2014年11月2日 星期日

flexlink

CISCO FLEXLINK

FlexLinks Access Model
FlexLinks are an alternative to the looped access layer topology. FlexLinks provide an active-standby
pair of uplinks defined on a common access layer switch. After an interface is configured to be a part of an active-standby FlexLink pair, spanning tree is turned off on both links and the secondary link is placed in a standby state, which prevents it from being available for packet forwarding. FlexLinks operate in single pairs only, participate only in a single pair at a time, and can consist of mixed interface types with mixed bandwidth. FlexLinks are configured with local significance only because the opposite end of a FlexLink is not aware of its configuration or operation. FlexLinks also has no support for preempt, or an ability to return to the primary state automatically after a failure condition is restored.
The main advantage of using FlexLinks is that there is no loop in the design and spanning tree is not enabled. Although this can have advantages in reducing complexity and reliance on STP, there is the drawback of possible loop conditions that can exist, which is covered in more detail later in this chapter. Other disadvantages are a slightly longer convergence time than R-PVST+, and the inability to balance traffic across both uplinks. Failover times measured using FlexLinks were usually under two seconds.


NNote:
When FlexLinks are enabled on the access layer switch, it is locally significant only. The aggregation switch ports to which FlexLinks are connected do not have any knowledge of this state, and the link state appears as up and active on both the active and standby links. CDP and UDLD packets still traverse and operate as normal. Spanning tree is disabled (no BPDUs flow) on the access layer ports configured for FlexLink operation, but spanning tree logical and virtual ports are still allocated on the aggregation switch line card. VLANs are in the forwarding state as type P2P on the aggregation switch ports.


Figure 6 shows the FlexLinks access topology.
Figure 6 FlexLinks Access Topology

The configuration steps for FlexLinks are as follows. FlexLinks are configured only on the primary interface.
ACCESS1#conf t
ACCESS1(config-if)#interface tenGigabitEthernet 1/1
ACCESS1(config-if)#switchport backup interface tenGigabitEthernet 1/2
ACCESS1(config-if)#
    May 2 09:04:14: %SPANTREE-SP-6-PORTDEL_ALL_VLANS: TenGigabitEthernet1/2 deleted from all Vlans
    May 2 09:04:14: %SPANTREE-SP-6-PORTDEL_ALL_VLANS: TenGigabitEthernet1/1 deleted from all Vlans
ACCESS1(config-if)#end

To view the current status of interfaces configured as FlexLinks:
ACCESS1#show interfaces switchport backup

Switch Backup Interface Pairs:

Active Interface Backup Interface State
————————————————————————
TenGigabitEthernet1/1 TenGigabitEthernet1/2 Active Up/Backup Standby

Note that both the active and backup interface are in up/up state when doing a “show interface" command:
ACCESS1#sh interfaces tenGigabitEthernet 1/1
TenGigabitEthernet1/1 is up, line protocol is up (connected)
 Hardware is C6k 10000Mb 802.3, address is 000e.83ea.b0e8 (bia 000e.83ea.b0e8)
Description: to_AGG1
 MTU 1500 bytes, BW 10000000 Kbit, DLY 10 usec,
 reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
 Keepalive set (10 sec)
 Full-duplex, 10Gb/s
input flow-control is off, output flow-control is off
ARP type: ARPA, ARP Timeout 04:00:00
Last input 00:00:09, output 00:00:09, output hang never
Last clearing of “show interface" counters 00:00:30
Input queue: 0/2000/0/0 (size/max/drops/flushes); Total output drops: 0
 Queueing strategy: fifo
Output queue: 0/40 (size/max)
5 minute input rate 32000 bits/sec, 56 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
1150 packets input, 83152 bytes, 0 no buffer
 Received 1137 broadcasts (1133 multicasts)
0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
0 watchdog, 0 multicast, 0 pause input
0 input packets with dribble condition detected
 26 packets output, 2405 bytes, 0 underruns
0 output errors, 0 collisions, 0 interface resets
 0 babbles, 0 late collision, 0 deferred
0 lost carrier, 0 no carrier, 0 PAUSE output
0 output buffer failures, 0 output buffers swapped out

ACCESS1#sh interfaces tenGigabitEthernet 1/2
TenGigabitEthernet1/2 is up, line protocol is up (connected)
 Hardware is C6k 10000Mb 802.3, address is 000e.83ea.b0e9 (bia 000e.83ea.b0e9)
Description: to_AGG2
 MTU 1500 bytes, BW 10000000 Kbit, DLY 10 usec,
 reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
 Keepalive set (10 sec)
 Full-duplex, 10Gb/s
input flow-control is off, output flow-control is off
ARP type: ARPA, ARP Timeout 04:00:00
Last input 00:00:51, output 00:00:03, output hang never
Last clearing of “show interface" counters 00:00:33
Input queue: 0/2000/0/0 (size/max/drops/flushes); Total output drops: 0
 Queueing strategy: fifo
Output queue: 0/40 (size/max)
5 minute input rate 32000 bits/sec, 55 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
1719 packets input, 123791 bytes, 0 no buffer
 Received 1704 broadcasts (1696 multicasts)
0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
0 watchdog, 0 multicast, 0 pause input
0 input packets with dribble condition detected
 7 packets output, 1171 bytes, 0 underruns
0 output errors, 0 collisions, 0 interface resets
 0 babbles, 0 late collision, 0 deferred
0 lost carrier, 0 no carrier, 0 PAUSE output
0 output buffer failures, 0 output buffers swapped out
ACCESS1#

Note that both the spanning-tree is no longer sending BPDUs in an effort to detect loops on interfaces in the FlexLink pair:
ACCESS1#sh spanning-tree interface tenGigabitEthernet 1/1
no spanning tree info available for TenGigabitEthernet1/1
ACCESS1#sh spanning-tree interface tenGigabitEthernet 1/2
no spanning tree info available for TenGigabitEthernet1/2

CDP and UDLD packets are still transmitted across Flexlinks as shown below:
ACCESS1#show cdp neighbor
Capability Codes: R – Router, T – Trans Bridge, B – Source Route Bridge
 S – Switch, H – Host, I – IGMP, r – Repeater, P – Phone

Device ID Local Intrfce Holdtme Capability Platform Port ID
Aggregation-1.cisco.com
 Ten 1/1 156 R S WS-C6509 Ten 7/4
Aggregation-2.cisco.com
 Ten 1/2 178 R S WS-C6509 Ten 7/4

ACCESS1#show udld neighbor
Port Device Name Device ID Port ID Neighbor State
—- ———– ——— ——- ————–
Te1/1 TBM06108988 1 Te7/4 Bidirectional
Te1/2 SCA0332000T 1 Te7/4 Bidirectional
ACCESS1#

FLEXLINK

下面討論思科FlexLink的操作和使用。
問:什麼是FlexLink?
答:FlexLink 能夠提供第2層永續性,一般在接入交換機和分佈交換機之間運行。它的收斂時間優於生成樹協定/快速生
成樹協定/IEEE 802.1w。FlexLink 在Cisco Catalyst 3000 和 Cisco Catalyst 6000 系列交換機上實施,收斂時
間低於100ms。換言之,從主鏈路的故障檢測,到通過備用鏈路轉發流量,總收斂時間低於100ms。FlexLink 成對部
署,即需要兩個埠。其中一個埠為主埠,另一個埠為從埠。這兩個埠可以是接入埠、EtherChannel埠或中繼埠。

問:FlexLink 是否關閉 Cisco Catalyst 3560-E 上的生成樹協議?

答:不會,FlexLink 只關閉FlexLink 對上的生成樹協定。換言之,只有為 FlexLink (主、從)配置的上行鏈路埠,
才會關閉生成樹協定。為避免網路迴圈,建議不關閉其餘埠上的生成樹協議。
問:備用埠的阻塞方式是否與生成樹協定相同?
答:不必相同。FlexLink 的最新增強允許備用埠對某些VLAN開放。與多生成樹協議相似,這些VLAN都由主埠提供備份。
這種方式稱為負載均衡,即允許使用者使用兩條主鏈路,而不是一條主用、一條備用。某些VLAN能將某條鏈路作為
主用鏈路,另一些VLAN則可以將該條鏈路作為備用鏈路。
問:FlexLink 是否支援負載均衡模式?
答:是的,FlexLink 支援VLAN均衡配置。在雙穴的配置中,某些VLAN 將一條鏈路作為主用鏈路(鏈路A),將另一條
鏈路作為備用鏈路(鏈路B);另一些VLAN則將鏈路B作為主用鏈路,將鏈路A作為備用鏈路。
問:是否能用 FlexLink 建立環拓撲?
不能,FlexLink 的目的是取代生成樹協定,建立上行鏈路,因此,它不支持環拓撲。
問:能否通過 EtherChannel 埠、中繼埠或接入埠配置 FlexLink?
答:可以,FlexLink 能在所有埠類型上配置,包括EtherChannel 埠、中繼埠和接入埠。
問:主、從埠是否必須是同一種埠?
答:不需要。在埠對中,可以將 EtherChannel 埠作為主埠,將接入埠作為從埠,而且主、從埠的速度也可以不同。
問:故障切換之後,當高頻寬主鏈路修復後,是否具有優先使用權?
答:是的,可以打開優先使用權設置,但預設狀態下是關閉的。
優先使用權能根據埠的頻寬配置。當然,為避免頻繁的
主從切換,也能在主鏈路恢復後延期切換。如果需要強迫使用優先使用權,應忽略延期計時器。
問:FlexLink 是否建立了調整機制,以便用戶能夠避免鏈路頻繁變動?
答:是的,為避免頻繁的主從切換,可以在主鏈路恢復後延期切換。
問:是否需要啟用 FlexLink 的MAC 移動更新(MMU)?
答:不需要,但是,為了加快雙向收斂,建議使用MMU。MMU 能夠向分佈層的交換機通報 MAC 表變更情況。
注意:Cisco
Catalyst 6000 產品線目前不支援MMU,但已列入了發展計畫。
問:運行 FlexLink 時是否需要關閉生成樹協定?
答:不需要,預設狀態下,生成樹協定只在 FlexLink 對上關閉,交換機的其它埠和VLAN上將繼續使用該協議。

出處:http://glw168.wordpress.com/2012/09/27/cisco-flexlink/

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