BGP
Quick Notes
Points to remember
A
transit AS is an AS that routes traffic from one external AS to another
external AS
The “show ip bgp” command is used to display
entries in the BGP routing table.
The AS-PATH attribute is used to prevent
BGP routing loops. When receiving an BGP advertisement, the router checks the
AS-PATH attribute, if it see it’s own AS number in the AS-Path then it is a
routing loop so the router will not install this route in its BGP table.
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. The default value for local preference is 100.Unlike the weight
attribute, which is only relevant to the local router, local preference is an
attribute that routers exchange in the same AS. The local preference is set
with the “bgp default local-preference value” command.
MD5 Authentication: You can
configure MD5 authentication between two BGP peers, MD5 authentication must be
configured with the same password on both BGP peers; otherwise, the connection
between them will not be made. If a router has a password configured for a
peer, but the other peer does not, a message “No MD5 digest from…” will
appear on the console while the routers attempt to establish a Multicast Source
Discovery Protocol (MSDP) session between them. Similarly, if the two routers
have different passwords configured, a message “Invalid MD5 digest from…”
will appear on the screen.
Aggregate-Address command: When
the aggregate-address command is used within BGP routing, the aggregated
address is advertised, along with the more specific routes. The exception to
this rule is through the use of the summary-only command. The “summary-only”
keyword suppresses the more specific routes and announces only the summarized
route.
The weight attribute is a special Cisco
attribute that is used in the path selection process when there is more than
one route to the same destination. The higher the weight value, the better the
path. The default weight is 0.The weight attribute is local to the router and
not propagated to other routers.
Below is the list of BGP states in order,
from startup to peering:
1 – Idle: the initial state of a BGP
connection. In this state, the BGP speaker is waiting for a BGP start event,
generally either the establishment of a TCP connection or the re-establishment
of a previous connection. Once the connection is established, BGP moves to the
next state.
2 – Connect: In this state, BGP is waiting
for the TCP connection to be formed. If the TCP connection completes, BGP will
move to the OpenSent stage; if the connection cannot complete, BGP goes to
Active
3 – Active: In the Active state, the BGP
speaker is attempting to initiate a TCP session with the BGP speaker it wants
to peer with. If this can be done, the BGP state goes to OpenSent state.
4 – OpenSent: the BGP speaker is waiting to
receive an OPEN message from the remote BGP speaker
5 – OpenConfirm: Once the BGP speaker receives
the OPEN message and no error is detected, the BGP speaker sends a KEEPALIVE
message to the remote BGP speaker
6 – Established: All of the neighbor
negotiations are complete. You will see a number (2 in this case), which tells
us the number of prefixes the router has received from a neighbor or peer
group.
Memorizing the BGP decision process steps
is very useful and you should remember them. The table below lists the complete
path selection process:
1.
Weight (Bigger is better)
2.
Local preference (Bigger is better)
3. Self
originated (Locally injected is better than iBGP/eBGP learned)
4.
AS-Path (Smaller is better)
5.
Origin
6. MED
(Smaller is better)
7.
External (Prefer eBGP over iBGP)
8. IGP
cost (Smaller is better)
9. EBGP
Peering (Older is better)
10. Router-
ID
There are 4 BGP attribute types:
Well-known Mandatory:
recognized by all implementations of BGP and must appear in a BGP update
message. If missing, a notification error will be generated.
Well-Known Discretionary:
recognized by all implementations of BGP but may not be sent in the BGP update
message (include LOCAL_PREF, ATOMIC_AGGREGATOR).
Optional Transitive: may or
may not be recognized by all BGP implementations. Because the attribute is
transitive, BGP accepts and advertises the attribute even if it is not
recognized (include Community attribte).
Optional Nontransitive: may or
may not be recognized by all BGP implementations. Whether or not the receiving
BGP router recognizes the attribute, it is nontransitive and is not passed
along to other BGP peers (include MED).
Which two conditions can cause BGP neighbor
establishment to fail?
·
There is an access list blocking all TCP traffic
between the two BGP neighbors.
·
The BGP neighbor is referencing an incorrect
autonomous system number in its neighbor statement.
Use of Static Route in BGP: When
two EBGP neighbors want to establish neighbor relationship without using the
directly connected interfaces (for example, use loopback interface), they must
tell each other how to reach their interfaces. A static route is the most
simple way to do this, especially when they are in different ASs.
In BGP,
“Connections established” is the
number of times a TCP and BGP connection have been successfully established
while “dropped” is the number of
times that a valid session has failed or been taken down.
