Non-Transitive or Transitive? – BGP Path Attributes

ID-100217670I was reading about BGP Path Attributes and came across this table (thanks to http://netcerts.net/bgp-path-attributes-and-the-decision-process/):

LIST OF BGP PATH ATTRIBUTES
Attribute Name Category / Class
ORIGIN Well-Known Mandatory
AS_PATH Well-Known Mandatory
NEXT_HOP Well-Known Mandatory
LOCAL_PREF Well-Known Discretionary
ATOMIC_AGGREGATE Well-Known Discretionary
AGGREGATOR Optional Transitive
COMMUNITY Optional Transitive
MULTI_EXIT_DISC (MED) Optional Non-Transitive
ORIGINATOR_ID Optional Non-Transitive
CLUSTER LIST Optional Non-Transitive
MULTIPROTOCOL Reachable NLRI Optional Non-Transitive
MULTIPROTOCOL Unreachable NLRI Optional Non-Transitive

I didn’t quite understand what “non-transitive” meant, so I labbed it up to find out…

 


I am using the topology from the INE BGP course, so I figured I would change the MED and see how far the change populated.

Here is the topology (credit to Keith Barker at INE for an excellent course, and also to Tom at CCIE4all for some config: http://ccie4all.wordpress.com/2012/11/05/ccip-bgp-gns3-topology/):

BGP Topology

 

So as it stand now, R6 is advertising the route 6.6.6.0/24 via a network statement, and R8 and R9 can see this:

R8#sh ip bgp 6.6.6.0
BGP routing table entry for 6.6.6.0/24, version 8
Paths: (1 available, best #1, table Default-IP-Routing-Table)
Flag: 0x820
Advertised to update-groups:
1
3
192.168.68.6 from 192.168.68.6 (6.6.6.6)
Origin IGP, metric 0, localpref 100, valid, external, best

R9#sh ip bgp 6.6.6.0
BGP routing table entry for 6.6.6.0/24, version 8
Paths: (1 available, best #1, table Default-IP-Routing-Table)
Flag: 0x820
Advertised to update-groups:
2
3
192.168.89.8 from 192.168.89.8 (8.8.8.8)
Origin IGP, metric 0, localpref 100, valid, internal, best

On R6 I now set up a route-map to change the metric (MED) of all routes advertised to R8:

route-map CHANGEMETRIC permit 10
set metric 10

router bgp 67

neighbor 192.168.68.8 route-map CHANGEMETRIC out

I then did

clear ip bgp * soft out

to ensure the change propagated.

This showed on R8, and also on R9:

R8#sh ip bgp 6.6.6.0
BGP routing table entry for 6.6.6.0/24, version 11
Paths: (1 available, best #1, table Default-IP-Routing-Table)
Flag: 0x4820
Advertised to update-groups:
1
3
192.168.68.6 from 192.168.68.6 (6.6.6.6)
Origin IGP, metric 10, localpref 100, valid, external, best

R9#sh ip bgp 6.6.6.0
BGP routing table entry for 6.6.6.0/24, version 12
Paths: (1 available, best #1, table Default-IP-Routing-Table)
Flag: 0x4820
Advertised to update-groups:
2
3
192.168.89.8 from 192.168.89.8 (8.8.8.8)
Origin IGP, metric 10, localpref 100, valid, internal, best

Note that the metric is now 10 on both of these routers.

This proves that “non-transitive” means “will not transit AS’s” rather than routers. A router will advertised a received non-transitive attribute to it’s iBGP neighbours, but will not advertise them to it’s eBGP neighbours.

Posted in BGP, Cisco.

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