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HP 5500 Ei 5500 Si Switch Series Configuration Guide

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    							 212 
Configuring the default local preference 
The local preference is used to determine the best route for traffic leaving the local AS. When a BGP 
router obtains from several IBGP peers multiple routes to the same destination but with different next hops, 
it considers the route with the highest local preference as the best route.  
This task allows you to specify the default loca l preference for routes sent to IBGP peers. 
To specify the default local preference: 
 
Step Command  Remarks 
1.  Enter system view. 
system-view N/A 
2.  Enter BGP view. 
bgp as-number   N/A 
3.  Configure the default local preference. default local-preference  value Optional. 
100 by default. 
 
Configuring the MED attribute 
MED is used to determine the best route for traffic 
going into an AS. When a BGP router obtains from 
E B G P  p e e r s  m u l t i p l e  r o u t e s  t o  t h e  s a m e  d e s t i n a t i o n  b u t  wi t h  d i f f e r e n t  n e x t  h o p s ,  i t  c o n s i d e r s  t h e  r o u t e  w i t h  
the smallest MED value as the best route if other conditions are the same. 
Configuring the default MED value 
 
Step Command  Remarks 
1.  Enter system view. 
system-view N/A 
2.  Enter BGP view. 
bgp as-number   N/A 
3.  Configure the default MED value. 
default med med-value  Optional. 
0 by default. 
 
Enabling the comparison of MED of routes from
 different ASs 
 
Step Command  Remarks 
1.  Enter system view. 
system-view N/A 
2.  Enter BGP view. 
bgp as-number   N/A 
3.  Enable the comparison of MED of 
routes from different ASs.  compare-different-as-med 
Not enabled by default. 
 
Enabling the comparison of MED of routes from each AS 
Route learning sequence may affect optimal route selection.
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Figure 86 Route selection based on MED  
 
 
As shown in Figure 86, Router D learns network 10.0.0.0 from  both Router A and Router B. Because 
Router B has a smaller router ID, the route learned from it is optimal.  
     Network            NextHop         MED        LocPrf     PrefVal Pa\
th/Ogn 
 *>i  10.0.0.0         2.2.2.2        50                      0       30\
0e 
 * i                   3.3.3.3        50                      0       20\
0e 
When Router D learns network 10.0.0.0 from Router C, it compares the route with the optimal route in its 
routing table. Because Router C and Router B reside  in different ASs, BGP will not compare the MEDs of 
the two routes. Router C has a smaller router ID than Router B, the route from Router C becomes optimal.   
     Network            NextHop         MED        LocPrf     PrefVal Pa\
th/Ogn 
 *>i  10.0.0.0         1.1.1.1        60                      0       20\
0e 
 * i  10.0.0.0         2.2.2.2        50                      0       30\
0e 
 * i                   3.3.3.3        50                      0       20\
0e 
However, Router C and Router A reside in the same AS, and Router C has a greater MED, so network 
10.0.0.0 learned from Router C cannot be optimal. 
You can configure the  bestroute compare-med  command on Router D. After that, Router D puts routes 
received from the same AS into a group. Router D then selects the route with the lowest MED from the 
same group, and compares routes from different groups. This mechanism avoids the above-mentioned 
problem. The following output is the BGP routing table  on Router D after the comparison of MED of routes 
from each AS is enabled. Network 10.0.0.0  learned from Router B is the optimal route. 
     Network            NextHop         MED        LocPrf     PrefVal Pa\
th/Ogn 
 *>i  10.0.0.0         2.2.2.2        50                      0       30\
0e 
 * i  10.0.0.0         3.3.3.3        50                      0       20\
0e 
 * i                   1.1.1.1        60                      0       20\
0e 
BGP load balancing cannot be implemented because lo ad balanced routes must have the same AS-path 
attribute. 
To enable the comparison of MED of routes from each AS: 
 
Step Command  Remarks 
1.  Enter system view. 
system-view N/A 
2.  Enter BGP view. 
bgp as-number   N/A 
3.  Enable the comparison of MED of 
routes from each AS.  bestroute compare-med Optional. 
Not enabled by default.
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    							 214 
Enabling the comparison of MED of routes from confederation peers 
The MED attributes of routes from confederation peers are not compared if their AS-path attributes 
contain AS numbers that do not belong to the confederation, such as these three routes: AS-path 
attributes of them are 65006 65009, 65007 65009, and 65008 65009; and MED values of them are 
2, 3, and 1. Because the third route contains an AS  number that does not belong to the confederation, 
the first route becomes the optimal route. 
To enable the comparison of MED of routes from confederation peers: 
 
Step Command  Remarks 
1.  Enter system view. 
system-view N/A 
2.  Enter BGP view. 
bgp as-number   N/A 
3.  Enable the comparison of MED of 
routes from confederation peers.  bestroute med-confederation Optional. 
Not enabled by default. 
 
Configuring the next hop attribute 
By default, when advertising routes to an IBGP peer or
 peer group, a BGP router does not set itself as the 
n e x t  h o p.  H o w e v e r,  t o  e n s u r e  a  B G P  p e e r  c a n  f i n d  t h e  c o r r e c t  n e x t  h o p  i n  s o m e  c a s e s ,  y o u  m u s t  c o n f i g u r e  
the router as the next hop for routes sent to the peer.  
For example, as shown in  Figure 87, R
outer A and Router B establish an EBGP neighbor relationship, and 
Router B and Router C establish an IBGP neighbor  relationship. When Router B advertises a network 
learned from Router A to Router C, if Router C has no route to IP address 1.1.1.1/24, you must configure 
Router B to set itself as the next hop (3.1.1.1 /24) for the route to be sent to Router C. 
Figure 87  Next hop attribute configuration 1 
 
 
If a BGP router has two peers on a common broadcast network, it does not set itself as the next hop for 
ro u te s  s e n t  to  a n  E B G P  p e e r  by  d e fa u l t.  A s  s h own  i n   Figure 88, R
 outer A and Router B establish an EBGP 
neighbor relationship, and Router B and Router C esta blish an IBGP neighbor relationship. They are on 
the same broadcast network 1.1.1.0/24. When Router B sends EBGP routes to Router A, it does not set 
i t s e l f  a s  t h e  n ex t  h o p  by  d e f a u l t.  H oweve r,  yo u  c a n  c o n fi g u re  Ro u t e r  B  t o  s e t  i t  a s  t h e  n ex t  h o p  (1.1.1. 2 / 24 )  
for routes sent to Router A by using the  peer next-hop-local command as needed.  
Figure 88  Next hop attribute configuration 2
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    							 215 
If you have configured BGP load balancing on a BGP router, the router will set it as the next hop for routes 
s e n t  t o  a n  I B G P  p e e r  o r  p e e r  g r o u p.  T h i s  i s  d o n e  r e g a r d l e s s  o f  w h e t h e r  t h e   peer next-hop-local command 
is configured.  
To configure the next hop attribute: 
 
Step Command  Remarks 
1.  Enter system view. 
system-view N/A 
2.
  Enter BGP view. 
bgp as-number   N/A 
3.  Specify the router as the next hop of 
routes sent to a peer or peer group.  peer { group-name 
| ip-address  } 
next-hop-local  Optional. 
By default, the router sets it 
as the next hop for routes 
s e n t  t o  a n  E B G P  p e e r  o r  p e e r  
group, but does not set it as 
the next hop for routes sent 
to an IBGP peer or peer 
group. 
 
Configuring the AS-PATH attribute 
Permitting local AS number to appear in
 routes from a peer or peer group 
BGP checks whether the AS_PATH attribute of a route fr om a peer contains the local AS number. If so, it 
discards the route to avoid routing loops. 
To permit local AS number to appear in routes fr om a peer or peer group and specify the appearance 
times. 
 
Step Command  Remarks 
1.  Enter system view. 
system-view N/A 
2.  Enter BGP view. 
bgp as-number   N/A 
3.  Permit local AS number to appear in 
routes from a peer or peer group and 
specify the appearance times.  peer 
{ group-name  | ip-address  } 
allow-as-loop  [ number ]  Optional. 
By default, the local AS 
number is not allowed. 
 
Disabling BGP from considering AS_P
ATH during best route selection 
 
Step Command  Remarks 
1.  Enter system view. 
system-view N/A 
2.  Enter BGP view. 
bgp as-number   N/A 
3.  Disable BGP from considering 
AS_PATH during best route selection.  bestroute as-path-neglect Optional. 
By default, BGP considers 
AS_PATH during best route 
selection.
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    							 216 
Specifying a fake AS number for a peer or peer group 
When Router A in AS 2  is moved to AS 3, you can configure Router A to specif y a fake AS number of 2  
for created connections to EBGP peers or peer groups. In this way, these EBGP peers still think Router A 
is in AS 2 and need not change their configurations. This feature ensures uninterrupted BGP services. 
To specify a fake AS number for a peer or peer group: 
 
Step Command  Remarks 
1.  Enter system view. 
system-view N/A 
2.  Enter BGP view. 
bgp as-number   N/A 
3.  Specify a fake AS number for a peer or 
peer group.  peer {
 group-name  | ip-address  } 
fake-as  as-number   Optional. 
Not specified by default. 
This command is only 
applicable to an EBGP peer 
or peer group. 
 
Configuring AS number substitution 
In L3VPN, if EBGP is used between PE and CE, sites 
in different geographical areas must have different 
AS numbers assigned to ensure correct route advertisement. If different CEs use the same AS number, you 
must configure the relevant PE to replace the AS number of the CE as its own AS number. This feature is 
used for route advertisement only. 
Figure 89  AS number substitution configuration 
 
 
A s  s h own  i n  t h e  a b ove  fi g u re,  C E  1  a n d  C E  2  u s e  t h e  s a m e  AS  n u m b e r  o f  8 0 0 .  I f  AS  n u m b e r  s u b s t i t u t i o n  
for CE 2 is configured on PE 2, and PE 2 receives a BGP update sent from CE 1, PE 2 replaces AS number 
800 as its own AS number 100. Similar configuration must also be made on PE 1.  
To configure AS number substitution for a peer or peer group: 
 
Step Command Remarks 
1.  Enter system view. 
system-view N/A 
2.  Enter BGP view. 
bgp as-number   N/A 
3.  Replace the AS number of a peer or 
peer group in the AS_PATH attribute as 
the local AS number.  peer { group-name 
| ip-address  } 
substitute-as  Not configured by default. 
Improper AS number 
substitution configuration 
may cause route loops; use 
this command with caution.
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    							 217 
Removing private AS numbers from updates to a peer or peer group 
 
Step Command  Remarks 
1.  Enter system view. 
system-view N/A 
2.  Enter BGP view. 
bgp as-number   N/A 
3.  Configure BGP to remove private AS 
numbers from the AS_PATH attribute of 
updates to a peer or peer group.  peer { group-name 
| ip-address  } 
public-as-only  By default, BGP updates 
carry private AS numbers. 
 
Ignoring the first AS numb
er of EBGP route updates 
Typically, BGP checks the AS_PATH attribute of a route update received from a peer. If the first AS number 
is not that of the BGP peer, the BGP  router discards the route update.  
To ignore the first AS number of EBGP route updates:  
 
Step Command  Remarks 
1.  Enter system view. 
system-view N/A 
2.  Enter BGP view. 
bgp as-number   N/A 
3.  Configure BGP to ignore the first AS 
number of EBGP route updates.  ignore-first-as By default, BGP checks the 
first AS number of EBGP 
route updates.  
 
Tuning and optimizing BGP networks 
Configuration prerequisites 
BGP connections must be created. 
Configuring the BGP keepalive interval and holdtime 
After establishing a BGP connection, two routers send
 keepalive messages periodically to each other to 
keep the connection. If a router receives no keepalive or update message from the peer within the 
holdtime, it tears down the connection. 
You can configure the keepalive interval and holdtime gl obally or for a specific peer or peer group. The 
actual keepalive interval and holdti me depend on the following cases: 
•   If the holdtime settings on the local and peer routers are different, the smaller one is used. The 
holdtime is no less than three seconds unless it is set to 0. 
•   If the keepalive interval is 0 and the negotiated  holdtime is not 0, the actual keepalive interval 
equals one-third of the holdtime. The maximum keepalive interval must be one third of the holdtime 
and no less than one second. 
•   If the keepalive interval is not 0, the actual keepalive interval is the smaller one between one third 
of the holdtime and the keepalive interval.  
Follow these guidelines when you configure BGP keepalive interval and holdtime: 
•   The intervals set with the  peer timer command are preferred to those set with the  timer command.
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    							 218 
•  If the router has established a neighbor relationship with a peer, you must reset the BGP connection 
to validate the new set timers. 
•   The  timer  command takes effect for only new connections. 
•   After  peer timer  command is executed, the peer connection is closed at once, and a new 
connection to the peer is negotiated  using the configured hold time. 
To configure BGP keepalive interval and holdtime: 
 
Step Command Remarks 
1.   Enter system view. 
system-view  N/A 
2.  Enter BGP view. 
bgp as-number   N/A 
3.  Configure BGP keepalive interval 
and holdtime. 
• Configure the global keepalive 
interval and holdtime: 
timer  keepalive  keepalive  hold 
holdtime 
•  Configure the keepalive interval 
and holdtime for a peer or peer 
group: 
peer {  group-name  | 
ip-address } timer  keepalive 
keepalive  hold holdtime   Optional. 
By default, the keepalive 
interval is 60 seconds, and 
holdtime is 180 seconds. 
 
Configuring the interval for sending the same update  
Step Command 
Remarks 
1.  Enter system view. 
system-view N/A 
2.  Enter BGP view. 
bgp as-number   N/A 
3.  Configure the interval for sending the 
same update to a peer or peer group.
 
peer { group-name  | ip-address  } 
route-update-interval  interval  Optional. 
The intervals for sending the 
same update to an IBGP 
peer and an EBGP peer 
default to 15 seconds and 
30 seconds.  
 
Configuring BGP soft-reset 
After modifying the route selection policy, reset BGP connections to make the new one take effect.  
The current BGP implementation supports the route-re
fresh feature that enables dynamic route refresh 
without tearing down BGP connections.  
However, if a peer not supporting route-refresh  exists in the network, you must configure the peer 
keep-all-routes  command to save all routes from the peer, which are used during applying the new route 
selection policy.
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    							 219 
Configuring automatic soft-reset 
After route refresh is enabled for peers and a policy is modified, the router advertises a route-refresh 
message to the peers, which then resend their routing information to the router. After receiving the routing 
information, the router performs dynami c route update by using the new policy. 
To enable BGP route refresh for a peer or peer group: 
 
Step Command  Remarks 
1.  Enter system view. 
system-view N/A 
2.  Enter BGP view. 
bgp as-number   N/A 
3.  Enable BGP route refresh for a peer or 
peer group.  peer { group-name
 | ip-address } 
capability-advertise  
route-refresh  Optional. 
Enabled by default. 
 
Configuring manual soft-reset 
If a BGP peer does not support route-refresh, you must
 save updates from the peer on the local router by 
using the peer keep-all-routes  command, and use the refresh bgp command to refresh the BGP routing 
table. 
If the BGP peer does not support route-refresh and the  peer keep-all-routes command is not configured 
for it, you need to decide whether to manually disco nnect the peer to learn routes again according to the 
impact of the new policy. 
Following these steps to save all route  updates from a peer or peer group: 
 
Step Command  Remarks 
1.  Enter system view. 
system-view N/A 
2.  Enter BGP view. 
bgp as-number   N/A 
3.  Disable BGP route-refresh and 
multi-protocol extension capability for 
a peer or peer group.  peer { group-name
 | ip-address } 
capability-advertise  
conventional  Enabled by default. 
4.
  Save all routes from a peer or peer 
group.  peer {
 group-name  | ip-address  } 
keep-all-routes  Not saved by default. 
5.
  Return to user view. 
return N/A 
6.  Perform manual soft reset on BGP 
connections.  refresh bgp
 { all  |  ip-address | 
group  group-name  | external | 
internal  } { export |  import }  N/A 
 
Enabling the BGP ORF capability 
The BGP Outbound Route Filtering (ORF) feature allows 
a BGP speaker to send its BGP peer a set of ORFs 
through route-refresh messages. The peer then applies the ORFs, in addition to its local routing policies 
(if any), to filter updates to the BGP speaker, re ducing the number of exchanged Update messages and 
saving network resources.  
After you enable the BGP ORF capability, the local BGP router negotiates the ORF capability with the 
BGP peer through Open messages (determines whether to carry ORF information in messages, and if yes,
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    							 220 
whether to carry non-standard ORF information in the packets). After completing the negotiation process 
and establishing the neighboring relationship, the BGP router and its BGP peer can exchange ORF 
information through specific route-refresh messages.  
For the parameters configured on both si des for ORF capability negotiation, see Tabl e  8. 
T

o enable the BGP ORF capability:  
 
Step Command Remarks 
1.   Enter system view. 
system-view  N/A 
2.  Enter BGP view. 
bgp as-number   N/A 
3.  Enable BGP route refresh for a 
peer or peer group.  peer 
{ group-name  | ip-address } 
capability-advertise route-refresh  Enabled by default. 
4.
  Enable the non-standard ORF 
capability for a BGP peer or 
peer group.  peer 
{ group-name  | ip-address } 
capability-advertise orf 
non-standard  Optional. 
By default, standard BGP ORF 
capability defined in RFC 5291 
and RFC 5292 is supported.  
If the peer supports only 
non-standard ORF, you need to 
configure this command.  
5.
  Enable the ORF capability for 
a BGP peer or peer group.  peer { group-name
 |  ip-address  } 
capability-advertise orf ip-prefix  
{  both  | receive  | send  }  Disabled by default. 
 
Table 8
 Description of the both, send, and receive parameters and the negotiation result 
Local parameter  Peer parameter Negotiation result 
send •  receive 
• both  The ORF sending capability is enabled locally and the 
ORF receiving capability is enabled on the peer.  
receive •
 send 
• both  The ORF receiving capability is enabled locally and the 
ORF sending capability is enabled on the peer. 
both both 
Both the ORF sending and receiving capabilities are 
enabled locally and on the peer.  
 
Enabling 4-byte AS number suppression 
When a switch that supports 4-byte AS numbers sends an Open message for peer relationship 
establishment, the Optional parameters field of the message indicates that the AS number occupies four 
bytes—in the range of 1 to 4294967295. If the peer device does not support 4-byte AS numbers (for 
examples, it supports only 2-byte AS numbers), th
e peer relationship cannot be established.  
After you enable the 4-byte AS number suppression function, the peer device can then process the Open 
message even though it does not support 4-byte AS  numbers, and the BGP peer relationship can be 
established.  
If the peer device supports 4-byte AS numbers,  do not enable the 4-byte AS number suppression  function; 
otherwise, the BGP peer relati onship cannot be established. 
To enable 4-byte AS number suppression:
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    							 221 
 
Step Command  Remarks 
1.  Enter system view. 
system-view N/A 
2.  Enter BGP view. 
bgp as-number   N/A 
3.  Enable 4-byte AS 
number suppression.  peer 
{ group-name  | ip-address } 
capability-advertise suppress-4-byte-as  Disabled by default.  
 
Setting the DSCP value for BGP packets 
An IPv4 packet header contains an 8-bit Type of Servic
e (TOS) field. As defined in RFC 2474, the first six 
bits set the Differentiated Services Code Point (DSCP) value and the last two bits are reserved. Network 
devices use the DSCP value as a reference to determine the packet priority for transmission. 
You can set the DSCP value for BGP packets. 
To configure the DSCP value for packets  sent to a BGP peer or peer group: 
 
Step Command Remarks 
1.  Enter system view. 
system-view  N/A 
2.  Enter BGP view or 
BGP-VPN view. 
• Enter BGP view: 
bgp  as-number 
• Enter BGP-VPN instance view: 
a.  bgp  as-number  
b.  ipv4-family vpn-instance  
vpn-instance-name   Use either approach.
 
3.  Set the DSCP value 
for the BGP packets 
sent to the specified 
BGP peer or peer 
group.  peer { group-name
 | ip-address } dscp  
dscp-value   Optional. 
By default, the DSCP value in BGP 
packets is 48. 
 
Enabling quick EBGP session reestablishment 
If the router receives no keepalive messages from a 
BGP peer within the holdtime, it disconnects from the 
peer. 
With quick EBGP connection reestablishment enabled, the router will reestablish a session to the EBGP 
peer immediately when the link to a di rectly connected EBGP peer is down. 
To enable quick EBGP session reestablishment: 
 
Step Command  Remarks 
1.  Enter system view. 
system-view N/A 
2.  Enter BGP view. 
bgp as-number   N/A 
3.  Enable quick EBGP session 
reestablishment.  ebgp-interface-sensitive Optional. 
Not enabled by default.
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