[mask requirements of OSPF and Isis in multi access network]

Thinking questions

I'm learning ISIS and OSPF in , We know ISIS and OSPF Ask for in MA The mask length should be consistent in the environment ,

If this MA Environment , Mask length is inconsistent , What will happen ？

Today we use ISIS and OSPF Two experimental environments , Let's talk about the environment where the mask length is inconsistent , Problems with routing delivery .

Experimental environment 1 ：ISIS

Thinking questions 1

ISIS Multiple access is not required, and the mask within the network is consistent , Real node LSP It carries all its direct leaf prefixes and prefix lengths , What will happen ？

Experimental environment ：

R1-R2-R3 All running ISIS, All are L2 Router , Belong to the same area , There is a loopback port behind each device to simulate the network behind it , The interface connecting the switch and the loop return port are enabled under the interface isis .

R1 -2-3 Routing table

reflection ：

R3 Belt source 3.3.3.3 Go to ping2.2.2.2, Certainly can ping through , that R3 Belt source 10.1.12.3/27 Yes no ping through 2.2.2.2？

answer ： It doesn't work .

The analysis process ：

R3 Routing table to 2.2.2.2/32 The next jump is 10.1.12.2/24,R3 Routing table to 10.1.12.0/24 The next jump is 10.1.12.2

however R3 There is one in the routing table 10.1.12.0/30 The routing entry for .

R3 Routing table ：

R2 Routing table to 3.3.3.3/32 The next jump is 10.1.12.3/27,R2 Routing table to 10.1.12.0/27 The next jump is 10.1.12.3, however R2 There is one in the routing table 10.1.12.0/30 The routing entry for .

R2 Routing table ：

When R3 Belt source 3.3.3.3 Go to ping2.2.2.2 when , The next jump is 10.1.12.2.R2 When returning the package , Give back 3.3.3.3, The next jump is 10.1.12.3. It's no problem .（ Keep an eye on this route ）

When R3 Belt source 10.1.12.3 Go to ping2.2.2.2 when , The next jump is 10.1.12.2.R2 Check the routing table when returning packets , Go to 10.1.12.3 Network segment routing , Match the longest mask rule to send messages , It's a match 10.1.12.0/30 Of ISIS route , Next jump 10.1.12.1, When R1 Received this message , Find the destination address of the message 10.1.12.3 Is the broadcast address of my direct address , The router isolates broadcast packets and loses packets .

10.1.12.0/30 This network segment has only 2 Two available addresses ,

10.1.12.0/30 - network number / Prefix

10.1.12.1/30 - Available addresses

10.1.12.2/30 - Available addresses

10.1.12.3/30 - Broadcast address

therefore ,ISIS Inconsistent masks cause packet loss due to the longest mask matching principle .

Experimental environment file ： The simulator is Huawei ensp

isis experiment .rar

Experimental environment II ：OSPF

Thinking questions 2：

ospf It stipulates that the mask in the broadcast network must be consistent

OSPF What will happen if the mask consistency is not required in the multi-channel access network ？

Experimental environment

This experiment can only analyze , Because the masks are inconsistent , MA The Internet OSPF Can't build a neighborhood

The analysis process

   If  ospf  If the mask is inconsistent, the neighbor can be established , and  DR  Declare that the mask of this network segment is  / 29, Neighbors in other broadcast domains cannot calculate the route  10.1.123.10/32  Network segment .

  because  10.1.123.0/29  The address contained is the smallest  10.1.123.1, Maximum  10.1.123.6,

10.1.123.0/29 Subnet partition ：

10.1.123.00000 000/29=10.1.123.0/29=== network number / Prefix

10.1.123.00000 111/29=10.1.123.7/29=== Broadcast address

The host address 10.1.123.10/32 It is beyond the scope of this network

In this way, some routes are missing , such as PC That address .

Routers in other network segments can only calculate some routes , Unable to calculate the route under your entire broadcast domain . therefore OSPF The mask must be consistent , In this way, no matter who will be in the future in this broadcast domain DR, The routes calculated by the neighbors of other network segments are the same network number , There will be no missing route .

Just like choosing the village head in the village , Our thoughts must be the same before we choose you , choose DR, The mask must be the same to recognize you as DR. Otherwise, the example above may appear .

and ISIS The agreement is not afraid of such problems , because isis Real node LSP Autonomously describes the mask information of direct network segment routing , Other routers will calculate directly after receiving it , and ospf By DR Description mask , It may lead to route loss .

therefore ISIS It is not required that the mask must be consistent , Real node LSP The mask information is described independently , No pseudo nodes are needed to describe , Therefore, it will not lead to missing routes , But it may cause the route to be calculated , however ping no .

When someone asks ：ospf What will happen if the mask is inconsistent ？

First of all, inconsistent masks on broadcast links will cause DBD and LSR The message cannot be sent , Can't build neighbors . Secondly, the masks are inconsistent. If the pseudo node declares that the broadcast network mask is its own mask , It will cause other broadcast networks 、P2P Network lost route .

summary ：

ISIS　　 If the masks are inconsistent , Will build neighbors normally , However, the route will appear although it is calculated , however ping The phenomenon of impassability .

OSPF 　 If the masks are inconsistent , MA The network cannot establish a neighbor relationship .