关于“clear arp”的一些问题
7月 11th, 2010,由 frank撰写
当在汇聚交换机上“clear arp”会发现arp表项会自动update，而无法删除。已经确认汇聚层交换机与接入层交换机之间没有任何流量，感觉交换机会自动发送arp信息。
经确认，发现是问题与Adj表有关。当交换机开启CEF转发后，有两个表比较重要，一个是FIB表，另一个就是Adj表。Adj表中的信息是由ARP表中提取而成，Adj表又关联了端口。当“clear arp”后，Adj表项仍然存在，所以交换机会自动发送ARP更新，如下所示：
vlan1: 1.1.1.1 vlan1: 1.1.1.2 3560-1（f0/1）---------(f0/1)3560-2
c3560-1#sh arp Protocol Address Age (min) Hardware Addr Type Interface Internet 1.1.1.1 - 000f.2406.bbc0 ARPA Vlan1 Internet 1.1.1.2 2 000f.2406.b840 ARPA Vlan1 c3560-1# c3560-1# c3560-1#sh mac add dy Mac Address Table ------------------------------------------- Vlan Mac Address Type Ports ---- ----------- -------- ----- 1 000f.2406.b803 DYNAMIC Fa0/1 1 000f.2406.b840 DYNAMIC Fa0/1 Total Mac Addresses for this criterion: 2 c3560-1# c3560-1#sh int vlan 1 | i Hard Hardware is EtherSVI, address is 000f.2406.bbc0 (bia 000f.2406.bbc0) c3560-1# c3560-1#sh ip cef 1.1.1.2 de 1.1.1.2/32, epoch 2 Adj source: IP adj out of Vlan1, addr 1.1.1.2 attached to Vlan1 c3560-1# c3560-1#sh adj de Protocol Interface Address IP Vlan1 1.1.1.2(7) 0 packets, 0 bytes epoch 5 sourced in sev-epoch 11 Encap length 14 000F2406B840000F2406BBC00800 ARP c3560-1# c3560-1#sh debug ARP: ARP packet debugging is on Adjacency system debugging is on filter: addr 1.1.1.2 (matches 46) IP CEF: IP CEF events debugging is on c3560-1# c3560-1# c3560-1#clear arp c3560-1# c3560-1# *Mar 2 22:21:10.027: IP ARP: sent req src 1.1.1.1 000f.2406.bbc0, dst 1.1.1.2 000f.2406.b840 Vlan1 *Mar 2 22:21:10.027: ADJ: IP adj out of Vlan1, addr 1.1.1.2 no src set: source ARP removed OK *Mar 2 22:21:10.027: ADJ: IP adj out of Vlan1, addr 1.1.1.2 (incomplete) no src set: changed bundle from IPv4 no fixup adj oce to IPv4 incomplete ad *Mar 2 22:21:10.027: ADJ: IP adj out of Vlan1, addr 1.1.1.2 (incomplete) no src set: made incomplete *Mar 2 22:21:10.035: ADJ: IP adj out of Vlan1, addr 1.1.1.2 (incomplete) no src set: delete *Mar 2 22:21:10.035: ADJ-ipv4 bundle changed from IPv4 no fixup adj oce to IPv4 no fixup, no redirect adjoce *Mar 2 22:21:10.035: IP ARP: sent rep src 1.1.1.1 000f.2406.bbc0, dst 1.1.1.1 ffff.ffff.ffff Vlan1 *Mar 2 22:21:10.035: IP ARP: rcvd rep src 1.1.1.2 000f.2406.b840, dst 1.1.1.1 Vlan1 *Mar 2 22:21:10.035: ADJ-ipv4 bundle changed from IPv4 no fixup, no redirect adj oce to IPv4 no fixup adj oce *Mar 2 22:21:10.035: ADJ: IP adj out of Vlan1, addr 1.1.1.2 (incomplete) no src set: set bundle to IPv4 incomplete adj oce *Mar 2 22:21:10.035: ADJ: IP adj out of Vlan1, addr 1.1.1.2 (incomplete) no src set: init/update from interface *Mar 2 22:21:10.035: ADJ: IP adj out of Vlan1, addr 1.1.1.2 (incomplete) no src set: allocated, setup and inserted OK *Mar 2 22:21:10.035: ADJ: IP adj out of Vlan1, addr 1.1.1.2 (incomplete) src ARP: source ARP added OK *Mar 2 22:21:10.035: ADJ: IP adj out of Vlan1, addr 1.1.1.2 (incomplete) src ARP: computed macstring (len 14): OK *Mar 2 22:21:10.035: ADJ: IP adj out of Vlan1, addr 1.1.1.2 src ARP: changed bundle from IPv4 incomplete adj oce to IPv4 no fixup adj oce *Mar 2 22:21:10.035: ADJ: IP adj out of Vlan1, addr 1.1.1.2 src ARP: made complete (macstring len 0 to 14/0 octets) *Mar 2 22:21:10.035: ADJ: IP adj out of Vlan1, addr 1.1.1.2 src ARP: received platform space change notification *Mar 2 22:21:10.035: IP ARP: creating entry for IP address: 1.1.1.2, hw: 000f.2406.b840 c3560-1# c3560-1# c3560-1#sh arp Protocol Address Age (min) Hardware Addr Type Interface Internet 1.1.1.1 - 000f.2406.bbc0 ARPA Vlan1 Internet 1.1.1.2 0 000f.2406.b840 ARPA Vlan1 c3560-1# c3560-1# c3560-1#clear adj !---Adj对应的端口不down，Adj表项是不会被清除的 c3560-1# c3560-1# *Mar 2 22:28:17.074: ADJ: IP adj out of Vlan1, addr 1.1.1.2 src ARP: found existing adj on add *Mar 2 22:28:17.074: ADJ: IP adj out of Vlan1, addr 1.1.1.2 src ARP: computed macstring (len 14): OK *Mar 2 22:28:17.074: ADJ-sev: request async walk @ 11 - walk [purge adjs from old HA epochs (ignore sev epoch)] filter [na/na] req [11] flags [3] *Mar 2 22:28:17.074: ADJ-sev: start @ 11 - walk [purge adjs from old HA epochs (ignore sev epoch)] filter [na/na] req [11] flags [3] *Mar 2 22:28:17.083: ADJ-sev: end @ 11 - walk [purge adjs from old HA epochs (ignore sev epoch)] filter [na/na] req [11] flags [3] [OK] c3560-1# c3560-1#sh arp Protocol Address Age (min) Hardware Addr Type Interface Internet 1.1.1.1 - 000f.2406.bbc0 ARPA Vlan1 Internet 1.1.1.2 7 000f.2406.b840 ARPA Vlan1 c3560-1# c3560-1# c3560-1# c3560-1# c3560-1# !---Shutdown F0/1，使Adj绑定的vlan1 down，才能清掉ARP表项 *Mar 2 22:32:19.572: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/1, changed state to down *Mar 2 22:32:19.589: %LINEPROTO-5-UPDOWN: Line protocol on Interface Vlan1, changed state to down *Mar 2 22:32:19.589: ADJ-sev: request async walk @ 12 - walk [remove adjacency sources] filter [na/Vlan1] req [11] flags [15] *Mar 2 22:32:19.589: ADJ-sev: start @ 12 - walk [remove adjacency sources] filter [na/Vlan1] req [11] flags [15] *Mar 2 22:32:19.589: ADJ: IP adj out of Vlan1, addr 1.1.1.2 no src set: source ARP removed OK *Mar 2 22:32:19.589: ADJ: IP adj out of Vlan1, addr 1.1.1.2 (incomplete) no src set: changed bundle from IPv4 no fixup adj oce to IPv4 incomplete ad *Mar 2 22:32:19.589: ADJ: IP adj out of Vlan1, addr 1.1.1.2 (incomplete) no src set: made incomplete *Mar 2 22:32:19.589: ADJ: IP adj out of Vlan1, addr 1.1.1.2 (incomplete) no src set: delete *Mar 2 22:32:19.589: ADJ-ipv4 bundle changed from IPv4 no fixup adj oce to IPv4 no fixup, no redirect adj oce *Mar 2 22:32:19.589: ADJ-sev: end @ 12 - walk [remove adjacency sources] filter [na/Vlan1] req [11] flags [15] [OK] *Mar 2 22:32:19.597: IP ARP: sent req src 1.1.1.1 000f.2406.bbc0, dst 1.1.1.2 000f.2406.b840 Vlan1 *Mar 2 22:32:19.597: IP ARP: sent rep src 1.1.1.1 000f.2406.bbc0, dst 1.1.1.1 ffff.ffff.ffff Vlan1 *Mar 2 22:32:20.587: %LINK-3-UPDOWN: Interface FastEthernet0/1, changed state to down *Mar 2 22:32:20.587: ADJ-sev: request async walk @ 13 - walk [remove adjacency sources] filter [na/FastEthernet0/1] req [12] flags [15] *Mar 2 22:32:20.587: ADJ-sev: start @ 13 - walk [remove adjacency sources] filter [na/FastEthernet0/1] req [12] flags [15] *Mar 2 22:32:20.587: ADJ-sev: end @ 13 - walk [remove adjacency sources] filter [na/FastEthernet0/1] req [12] flags [15] [OK]本文出自 Frank's Blog