the link below.

http://www.faqs.org/rfcs/rfc3021.html

HTH

Sundar

5 REPLIES
Hall of Fame Super Silver

Re: /31 on Catalyst 3750 routed port

Mason

You may know for sure that the connection is functionally a point to point, but the 3750 knows that its interfaces are inherently capable of multiaccess (broadcast) functionality. I am not aware of any configuration option to make the 3750 interface point to point instead of broadcast.

This is still an Ethernet network interface and it will need to do certain functions (ARP is one that comes to mind) that are based on being sent to broadcast destination addresses. You may get the /31 to work (the message was a "warning" not an "error") but the interface is still inherently broadcast capable.

HTH

Rick

Re: /31 on Catalyst 3750 routed port

Hi,

It doesn't appear the error is platform specific. I am getting the same error in my lab 2600 when configuring a /31 bit mask.

You would use the /31 bit mask only on a p-t-p interface hence, you don't need to tell the interface it's a p-t-p interface. I don't know of any generic command to tell the broadcast interface that it's a p-t-p interface. There is a OSPF command but that doesn't have anything to do with this setup.

R2(config)#int e0/0

R2(config-if)#ip add 1.1.1.0 255.255.255.254

% Warning: use /31 mask on non point-to-point interface cautiously

R2(config-if)#^Z

R2#show run

*Mar 1 18:20:21.795: %SYS-5-CONFIG_I: Configured from console by console int e

/0

Building configuration...

Current configuration : 78 bytes

!

interface Ethernet0/0

ip address 1.1.1.0 255.255.255.254

half-duplex

end

R2#show ip arp

Protocol Address Age (min) Hardware Addr Type Interface

Internet 1.1.1.0 - 0004.dd85.eee0 ARPA Ethernet0/0

R2#ping 1.1.1.1

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 1.1.1.1, timeout is 2 seconds:

.!!!!

Success rate is 80 percent (4/5), round-trip min/avg/max = 1/3/4 ms

R2#show ip arp

Protocol Address Age (min) Hardware Addr Type Interface

Internet 1.1.1.1 0 0004.dd85.e9c0 ARPA Ethernet0/0

Internet 1.1.1.0 - 0004.dd85.eee0 ARPA Ethernet0/0

HTH

Sundar

Re: /31 on Catalyst 3750 routed port

Hi,

Forgot to mention a couple of things in my previous post. Though you are using a /31 bit mask on the ethernet it still supports local-link (limited) broadcast and broadcast (all 1's). However, directed broadcast isn't supported with this configuration. Local-link is needed for the interface to resolve the IP-ARP of other devices on the same multiaccess segment.

For more info on this check out the RFC 3021 at the link below.

http://www.faqs.org/rfcs/rfc3021.html

HTH

Sundar

New Member

Re: /31 on Catalyst 3750 routed port

Thanks to all responders on this. This last post describing the link-local actually made everything click for me.

I've been mostly concerned about how broadcast traffic on the point-to-point link would work (ARP, L3 broadcasts, etc) but was stuck thinking in terms of L3 broadcasts when most of the functions I care about are either L2 broadcasts or L3 Multicasts (routing protocols, etc).

Also, thanks for trying this on a 2600 router - it's helpful to know the behaviour is the same on a router as on the switch.

-Mason

Cisco Employee

Re: /31 on Catalyst 3750 routed port

Well, I think this warning is platform specific. remember, we are talking about ethernet interface,which is broadcast/multiaccess by default, thus we are getting the message "use /31 mask on non point-to-point interface". I believe if we try this (I'm not able to for some reasons) on a serial interface which is a point to point interface (can be configured to enable broadcast as well),we should not get this warning.

Please post as applicable.

Thanks

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