Hcip 13th day notes

IBGP Horizontal segmentation ： That is, when the router starts from a IBGP The peer learns a routing message , He will not announce this route to other IBGP peers .

Routing reflectors ： Router Reflector — RR
We can put one IBGP The device is configured as a routing reflector （RR）, A device configured as a routing reflector will reflect the learned routing information under certain conditions .

Under certain conditions, it means ： After we specify a device to become a routing reflector RR At the same time , One or more neighbors must be designated as his customers ,RR And customers , We call it reflection cluster , Set one for each cluster cluster ID----- In this reflection cluster RR Of RID. Other neighbors not defined as customers are called non customers .

Reflection rules ：
1、 If the routing reflector learns one from its own customer IBGP route , Then he will reflect to all his customers and non customers .
2、 If the routing reflector learns one from its own non customers IBGP route , Then he will reflect it to all his customers , But not reflected to non customers .----- Non non non transmission .
3、 When the router is performing route reflection , Refers to reflection of oneself BGP The best route in routing .----- and BGP The routing rules are the same .

because IBGP Horizontal segmentation is mainly to prevent routing loops , The reflection of the route reflector is equivalent to breaking IBGP Rules for horizontal segmentation , It is possible that a routing loop . therefore , To avoid routing loops , The routing reflector was designed , Two attributes are specifically introduced , To avoid loop generation .-------originator_ID（ Originator ID）,cluster_list（ Cluster list ）

Originator ID： Reflect the originator of this route information RID----- When this route passes through RR After reflection , This attribute will be added to the reflected routing information . If RR The originator already exists in a routing message ID attribute , Do not modify this property . When a device receives a routing message , Discover the originator inside ID It's local RID, You will not receive this routing information , Avoid routing backhaul .

If one AS If there are multiple reflections inside , that , There must be multiple reflection clusters , Then each RR When reflecting routing information , Local clusters will be added to the cluster list attribute in the routing information ID.

Be careful ： These two properties , It's just for IBGP Internal anti ring , therefore , When these routes are passed to EBGP Peer time , These attributes will not be carried .
RR During route reflection , In addition to adding the above two attributes to the routing information , Other properties remain unchanged .

RR Configuration of ：
[r3-bgp]peer 2.2.2.2 reflect-client ---- Appoint R2 As a reflector customer , meanwhile R3 Become yourself RR.

The federal
The federal ： The federal EBGP Relationships inherit only ENGP Rules for routing between , But not like ENGP Modify routing properties like peers , It's a special being .
The federal EBGP The anti ring can be directly used EBGP Horizontal segmentation mechanism to prevent ring , Directly in AS_PATH Add a member to the property AS To prevent the ring , It's just , Federal AS The number will be enclosed in parentheses , To distinguish .---- This attribute is mainly used for AS Use of internal anti ring , therefore , Nor will it be passed on to EBGP peers .

[r2]bgp 64512 ---- Federal members start BGP The process needs to use a small number （ Of federal members AS Number ） start-up
[r2-bgp]router-id 2.2.2.2
[r2-bgp]confederation id 2 ---- Federal members must declare their large size in the process （ Where it really is AS Number ）
[r2-bgp]peer 12.0.0.1 as-number 1
[r2-bgp]peer 3.3.3.3 as-number 64512— Establish... With members of the Federation IBGP Use the trumpet when you're in a neighborhood
[r2-bgp]peer 3.3.3.3 connect-interface LoopBack 0

[r3]bgp 64512
[r3-bgp]router-id 3.3.3.3
[r3-bgp]confederation id 2
[r3-bgp]peer 2.2.2.2 as-number 64512
[r3-bgp]peer 2.2.2.2 connect-interface LoopBack 0
[r3-bgp]confederation peer-as 64513---- Need to establish a federal EBGP Peer relationship , You need to declare the other party's trumpet first
[r3-bgp]peer 4.4.4.4 as-number 64513
[r3-bgp]peer 4.4.4.4 connect-interface LoopBack 0
[r3-bgp]peer 4.4.4.4 ebgp-max-hop 2— Because the Federation EBGP According to... Between EBGP Peer to peer transmission rules , therefore , default TTL value 1, And there is direct connection detection , therefore , It needs to be modified TTL The way of value , Let it turn off direct detection .

Federal settlement IBGP The drawback of horizontal segmentation is that there can only be one pair within a federation IBGP Peer relationship , because IBGP Routes between can only be passed once , therefore , In practical engineering , Federation and routing reflectors can often be used in combination .

BGP The road selection principle
BGP The prerequisite for choosing a route ： Discard all unavailable routing information

1、 optimization preferred-valie Maximum route of attribute value
This property is Huawei's private property , It can be understood as weight .
PV Attribute is the attribute with the highest routing priority on the device , The bigger the attribute, the better -（ The larger the value, the better ）– This property is not passed , Modifications can only be made on this device , It can only work on this device , The route entry passed out will not carry this attribute , therefore , After the other party receives the route , This attribute will still be added according to the default value .
Therefore, this attribute will be the most convenient attribute for interference routing on this device .

The first modification method ：
[r4-bgp]peer 3.3.3.3 preferred-value 100------- With this command, you can modify the routing information sent by the corresponding neighbor PV value

Load sharing ： That is, different flows take different lines , Share the pressure of a single line .

The second method ： Modify attributes through traffic fetching
1）、 The prefix list captures traffic
[r4]ip ip-prefix pv permit 10.0.0.0 24
2）、 Routing policy modification properties
[r4]route-policy pv permit node 10
Info: New Sequence of this List.
[r4-route-policy]if-match ip-prefix pv
[r4-route-policy]apply preferred-value 100
[r4-route-policy]q[r4]route-policy pv permit node 20---- Add empty table , Discharge the remaining flow
Info: New Sequence of this List.
[r4-route-policy]q3）、 stay BGP Call routing policy
3） stay BGP Call routing policy
[r4-bgp]peer 3.3.3.3 route-policy pvimport ---- Be careful , When calling, you need to choose the direction ; The direction of entry affects itself , Go out and influence others .

2、 optimization local-preference Route with the largest attribute
LP— Local preference , The default value is 100, The bigger the better （ The larger the value, the better ）.LP Property can be passed , But only in IBGP Between neighbors , Pass on to EBGP Neighbor will not carry this attribute , therefore , This property is in IBGP The most common attribute of internal routing .

The first modification method ：
[r3-bgp]default local-preference 200— take R3 In the routing information sent LP Value is modified to 200

The second method of modification ：
[r3]ip ip-prefix lp permit 10.0.0.0 24

[r3]route-policy lp permit node 10
Info: New Sequence of this List.
[r3-route-policy]if-match ip-prefix lp
[r3-route-policy]apply local-preference 200
[r3-route-policy]q
[r3]route-policy lp permit node 20
Info: New Sequence of this List.
[r3-bgp]peer 4.4.4.4 route-policy lp export ---- in the light of 4.4.4.4 Execute the routing strategy of out direction

3、 What you start is greater than what you learn from others , The priority of multiple methods of routing published by yourself is ： Manual aggregation > Automatic aggregation >network> Reissue .

4、 optimization AS_PATH Attribute the shortest route
AS_PATH The basic principle of route selection — Record AS The fewer the number, the better
1） We turn on when aggregating routes AS_SET After function , Will be different AS Detailed routing AS The number is enclosed in curly braces and placed in AS_PATHS Properties of the . But pay attention to , When comparing routes , No matter how many... Are in braces AS Number , We all treat it as one .
2） Federal time , We also need to use... Within the Federation AS Conduct anti ring , We use parentheses to indicate . In this case , We're doing AS_PATH When comparing lengths , The contents in parentheses will not be considered .

The first modification method ：（ stay R1 In the direction of exit ）
[r1]ip ip-prefix as permit 10.0.0.0 24

The second method of modification ：（ stay R2 In the direction of ）
[r1]route-policy as permit node 10
Info: New Sequence of this List.
[r1-route-policy]apply as-path 11 22 33 ?
INTEGER<1-4294967295> AS number in asplain format (number<1-4294967295>)
STRING<3-11> AS number in asdot format (number<1-65535>.number<0-65535>)
additive Append to original As Number ---- In the original AS Add... On the basis of No AS Number
overwrite Overwrite original As Number ---- Know the original AS Number , Then add AS Number
[r1-route-policy]apply as-path 11 22 33 additive ---- In consideration of anti ring , It is recommended to use additive

[r1]route-policy as permit node 20 — Empty table to put through all
Info: New Sequence of this List.
[r1-route-policy]

[r1-bgp]peer 12.0.0.2 route-policy as export — Out direction call affects AR2

[r2-route-policy]apply as-path 1 1 1 additive ---- We just added AS Number , May exist in real networks . because AS_PATH Property can also be used to prevent ring , therefore , Writing like this may lead to these AS Unable to accept the routing information . therefore , To avoid that , We can add AS Change all the numbers to the ones that actually leave AS Number , After all , The route selection is only related to the length , Nothing to do with the content .

5、 Preferred origin code （origin） Attribute optimal routing
 
OGN ： Origin code
1） adopt Network Published routes ----I ----I Indicates that the route originated from IGP agreement （ Including static routing and direct routing ）.
2） adopt EGP Protocol published routing ----e ---- refer to BGP The external gateway protocol before the protocol –EGP agreement , Because at present, the agreement is basically not used , therefore ,e Markers are rare .
3） Routes published in addition to the above two ways ----？---- The origin code mark of republishing route is ？
 
The priority relationship of these three origin codes is ：I > E > ?
[r1]route-policy ogn permit node 10
Info: New Sequence of this List.
[r1-route-policy]if-match ip-prefix ogn
[r1-route-policy]apply origin ?
egp Remote EGP
Igp Local IGP
incomplete Unknown heritage
[r1-route-policy]apply origin incomplete
 
6、 optimization MED The route with the smallest attribute value
MED： Multi exit authentication attribute —MED The initial value of the property , Is inherited IGP The overhead value of the protocol or static route or direct route in the routing table .
This attribute can affect other AS How the traffic flows into yourself AS in
BGP When the agreement is announced , You can declare any routing entry in the routing table , By default, the cost values of these routes are inherited to BGP The routing MED Properties of the . If locally announced BGP Route to EBGP neighbor , Will carry MED value , Easy EBGP Neighbor's AS Internal equipment for routing . If it passes locally IBGP What the neighbors learned BGP route , Will also be passed on to their own EBGP neighbor , But it will MED Value return 0 Pass on . Because these metrics are not locally generated .
summary ： There is EBGP All devices in the neighborhood are advised to declare internal AS The routing .
 
Be careful ： We're doing MED When comparing values , Multiple identical route information must come from the same AS Can only be , namely AS_PATH The leftmost of the properties AS The numbers have to be the same , Otherwise, there is no comparability , We will skip this comparison directly .
 
[r2]route-policy med permit node 10
Info: New Sequence of this List.
[r2-route-policy]if-match ip-prefix med
[r2-route-policy]apply cost 100
[r2-route-policy]q
 
[r4]ping -r -a 4.4.4.4 1.1.1.1 — Record orders , It can record the path information of data transmission