Catalog

Single arm routing

Link type

A subinterface

Single arm routing implements different VLAN The principle of inter communication

Three layer switching technology

 vrrp Overview of the agreement

VRRP message :

summary

Single arm routing

Single arm routing （router-on-a-stick） It refers to configuring sub interfaces on an interface of a router （ or “ Logical interface ”, There is no real physical interface ） The way , Achieve the original isolation of different VLAN（ Virtual LAN ） Interconnection between . Single arm routing implements different VLAN Inter communication .

Link type

The port connecting the switch to the host is access link
The port connecting the switch to the router is trunk link

A subinterface

The physical interface of a router can be divided into multiple logical interfaces
Each sub interface corresponds to a VLAN Gateway of network segment

Single arm routing implements different VLAN The principle of inter communication

Router repackaging MAC Address , transformation VLAN label

Three layer switching technology

Layer 3 switching technology can realize VLAN Inter communication
Three layer exchange = Two layer exchange + Three layer forwarding

1. When the layer 3 device receives a data frame , The original data frame will be removed , Repackage the new source MAC Address and destination MAC Address , And because the information of the frame header changes , The last frame check CRC It should also change .
2. Multiple packets in this stream , Only the first packet is processed by the layer 3 engine of the layer 3 switch , The way of processing is the software way , Same as router , The three-tier engine has acquired new 2 Layer encapsulates information , Route this packet .
3. After the first packet forwarding is completed , Create a... In the hardware MLS entry , Re encapsulation and fast forwarding performed by hardware for subsequent packets .2 The layer data frame will be re encapsulated into the frame format of the next network segment to be forwarded .
This is it. MLS“ One time routing , Exchange many times " Principle .

be based on CEF ( A topology based forwarding model ) Of MLS, The key is two forwarding information tables , Forward information base (FIB) It corresponds to the routing table one by one , It is an image of the routing table . When the routing table is updated ,FIB then change , among FIB Contains adjacent hosts IP Address and VLANID Correspondence of . The adjacency relationship table contains adjacency hosts and switches MAC The correspondence of addresses is used to provide layer-2 rewriting information .

be based on CEF Of MLS Forwarding process , That is, send unicast packets , By looking for FIB And adjacency table , Reseal the data frame , Forward from the corresponding port .

 vrrp Overview of the agreement

It provides a device backup mechanism on the LAN .VRRP It's a fault-tolerant protocol , It ensures that when the host's next hop router breaks down , In time
Replaced by another router , So as to ensure the continuity and reliability of communication .

VRRP message :

VRRP A message is a multicast message , from Master The router sends out a notification of its existence , Use VRRP The message can detect each of the virtual router
Parameters , For the selection of the main router .VRRP The message number is encapsulated in IP In the message , It is transmitted by multicast . in other words VRRP agreement
Follow ICMP agreement , Working on the third floor ( The routing layer ), It doesn't have a port .
(1) Version: VRRP Agreement version No ,RFC3768 Defines the version number 2
(2)Type:
To specify vRRP Message type ,RFC3768 Only one kind is ordered VRRP message , namely VRRP Notification message , So this field is always 1
( If you receive VPPR The type of this field in the notification message is non 1 It will be discarded )
(3) Virtual Rtr ID:
  That is, the big VRID. One VRID Uniquely identifies a virtual router , The value range is [1,255].
( Therefore, the interface of a router can run at most 255 individual VRP Group )
（4）Priority
priority . Emptiness is not affected . If this field is 0 It means that this is Master The router failed to send VRRP Notification message , When Backup After the router receives this notification message , Will wait for (Skew time(256-Backup This field is used in the router to determine Master Routers and Backup Circuit appliance , A higher value indicates a higher priority , The value is 11.2551, The default value is 100. Suppose that the virtual router IP Address with VRRP The address of a router in the group is the same , Then the router sends out VRRP The fields of the message are always 255, Even if it is artificially specified as other values, it is also determined by the priority of the filter )/256) Second time , Then switch yourself to Master Router .

(5)CountIPAddrs:VRRP The... Contained in the notification message IP number , That is, by a VRRP Assigned by the virtual router IP Number of addresses .

（6)Auth Type: Authentication type field , A virtual router can only use one authentication type , if Backup The authentication type field in the notification message segment received by the router is unknown or does not match the local configuration , The packet is discarded .
 

summary

It can realize the function of one active and multiple standby , So as to solve the single point of failure ,
VRRP Will combine multiple routers into one Virtual routing group VRID,VRRP Will generate a virtual router （ Contains virtual IP and
fictitious MAC, Users in the LAN don't care which is the primary and which is the standby , They only use virtual routers IP As their gateway ）
Actually virtual IP It's loaded on master Router , That is, the actual packet , It's through master To forward to master backup
It is the priority that determines which is master , The one with the highest priority is master 
,backup It's just for monitoring master From time to time vrrp message , If the timeout is not received master It's from vrrp message backup will
preemption master fictitious IP Will also drift to backup On .