简介
本文档介绍如何在网段路由中配置独立于拓扑的无环备用(TI-LFA)。重点将介绍Cisco IOS® XR上的TI-LFA配置和验证。
先决条件
要求
Cisco 建议您了解以下主题:
使用的组件
本文档中的信息基于Cisco IOS XR。
本文档中的信息都是基于特定实验室环境中的设备编写的。本文档中使用的所有设备最初均采用原始(默认)配置。如果您的网络处于活动状态,请确保您了解所有命令的潜在影响。
背景信息
IP-Fast重路由
TI-LFA在保持IPFRR解决方案简单性的同时,为这些限制提供了解决方案。
TI-LFA
分段路由中的TI-LFA可解决快速重路由(FRR)中的挑战,而之前的传统LFA路径计算不能实现这些挑战。
TI-LFA是一种基于分段路由的LFA FRR,它具有:
- 简单性
- 完全自动化
- 无定向LDP会话
- 无RSVP-TE隧道
- 增量部署
- 收敛后路径中的最佳备份路径
- 防止临时拥塞和次优路由
当FRR未启用时,RIB中未安装备份路由。
RP/0/0/CPU0:R1#show route 192.0.2.6
Routing entry for192.0.2.6/32
Known via "isis 111", distance 115, metric 30, labeled SR, type level-2
Routing Descriptor Blocks
198.51.100.2, from 192.0.2.6, via GigabitEthernet0/0/0/0
Route metric is 30
No advertising protos.
PQ节点
rLFA和TI-LFA在计算修复路径时使用术语,如P空间、Q空间或PQ节点。有关详细信息,请参阅“验证”部分,
P空格:根据最短路径树(SPT)算法,R1可以到达的一组路由器,在收敛前不经过故障路径。
Q空格:按照SPT算法,R6可以到达的一组路由器,在收敛前状态下,无需通过故障路径。
PQ空间:R1的P空间与R6的Q空间的交集。
配置
在IGP(中间系统到中间系统(ISIS)、开放最短路径优先(OSPF))接口配置模式下启用TI-LFA的简单CLI,如下所示。
ISIS
router isis 111
interface GigabitEthernet0/0/0/0
address-family ipv4 unicast
fast-reroute per-prefix
fast-reroute per-prefix ti-lfa
OSPF
router ospf 111
area 0
interface GigabitEthernet0/0/0/0
fast-reroute per-prefix
fast-reroute per-prefix ti-lfa enable
验证
使用本部分可确认配置能否正常运行。
修复节点是直接邻居(LFA)
R1(192.0.2.1)通常计算其通向R6(192.0.2.6)的最低开销路径并安装在RIB中。流量将通过R1 —R2 — R3 — R6(主路径)从R1转发到R6。
如果没有LFA,如果R1 — x — R2之间出现链路故障,R1 > R6之间的流量将被丢弃几毫秒,直到R1重新计算并找到通过R4的另一条路由。
启用LFA后,R1会预装通过R4到R6的路由作为备份。
在LFA的情况下,此标准应满足备份路由安装要求,
- R4到R6的最低开销路径不应通过R1
- 从R4到R6的总开销应低于当前主路径(即从R1 > R6的开销),如图所示。
RP/0/0/CPU0:R1#show route 192.0.2.6/32
Routing entry for192.0.2.6/32
Known via "isis 111", distance 115, metric 30, labeled SR, type level-2
Routing Descriptor Blocks
198.51.100.2, from 192.0.2.6, via GigabitEthernet0/0/0/0, Protected !Primary Path
Route metric is 30
198.51.100.21, from 192.0.2.6, via GigabitEthernet0/0/0/1, Backup (Local-LFA) !Backup Path
Route metric is 32
No advertising protos.
RP/0/0/CPU0:R1#show isis fast-reroute 192.0.2.6/32 detail
L2 192.0.2.6/32 [30/115] medium priority
via 198.51.100.2, GigabitEthernet0/0/0/0, R2, SRGB Base: 48000, Weight: 0
FRR backup via 198.51.100.21, GigabitEthernet0/0/0/1, R4, SRGB Base: 48000, Weight: 0, Metric: 32
P: No, TM: 32, LC: No, NP: Yes, Yes, SRLG: Yes
src R6.00-00, 192.0.2.6, prefix-SID index 6, R:0 N:1 P:0 E:0 V:0 L:0
RP/0/0/CPU0:R1#show cef 192.0.2.6/32 detail
192.0.2.6/32, version 1056, labeled SR, internal 0x1000001 0x81 (ptr 0xa12dbd34) [1], 0x0 (0xa12c12fc), 0xa28 (0xa170e1dc)
local adjacency 198.51.100.2
Prefix Len 32, traffic index 0, precedence n/a, priority 1
via 198.51.100.2/32, GigabitEthernet0/0/0/0, 11 dependencies, weight 0, class 0, protected [flags 0x400]
path-idx 0 bkup-idx 1 NHID 0x0 [0xa175c4b8 0x0]
next hop 198.51.100.2/32
local label 48006 labels imposed {48006}
via 198.51.100.21/32, GigabitEthernet0/0/0/1, 11 dependencies, weight 0, class 0, backup (Local-LFA) [flags 0x300]
path-idx 1 NHID 0x0 [0xa166e338 0x0]
next hop 198.51.100.21/32
local adjacency
local label 48006 labels imposed {48006}
Load distribution: 0 (refcount 2)
Hash OK Interface Address
0 Y GigabitEthernet0/0/0/0 198.51.100.2
修复节点是PQ节点(rLFA)
在大多数情况下,LFA修复路径不可用(因为它们无法满足前面提到的标准)。
rLFA通过选择节点作为修复节点来解决LFA的限制,即使它们不是指向R1的邻居。在您识别PQ节点时,rLFA起作用。
R1的P空间包括R1可以到达的所有节点,而无需在R1 > R2处于预收敛状态(当所有前缀都安装且没有其他更新时,该状态即可实现)。
R6的Q空间包括R6可以到达的所有节点,而无需在收敛前通过R1> R2。
由于R5是P和Q-space的唯一节点,因此R5被选为PQ节点,并将作为链路R1 > R2的修复节点,如图所示。
注意:度量从12更改为30(R4 — R5),以演示rLFA行为。
RP/0/0/CPU0:R1#show route 192.0.2.6/32
Routing entry for 192.0.2.6/32
Known via "isis 111", distance 115, metric 30, labeled SR, type level-2
Routing Descriptor Blocks
198.51.100.2, from 192.0.2.6, via GigabitEthernet0/0/0/0, Protected !Primary path
Route metric is 30
198.51.100.21, from 192.0.2.6, via GigabitEthernet0/0/0/1, Backup (TI-LFA) !Backup path
Repair Node(s): 192.0.2.5
Route metric is 50
No advertising protos.
RP/0/0/CPU0:R1#show isis fast-reroute 192.0.2.6/32 detail
L2 192.0.2.6/32 [30/115] medium priority
via 198.51.100.2, GigabitEthernet0/0/0/0, R2, SRGB Base: 48000, Weight: 0
Backup path: TI-LFA (link), via 198.51.100.21, GigabitEthernet0/0/0/1 R4, SRGB Base: 48000, Weight: 0
P node: R5.00 [192.0.2.5], Label: 48005
Prefix label: 48006
Backup-src: R6.00
P: No, TM: 50, LC: No, NP: No, No, SRLG: Yes
src R6.00-00, 192.0.2.6, prefix-SID index 6, R:0 N:1 P:0 E:0 V:0 L:0
RP/0/0/CPU0:R1#show cef 192.0.2.6/32 detail
192.0.2.6/32, version 1166, labeled SR, internal 0x1000001 0x81 (ptr 0xa12dc41c) [1], 0x0 (0xa12c19e0), 0xa28 (0xa170e1b0)
local adjacency 198.51.100.2
via 198.51.100.2/32, GigabitEthernet0/0/0/0, 10 dependencies, weight 0, class 0, protected [flags 0x400]
path-idx 0 bkup-idx 1 NHID 0x0 [0xa175c4b8 0x0]
next hop 198.51.100.2/32
local label 48006 labels imposed {48006}
via 198.51.100.21/32, GigabitEthernet0/0/0/1, 10 dependencies, weight 0, class 0, backup (TI-LFA) [flags 0xb00]
path-idx 1 NHID 0x0 [0xa166e338 0x0]
next hop 198.51.100.21/32, Repair Node(s): 192.0.2.5
local adjacency
local label 48006 labels imposed {48005 48006}
Load distribution: 0 (refcount 3)
Hash OK Interface Address
0 Y GigabitEthernet0/0/0/0 198.51.100.2
修复是Q节点,最后一个P节点(TI-LFA)的邻居
如果R4 > R5之间的度量提高到50,则rLFA将无法提供保护。R1的链路R1 > R2的P空间仅包括R3。R6的链路R1的Q空间包括R3、R4和R2r5.没有可用的修复节点。
因此,即使rLFA也不能保证所有网络拓扑中的备份路径。 TI-LFA解决了rLFA的局限性,并为链路R1 > R2提供备份路径,如图所示。
注意:度量从30更改为50(R4 — R5),以演示分段路由TI-LFA行为(对于rLFA不可能)。
RP/0/0/CPU0:R1#show route 192.0.2.6/32
Routing entry for 192.0.2.6/32
Known via "isis 111", distance 115, metric 30, labeled SR, type level-2
Routing Descriptor Blocks
198.51.100.2, from 192.0.2.6, via GigabitEthernet0/0/0/0, Protected !Primary Path
Route metric is 30
198.51.100.21, from 192.0.2.6, via GigabitEthernet0/0/0/1, Backup (TI-LFA) !Backup Path
Repair Node(s): 192.0.2.4, 192.0.2.5
Route metric is 70
No advertising protos.
RP/0/0/CPU0:R1#show isis fast-reroute 192.0.2.6/32 detail
L2 192.0.2.6/32 [30/115] medium priority
via 198.51.100.2, GigabitEthernet0/0/0/0, R2, SRGB Base: 48000, Weight: 0
Backup path: TI-LFA (link), via 198.51.100.21, GigabitEthernet0/0/0/1 R4, SRGB Base: 48000, Weight: 0
P node: R4.00 [192.0.2.4], Label: ImpNull
Q node: R5.00 [192.0.2.5], Label: 24003
Prefix label: 48006
Backup-src: R6.00
P: No, TM: 70, LC: No, NP: No, No, SRLG: Yes
src R6.00-00, 192.0.2.6, prefix-SID index 6, R:0 N:1 P:0 E:0 V:0 L:0
RP/0/0/CPU0:R1#show cef 192.0.2.6/32 detail
192.0.2.6/32, version 1192, labeled SR, internal 0x1000001 0x81 (ptr 0xa12dc41c) [1], 0x0 (0xa12c165c), 0xa28 (0xa170e310)
local adjacency 198.51.100.2
via 198.51.100.2/32, GigabitEthernet0/0/0/0, 12 dependencies, weight 0, class 0, protected [flags 0x400]
path-idx 0 bkup-idx 1 NHID 0x0 [0xa175c170 0xa175c4b8]
next hop 198.51.100.2/32
local label 48006 labels imposed {48006}
via 198.51.100.21/32, GigabitEthernet0/0/0/1, 12 dependencies, weight 0, class 0, backup (TI-LFA) [flags 0xb00]
path-idx 1 NHID 0x0 [0xa166e16c 0xa166e338]
next hop 198.51.100.21/32, Repair Node(s): 192.0.2.4, 192.0.2.5
local adjacency
local label 48006 labels imposed {ImplNull 24003 48006}
Load distribution: 0 (refcount 7)
Hash OK Interface Address
0 Y GigabitEthernet0/0/0/0 198.51.100.2
RP/0/0/CPU0:R1#
故障排除
本部分提供了可用于对配置进行故障排除的信息。
请参阅;网段路由故障排除 — Cisco Systems
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