3Com Router User Manual

							OSPF Configuration Example447
dial-up mode, although the PPP protocol is encapsulated on the link layer, it is 
still NBMA type. The peer must be specified manually. Use the 
ospf peer 
ip-address command.
■If the network type is broadcast network or NBMA, at least the priority of one 
interface must be over 0.
■If an area is configured to a stub area, all routers connected with this area must 
be configured to stub areas. 
■The interface type of two adjacent routers must be the same.
■If two or more areas are configured, at least one area must be configured into 
a backbone area (area 0).
■Make sure the backbone area is connected with all areas.
■A virtual connection cannot go through stub area.
2Global troubleshooting: If the previous steps are correct but OSPF still cannot find 
the remote route, check the following features of the configuration.
■If two or more areas are configured for one router, at least one area must be 
configured as a backbone area (the area-id of one area must be 0 or a virtual 
link must be configured).
As shown in the following diagram, only one area is configured on Router A and 
Router D and two areas are configured respectively for Router B (area0, area1) and 
Router C (area1, area2). One area in Router B is 0, which satisfies the requirement. 
However, none of the two areas in Router C is 0. In such a case, a virtual link must 
be set up between Router C and Router B.
Make sure area 2 and area 0 (backbone area) are connected. 
Figure 151   OSPF Area Schematic Diagram
■The virtual link cannot go through a stub area and the backbone area (area 0) 
cannot be configured as stub area. Therefore, if a virtual link is configured 
between Router B and Router C, area 1 cannot be configured as a stub area, 
nor can area 0. In the above diagram, only area 2 can be configured as a stub 
area.
■The router in the stub area (Router D) cannot receive an external route. 
■Make sure the backbone areas are connected.
Rout er  A
Ro u t e r  B
Rout er  CRout er  D ar ea 0
ar ea 1ar ea 2 
						

							448CHAPTER 29: CONFIGURING OSPF 
						

							30
CONFIGURING BGP
This chapter covers the following topics:
■BGP Overview
■Configuring BGP 
■Displaying and Debugging BGP 
■BGP Configuration Example
BGP OverviewBorder Gateway Protocol (BGP) is an inter-AS dynamic route discovery protocol. Its 
primary function is to exchange loop-free routing information between ASs 
automatically and to construct the topology diagram of an AS through the 
exchange of path reachability information, including AS numbers. It constructs the 
topological diagrams of the ASs to eliminate route loops and carry out user 
configured strategies. The BGP protocol is usually used between ISPs.
The current version of BGP is BGP- 4. It applies to the distributed structure and 
supports classless interdomain routing (CIDR). BGP-4 has become the standard of 
Internet external routing protocol. It features the following:
■BGP is an external routing protocol, oriented to control route spreading and 
select best route rather than find and calculate route. This is different from the 
internal routing protocol.
■Completely resolves the route loop problem by carrying AS path information.
■Uses TCP as the transmission layer protocol, improving the reliability of the 
protocol.
■BGP-4 supports classless interdomain routing (CIDR), or supernetting. CIDR 
judges the IP address in a totally new way. It no long recognizes network class 
A, network class B, or network class C. For example, with CIDR, an illegal class 
C network address 192.213.0.0 (255.255.0.0) is indicated as 192.213.0.0/16, 
which is a legal supernetwork. /16 means that the subnet mask is 16bit starting 
from the left of the address. The introduction of CIDR simplifies the route 
aggregation. Route aggregation is the combination of several routes. Thus one 
route instead of several routes are distributed and the routing table is 
simplified.
■When a route is updated, BGP only sends the incremental route. In this way, 
BGP occupies much less bandwidth in transmitting routes. It applies to the 
transmission of a large amount of routing information on the Internet.
■For political and economic reasons, each AS must filter, select and control the 
routes. BGP-4 provides abundant routing strategies for easy expansion of BGP 
to support new developments of the Internet. 
						

							450CHAPTER 30: CONFIGURING BGP
The BGP system runs on a specific router as a high layer protocol. At system 
startup, the whole BGP routing table is transmitted for the exchange of routing 
information. Later on, only an update message is transmitted for updating the 
routing table. In the system, keep-alive messages are received and transmitted to 
check whether the connection between routers is normal.
The router transmitting the BGP message is called the BGP speaker. It receives and 
generates new routing information from time to time and advertises to other BGP 
speakers. When a BGP speaker receives a new route advertisement from other 
ASs, if this route is better than the existing route, or if there is no acceptable route 
currently, the BGP speaker broadcasts this route to all other BGP speakers in the 
AS. BGP speakers are peers to each other and several related peers form a peer 
group.
BGP runs on the router in two modes:
■IBGP (Internal BGP)
■EBGP (External BGP)
IBGP is run when routers in an autonomous system exchange network reachable 
information. When routers of different ASs exchange network reachable 
information, they use EBGP. 
The BGP protocol system is driven by messages that can be divided into 4 
categories:
■Open message. This is the first transmitted message after the connection is 
created. It is used to create a connection between BGP peers
■Update message is the most important message in BGP system, and is used to 
switch routing information among the peers. Update message consists of three 
parts: unreachable route, path attributes and Network Layer Reachability 
Information (NLRI).
■Notification message notifies errors.
■Keep-alive message is used to check the validity of the connection.
Configuring BGP A BGP configuration includes tasks described in the following sections:
■Enabling BGP
■Configuring Networks for BGP Distribution
■Configuring Peers
A BGP advanced configuration includes:
■Setting the MED for the AS
■Allow Comparing Path MED
■Configuring the Local Preference
■Configuring BGP Timers
■Configuring a BGP Peer Group
■Creating Aggregate Addresses
■Configure BGP Route Reflector 
						

							Configuring BGP 451
■Configuring a BGP Community
■Configuring a BGP AS Confederation Attribute
■Configuring Route Dampening
■Configuring Synchronization of BGP and IGP
■Configuring the Interactions between BGP and an IGP 
■Defining an Access List Entry, an AS Path-list Entry, a Routing Policy
■Configuring a Route Filter for BGP
■Resetting BGP Connections
Enabling BGPSpecify the local AS number when BGP is enabled. After BGP is enabled, the local 
router continuously monitors whether any incoming BGP connection request is 
received from the peer routers. To make the local router send BGP connection 
requests to the peer routers, use the 
peer command. When BGP is turned off, 
BGP protocol closes all BGP connections that have been created.
Perform the following configurations in system view.
Ta b l e 508   Enable BGP
By default, BGP is disabled.
Configuring Networks 
for BGP DistributionPerform the following configurations in BGP view.
Ta b l e 509   Configure Networks for BGP Distribution
By default, no network is configured for BGP distribution.
Configuring PeersThe routers that exchange BGP packets are called peers to each other. Peers can 
be directly connected routers or indirectly connected routers but should be 
connected by other directly connected router or routers. 
BGP peer basic configuration includes setting the AS number of the peer.
Perform the following peer configuration in BGP view.
Ta b l e 510   Configure AS Number of the Peer
OperationCommand
Enable BGP and enter into the BGP viewbgp [ as-number ]
Turn off the BGPundo bgp
OperationCommand
Specify a list of networks associated with 
BGPnetwork ip-address [ mask 
address-mask ] [ route-policy 
policy-name ]
Delete a list of networks associated with 
BGPundo network ip-address [ mask 
address-mask ] 
OperationCommand
Configure AS number of the peer peer peer-address as-number 
as-number 
						

							452CHAPTER 30: CONFIGURING BGP
BGP peer advanced configuration includes the following steps:
1Configure the connection between EBGP peers that are connected indirectly.
Ta b l e 511   Configure Connection Between EBGP Peers Connected Indirectly
By default, the BGP connection can be established with a directly connected peer 
router.
2Configure the BGP version of the peer.
Ta b l e 512   Configure the BGP Version of the Peer
By default, software accepts BGP Version 4.
3Set the timers for the BGP peer.
Ta b l e 513   Set the Timers for BGP Peer
By default, the value of keepalive-interval is 60 seconds, the value of 
holdtime-interval is 180 seconds.
Caution: The timer configured with this command is of higher preference than 
that configured with the timers command.
4Configure the BGP route-update interval.
Ta b l e 514   Configure BGP Route-update Interval
By default, the BGP route-update interval is 5 seconds.
5Configure to send community attribute to the peer.
Delete a BGP peer undo peer peer-address as-number 
as-number
OperationCommand
Configure connection between EBGP 
peers connected indirectlypeer peer-address ebgp-max-hop [ 
max-hop-count ]
Return to the default BGP connections to 
external peersundo peer peer-address ebgp-max-hop [ 
max-hop-count ]
OperationCommand
Configure the BGP version of the peerpeer peer-address version 
version-number
Unconfigure the BGP version of the peerundo peer peer-address version
OperationCommand
Set the timers for BGP peerpeer peer-address timers 
keepalive-interval holdtime-interval
Set the timers for BGP peer to default 
valueundo peer peer-address timers
OperationCommand
Configure BGP route-update intervalpeer peer-address 
route-update-interval seconds
Restore BGP route-update intervalundo peer peer-address 
route-update-interval 
						

							Configuring BGP 453
Ta b l e 515   Configure to Send Community Attribute to the Peer
By default, the community attributes are not sent to the peer.
6Configure the peer to be the client of the route reflector. 
Ta b l e 516   Configure the Peer to be the Client of the Route Reflector
7Configure to distribute default route to the peer.
Ta b l e 517   Configure to Distribute Default Router to the Peer
By default, the local router does not advertise the default route to any peer. A next 
hop should be sent to the peer unconditionally as the default route.
8Set the own IP address as the next hop when the peer distributes routes.
Set the routers own IP address as the next hop when the peer distributes routes.
Ta b l e 518   Set the Own IP Address as the Next Hop When the Peer Distributes Route
By default, its own address is not the next hop when the peer distributes routes.
9Create a routing policy for the peer. 
Ta b l e 519   Create a Routing Policy for the Peer
OperationCommand
Configure to send community attribute to 
the peerpeer peer-address 
advertise-community
Cancel sending community attribute to 
the peerundo peer peer-address 
advertise-community
OperationCommand
Configure the peer to be the client of the 
route reflectorpeer peer-address reflect-client
Cancel the configuration of making the 
peer as the client of the BGP route 
reflectorundo peer peer-address 
reflect-client
OperationCommand
Configure to distribute default route to 
the peerpeer peer-address 
default-route-advertise
Configure not to distribute default route 
to the peerundo peer peer-address 
default-route-advertise
OperationCommand
Set the own IP address as the next hop 
when the peer distributes routepeer peer-address next-hop-local
Not to Set the own IP address as the next 
hop when the peer distributes routeundo peer peer-address 
next-hop-local
OperationCommand
Create a routing policy for the peerpeer peer-address route-policy 
policy-name { import | export }
Remove a routing policy to import or 
export routesundo peer peer-address route-policy 
policy-name { import | export } 
						

							454CHAPTER 30: CONFIGURING BGP
By default, the route from the peer or peer group is not designated with any route 
policy.
10Create an filtering policy based on access list for the peer.
Ta b l e 520   Create a Fltering Policy Based on Access List for the Peer
By default, no route filtering policy based on IP ACL for a peer is set.
11Create BGP route filtering based on the AS path for the peer. By default, a BGP 
filter is disabled.
Ta b l e 521   Create a BGP Route Filtering Based on AS Path for the Peer
Setting the MED for the 
ASThe multi-exit discriminator (MED) is the external metric of a route. It is different 
from the local preference attribute. MED is switched between ASs and the MED 
that has entered the AS does not leave the AS. AS uses local attributes for its own 
out-site selection processing while MED attribute is used to select the best route. 
The route with smallest MED value is selected. When a router running BGP gets 
routes with the same destination address but a different next hop through 
different external peers, it makes a preference selection based on the MED values. 
To operate the MED attribute, an access control list is used to indicate what 
network will be operated.
Perform the following configurations in BGP view.
Ta b l e 522   Configure the BGP MED Metric
Allow Comparing Path 
MEDThis command is used to compare MED values from different AS neighboring 
routes and to select the best route. The route with smaller MED value is selected.
Perform the following configurations in BGP view.
Ta b l e 523   Allow Comparing Path MED
OperationCommand
Create an filter policy based on access list 
for the peerpeer peer-address filter-policy 
acl-number { import | export }
Remove an filter policy based on the 
access list for the peerundo peer peer-address filter-policy 
acl-number { import | export }
OperationCommand
Create a BGP route filtering based on AS 
path for the peerpeer peer-address acl 
aspath-list-number { import | export 
}
Delete  a BGP route filtering based on AS 
path for the peerundo peer peer-address acl 
aspath-list-number { import | export 
}
OperationCommand
Configure MED for an AS default-med med
Restore the default MED of an AS undo default-med
OperationCommand 
						

							Configuring BGP 455
By default, MED values from different AS neighboring routes are not compared 
when determining the best route.
This configuration should not be used unless it is certain that different ASs uses 
the same IGP and routing modes.
Configuring the Local 
PreferenceConfiguring different local preferences affects BGP routing selection. When a 
router running BGP gets routes with the same destination address but different 
next hops through different internal peers, it selects the route of the highest local 
preference to this destination.
Perform the following configurations in BGP view.
Ta b l e 524   Configure the Local Preference
By default, the value of local preference is 100.
Configuring BGP TimersThe interval of sending keepalive messages required by RFC and BGP holdtime are 
important parameters in BGP protocol.
When a router has created a BGP connection successfully with the other router, it 
sends keepalive messages to this router with the time interval set by the 
keepalive-interval attribute to indicate whether the connection channel is 
normal. Generally, the time interval for sending a keepalive message is one third of 
the value for the 
holdtime attribute.
The value of the holdtime-interval attribute is the time interval for continuously 
receiving keepalive and update messages. If a keepalive or update message is 
received, the holding timer is reset. If a router has not received any messages from 
the opposite router for a specific period of holding time, this BGP connection is 
considered broken and is cut off. The router can negotiate with the 
interconnected router to set a shorter holding time.
Perform the following configurations in BGP view.
Ta b l e 525   Configure the Keepalive Timer and Holdtime Tmer for BGP
By default, the value for the keepalive-interval attribute is 60 seconds and may 
have a value ranging from 1 to 4294967295 seconds. The default value for the 
Allow comparing path MEDcompare-different-as-med
Prohibit comparing path MEDundo compare-different-as-med
OperationCommand
Configure the local preferencedefault local-preference value
Restore the local preference value to its 
default valueundo default local-preference
OperationCommand
Configure the keepalive timer and 
holdtime timer for BGP timers keepalive-interval 
holdtime-interval
Restore BGP network timers to their 
default valueundo timers 
						

							456CHAPTER 30: CONFIGURING BGP
holdtime-interval attribute is 180 seconds and may have a time interval 
ranging from 3 to 42949675 seconds.
Configuring a BGP Peer 
GroupThe BGP peer group command can be used for user configuration. When starting 
several peers of the same configuration, you can first create and configure one 
peer group, then add other peer groups into this group to get the same 
configuration. 
Configuring a basic BGP peer group includes creating a peer group and adding a 
peer to the group.
Perform the following peer group configuration in BGP view.
1Create a peer group
By default, a IBGP peer is added to the default peer group and no configuration is 
necessary. The configuration of route updating strategy to any IBGP peer is only 
applicable to other IBGP peers in the group. If the router is not configured as a 
route reflector, all IBGP peers are in one group. Otherwise, all route reflection 
clients are in one group and non-clients are in another group.
The members of an external peer group must be in the same network segment, 
otherwise some EBGP peers may discard the route updating information you have 
sent.
All peers in this group must be configured with an AS number, if this group is not 
configured with an AS number. If you add an AS number to the peer group, any 
peer in this group cannot be configured with an AS number different from this 
peer group AS number.
The members of the peer group cannot be configured with a route updating 
strategy different from that of the group but different access strategy is permitted.
Ta b l e 526   Create a Peer Group
By default, no peer group is created.
2Add a peer to the BGP peer group
Add one BGP peer into the peer group to create a peer group. When the 
configuration of the peer group is changed, the configuration of each peer should 
also be changed accordingly. IBGP peer and EBGP peer cannot be in the same 
group. 
Ta b l e 527   Add a Peer to the BGP Peer Group
By default, there is no BGP peer in a peer group.
OperationCommand
Create a peer grouppeer group-name group
Delete a specified peer groupundo peer group-name group
Reset the connection of all members in 
the peer groupreset bgp group group-name
OperationCommand
Add a peer to the BGP peer grouppeer peer-address group group-name
Delete a member from the BGP peer 
groupundo peer peer-address group 
group-name