BGP uses TCP port 179 for transport. Router with the higher BGP router-id
initiates BGP session from a random port.
2. The interface from which the BGP router ID is taken does not have to be
running BGP. Any valid IP address can be used as BGP router-id, even an
address that is not locally configured on the router.
3. The BGP router-id must be the same as the OSPF router-id for redistributing
the routes from OSPF to BGP or vice versa.
4. If the 'network â€¦' command is configured with the 'mask' option under the
BGP process, then an exact match (network/mask) must exist in the IP
routing table in order to advertise this route into BGP regardless of
'auto-summary' / 'no auto-summary' command. But the 'network â€¦'
command configured without the 'mask' assumes the default classful mask
and if 'auto-summary' is configured then BGP will advertise a classful
network only if any subnets of the classful network exist in the IP routing
table. Again if the 'network â€¦' command is configured without the 'mask'
option and if 'no auto-summary' is configured, then that router must have
the exact classful network in the IP routing table in order to advertise it in
5. To accept and attempt BGP connections to the external peers residing on
networks that are not directly connected, we need to use either 'neighbor
ebgp-multihop â€¦' or 'neighbor ttl-security â€¦' command. These two
commands are mutually exclusive. We can use another command 'neighbor
disable-connected-check' to accomplish the same task if the BGP neighbor is
6. The synchronization rule states that an iBGP learned prefix cannot be
considered best unless there is a matching IGP route for that BGP prefix. BGP
only advertises what it considers the best path. This issue can be resolved (1)
by redistributing BGP routes into the IGP, (2) by creating a full-mesh of IBGP
routers and disabling the synchronization, or (3) by creating a GRE tunnel.
When BGP is synchronizing with OSPF, the router ID must match in both
protocols in order to make it work.
7. When a prefix is received from an eBGP neighbor, it is advertised to both
eBGP & iBGP neighbors. When a prefix is received from an iBGP neighbor, it is
advertised ONLY to eBGP neighbors and not to any iBGP neighbors. To
advertise iBGP leaned routes to other iBGP peers requires the use of
route-reflectors or confederations or a full-mesh of iBGP peers.
8. While sending BGP updates, EBGP peers modify the next-hop value to its own
IP address. But iBGP peers do not modify it.
9. The â€˜default-information originateâ€™ command, however, requires explicit
redistribution of the route 0.0.0.0. . Default routes can be injected into BGP in
one of three ways: (1) using the 'network â€¦' command (default route must
exist in the local routing table), (2) using the 'default-information originate'
command (a redistribution statement must also be configured to redistribute
the default route from the local routing table to the BGP table), and (3) using
the 'neighbor â€¦ default-originate [route-map route-map-name]' command
(this method does not even check for the existence of a default route in the
IP routing table). The 'default-information originate' command should not be
configured with the 'neighbor â€¦ default-originate' command on the same
10. 'weight' and 'local-preference' are set inbound and they affect outbound
traffic. But 'as-path' and 'med' are set outbound and they affect inbound
11. The weights assigned with the 'set weight â€¦â€™ route-map command overrides
the weights assigned using the 'neighborâ€¦ weight â€¦' command.
12. Origin code 'i' is default on the BGP routes advertised by 'network ...',
'aggregate-address ...' (if all subnet has 'i'), and 'neighbor â€¦
default-originate' commands. And origin code '?' is default on the BGP routes
advertised by 'redistribute ...', 'aggregate-address ...' (if any single subnet
has '?', but can be changed using â€˜attribute-mapâ€™ option),
'default-information originate', and 'bgp inject-map ...' commands.
13. When BGP originates a route with the â€˜network â€¦â€™ command, MED is copied
from the metric of the original route.
14. BGP MED values are not passed beyond the receiving (neighbor) AS.
15. Enabling the â€˜bgp deterministic-medâ€™ command ensures the comparison of
the MED variable when choosing routes advertised by different peers in the
same autonomous system. Enabling the â€˜bgp always-compare-medâ€™
command ensures the comparison of the MED for paths from neighbors in
different autonomous systems.
16. The default behavior of BGP routers that run Cisco IOS software is to treat
routes without the MED attribute as having a MED of 0, making the route
that lacks the MED variable the most preferred. The 'bgp bestpath med
missing-as-worst' command can be configured to treat the route that missing
MED as the least preferred one.
17. â€˜bgp bestpath as-path ignoreâ€™ is a hidden command in Cisco IOS which allows
BGP to not consider the AS path during best path route selection.
18. There are two ways to create an aggregate address under BGP. The first is to
create a static route to null interface in the routing table for the aggregate
address and then advertise it with the â€˜network â€¦â€™ command. The second
way is to use the â€˜aggregate-address â€¦â€™ command.
19. By default when aggregation is configured in BGP, the 'atomic-aggregate'
attribute is attached to the aggregate address if the 'as-set' argument is not
used in the 'aggregate-address â€¦' command. The 'as-set' argument reveals
the AS numbers which can prevent a routing loop, and once 'as-set' is
configured along with the 'aggregate-address â€¦' command, the
'atomic-aggregate' attribute is automatically removed.
20. A router reflector and its clients are known collectively as a cluster. If the
cluster contains a single route reflector, the cluster ID is the router ID of the
route reflector. If the cluster contains multiple route reflectors, each RR must
be manually configured with a cluster ID.
21. A client router in a route reflection cluster can peer with external neighbors,
but the only internal neighbor it can peer with is a route reflector in its
cluster or other clients in the cluster. Clients cannot peer with routers outside
of their own cluster. However, the RR itself can peer with both internal and
external neighbors outside of the cluster and can reflect their routes to its
22. In case of route reflection, (1) routes from EBGP are advertised to EBGP,
client, non-client (2) routes from client are advertised to EBGP, client,
non-client (3) routes from non-client are advertised to EBGP, client.
23. When the 'no bgp client-to-client reflection' command is configured the RR
does not reflect routes from one client to another. It does, however, continue
to reflect routes from clients to peers outside of the cluster, and from peers
outside of the cluster to clients.
24. Standard and extended BGP communities are removed from the reflected
routes unless the 'neighbor ... send-community [both]' is configured on the
route reflector. The link bandwidth community is removed from reflected
route if the route-reflector performs IBGP multipath load-sharing for that
25. The â€œneighbor â€¦ nexthop-selfâ€� on router reflectors only affects the next hop
of eBGP learned routes because the next hop of reflected routes should not
be changed. To avoid a common configuration error for reflected routes, the
â€œset ip next-hopâ€� command should not be used in a route map to BGP route
26. Unlike route reflector environments in which only the route reflector itself
has to support route reflection, all routers within a confederation must
support the confederation functionality.
27. EBGP routes external to the confederation are preferred over EBGP routes to
member autonomous systems, which are preferred over iBGP routes.
28. AS_PATH types are AS_SEQUENCE, AS_CONFED_SEQUENCE, AS_SET, and
AS_CONFED_SET. AS_SEQUENCE is an ordered set of AS numbers, and
AS_SET is an unordered set of AS numbers. AS_CONFED_SEQUENCE and
AS_CONFED_SET are the same as AS_SEQUENCE and AS_SET but are used
only within BGP confederations.
29. When 'bgp bestpath med confed' command is configured, the router picks
the confederation-internal path with the lowest MED and ignores the path
with the external AS number.
30. BGP private autonomous system numbers are from 64,512 to 65,535
31. BGP prefixes can be filtered using (1) 'distribute-list', (2) 'prefix-list', (3)
'filter-list', (4) 'policy-list', (5) community/extended community lists, (6)
32. For BGP, the â€˜distance â€¦â€™ command sets the administrative distance of the
External BGP (eBGP) route. This command only affects the routing table and
not the BGP table.
33. The 'network â€¦ backdoor' command has the same effect as the 'network â€¦'
command. The EBGP route is treated as a local BGP route, and the
administrative distance is changed to 200. The difference is that the address
specified by the network backdoor command is not advertised to EBGP peers.
34. iBGP routes are not redistributed into an IGP unless you use "bgp
redistribute-internal" command under BGP routing process.
35. 'bgp inject-map ... exist-map ...' command injects prefixes in the local BGP
RIB when a valid parent route exists. Only prefixes that are equal to or more
specific than the aggregate route (existing prefix) can be injected. exist-map
(route-map) must contain a 'match ip address prefix-list ...' command
statement to specify the aggregate prefix and a 'match ip route-source
prefix-list ...' command statement to specify the route source. If the parent
route is a default route, we can inject any route out of it.
36. A BGP neighbor cannot be configured to work with both peer groups and
peer templates. BGP peer templates and BGP peer groups are mutually
37. Peer session template can inherit only one session template directly, but peer
policy template can inherit multiple policy templates.
38. When the maximum number (as set by the â€˜neighbor â€¦ maximum-prefix ...â€™
command) of prefixes are reached, the string "PfxRcd" appears in the entry,
the neighbor goes to shutdown state, and the connection becomes idle.
39. No penalty is applied to a BGP peer reset when route dampening is enabled.
Although the reset withdraws the route, no penalty is applied in this instance.
40. In case of iBGP multipath load sharing, when multiple iBGP paths installed in
a routing table, a route reflector will advertise only one of the paths (one
41. For multiple paths to the same destination to be considered as multipaths, all
attributes including weight, local preference, autonomous system path
(entire attribute and not just length), origin code, MED, and IGP distance
must be same. But if 'bgp bestpath as-path multipath-relax' command is
configured, the AS paths still have to be the same length, but don't have to
42. Though BGP Multipath allows the installation of multiple BGP paths (for load
sharing purpose) into the IP routing table for the same prefix, it does not
affect the bestpath selection. A router still designates one of the paths as the
best path and advertises this best path to its neighbors.
43. 'neighbor â€¦ dmzlink-bw' command can be used with eBGP and iBGP
multipath features to enable unequal cost load balancing over multiple links.
BGP can originate the link bandwidth community only for directly connected
links to eBGP neighbors.
44. The 'bgp update-delay ...' command is used to tune the maximum time the
software will wait after the first neighbor is established until it starts
calculating best paths and sending out advertisements.
45. The â€œneighbor â€¦ local-as â€¦â€� command is valid only if the peer is a true eBGP
peer. It does not work for two peers in different sub-ASs in a confederation.
46. In a route-map, a continue clause can be executed, without a successful
match, if a route map entry does not contain a match clause. But if a match
clause exists, the continue clause is executed only if a match occurs. If no
successful matches occur, the continue clause is ignored. The continue
statement proceeds to the specified route map entry only after configured
set actions (if any) are performed.
47. When multiple values are configured in the same community list statement, a
logical AND condition is created. All community values must match to satisfy
an AND condition. When multiple values are configured in separate
community list statements, a logical OR condition is created. The first list that
matches a condition is processed.
48. While redistributing OSPF into BGP, by default only OSPF intra-area and
inter-area routes are redistributed into BGP.
49. When a BGP router with synchronization enabled has also a OSPF route
(redistributed from BGP) for a iBGP-learned route, then the OSPF ASBR
router-id must match the originating BGP router-id in order to synchronize
BGP route with OSPF route.
50. An â€œupdate groupâ€� is a group of peers with a common outbound policy which
will be converged as if they are in a peer-group.
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