Shulou(Shulou.com)06/01 Report--

In the network, we use static route priority for active and standby routing, so we can rest easy if we have a spare tire, but do you really know when the active line will fail?

First of all, it involves the condition of static route adding table, and the condition of static route adding table is that the next hop is reachable (double UP).

1.1 under normal circumstances

Active link failure:

When the next hop of the active static route is physically directly connected to the interface DOWN, the active route fails, and the standby route adds a table and the data switches to the standby link.

Active link recovery

When the next hop of the active static route physically directly connects the interface from DOWN to UP, the active route takes effect, and the active route adds a table and the data switches to the active link.

1.2 under special circumstances

When the network device and the peer network device are separated by a layer 2 switch, the link cannot sense the next hop's non-directly connected interface DOWN or UP (the active static route will not disappear), and the data is still sent to the active line.

Note: this situation is very common in the existing network, where there are multiple devices between the exit router and the operator's layer 3 equipment, and a device in the middle fails, and the router cannot sense the line failure, so it cannot switch from the active link to the standby link.

1.2.1 method 1: manual switching of network management

At this time, the normal network management finds that it cannot get on the network, and whether the IP of the next hop on the main line of the active router PING can be reached. If the PING is not available, the active interface will be manually shutdown at this time, and the active route will become invalid.

Manual detection and switching requires manual real-time operation is obviously unscientific.

Example:

The default AD value (administrative distance) of the active static route is 1

Standby static route configuration AD value (administrative distance) is 10

Note: the smaller the AD value (administrative distance), the better.

The following figure shows that when the R1 device interface fails, the DOWN or UP of the peer interface cannot be sensed for R3. At this time, the routing of the active line still adds the table, and the data main line sends the packet. At this time, the fault packet of the opposite end link is not reachable.

At this time, the simple way is whether the next-hop address of the PING static route can be connected, and there is no switching standby.

1.2.2 method 2: IP SLA detection

At this time, we use IP SLA technology. IP SLA has been automatically sending ICMP (PING also uses ICMP protocol, and PING is an application) to the next hop of IP. When the link fails, the ICMP of SLA does not respond. In this case, the active route associated with SLA automatically expires, and the backup path automatically adds tables, effectively achieving the purpose of automatic switching.

Example:

First, how to achieve automatic switching?

After IP SLA configuration, ICMP packets can be sent automatically to detect the link condition. When the next hop of the active route is unreachable, the active route fails, the backup route adds a table, and the packet switches the alternate path.

II. SLA configuration steps

1. Define SLA detection protocol, destination address, and specific parameters.

two。 Call SLA in track (because static routes can only call track).

3. Let sla take effect immediately.

4. Track is called in a static route.

III. Specific configuration of SLA

Define SLA

Ip sla 1

Icmp-echo 12.1.1.1

The timeout 500timeout is 500ms.

Threshold 500threshold is 500ms

Frequency 1 frequency one second

Track calls SLA

Track 1 rtr 1 defines track 1 calls SLA 1

SLA takes effect immediately

Ip sla schedule 1 life forever start-time now sla 1 is effective immediately

Static route calls track

Ip route 114.114.114.114 255.255.255.255 12.1.1.1 track 1

1.3 Experimental simulation

Environment: build a test environment in GNS3

Usage knowledge: basic principles of static routing, static routing table conditions, AD value routing principles, IP SLA,

Build the following topology

Detailed configuration:

1. Configure SLA

Ip sla 1

Icmp-echo 12.1.1.1

Timeout 500

Threshold 500

Frequency 1

2. Call SLA in track

Track 1 rtr 1

3. Enable SLA 1 now

Ip sla schedule 1 life forever start-time now

4. Configure active and standby static rout

Ip route 114.114.114.114 255.255.255.255 12.1.1.1 track 1

Ip route 114.114.114.114 255.255.255.255 23.1.1.3 10

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