HP 5500 Ei 5500 Si Switch Series Configuration Guide

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Use the AS_PATH attribute for route selection and filtering. BGP gives priority to the route with the 
shortest AS_PATH length, if other factors are the same. As shown in  Figure 76, the BGP rout
er in 
AS50 gives priority to the route passing AS40  for sending data to the destination 8.0.0.0. 
In some applications, you can apply  a routing policy to control BGP route selection by modifying 
the AS_PATH length. 
By configuring an AS path filtering list, you can fi lter routes based on AS numbers...

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Figure 78 MED attribute 
 
 
In general, BGP compares MEDs of routes received from the same AS only.  
 NOTE: 
The current implementation supports using the  compare-different-as-med command to force BGP to 
compare MED values of routes received from different ASs. 
 
•   LOC AL _PR EF 
The LOCAL_PREF attribute is exchanged between IBGP  peers only; therefore, it is not advertised to 
any other AS. It indicates the priority of a BGP router. 
LOCAL_PREF is used to determine the best route fo r...

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usage and facilitates management and maintenance. Well-known community attributes are as 
follows: 
{  Internet —By default, all routes belong to the Internet community. Routes with this attribute can 
be advertised to all BGP peers. 
{  No_Export —After received, routes with this attribute cannot be advertised out the local AS or 
out the local confederation, but can be advertised to other sub-ASs in the confederation. For 
confederation information, see  Settlements for problems in large scale...

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•  IGP routing protocols such as RIP and OSPF comp ute metrics of routes, and then implement load 
balancing over routes with the same metric and to  the same destination. The route selection criterion 
is metric. 
•   BGP has no route computation algorithm, so it  cannot implement load balancing according to 
metrics of routes. However, BGP has abundant route selection rules, through which, it selects 
available routes for load balancing and adds  load balancing to route selection rules. 
BGP...

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•  A BGP speaker advertises all routes to a newly connected peer. 
IBGP and IGP synchronization 
Routing information synchronization between IBGP an d IGP avoids giving wrong directions to routers 
outside of the local AS. 
If a non-BGP router works in an AS, it can discar d a packet because a destination is unreachable. As 
shown in  Figure 81, R
outer E has learned a route of 8.0.0.0/8 from Router D via BGP. Router E then sends 
a packet to 8.0.0.0/8 through Router D, which finds from its routing...

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In most cases, BGP is used in complex networks, where route changes are more frequent. To solve the 
problem caused by route flaps, BGP route dampening is used to suppress unstable routes.  
BGP route dampening, as shown in  Figure 82, u
 ses a penalty value to judge the stability of a route. The 
bigger the value, the less stable th e route. Each time a route flap occurs, BGP adds a penalty value (1000, 
which is a fixed number and cannot be changed) to the route. When the penalty value of the...

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Route reflector 
I BG P  pe ers  must be  fu l ly mes he d to  mai ntai n c onne ctivi t y. I f  n  routers  exist i n an AS,  the  nu mber  of  I BG P 
connections is n (n-1)/2, and large amounts of network and CPU resources are consumed. 
Using route reflectors can resolve this issue. In an AS, a router acts as a route reflector, and other routers 
act as clients connecting to the route reflector. The route reflector forwards routing information between 
clients, so BGP sessions between clients...

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 NOTE: 
After route reflection is disabled between clients,  routes can still be reflected between a client and a 
non-client. 
 
Confederation 
Confederation is another method to manage growing IB G P  c o n n e c t i o n s  i n  A S s .  T h i s  m e t h o d  s p l i t s  a n  A S  
into multiple sub-ASs. In each sub-AS, IBGP peers are fully meshed, and, as shown in  Figure 85, 
in

tra-confederation EBGP connections are established between sub-ASs. 
Figure 85  Confederation network diagram...

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session. If neither party has the GR capability, the session established between them will not be GR 
capable. 
3.  W h e n  a n  a c t i v e / s t a n d b y  s w i t c h o v e r  o c c u r s  o n  t h e  G R  R e s t a r t e r ,  s e s s i o n s  o n  i t  w i l l  g o  d o w n .  T h e n ,  
GR-capable peers will mark all routes  associated with the GR Restarter as stale. However, during 
the configured GR Time, they still use these routes for packet forwarding. 
4.  After the restart is...

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•  RFC 1997,  BGP Communities Attribute  
•   R F C  2796,   BGP Route Reflection  
•   RFC 3065,  Autonomous System Confederations for BGP  
•   RFC 4271,  A Border Gateway Protocol 4 (BGP-4) 
•   RFC 5291,  Outbound Route Filtering Capability for BGP-4  
•   RFC 5292,  Address-Prefix-Based Outbound Route Filter for BGP-4 
•   draft-ietf-idr-restart-08,  Graceful Restart Mechanism for BGP  
BGP configuration task list 
 
Task Remarks 
Configuring BGP basic 
functions Creating a BGP connection...