Cisco CCNP ROUTE BGP Attributes
The above slide lists different BGP attributes.
Cisco CCNP ROUTE BGP Attributes
Continuation of BGP attributes.
Cisco CCNP ROUTE Path Selection
Like all other Routing protocols BGP uses a set of metrics to determine its
best route. BGP uses Attributes.
It evaluates the attributes in the following order:
- If next hop is inaccessible – ignore route
- Highest weight (Cisco proprietary)
- Highest local preference
- Routes originated by self
- Shortest AS path
- Lowest Origin
- Lowest Multi Exit Discriminator
- EBGP over Confederation EBGP over IBGP
- Minimum cost to Next Hop
- Neighbor that’s been up the longest
- Lowest BGP identifier
Cisco CCNP ROUTE ORIGIN
Prefixes are learned via the following methods and injected into BGP:
- • Directly connected interfaces
- • Manually configured static routes
- • Dynamic IGPs
- • Dynamic EGP
IGP (i)
- • Routes advertised via NETWORK command
- • Aggregates advertised via AGGREGATE ADDRESS
EGP (e)
- • Learned via the External Gateway Protocol
Incomplete (?)
- • Static or Dynamic Routes advertised via REDISTRIBUTE command
Cisco CCNP ROUTE AS-PATH
Sequence of autonomous systems through which this announcement has passed
AS-SEQUENCE – an ordered list of AS through which a route has passed
AS-SET – an un-ordered list of AS through which one or more individual routes of an aggregate route have passed
Originating AS adds it’s number to start the process when it passes a route to an EBGP peer
Each AS ther after PREPENDS it’s AS number as it passes it on to EBGP peers
AS-PATH is not modified when routes are passed between IBGP peers
Cisco CCNP ROUTE Route Loop Detection
Because each AS in the path has added it’s number to the AS PATH attribute, the arrival of an update already containing an AS’s number is an indication of a route loop and can be refused
Cisco CCNP ROUTE Next Hop Attribute
Cisco CCNP ROUTE Mutli Exit Discriminator
Indicates to EXTERNAL neighbors the preferred path INTO an AS
Low values are preferred
MEDs are exchanged between two AS but are not propagated to other AS
MEDs from different AS are not usually comparable but we can force this with bgp always-compare-med
In the above diagram AS 64050 wants AS 64000 to use the local link between them. Routes originating in the NY area should have a low MED when being sent on the New York link. The SF originated routes are treated the same way on the SF link to ensure traffic is routed over that link. If one side ignored MED, it would be possible to place an unfair burden for carrying long haul traffic on the side that accepted the MED values MED is usually ignored
In order to ensure that the MED value is used, configure the following IOS command:
(config-router)# bgp always-compare-med
Cisco CCNP ROUTE Local Preference
Indicates to EXTERNAL neighbors the preferred path INTO an AS
Low values are preferred
MEDs are exchanged between two AS but are not propagated to other AS
MEDs from different AS are not usually comparable but we can force this with bgp always-compare-med
In the above diagram AS 64050 wants AS 64000 to use the local link between them. Routes originating in the NY area should have a low MED when being sent on the New York link. The SF originated routes are treated the same way on the SF link to ensure traffic is routed over that link. If one side ignored MED, it would be possible to place an unfair burden for carrying long haul traffic on the side that accepted the MED values MED is usually ignored
In order to ensure that the MED value is used, configure the following IOS command:
(config-router)# bgp always-compare-med
Cisco CCNP ROUTE Local Preference
Used to set the exit point of an AS when it learns about the same route via multiple points
Value is significant only to the local AS which shares it among all IBGP sessions
High values indicate preferred exit points
Cisco CCNP ROUTE WEIGHT
The weight attribute is a Cisco proprietary attribute within BGP. The default value of weight is 0 and its range is from 0 to 65535. A change of the weight on one router will not propagate to other routers. The route with maximum weight value will be considered as the best route.
Cisco CCNP ROUTE BGP routing table
Notice the carat indicate the next hop in use. The 172.16.0.0 network has chosen the 192.168.28.1 path because of the shortest as-path. The 172.30.0.0 network has chosen the 192.168.50.1 path because of local preference.
Best (>) pathways for networks 172.16.0.0/16 and 172.24.0.0/16 have not changed.
Best (>) pathway for network 172.30.0.0 has changed to a new next hop of 192.168.28.1 due to the
next hop of 192.168.28.1 having a higher local preference, 400.
In AS 65001, the percentage of traffic going to 172.24.0.0 is 30%, 172.30.0.0 is 20%, and 172.16.0.0
is 10%.
30% of all traffic will go to the next hop of 172.20.50.1 (AS 65005), and 30% of all traffic will go to
the next hop of 192.168.28.1 (AS 65002).