Explain Border
Gateway Protocol (BGP) ?
Border Gateway Protocol advertises, learns and chooses the
best paths inside the global Internet. When two ISPs connect, they typically
use BGP to exchange routing information. Enterprises also sometimes uses BGP to
exchange routing information with ISPs, allowing the Enterprise routers to
learn Internet routes. when we have multiple Internet connections and we want
to influence some packets to take one path and some packets to take another we
use BGP.
Can Routers on
different subnet become BGP neighbors?
BGP does not require neighbors to be attached to the same
subnet. Instead, BGP routers use a TCP connection between the routers to pass
BGP messages allowing neighboring routers to be on the same or different
subnet.
What TCP port number
BGP use for connection?
BGP uses TCP port 179 for the connection.
Difference between
eBGP and iBGP neighbor?
In iBGP, neighborship is formed between routers within the
same AS (autonomous system) whereas in eBGP, neighborship is formed between
routers within different AS.
What Administrative Distance BGP uses for iBGP & eBGP ?
AD for iBGP = 200, AD for eBGP = 20.
Explain Loop
prevention mechanism in BGP?
BGP uses two mechanism to prevent loops:-
1. When a router learns routes from an iBGP peer, that
router does not advertise the same routes to another iBGP peer.
2. By using AS_PATH - When advertising to an eBGP peer, a
BGP router adds its own ASN to the AS_PATH. If a BGP router receives an update
and the route advertisement lists an AS_PATH with its own ASN, the router
ignores that route.
Note - A BGP router does not add its ASN when advertising to
an iBGP peer.
Do we need to follow
3 way handshake process to establish BGP communication?
Yes
What is the
difference between hard reset and soft reset in BGP?
In case of hard reset the local router brings down the
neighborship, brings down the underlying TCP connection and all the BGP table
entries learned from that neighbor are removed.
#clear ip bgp * command is used for hard reset.
In case of soft reset, the router does not bring down the
BGP neighborship or the underlying TCP connection. However, the local router
resends outgoing Updates and reprocesses incoming Updates adjusting the BGP
table based on the current configuration. #clear ip bgp * soft command is used
for soft reset.
What are different
BGP Message Types?
1. Open - It is Used to establish a neighbor relationship
and exchange parameters, including autonomous system number and authentication
values.
2. Keepalive - It is
Sent periodically to maintain the neighbor relationship. If the
Keepalive message is not received within the negotiated Hold timer than BGP
neighborship will be turned down.
3. Update - It exchanges Path Attributes and the associated
prefix/length (NLRI) that use those attributes.
4. Notification - It is Used to report BGP error. It results
in a reset of neighbor relationship.
Explain various
states of BGP?
1. Idle - The BGP process is either administratively down or
waiting for the the next retry attempt.
2. Connect - The BGP process is waiting for the TCP
connection to be completed. If it is successful, it will continue to the
OpenSent state. In case it fails, it will continue to the Active state.
3. Active - BGP will try another TCP three-way handshake to
establish a connection with the remote BGP neighbor. If it is successful, it
will move to the OpenSent state.
4. Opensent - The TCP connection exists, and a BGP Open
message has been sent to the peer, but the matching Open message has not yet
been received from the other router.
5. Openconfirm - An Open message has been both sent to and
received from the other router.
Next step is to receive a BGP Keepalive message (to confirm
that all neighbor-related parameters match) or a BGP Notification message (to
learn that there is some mismatch in neighbor parameters).
6. Established - All neighbor parameters matched, the
neighbor relationship has been established and the peers can now exchange
Update messages
Explain BGP Path
Attributes?
BGP supports a wide variety of Path Attributes. BGP use these
path attributes to examine the competing BGP paths (routes) in BGP table to choose the best path(route).
1. Next Hop - It lists the next-hop IP address used to reach
a prefix. If Next hop is reachable? If no route to reach Next Hop, router
cannot use this route.
2. Weight - It is a numeric value set by a router when
receiving updates to influence the route for a prefix. It is not advertised to
any BGP peers. Bigger is preferred
3. Local Preference - It is a numeric value set and
communicated within a single AS for the purpose of choosing best route for all
routers in that AS to reach a certain network. Bigger is preferred
4. Locally injected routes - Locally injected routes (routes
injected using network command) are better than iBGP/eBGP learned.
5. AS Path - It is the number of ASNs in the AS Path.
Smaller is preferred.
6. Origin - Preferred I over E & E over ?. It implies
that the route was injected into BGP as I (IGP), E (EGP) or ? (incomplete
information).
7. Multi-Exit Discriminator (MED) - Allows an AS to tell a
neighboring AS the best path to forward packets
into the first AS. Smaller is preferred.
8. Neighbor type - eBGP is preferred over iBGP.
9. IGP metric - Route
with nearest IGP neighbor (lowest IGP metric) is preferred.
10. eBGP route - Oldest (longest known) route is preferred.
11. Neighbor Router ID - Lowest is preferred.
12. Neighbor IP address - Lowest is preferred.
Trick to Remember -
N WLLA OMNI
Explain BGP Weight
attribute?
The weight attribute is a Cisco proprietary attribute that
is used in the path selection process when there is more than one route to the
same destination. A path with the Higher weight value is preferred. The default
value for weight is 0. The weight attribute is local to the router and is not
propagated to any BGP peers. Weight attribute is set by a router when receiving
Updates influencing that one router’s route for a prefix.
Explain BGP Local
preference?
Local preference is an indication to the AS about which path
has preference to exit the AS in order to reach a certain network. A path with
a higher local preference is preferred more. By default value for local
preference is 100 and can be changed manually. Unlike the weight attribute,
which is only relevant to the local router, local preference attribute is
communicated throughout a single AS for the purpose of influencing the choice
of best path to exit the AS.
Explain BGP MED?
The purpose of MED is to influence how other autonomous
systems enters into your AS to reach a certain prefix. BGP MED is an attribute
which is not propagated throughout the whole network but just to adjacent AS.
The lower the MED the more the path will be preferred.
What is Recursive
Lookup?
The router looks up the BGP route and the next hop to reach
a destination in the remote AS. Then the router looks up the route to reach the
next hop. In this way router has to perform lookup twice to reach to a
destination, this process is called recursive lookup.
What is route
reflector and why it is required?
In BGP, route learned from an iBGP neighbor will not be
advertised to another iBGP neighbor. To overcome this situation route reflector
is used. It acts as a route reflector server and makes IBGP neighbors as route
reflector clients enabling route advertisements between them.
What is the
difference between Local Preference and MED?
The Local Preference attribute is to influence your own AS
how to get or exit to another AS.
MED is to influence other AS how to enter your own AS.
What is the command
to administratively disable BGP neighborship?
# neighbor neighbor-ip shutdown
# no neighbor neighbor-ip shutdown (to enable it again)
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