Hi Martin, Keepalives are sent on the Catalyst 2940, 2950, 2950-LRE, 2955, 2970, 3550, 3560 or 3750 switch to prevent loops in the network. The primary reason for the keepalives is to prevent loops as a result of Type 2 cabling which does cause a loop in some situations. A loop is detected when the switch receives back it's own keepalive pakcet.
Keepalives are sent on ALL interfaces by default in 12.1EA based software. Starting in 12.2SE based releases, keepalives are NO longer sent by default on fiber and uplink interfaces. Best regards, Andras On Sun, Jul 31, 2011 at 3:51 AM, Martin T <[email protected]> wrote: > I have a following connection: > > T60[eth0] <-> [Fa0/2]WS-C2950C-24 > > ..and port Fa0/2 in the switch in configured like this: > > WS-C2950C-24#sh run int Fa0/2 > Building configuration... > > Current configuration : 149 bytes > ! > interface FastEthernet0/2 > description -> T60 > switchport mode access > switchport nonegotiate > no cdp enable > spanning-tree bpdufilter enable > end > > WS-C2950C-24# > > ..and "keepalive" signals are sent after every 10s: > > WS-C2950C-24#sh int Fa0/2 | i Keepalive > Keepalive set (10 sec) > WS-C2950C-24# > > Now if I tcpdump those frames, they look like this: > > root@martin-ThinkPad-T60:~# tcpdump -i eth0 -e -XX -c 4 > tcpdump: WARNING: eth0: no IPv4 address assigned > tcpdump: verbose output suppressed, use -v or -vv for full protocol decode > listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes > 03:26:35.984629 00:12:7f:13:8f:c2 (oui Unknown) > 00:12:7f:13:8f:c2 > (oui Unknown), ethertype Loopback (0x9000), length 60: > 0x0000: 0012 7f13 8fc2 0012 7f13 8fc2 9000 0000 ................ > 0x0010: 0100 0000 0000 0000 0000 0000 0000 0000 ................ > 0x0020: 0000 0000 0000 0000 0000 0000 0000 0000 ................ > 0x0030: 0000 0000 0000 0000 0000 0000 ............ > 03:26:45.984971 00:12:7f:13:8f:c2 (oui Unknown) > 00:12:7f:13:8f:c2 > (oui Unknown), ethertype Loopback (0x9000), length 60: > 0x0000: 0012 7f13 8fc2 0012 7f13 8fc2 9000 0000 ................ > 0x0010: 0100 0000 0000 0000 0000 0000 0000 0000 ................ > 0x0020: 0000 0000 0000 0000 0000 0000 0000 0000 ................ > 0x0030: 0000 0000 0000 0000 0000 0000 ............ > 03:26:55.984277 00:12:7f:13:8f:c2 (oui Unknown) > 00:12:7f:13:8f:c2 > (oui Unknown), ethertype Loopback (0x9000), length 60: > 0x0000: 0012 7f13 8fc2 0012 7f13 8fc2 9000 0000 ................ > 0x0010: 0100 0000 0000 0000 0000 0000 0000 0000 ................ > 0x0020: 0000 0000 0000 0000 0000 0000 0000 0000 ................ > 0x0030: 0000 0000 0000 0000 0000 0000 ............ > 03:27:05.984651 00:12:7f:13:8f:c2 (oui Unknown) > 00:12:7f:13:8f:c2 > (oui Unknown), ethertype Loopback (0x9000), length 60: > 0x0000: 0012 7f13 8fc2 0012 7f13 8fc2 9000 0000 ................ > 0x0010: 0100 0000 0000 0000 0000 0000 0000 0000 ................ > 0x0020: 0000 0000 0000 0000 0000 0000 0000 0000 ................ > 0x0030: 0000 0000 0000 0000 0000 0000 ............ > 4 packets captured > 4 packets received by filter > 0 packets dropped by kernel > root@martin-ThinkPad-T60:~# > > As you can see, they are sent by switch port after every 10s. The > source and destination MAC address are the same and ethertype is > 0x9000 and it looks like the frame is just padded with zeros. I can > change the keepalive messages interval between 1s and 32767s or > disable keepalive frames by "no keepalive" or "keepalive 0". > What are those "keepalive" frames used for? Some historical > configuration setting? What should my T60 NIC do with those frames as > at the moment it responds nothing? > > > regards, > martin > _______________________________________________ > cisco-nsp mailing list [email protected] > https://puck.nether.net/mailman/listinfo/cisco-nsp > archive at http://puck.nether.net/pipermail/cisco-nsp/ > _______________________________________________ cisco-nsp mailing list [email protected] https://puck.nether.net/mailman/listinfo/cisco-nsp archive at http://puck.nether.net/pipermail/cisco-nsp/
