BGP routing optimization rules and notification principles

BGP Routing preference rules

When there are multiple routes to the same destination network segment ,BGP Route optimization is carried out in the following order ：
Premise ： Discard the next hop unreachable route

1. optimization Preferred-Value（ Preferred protocol preferred value ） The route with the largest attribute value , The default is 0, Unique to Huawei .
2. optimization Local_Preference The route with the largest attribute value . The local priority attribute is in the whole AS Intransition , But not to AS outside , The higher the value, the higher the priority , Generally used as AS The internal router selects an optimal exit to the outside . Default local priority 100 .
3. Local （local） Starting Of BGP Routing is better than the routing learned from other peers , Local origin priority ： Preferred manual aggregation > Automatic aggregation >network>import> What we learned from the equivalent
4. optimization AS_Path Property value The shortest The routing
5. optimization Origin Attribute optimal routing .Origin The attribute values are ranked from high to low in priority ：IGP、EGP And Incomplete
6. optimization MED The route with the smallest attribute value
7. Preferably from EBGP Peer to peer routing （EBGP Routing priority is higher than IBG）
8. Choose to Next_Hop Of IGP The route with the smallest metric
9. optimization Cluster_List The shortest route
10. optimization Router ID（Orginator_ID） Minimal device advertised routing
11. It is preferred to have a minimum IP Route of peer advertisement of address

• The above rules are arranged in order ,BGP When routing optimization , Start with the first rule , If you cannot make a judgment according to the first rule , For example, routed Preferred-Value Same property value , Then continue to execute the next rule , If according to the current rules ,BGP Can decide the best route , No further execution .

Memory formula ：PL-lao-man, front 2 The larger the value, the better , The smaller the value of the last few items, the better , At present 8 When all the attributes of the bar are the same , Routing load sharing can be formed
notes It means ：
When a route to the same destination in the public network forms load sharing , The system will first determine the type of optimal route . If the optimal route is IBGP Routing is just IBGP Routing participates in load sharing , If the optimal route is EBGP Routing is just EBGP Routing participates in load sharing , That is, those who arrive at the same destination in the public network IBGP and EBGP Routing cannot form load sharing .

BGP Notification principle

1、 from EBGP Routes obtained by peers
a Will be posted to EBGP peers
b Will be posted to IBGP peers
2、 from IBGP Routes obtained by peers
a Will not be published to IBGP peers
b Whether to publish to EBGP peers , It depends on whether it is turned on BGP Sync
IBGP And IGP The concept of synchronization ：BGP Spaker Will not be from IBGP The routing information obtained by the peer is announced to it EBGP peers , Unless the routing information can also pass IGP get .（ stay IBGP Routes are added to the routing table and published to EBGP Before the peer , Will check IGP Routing table , Only in IGP Also know this route , It will be added to the routing table , And publish to EBGP peers ）（ This is to prevent routing black holes , But on Huawei equipment , The default is off , And it cannot be opened , Mainly worried about IGP Can't pass BGP A huge amount of routing information , At this time, you can consider IBGP Full connection ,RR And the alliance ）
3、 Only will BGP The optimal route of is published to the peer
a It works + The optimal , namely *+>
b If the route is invalid , Then it's not optimal
c Whether the route is valid , There are two conditions ： Whether the next jump can reach ,BGP And IGP Whether synchronization is on
4、 Send only updated routes