Wednesday 24 April 2019
Thursday 14 March 2019
BGP Series 17: Other BGP Attributes: Origin Code, IGP Metric to Next-hop, BGP Multipath :
Other BGP Attributes: Origin Code, IGP Metric to Next-hop, BGP Multipath :
Origin Code:
BGP routes with origin attribute of ‘i’ > ‘e’ > ‘?’ is the order of preference
Usually, we will never see any routes with ‘e’ as origin code since that meant the routes originated on the old EGP routing protocol which is unused
In modern scenarios, we will see either routes having Origin code as
- ‘i’ which means the routes originated via IGP and was injected into BGP via “network” command or “aggregate-address” command or “neighbor default-originate” command
- ‘?’ which means the origin is unclear as the routes were redistributed into BGP
Wednesday 6 March 2019
BGP Series 16: BGP Attributes: AS_PATH and MED
BGP Attributes: AS_PATH and MED:
.
AS_PATH Attribute:
It is a BGP attribute
Shortest length of AS_PATH is preferred when weight and local preference is same
It can be used to influence both inbound and outbound traffic
With AS_PATH prepending, we can make the AS_PATH longer for a path that is less preferred. Ideally, the local AS is prepended
Example:
In below example, we own AS-1 and both our routers ‘a’ and ‘b’ are advertising net-X to downstream neighbor...But, we want the downstream neighbor to choose ‘b’ .
Since I can’t change Weight in router ‘c’ which is not owned by me and I cant use local-pref since it is in a different AS
Thursday 28 February 2019
BGP Series 15: BGP Attributes: Weight and Local Preference
BGP Attributes: Weight and Local Preference:
.
Usually if we want to modify some attributes for best path selection, we will mosify one of these: WEIGHT, LOC_PREF, AS_PATH
Influencing BGP Weight:
It is not actually an attribute since it is not part of a bgp packet
It was previously Cisco proprietary (now used by others like Arista as well) and configured locally on a router
It is locally significant to the router and is not advertised to any other BGP neighbor as part of BGP Update
It can be used to influence the choice of outbound routes
It is set on inbound routes (.ie. for updates coming into the router, a weight is set)
Tuesday 19 February 2019
BGP Series 14: BGP Path Attributes and BGP Best Path Algorithm
BGP Path Attributes and BGP Best Path Algorithm:
.
BGP uses different path attributes to calculate the best path to a certain destination
But, they can also be used for a variety of things other than best path calculation such as AS_PATH attribute for loop-detection, …
Each path-attribute describes something about the path
The most common bgp path attributes are: